- Similarities and differences between three coexisting spaceborne radars in
global rainfall and snowfall estimation
- Authors: Guoqiang Tang; Yixin Wen, Jinyu Gao, Di Long, Yingzhao Ma, Wei Wan, Yang Hong
Abstract: Precipitation is one of the most important components in the water and energy cycles. Radars are considered the best available technology for observing the spatial distribution of precipitation either from the ground since the 1980s or from space since 1998. This study, for the first time ever, compares and evaluates the only three existing spaceborne precipitation radars, i.e., the Ku-band precipitation radar (PR), the W-band Cloud Profiling Radar (CPR), and the Ku/Ka-band Dual-frequency Precipitation Radar (DPR). The three radars are matched up globally and intercompared in the only period which they co-exist: 2014-2015. In addition, for the first time ever, TRMM PR and GPM DPR are evaluated against hourly rain gauge data in Mainland China. Results show that DPR and PR agree with each other and correlate very well with gauges in Mainland China. However, both show limited performance in the Tibetan Plateau (TP) known as the Earth's third pole. DPR improves light precipitation detectability, when compared with PR, whereas CPR performs best for light precipitation and snowfall. DPR snowfall has the advantage of higher sampling rates than CPR; however, its accuracy needs to be improved further. The future development of spaceborne radars is also discussed in two complementary categories: (1) multi-frequency radar instruments on a single platform and (2) constellations of many small cube radar satellites, for improving global precipitation estimation. This comprehensive intercomparison of PR, CPR, and DPR sheds light on spaceborne radar precipitation retrieval and future radar design.
- Modeling blowing snow accumulation downwind of an obstruction: The Ohara
Eulerian particle distribution equation
- Authors: N. J. Kinar
Abstract: An equation [Ohara, 2017] was proposed to model the height of blowing snow accumulation downwind of an obstacle such as vegetation, a snow fence, a building, or a topographic feature. The equation does not require aerodynamic flow condition parameters such as wind speed, allowing for the spatial distribution of snow to be determined at locations where meteorological data is not available. However, snow particle diffusion, drift and erosion coefficients must be estimated for application of the equation. These coefficients can be used to provide insight into the relative magnitude of blowing snow processes at a field location. Further research is required to determine efficient methods for coefficient estimation. The equation could be used with other models of wind-transported snow to predict snow accumulation downwind of an obstacle without the need for wind speed adjustments or correction equations. Applications for this equation include the design of snow fences, and the use of this equation with other hydrological models to predict snow distribution, climate change, drought, flooding, and avalanches.
- Observational breakthroughs lead the way to improved hydrological
- Authors: Dennis P. Lettenmaier
Abstract: New data sources are revolutionizing the hydrological sciences. The capabilities of hydrological models have advanced greatly over the last several decades, but until recently model capabilities have outstripped the spatial resolution and accuracy of model forcings (atmospheric variables at the land surface) and the hydrologic state variables (e.g., soil moisture; snow water equivalent) that the models predict. This has begun to change, as shown in two examples here: soil moisture and drought evolution over Africa as predicted by a hydrology model forced with satellite-derived precipitation, and observations of snow water equivalent at very high resolution over a river basin in California's Sierra Nevada.
- Solving water quality problems in agricultural landscapes: New approaches
for these nonlinear, multiprocess, multiscale systems
- Authors: Patrick Belmont; Efi Foufoula-Georgiou
Abstract: Changes in climate and agricultural practices are putting pressure on agroenvironmental systems all over the world. Predicting the effects of future management or conservation actions has proven exceptionally challenging in these complex landscapes. We present a perspective, gained from a decade of research and stakeholder involvement in the Minnesota River Basin, where research findings have influenced solutions and policy in directions not obvious at the outset. Our approach has focused on identifying places, times, and processes of accelerated change and developing reduced complexity predictive frameworks that can inform mitigation actions.
- The essential value of long-term experimental data for hydrology and water
- Authors: Doerthe Tetzlaff; Sean K. Carey, James P. McNamara, Hjalmar Laudon, Chris Soulsby
Abstract: Observations and data from long-term experimental watersheds are the foundation of hydrology as a geoscience. They allow us to benchmark process understanding, observe trends and natural cycles, and are prerequisites for testing predictive models. Long-term experimental watersheds also are places where new measurement technologies are developed. These studies offer a crucial evidence base for understanding and managing the provision of clean water supplies, predicting and mitigating the effects of floods, and protecting ecosystem services provided by rivers and wetlands. They also show how to manage land and water in an integrated, sustainable way that reduces environmental and economic costs.
- Science, politics, and rationality in a partisan era
- Authors: James W. Kirchner
Abstract: Science plays an essential role in public policy by outlining the factual foundations of policy debates. As a result, science often becomes a political football, with partisans dismissing or misrepresenting scientific findings that conflict with their political views. Here I argue that scientists can most effectively speak out, not as activists supporting particular political causes, but instead as advocates for a fundamentally rational public discourse, one that starts from the facts – not from whatever one might choose to believe – and then explores how society should respond to the challenges that they pose.
- The frontier beneath our feet
- Authors: Gordon E. Grant; William E. Dietrich
Abstract: Following the simple question as to where water goes when it rains leads to one of the most exciting frontiers in earth science: the critical zone – Earth's dynamic skin. The critical zone extends from the top of the vegetation canopy through the soil and down to fresh bedrock and the bottom of the groundwater. Only recently recognized as a distinct zone, it is challenging to study because it is hard to observe directly, and varies widely across biogeoclimatic regions. Yet new ideas, instruments, and observatories are revealing surprising and sometimes paradoxical insights, underscoring the value of field campaigns and long-term observatories. These insights bear directly on some of the most pressing societal problems today: maintaining healthy forests, sustaining streamflow during droughts, and restoring productive terrestrial and aquatic ecosystems. The critical zone is critical because it supports all terrestrial life; it is the nexus where water and carbon is cycled, vegetation (hence food) grows, soil develops, landscapes evolve, and we live. No other frontier is so close to home.
- Water and life from snow: A trillion dollar science question
- Authors: Matthew Sturm; Michael A. Goldstein, Charles Parr
Abstract: Snow provides essential resources/services in the form of water for human use, and climate regulation in the form of enhanced cooling of the Earth. In addition, it supports a thriving winter outdoor recreation industry. To date, the financial evaluation of the importance of snow is incomplete and hence the need for accelerated snow research is not as clear as it could be. With snow cover changing worldwide in several worrisome ways, there is pressing need to determine global, regional, and local rates of snow cover change, and to link these to financial analyses that allow for rational decision-making, as risks related to those decisions involve trillions of dollars.
- Geological storage of captured carbon dioxide as a large-scale carbon
- Authors: Michael A. Celia
Abstract: Carbon capture and storage, or CCS, involves capture of CO2 emissions from power plants and other large stationary sources and subsequent injection of the captured CO2 into deep geological formations. This is the only technology currently available that allows continued use of fossil fuels while simultaneously reducing emissions of CO2 to the atmosphere. Although the subsurface injection and subsequent migration of large amounts of CO2 involve a number of challenges, many decades of research in the earth sciences, focused on fluid movement in porous rocks, provides a strong foundation on which to analyze the system. These analyses indicate that environmental risks associated with large CO2 injections appear to be manageable.
- Science, society, and the coastal groundwater squeeze
- Authors: Holly A. Michael; Vincent E.A. Post, Alicia M. Wilson, Adrian D. Werner
Abstract: Coastal zones encompass the complex interface between land and sea. Understanding how water and solutes move within and across this interface is essential for managing resources for society, such as clean water, and for ecosystem conservation. The increasingly dense human occupation of coastal zones disrupts natural groundwater flow patterns and degrades freshwater resources by both over-use and pollution. This pressure results in a ‘coastal groundwater squeeze', where the thin veneers of potable freshwater are threatened by contaminant sources at the land surface and saline groundwater at depth. Scientific advances in the field of coastal hydrogeology have enabled responsible management of water resources and protection of important ecosystems. To address the problems of the future, we must continue to make scientific advances, and groundwater hydrology needs to be firmly embedded in integrated coastal zone management. This will require interdisciplinary scientific collaboration, open communication between scientists and the public, and strong partnerships with policymakers.
- The food-energy-water nexus: Transforming science for society
- Authors: Bridget R. Scanlon; Ben L. Ruddell, Patrick M. Reed, Ruth I. Hook, Chunmiao Zheng, Vince C. Tidwell, Stefan Siebert
Abstract: Emerging interdisciplinary science efforts are providing new understanding of the interdependence of food, energy, and water (FEW) systems. These science advances, in turn, provide critical information for coordinated management to improve the affordability, reliability, and environmental sustainability of FEW systems. Here we describe the current state of the FEW nexus and approaches to managing resource conflicts through reducing demand and increasing supplies, storage, and transport. Despite significant advances within the past decade, there are still many challenges for the scientific community. Key challenges are the need for interdisciplinary science related to the FEW nexus; ground-based monitoring and modeling at local-to-regional scales; incorporating human and institutional behavior in models; partnerships among universities, industry, and government to develop policy relevant data; and systems modeling to evaluate tradeoffs associated with FEW decisions.
- Lattice Boltzmann simulation of immiscible two-phase flow with capillary
valve effect in porous media
- Authors: Zhiyuan Xu; Haihu Liu, Albert J. Valocchi
Abstract: A new algorithm for imposing the contact angle on solid surfaces is proposed in the Lattice Boltzmann color-gradient model. The capability and accuracy of this algorithm are validated by simulation of contact angles for a droplet resting on a flat surface and on a cylindrical surface. The color-gradient model with the proposed contact angle algorithm is then used to study the capillary valve effect in porous media. As a preliminary study, the capillary valve effect is explained by simulating immiscible two-phase displacement within a single-pore geometry. It is shown that the capillary valve effect is accurately captured by the present simulations. Further simulations of drainage and imbibition are also conducted to understand the capillary valve effect in an experiment-matched pore network micromodel. The simulated results are found to agree quantitatively with the experimental results reported in literature, except for a few differences which result from the exclusion of contact angle hysteresis in the proposed algorithm.
- A modeling approach to identify the effective forcing exerted by wind on a
pre-alpine lake surrounded by a complex topography
- Authors: G. Valerio; A. Cantelli, P. Monti, G. Leuzzi
Abstract: The representation of spatial wind distribution is recognized as a serious difficulty when modeling the hydrodynamics of lakes surrounded by a complex topography. To address this issue, we propose to force a 3D lake model with the wind field simulated by a high-resolution atmospheric model, considering as a case study a 61 km2 pre-alpine lake surrounded by mountain ranges that reach 1800 m above the lake's surface, where a comprehensive data set was available in the stratified season. The improved distributed description of the wind stress over the lake surface led to a significant enhancement in the representation of the main basin-scale internal wave motions, and hence provided a reference solution to test the use of simplified approaches. Moreover, the analysis of the power exerted by the computed wind field enabled us to identify measuring stations that provide suitable wind data to be applied uniformly on the lake surface in long-term simulations. Accordingly, the proposed methodology can contribute to reducing the uncertainties associated with the definition of wind forcing for modeling purposes and can provide a rational criterion for installing representative measurement locations in pre-alpine lakes.
- Machine learning algorithms for modeling groundwater level changes in
agricultural regions of the United States
- Authors: S. Sahoo; T. A. Russo, J. Elliott, I. Foster
Abstract: Climate, groundwater extraction, and surface water flows have complex nonlinear relationships with groundwater level in agricultural regions. To better understand the relative importance of each driver, and predict groundwater level change, we develop a new ensemble modeling framework based on spectral analysis, machine learning, and uncertainty analysis, as an alternative to complex and computationally expensive physical models. We apply and evaluate this new approach in the context of two aquifer systems supporting agricultural production in the United States: the High Plains aquifer (HPA) and the Mississippi River Valley alluvial aquifer (MRVA). We select input datasets by using a combination of mutual information, genetic algorithms, and lag analysis, and then use the selected datasets in a Multilayer Perceptron network architecture to simulate seasonal groundwater level change. As expected, model results suggest that irrigation demand has the highest influence on groundwater level change for a majority of the wells. The subset of groundwater observations not used in model training or cross-validation correlates strongly (R > 0.8) with model results for 88% and 83% of the wells in the HPA and MRVA, respectively. In both aquifer systems, the error in the modeled cumulative groundwater level change during testing (2003 to 2012) was less than 2 m over a majority of the area. We conclude that our modeling framework can serve as an alternative approach to simulating groundwater level change and water availability, especially in regions where subsurface properties are unknown.
- Changes in floodplain inundation under nonstationary hydrology for an
adjustable, alluvial river channel
- Authors: B.C. Call; P. Belmont, J.C. Schmidt, P. R. Wilcock
Abstract: Predicting the frequency and aerial extent of flooding in river valleys is essential for infrastructure design, environmental management, and risk assessment. Conventional flood prediction relies on assumptions of stationary flood distributions and static channel geometries. However, non-stationary flow regimes are increasingly observed and changes in flow and/or sediment supply are known to alter the geometry and flood conveyance of alluvial channels. Systematic changes in flows and/or channel geometry may amplify or attenuate the frequency and/or extent of flood inundation in unexpected ways. We present a stochastic, reduced complexity model to investigate such dynamics. The model routes a series of annual peak-discharges through a simplified reach-averaged channel-floodplain cross-section. Channel width, depth, and slope are permitted to adjust annually by a user-specified fraction towards equilibrium geometries predicted based on each year's peak-discharge and sediment supply. Modeled channel adjustments are compared with empirical observations for two rivers in Minnesota, USA that have experienced multiple large floods over the past six-years. The model is then run using six-hypothetical scenarios simulating non-stationary flow regimes with temporal adjustments in the mean and/or variance of the governing peak-flow distributions. Each scenario is run repeatedly while varying parameters that control the amount of fractional adjustment that channel geometries can make annually. Results indicate that the intra-annual mean horizontal width of floodplain inundation primarily depends on the governing peak-flow distribution's coefficient of variation, but the intra-annual frequency of floodplain inundation (i.e. the fraction of modeled years with inundation) primarily depends on the amount of channel adjustment permitted annually.
- Big ship data: Using vessel measurements to improve estimates of
temperature and wind speed on the Great Lakes
- Authors: Kevin Fries; Branko Kerkez
Abstract: The sheer size of many water systems challenges the ability of in-situ sensor networks to resolve spatiotemporal variability of hydrologic processes. New sources of vastly distributed and mobile measurements are, however, emerging to potentially fill these observational gaps. This paper poses the question: How can non-traditional measurements, such as those made by volunteer ship captains, be used to improve hydrometeorological estimates across large surface water systems? We answer this question through the analysis of one of the largest such data sets: an unprecedented collection of one million unique measurements made by ships on the North American Great Lakes from 2006-2014. We introduce a flexible probabilistic framework, which can be used to integrate ship measurements, or other sets of irregular point measurements, into contiguous datasets. The performance of this framework is validated through the development of a new ship-based spatial data product of water temperature, air temperature and wind speed across the Great Lakes. An analysis of the final data product suggests that the availability of measurements across the Great Lakes will continue to play a large role in the confidence with which these large surface water systems can be studied and modeled. We discuss how this general and flexible approach can be applied to similar data sets, and how it will be of use to those seeking to merge large collections of measurements with other sources of data, such as physical models or remotely sensed products.
- Implementation of a physiographic complexity-based multiresolution snow
- Authors: Elisabeth Baldo; Steven A. Margulis
Abstract: Using a uniform model resolution over a domain is not necessarily the optimal approach for simulating hydrologic processes when considering both model error and computational cost. Fine -resolution simulations at 100 m or less can provide fine-scale process representation, but can be costly to apply over large domains. On the other hand, coarser spatial resolutions are more computationally inexpensive, but at the expense of fine-scale model accuracy. Defining a multi-resolution (MR) grid spanning from fine-resolutions over complex mountainous areas to coarser resolutions over less complex regions can conceivably reduce computational costs, while preserving the accuracy of fine-resolution simulations on a uniform grid. A MR scheme was developed using a physiographic complexity metric (CM) that combines surface heterogeneity in forested fraction, elevation, slope and aspect. A data reduction term was defined as a metric (relative to a uniform fine-resolution grid) related to the available computational resources for a simulation. The focus of the effort was on the melt season where physiographic complexity is known to have a significant signature. MR simulations were run for different data reduction factors to generate melt rate estimates for three representative water years over a test headwater catchment in the Colorado River Basin. The MR approach with data reductions up to 47% led to negligible cumulative snowmelt differences compared to the fine-resolution baseline case, while tests with data reductions up to 60% showed differences lower than 2%. Large snow-dominated domains could therefore benefit from a MR approach to be more efficiently simulated while mitigating error.
- Physical effects of thermal pollution in lakes
- Authors: Love Råman Vinnå; Alfred Wüest, Damien Bouffard
Abstract: Anthropogenic heat emissions into inland waters influence water temperature and affect stratification, heat and nutrient fluxes, deep-water renewal and biota. Given the increased thermal stress on these systems by growing cooling demands of riparian/costal infrastructures in combination with climate warming, the question arises on how to best monitor and manage these systems. In this study, we investigate local and system-wide physical effects on the medium-sized perialpine Lake Biel (Switzerland), influenced by point-source cooling-water emission from an upstream nuclear power plant (heat emission ∼700 MW, ∼18 W m−2 lake-wide). We use one-dimensional (SIMSTRAT) and three-dimensional (Delft3D-Flow) hydrodynamic numerical simulations and provide model resolution guidelines for future studies of thermal pollution. The effects on Lake Biel by the emitted excess heat are summarized as: (i) clear seasonal trend in temperature increase, locally up to 3.4°C and system-wide volume-mean ∼0.3°C, which corresponds to one decade of regional surface water climate warming, (ii) the majority of supplied thermal pollution (∼60%) leaves this short residence time (∼58 days) system via the main outlet, whereas the remaining heat exits to the atmosphere, (iii) increased length of stratified period due to the stabilizing effects of additional heat, (iv) system-wide effects such as warmer temperature, prolonged stratified period and river-caused epilimnion flushing are resolved by both models whereas local raised temperature and short-circuiting was only identifiable with the three-dimensional model approach. This model-based method provides an ideal tool to assess man-made impacts on lakes and their downstream outflows.
- Time-dependent velocity-field controls on anomalous chemical transport in
- Authors: Alon Nissan; Ishai Dror, Brian Berkowitz
Abstract: Temporal variations in the subsurface velocity field are often (if not always) present in the real world to at least some degree. However, an accounting of their effects on chemical transport has been largely neglected. Here, we demonstrate experimentally effects of a time-varying velocity field on conservative chemical tracer transport in porous media, as compared to constant velocity conditions. We find that velocity-field fluctuations increase chemical tracer spreading and residence time, which intensify the anomalous nature of the transport. This behavior is modeled by a continuous time random walk particle tracking method formulated to account for time-dependent velocity fields. The model matches the experimental results with a parsimonious and consistent set of parameters. The model is then applied to study the effects of different magnitudes in velocity-field fluctuations, as well as different degrees of porous media heterogeneity, on 1-D and 2-D spatiotemporal propagation of an injected, point source, chemical plume. Increased intensity of velocity-field fluctuations, and increased porous medium heterogeneity, each serve to increase the extent of chemical spreading and anomalous behavior.
- An efficient and stable hydrodynamic model with novel source term
discretisation schemes for overland flow simulations
- Authors: Xilin Xia; Qiuhua Liang, Xiaodong Ming, Jingming Hou
Abstract: Numerical models solving the full 2D shallow water equations (SWEs) have been increasingly used to simulate overland flows and better understand the transient flow dynamics of flash floods in a catchment. However, there still exists key challenges that have not yet been resolved for the development of fully dynamic overland flow models, related to 1) the difficulty of maintaining numerical stability and accuracy in the limit of disappearing water depth; and 2) inaccurate estimation of velocities and discharges on slopes as a result of strong nonlinearity of friction terms. This paper aims to tackle these key research challenges and present a new numerical scheme for accurately and efficiently modelling large-scale transient overland flows over complex terrains. The proposed scheme features a novel surface reconstruction method (SRM) to correctly compute slope source terms and maintain numerical stability at the small water depth condition, and a new implicit discretisation method to handle highly nonlinear friction terms. The resulting shallow water overland flow model is first validated against analytical and experimental test cases and then applied to simulate a hypothetic rainfall event on the 42 km2 Haltwhistle Burn, UK.
- Mineralogical and transport controls on the evolution of porous media
texture using direct numerical simulation
- Authors: Sergi Molins; David Trebotich, Gregory H. Miller, Carl I. Steefel
Abstract: The evolution of porous media due to mineral dissolution and precipitation can change the bulk properties of subsurface materials. The pore-scale structure of the media, including its physical and mineralogical heterogeneity, exerts controls on porous media evolution via transport limitations to reactive surfaces and mineral accessibility. Here we explore how these controls affect the evolution of the texture in porous media at the pore scale. For this purpose, a pore-scale flow and reactive transport model is developed that explicitly tracks mineral surfaces as they evolve using a direct numerical simulation approach. Simulations of dissolution in single-mineral domains provide insights into the transport controls at the pore scale, while the simulation of a fracture surface composed of bands of faster-dissolving calcite and slower-dissolving dolomite provides insights into the mineralogical controls on evolution. Transport-limited conditions at the grain-pack scale may result in unstable evolution, a situation in which dissolution is focused in a fast-flowing, fast-dissolving path. Due to increasing velocities, the evolution in these regions is like that observed under conditions closer to strict surface control at the pore scale. That is, grains evolve to have oblong shapes with their long dimensions aligning with the local flow directions. Another example of an evolving reactive transport regime that affects local rates is seen in the evolution of the fracture surface. As calcite dissolves, the diffusive length between the fracture flow path and the receding calcite surfaces increases. Thus, the calcite dissolution reaction becomes increasingly limited by diffusion.
- Continuous monitoring of snowpack dynamics in alpine terrain by
above-ground neutron sensing
- Authors: Paul Schattan; Gabriele Baroni, Sascha E. Oswald, Johannes Schöber, Christine Fey, Christoph Kormann, Matthias Huttenlau, Stefan Achleitner
Abstract: The characteristics of an above-ground cosmic-ray neutron sensor (CRNS) are evaluated for monitoring a mountain snowpack in the Austrian Alps from 03/2014 to 06/2016. Neutron counts were compared to continuous point-scale snow depth (SD) and snow-water-equivalent (SWE) measurements from an automatic weather station with a maximum SWE of 600 mm (04/2014). Several spatially distributed Terrestrial Laser Scanning (TLS) based SD and SWE maps were additionally used. A strong non-linear correlation is found for both SD and SWE. The representative footprint of the CRNS is in the range of 230 to 270 m. In contrast to previous studies suggesting signal saturation at around 100 mm of SWE, no complete signal saturation was observed. These results imply that CRNS could be transferred into an unprecedented method for continuous detection of spatially-averaged SD and SWE for alpine snowpacks, though with sensitivity decreasing with increasing SWE. While initially different functions were found for accumulation and melting season conditions, this could be resolved by accounting for a limited measurement depth. This depth limit is in the range of 200 mm of SWE for dense snowpacks with high liquid water contents and associated snow density values around 450 kg m−3 and above. In contrast to prior studies with shallow snowpacks, inter-annual transferability of the results is very high regardless of pre-snowfall soil moisture conditions. This underlines the unexpectedly high potential of CRNS to close the gap between point-scale measurements, hydrological models and remote sensing of the cryosphere in alpine terrain.
- Characterizing landscape-scale erosion using 10Be in detrital fluvial
sediment: Slope-based sampling strategy detects the effect of widespread
- Authors: Lucas J. Reusser; Paul R. Bierman, Donna M. Rizzo, Eric W. Portenga, Dylan H. Rood
Abstract: Concentrations of in situ 10Be measured in detrital fluvial sediment are frequently used to estimate long-term erosion rates of drainage basins. In many regions, basin-averaged erosion rates are positively correlated with basin average slope. The slope dependence of erosion allows model-based erosion rate estimation for unsampled basins and basins where human disturbance may have biased cosmogenic nuclide concentrations in sediment.Using samples collected from southeastern North America, we demonstrate an approach that explicitly considers the relationship between average basin slope and erosion rate. Because dams and reservoirs are ubiquitous on larger channels in the field area, we selected 36 undammed headwater sub-basins (average area 10.6 km2) from which we collected river sand samples and measured 10Be concentrations. We used these data to train a predictive model that relates average basin slope and 10Be-inferred erosion rate.Applying our model to 28 basins in the same region previously studied with 10Be, we find it successfully predicts erosion rates for basins of different sizes if they are undammed or if samples were collected >25 km downstream of dams. For samples collected closer to dams, measured erosion rates exceed modeled erosion rates for two-thirds of the samples. In three of four cases where paired samples were collected upstream of reservoirs and downstream of the impounding dam, 10Be concentrations were lower downstream. This finding has implications for detrital cosmogenic studies, whether or not samples were collected directly downstream of dams, because dams obstruct most major rivers around the world, effectively trapping sediment that originated upstream.
- Ambient groundwater flow diminishes nitrate processing in the hyporheic
zone of streams
- Authors: Morvarid Azizian; Fulvio Boano, Perran L.M. Cook, Russell L. Detwiler, Megan A. Rippy, Stanley B. Grant
Abstract: Modeling and experimental studies demonstrate that ambient groundwater flow reduces the flux of water through the hyporheic zone, but the implications of this observation for stream N-cycling is not yet clear. Here we utilize a simple process-based model (the Pumping and Streamline Segregation or PASS model) to evaluate N-cycling over two scales of hyporheic exchange (fluvial ripples and riffle-pool sequences), ten ambient groundwater and stream flow scenarios (five gaining and losing conditions and two stream discharges), and three biogeochemical settings (identified based on a principal component analysis of previously published measurements in streams throughout the U.S.). Model-data comparisons indicate that our model provides realistic estimates for direct denitrification of stream nitrate, but over-predicts nitrification and coupled nitrification-denitrification. Riffle-pool sequences are responsible for most of the N-processing, despite the fact that fluvial ripples generate 3 to 11 times more hyporheic exchange flux. Across all scenarios, hyporheic exchange flux and the Damköhler Number emerge as primary controls on stream N-cycling; the former regulates trafficking of nutrients and oxygen across the sediment-water interface, while the latter quantifies the relative rates of organic carbon mineralization and advective transport in streambed sediments. Vertical groundwater flux modulates both of these master variables in ways that tend to diminish stream N-cycling. Thus, anthropogenic perturbations of ambient groundwater flows (e.g., by urbanization, agricultural activities, groundwater mining, and/or climate change) may compromise some of the key ecosystem services provided by streams.
- A Kolmogorov-Brutsaert structure function model for evaporation into a
- Authors: Gabriel Katul; Heping Liu
Abstract: In 1965, Brutsaert proposed a model that predicted mean evaporation rate E¯ from rough surfaces to scale with the 3/4 power-law of the friction velocity (u*) and the square-root of molecular diffusivity (Dm) for water vapor. In arriving at these results, a number of assumptions were made regarding the surface renewal rate describing the contact durations between eddies and the evaporating surface, the diffusional mass process from the surface into eddies, and the cascade of turbulent kinetic energy sustaining the eddy renewal process itself. The working hypothesis explored here is that E¯∼Dmu*3/4 is a direct outcome of the Kolmogorov scaling for inertial subrange eddies modified to include viscous-cutoff thereby by-passing the need for a surface renewal assumption. It is demonstrated that Brutsaert's model for E¯ may be more general than its original derivation implied.
- Noble gas signatures in the Island of Maui, Hawaii: Characterizing
groundwater sources in fractured systems
- Authors: Yi Niu; M. Clara Castro, Chris M. Hall, Stephen B. Gingerich, Martha A. Scholl, Rohit B. Warrier
Abstract: Uneven distribution of rainfall and freshwater scarcity in populated areas in the Island of Maui, Hawaii, renders water resources management a challenge in this complex and ill-defined hydrological system. A previous study in the Galapagos Islands suggests that noble gas temperatures (NGTs) record seasonality in this fractured, rapid infiltration groundwater system rather than the commonly observed mean annual air temperature (MAAT) in sedimentary systems where infiltration is slower thus, providing information on recharge sources and potential flow paths. Here, we report noble gas results from the basal aquifer, springs, and rainwater in Maui to explore the potential for noble gases in characterizing these complex fractured hydrologic systems. Most samples display a mass-dependent depletion pattern with respect to surface conditions consistent with previous observations both in the Galapagos Islands and Michigan rainwater. Basal aquifer and rainwater noble gas patterns are similar and suggest direct, fast recharge from precipitation to the basal aquifer. In contrast, multiple springs, representative of perched aquifers, display highly variable noble gas concentrations suggesting recharge from a variety of sources. The distinct noble gas patterns for the basal aquifer and springs suggest that basal and perched aquifers are separate entities. Maui rainwater displays high apparent NGTs, incompatible with surface conditions, pointing either to an origin at high altitudes with the presence of ice or an ice-like source of undetermined origin. Overall, noble gas signatures in Maui reflect the source of recharge rather than the expected altitude/temperature relationship commonly observed in sedimentary systems.
- Little impact of Three Gorges Dam on recent decadal lake decline across
China's Yangtze Plain
- Authors: Jida Wang; Yongwei Sheng, Yoshihide Wada
Abstract: The ubiquitous lakes across China's Yangtze Plain (YP) are indispensable freshwater resources sustaining ecosystems and socioeconomics for nearly half a billion people. Our recent survey revealed a widespread net decline in the total YP lake inundation area during 2000–2011 (a cumulative decrease of ∼10%), yet its mechanism remains contentious. Here, we uncover the impacts of climate variability and anthropogenic activities including i) Yangtze flow and sediment alterations by the Three Gorges Dam (TGD) and ii) human water consumption in agricultural, industrial, and domestic sectors throughout the downstream Yangtze Basin. Results suggest that climate variability is the dominant driver of this decadal lake decline, whereas studied human activities, despite varying seasonal impacts that peak in fall, contribute marginal fraction (∼10–20% or less) to the interannual lake area decrease. Given that the TGD impacts on the total YP lake area and its seasonal variation are both under ∼5%, we also dismiss the speculation that the TGD might be responsible for evident downstream climate change by altering lake surface extent and thus open water evaporation. Nevertheless, anthropogenic impacts exhibited a strengthening trend during the past decade. Although the TGD has reached its full-capacity water regulation, the negative impacts of human water consumption and TGD-related net channel erosion are already comparable to that of TGD's flow regulation, and may continue to grow as crucial anthropogenic factors to future YP lake conservation.
- Early formation of preferential flow in a homogeneous snowpack observed by
- Authors: Francesco Avanzi; Giacomo Petrucci, Margret Matzl, Martin Schneebeli, Carlo De Michele
Abstract: We performed X-ray microtomographic observations of wet-snow metamorphism during controlled continuous melting and melt-freeze events in the laboratory. Three blocks of snow were sieved into boxes and subjected to cyclic, superficial heating or heating-cooling to reproduce vertical water infiltration patterns in snow similarly to natural conditions. Periodically, samples were taken at different heights and scanned. Results suggest that wet-snow metamorphism dynamics are highly heterogeneous even in an initially homogeneous snowpack. Consistent with previous work, we observed an increase with time in the thickness of the ice structure, which is a measure of grain size. However, this was coupled with large temporal scatter between consecutive measurements of the specific surface area and of the statistical moments of grain thickness distributions. Because of marked differences in the right tail, grain thickness distributions did not show shape invariance with time, contrary to previous analyses. In our experiments, wet-snow metamorphism showed two strikingly different patterns: homogeneous coarsening superimposed by faster heterogeneous coarsening in areas that were affected by preferential percolation of water. Liquid water movement in snow and fast structural evolution may be thus intrinsically coupled by early formation of preferential flow at local scale. These observations suggest that further experiments are highly needed to fully understand wet-snow metamorphism and infiltration patterns in a natural snowpack.
- Development of an experimental approach to study coupled
soil-plant-atmosphere processes using plant analogs
- Authors: Andrew C. Trautz; Tissa H. Illangasekare, Ignacio Rodriguez-Iturbe, Katharina Heck, Rainer Helmig
Abstract: The atmosphere, soils, and vegetation near the land-atmosphere interface are in a state of continuous dynamic interaction via a myriad of complex interrelated feedback processes which collectively, remain poorly understood. Studying the fundamental nature and dynamics of such processes in atmospheric, ecological, and/or hydrological contexts in the field setting presents many challenges; current experimental approaches are an important factor given a general lack of control and high measurement uncertainty. In an effort to address these issues and reduce overall complexity, new experimental design considerations (2-dimensional intermediate-scale coupled wind tunnel-synthetic aquifer testing using synthetic plants) for studying soil-plant-atmosphere continuum soil moisture dynamics are introduced and tested in this study. Validation of these experimental considerations, particularly the adoption of synthetic plants, is required prior to their application in future research. A comparison of three experiments with bare soil surfaces or transplanted with a Stargazer lily/limestone block, were used to evaluate the feasibility of the proposed approaches. Results demonstrate that coupled wind tunnel-porous media experimentation, used to simulate field conditions, reduces complexity and enhances control while allowing fine spatial-temporal resolution measurements to be made using state-of-the-art technologies. Synthetic plants further help reduce system complexity (e.g. airflow) while preserving the basic hydrodynamic functions of plants (e.g. water uptake and transpiration). The trends and distributions of key measured atmospheric and subsurface spatial and temporal variables (e.g. soil moisture, relative humidity, temperature, air velocity) were comparable, showing that synthetic plants can be used as simple, idealized, non-biological analogs for living vegetation in fundamental hydrodynamic studies.
- A shallow water table fluctuation model in response to precipitation with
consideration of unsaturated gravitational flow
- Authors: Jina Jeong; Eungyu Park
Abstract: A precise estimation of groundwater fluctuation is studied by considering delayed recharge flux (DRF) and unsaturated zone drainage (UZD). Both DRF and UZD are due to gravitational flow impeded in the unsaturated zone, which may nonnegligibly affect groundwater level changes. In the validation, a previous model without the consideration of unsaturated flow is benchmarked. The model is calibrated using multi-year groundwater data, and consistent model parameter statistics are obtained and validated. The estimation capability of the new model is superior to the benchmarked model as indicated by the significantly improved representation of groundwater level with physically interpretable model parameters.
- Mixing layer and coherent structures in compound channel flows: Effects of
transverse flow, velocity ratio, and vertical confinement
- Authors: S. Proust; J. N. Fernandes, J.B. Leal, N. Rivière, Y. Peltier
Abstract: Turbulent mixing layers associated with streamwise uniform and non-uniform flows in compound channels (main channel with adjacent floodplains) are experimentally investigated. The experiments start with uniform flow conditions. The streamwise non-uniformity is then generated by imposing an imbalance in the upstream discharge distribution between main channel (MC) and floodplains (FPs), keeping the total discharge constant, which results in a transverse depth-averaged mean flow. This study firstly aims at assessing the effect of a transverse flow on the mixing layer and coherent structures that form at the MC/FP interfaces. A wide range of initial velocity ratio or dimensionless shear between MC and FP is tested. The study secondly aims at assessing the effect of this velocity ratio on the mixing layer, for a fixed vertical confinement of flow. The total discharge was then varied to quantify the confinement effect. The results show that, far from the inlet section, Reynolds-stresses increase with local velocity ratio for a fixed confinement, and decrease with confinement for a fixed velocity ratio. It is also shown that, irrespective of confinement, the existence of quasi-two-dimensional coherent structures is driven by velocity ratio and the direction and magnitude of transverse flow. These structures cannot develop if velocity ratio is lower than 0.3 and if a strong transverse flow towards the MC occurs. In the latter case, the transverse flow is the predominant contribution to momentum exchange (compared with turbulent mixing and secondary currents), convex mean velocity profiles are observed, preventing the formation of quasi-two-dimensional structures.
- Multiobjective reservoir operating rules based on cascade reservoir input
variable selection method
- Authors: Guang Yang; Shenglian Guo, Pan Liu, Liping Li, Chongyu Xu
Abstract: The input variable selection in multi-objective cascade reservoir operation is an important and difficult task. To address this problem, this study proposes the cascade reservoir input variable selection (CIS) method that searches for the most valuable input variables for decision-making in multiple-objectivity cascade reservoir operations. From a case study of Hanjiang cascade reservoirs in China, we derive reservoir operating rules based on the combination of CIS and Gaussian radial basis functions (RBFs) methods, and optimize the rules through Pareto-archived dynamically dimensioned search (PA-DDS) with two objectives: to maximize both power generation and water supply. We select the most effective input variables and evaluate their impacts on cascade reservoir operations. From the simulated trajectories of reservoir water level, power generation, and water supply, we analyze the multi-objective operating rules with several input variables. The results demonstrate that the CIS method performs well in the selection of input variables for the cascade reservoir operation, and the RBFs method can fully express the non-linear operating rules for cascade reservoirs. We conclude that the CIS method is an effective and stable approach to identifying the most valuable information from a large number of candidate input variables. While the reservoir storage state is the most valuable information for the Hanjiang cascade reservoir multi-objective operation, the reservoir inflow is the most effective input variable for the single-objective operation of Danjiangkou.
- Uncertainty analysis and risk-based design of detention basin without
- Authors: Yeou-Koung Tung
Abstract: Risk-based analysis provides an economically defensible framework for determining the optimal design of hydrosystems with the minimum total cost including project cost (installation plus operation/maintenance/repair) and failure induced expected damage cost. However, failure related damage function with good quality may not be widely available in practical applications for assessing annual expected damage cost. In addition to aleatory uncertainty representing natural randomness of hydrologic events, there exists a variety of epistemic uncertainties due to knowledge deficiency from the use of inadequate models, inaccurate model parameters, etc. The presence of epistemic uncertainties could affect the loads and capacity of hydrosystem facilities which, in turn, would affect the value of failure induced physical performance indicators. Using detention basin design as an example, this paper presents a systematic framework to integrate aleatory and epistemic uncertainties for the risk-based design under the condition of no monetary damage function. For illustration, aleatory uncertainty due to randomness of rainfall intensity and epistemic uncertainties caused by runoff coefficient and curve number are considered in risk-based design of an example detention basin.
- Lagrangian scheme to model subgrid-scale mixing and spreading in
heterogeneous porous media
- Authors: P. A. Herrera; J. M. Cortínez, A. J. Valocchi
Abstract: Small-scale heterogeneity of permeability controls spreading, dilution, and mixing of solute plumes at large scale. However, conventional numerical simulations of solute transport are unable to resolve scales of heterogeneity below the grid-scale. We propose a Lagrangian numerical approach to implement closure models to account for subgrid-scale spreading and mixing in Darcy-scale numerical simulations of solute transport in mildly heterogeneous porous media. The novelty of the proposed approach is that it considers two different dispersion coefficients to account for advective spreading mechanisms and local-scale dispersion. Using results of benchmark numerical simulations we demonstrate that the proposed approach is able to model subgrid-scale spreading and mixing provided there is a correct choice of block-scale dispersion coefficient. We also demonstrate that for short travel times it is only possible to account for spreading or mixing using a single block-scale dispersion coefficient. Moreover, we show that it is necessary to use time-dependent dispersion coefficients to obtain correct mixing rates. On the contrary, for travel times that are large in comparison to the typical dispersive time scale, it is possible to use a single expression to compute the block-dispersion coefficient, which is equal to the asymptotic limit of the block-scale macro-dispersion coefficient proposed by Rubin et al. . Our approach provides a flexible and efficient way to model subgrid-scale mixing in numerical models of large-scale solute transport in heterogeneous aquifers. We expect that these findings will help to better understand the applicability of the advection-dispersion-equation (ADE) to simulate solute transport at the Darcy-scale in heterogeneous porous media.
- Lagrangian simulation of mixing and reactions in complex geochemical
- Authors: Nicholas B. Engdahl; David A. Benson, Diogo Bolster
Abstract: Simulations of detailed geochemical systems have traditionally been restricted to Eulerian reactive transport algorithms. This note introduces a Lagrangian method for modeling multi-component reaction systems. The approach uses standard random walk based methods for the particle motion steps but allows the particles to interact with each other by exchanging mass of their various chemical species. The colocation density of each particle pair is used to calculate the mass transfer rate, which creates a local disequilibrium that is then relaxed back toward equilibrium using the reaction engine PhreeqcRM. The mass exchange is the only step where the particles interact and the remaining transport and reaction steps are entirely independent for each particle. Several validation examples are presented, which reproduce well-known analytical solutions. These are followed by two demonstration examples of a competitive decay chain and an acid-mine drainage system. The source code, entitled Complex Reaction on Particles (CRP), and files needed to run these examples are hosted openly on GitHub (https://github.com/nbengdahl/CRP), so as to enable interested readers to readily apply this approach with minimal modifications.
- Let hydrologists learn the latest computer science by working with
Research Software Engineers (RSEs) and not reinvent the waterwheel
ourselves. A comment to “Most Computational Hydrology is not
Reproducible, so is it Really Science?”
- Authors: R. W. Hut; N. C. van de Giesen, N. Drost
Abstract: The suggestions by Hutton et al. might not be enough to guarantee reproducible computational hydrology. Archiving software code and research data alone will not be enough. We add to the suggestion of Hutton et al. that hydrologists not only document their (computer) work, but that hydrologists use the latest best practices in designing research software, most notably the use of containers and open interfaces. To make sure hydrologists know of these best practices we urge close collaboration with Research Software Engineers (RSEs).
- Numerical simulation of backward erosion piping in heterogeneous fields
- Authors: Yue Liang; Tian-Chyi Jim Yeh, Yu-Li Wang, Mingwei Liu, Junjie Wang, Yonghong Hao
Abstract: Backward erosion piping (BEP) is one of the major causes of seepage failures in levees. Seepage fields dictate the BEP behaviors and are influenced by the heterogeneity of soil properties. To investigate the effects of the heterogeneity on the seepage failures, we develop a numerical algorithm and conduct simulations to study BEP progressions in geologic media with spatially stochastic parameters. Specifically, the void ratio e, the hydraulic conductivity k, and the ratio of the particle contents r of the media are represented as the stochastic variables. They are characterized by means and variances, the spatial correlation structures, and the cross-correlation between variables. Results of the simulations reveal that the heterogeneity accelerates the development of preferential flow paths, which profoundly increase the likelihood of seepage failures. To account for unknown heterogeneity, we define the probability of the seepage instability (PI) to evaluate the failure potential of a given site. Using Monte-Carlo simulation (MCS), we demonstrate that the PI value is significantly influenced by the mean and the variance of lnk and its spatial correlation scales. While the other parameters, such as means and variances of e and r, and their cross-correlation, have minor impacts. Based on PI analyses, we introduce a risk rating system to classify the field into different regions according to risk levels. This rating system is useful for seepage failures prevention and assists decision making when BEP occurs.
- A fully subordinated linear flow model for hillslope subsurface stormflow
- Authors: Yong Zhang; Boris Baeumer, Li Chen, Donald M. Reeves, HongGuang Sun
Abstract: Hillslope subsurface stormflow exhibits complex patterns when natural soils with multi-scale heterogeneity impart a spatiotemporally nonlocal memory on flow dynamics. To efficiently quantify such nonlocal flow responses, this technical note proposes a fully subordinated flow (FSF) equation where the time- and flow-subordination capture the temporal and spatial memory, respectively. Results show that the time-subordination component of the FSF model captures a wide range of delayed flow response due to various degrees of soil heterogeneity (especially for low-conductivity zones), while the model's flow-subordination term accounts for the rapid flow responses along preferential flow paths. In the FSF model, parameters defining spatiotemporal memory functions may be related to soil properties, while other parameters such as scalar factors controlling the overall advection and diffusion are difficult to predict and can be estimated from subsurface stormflow hydrographs. These parameters can be constants at the hillslope scale because the spatiotemporal subordination, an upscaling technique, can capture the impact of system heterogeneity on flow dynamics, leading to a linear FSF model that might be applicable for various slopes. Valid scale, limitation and extension of the FSF model, and modification of the model for other complex hydrological dynamics are also discussed.
- Bayesian calibration of groundwater models with input data uncertainty
- Authors: Tianfang Xu; Albert J. Valocchi, Ming Ye, Feng Liang, Yu-Feng Lin
Abstract: Effective water resources management typically relies on numerical models to analyze groundwater flow and solute transport processes. Groundwater models are often subject to input data uncertainty, as some inputs (such as recharge and well pumping rates) are estimated and subject to uncertainty. Current practices of groundwater model calibration often overlook uncertainties in input data; this can lead to biased parameter estimates and compromised predictions. Through a synthetic case study of surface-ground water interaction under changing pumping conditions and land use, we investigate the impacts of uncertain pumping and recharge rates on model calibration and uncertainty analysis. We then present a Bayesian framework of model calibration to handle uncertain input of groundwater models. The framework implements a marginalizing step to account for input data uncertainty when evaluating likelihood. It was found that not accounting for input uncertainty may lead to biased, overconfident parameter estimates because parameters could be over-adjusted to compensate for possible input data errors. Parameter compensation can have deleterious impacts when the calibrated model is used to make forecast under a scenario that is different from calibration conditions. By marginalizing input data uncertainty, the Bayesian calibration approach effectively alleviates parameter compensation and gives more accurate predictions in the synthetic case study. The marginalizing Bayesian method also decomposes prediction uncertainty into uncertainties contributed by parameters, input data and measurements. The results underscore the need to account for input uncertainty to better inform post-modeling decision making.
- Pore network extraction from pore space images of various porous media
- Authors: Z. X. Yi; M. Lin, W. B. Jiang, Z. B. Zhang, H. S. Li, J. Gao
Abstract: Pore network extraction, which is defined as the transformation from irregular pore space to a simplified network in the form of pores connected by throats, is significant to microstructure analysis and network modeling. A physically realistic pore network is not only a representation of the pore space in the sense of topology and morphology, but also a good tool for predicting transport properties accurately. We present a method to extract pore network by employing the centrally located medial axis to guide the construction of maximal-balls-like skeleton where the pores and throats are defined and parameterized. To validate our method, various rock samples including sand pack, sandstones and carbonates were used to extract pore networks. The pore structures were compared quantitatively with the structures extracted by medial axis method or maximal ball method. The predicted absolute permeability and formation factor were verified against the theoretical solutions obtained by lattice Boltzmann method and finite volume method, respectively. The two-phase flow was simulated through the networks extracted from homogeneous sandstones, and the generated relative permeability curves were compared with the data obtained from experimental method and other numerical models. The results show that the accuracy of our network is higher than that of other networks for predicting transport properties, so the presented method is more reliable for extracting physically realistic pore network.
- Overtopping-induced failure of noncohesive, homogenous fluvial dikes
- Authors: Ismail Rifai; Sebastien Erpicum, Pierre Archambeau, Damien Violeau, Michel Pirotton, Kamal El Kadi Abderrezzak, Benjamin Dewals
Abstract: Accurate predictions of breach characteristics are necessary to reliably estimate the outflow hydrograph and the resulting inundation close to fluvial dikes. Laboratory experiments on the breaching of fluvial sand dikes were performed, considering a flow parallel to the dike axis. The breach was triggered by overtopping the dike crest. A detailed monitoring of the transient evolution of the breach geometry was conducted, providing key insights into the gradual and complex processes involved in fluvial dike failure. The breach develops in two phases: (1) the breach becomes gradually wider and deeper eroding on the downstream side along the main channel, and (2) breach widening controlled by side slope failures, continuing in the downstream direction only. Increasing the inflow discharge in the main channel, the breach formation time decreases significantly and the erosion occurs preferentially on the downstream side. The downstream boundary condition has a strong influence on the breach geometry and the resulting outflow hydrograph.
- Introduction to special section on modeling highly heterogeneous aquifers:
- Authors: J. Jaime Gómez-Hernández; James. J. Butler, Aldo Fiori, Diogo Bolster, Vladimir Cvetkovic, Gedeon Dagan, David Hyndman
- Quantifying spatial groundwater dependence in peatlands through a
distributed isotope mass balance approach
- Authors: Elina Isokangas; Pekka M. Rossi, Anna-Kaisa Ronkanen, Hannu Marttila, Kazimierz Rozanski, Bjørn Kløve
Abstract: The unique biodiversity and plant composition of peatlands rely on a mix of different water sources: precipitation, runoff, and groundwater (GW). Methods used to delineate areas of ecosystem groundwater dependence, such as vegetation mapping and solute tracer studies, are indirect and lack the potential to assess temporal changes in hydrology, information needed in GW management. This paper outlines a new methodology for mapping groundwater-dependent areas (GDAs) in peatlands using a 2H and 18O isotope mass balance method. The approach reconstructs the initial isotopic composition of the peat pore water in the uppermost peat layer before its modification by evaporation. It was assumed that pore water in this layer subject to evaporation is a two-component mixture consisting of GW and precipitation input from the month preceding the sampling period. A Bayesian Monte Carlo isotope mixing model was applied to calculate the proportions of GW and rainwater in the sampled pore water and to assess uncertainties. The approach revealed large spatial variability in the contribution of GW to the pore water present in the top layer of peatland, covering the range from approximately 0 to 100%. Results show that the current GW protection zones determined by Finnish legislation do not cover the GDAs in peatlands and highlight a need for better classification of groundwater-dependent ecosystems and conceptualization of aquifer-ecosystem interactions. Our approach offers an efficient tool for mapping GDAs and quantifying the contribution of GW to peatland pore water. However, more studies are needed to test the method for different peatland types.
- FracFit: A robust parameter estimation tool for fractional calculus models
- Authors: James F. Kelly; Diogo Bolster, Mark M. Meerschaert, Jennifer D. Drummond, Aaron I. Packman
Abstract: Anomalous transport cannot be adequately described with classical Fickian advection-dispersion equations (ADE) with constant coefficients. Rather, fractional calculus models may be used, which capture salient features of anomalous transport (e.g., skewness and power law tails). FracFit is a parameter estimation tool based on space-fractional and time-fractional models used by the hydrology community. Currently, four fractional models are supported: (1) space-fractional advection-dispersion equation (sFADE), (2) time-fractional dispersion equation with drift (TFDE), (3) fractional mobile-immobile (FMIM) equation, and (4) temporally tempered Lévy motion (TTLM). Model solutions using pulse initial conditions and continuous injections are evaluated using stable distributions or subordination integrals. Parameter estimates are extracted from measured breakthrough curves (BTCs) using a weighted nonlinear least squares (WNLS) algorithm. Optimal weights for BTCs for pulse initial conditions and continuous injections are presented. Two sample applications are analyzed: (1) pulse injection BTCs in the Selke River and (2) continuous injection laboratory experiments using natural organic matter. Model parameters are compared across models and goodness-of-fit metrics are presented, facilitating model evaluation.
- Thermal effect of climate change on groundwater-fed ecosystems
- Authors: Erick R. Burns; Yonghui Zhu, Hongbin Zhan, Michael Manga, Colin F. Williams, Steven E. Ingebritsen, Jason Dunham
Abstract: Groundwater temperature changes will lag surface temperature changes from a changing climate. Steady-state solutions of the heat-transport equations are used to identify key processes that control the long-term thermal response of springs and other groundwater discharge to climate change, in particular changes in (1) groundwater recharge rate and temperature and (2) land-surface temperature transmitted through the vadose zone. Transient solutions are developed to estimate the time required for new thermal signals to arrive at ecosystems. The solution is applied to the volcanic Medicine Lake highlands, California, USA, and associated springs complexes that host groundwater-dependent ecosystems. In this system, upper-basin groundwater temperatures are strongly affected only by recharge conditions. However, as the vadose zone thins away from the highlands, changes in the average annual land-surface temperature also influence groundwater temperatures. Transient response to temperature change depends on both the conductive timescale and the rate at which recharge delivers heat. Most of the thermal response of groundwater at high elevations will occur within 20 years of a shift in recharge temperatures, but the large lower-elevation springs will respond more slowly, with about half of the conductive response occurring within the first 20 years and about half of the advective response to higher recharge temperatures occurring in approximately 60 years. This article is protected by copyright. All rights reserved.
- Using Dual-Domain Advective-Transport Simulation to Reconcile Multiple
Tracer Ages and Estimate Dual-Porosity Transport Parameters
- Authors: Ward E. Sanford; L. Niel Plummer, Gerolamo Casile, Ed Busenberg, David L. Nelms, Peter Schlosser
Abstract: Dual-domain transport is an alternative conceptual and mathematical paradigm to advection-dispersion for describing the movement of dissolved constituents in groundwater. Here we test the use of a dual-domain algorithm combined with advective pathline tracking to help reconcile environmental tracer concentrations measured in springs within the Shenandoah Valley, USA. The approach also allows for the estimation of the three dual-domain parameters: mobile porosity, immobile porosity, and a domain exchange rate constant. Concentrations of CFC-113, SF6, 3H, and 3He were measured at 28 springs emanating from carbonate rocks. The different tracers give three different mean composite piston-flow ages for all the springs that vary from 5 to 18 years. Here we compare four algorithms that interpret the tracer concentrations in terms of groundwater age: piston flow, old-fraction mixing, advective-flowpath modeling, and dual-domain modeling. Whereas the second two algorithms made slight improvements over piston flow at reconciling the disparate piston-flow age estimates, the dual-domain algorithm gave a very marked improvement. Optimal values for the three transport parameters were also obtained, although the immobile porosity value was not well constrained. Parameter correlation and sensitivities were calculated to help quantify the uncertainty. Although some correlation exists between the three parameters being estimated, a watershed simulation of a pollutant breakthrough to a local stream illustrates that the estimated transport parameters can still substantially help to constrain and predict the nature and timing of solute transport. The combined use of multiple environmental tracers with this dual-domain approach could be applicable in a wide variety of fractured-rock settings. This article is protected by copyright. All rights reserved.
- Lake and wetland ecosystem services measuring water storage and local
- Authors: Christina P. Wong; Bo Jiang, Theodore J. Bohn, Kai N. Lee, Dennis P. Lettenmaier, Dongchun Ma, Zhiyun Ouyang
Abstract: Developing interdisciplinary methods to measure ecosystem services is a scientific priority, however progress remains slow in part because we lack ecological production functions (EPFs) to quantitatively link ecohydrological processes to human benefits. In this study we tested a new approach, combining a process-based model with regression models, to create EPFs to evaluate water storage and local climate regulation from a green infrastructure project on the Yongding River in Beijing, China. Seven artificial lakes and wetlands were established to improve local water storage and human comfort; evapotranspiration (ET) regulates both services. Managers want to minimize the tradeoff between water losses and cooling to sustain water supplies while lowering the heat index (HI) to improve human comfort. We selected human benefit indicators using water storage targets and Beijing's HI, and the Variable Infiltration Capacity model to determine the change in ET from the new ecosystems. We created EPFs to quantify the ecosystem services as marginal values [Δfinal ecosystem service/Δecohydrological process]: (1) Δwater loss (lake evaporation/volume)/Δdepth and (2) Δsummer HI/ΔET. We estimate the new ecosystems increased local ET by 0.7 mm/day (20.3 W/m2) on the Yongding River. However ET rates are causing water storage shortfalls while producing no improvements in human comfort. The shallow lakes/wetlands are vulnerable to drying when inflow rates fluctuate, low depths lead to higher evaporative losses, causing water storage shortfalls with minimal cooling effects. We recommend managers make the lakes deeper to increase water storage, and plant shade trees to improve human comfort in the parks. This article is protected by copyright. All rights reserved.
- A decomposition-integration risk analysis method for real-time operation
of a complex flood control system
- Authors: Juan Chen; Ping-An Zhong, Yu Zhang, David Navar, William W.-G. Yeh
Abstract: Risk analysis plays an important role in decision making for real-time flood control operation of complex flood control systems. A typical flood control system consists of reservoirs, river channels, and downstream control points. The system generally is characterized by nonlinearity and large scale. Additionally, the input variables are mostly stochastic. Because of the dimensionality problem, generally, it would not be possible to carry out risk analysis without decomposition. In this paper, we propose a decomposition-integration approach whereby the original complex flood control system is decomposed into a number of independent subsystems. We conduct risk analysis for each subsystem and then integrate the results by means of combination theory of stochastic processes. We evaluate the propagation of uncertainties through the complex flood control system and calculate the risk of reservoir overtopping, as well as the risk of flooding at selected downstream control points. We apply the proposed methodology to a flood control system in the middle reaches of the Huaihe River basin in China. The results show that the proposed method is practical and provides a way to estimate the risks in real-time flood control operation of a complex flood control system.
- Snowmelt controls on concentration-discharge relationships and the balance
of oxidative and acid-base weathering fluxes in an alpine catchment, East
- Authors: Matthew J. Winnick; Rosemary W. H. Carroll, Kenneth H. Williams, Reed M. Maxwell, Wenming Dong, Kate Maher
Abstract: Although important for riverine solute and nutrient fluxes, the connections between biogeochemical processes and subsurface hydrology remain poorly characterized. We investigate these couplings in the East River, CO, a high-elevation shale-dominated catchment in the Rocky Mountains, using concentration-discharge (C-Q) relationships for major cations, anions, and organic carbon. Dissolved organic carbon (DOC) displays a positive C-Q relationship with clockwise hysteresis, indicating mobilization and depletion of DOC in the upper soil horizons and emphasizing the importance of shallow flow paths during snowmelt. Cation and anion concentrations demonstrate that carbonate weathering, which dominates solute fluxes, is promoted by both sulfuric acid derived from pyrite oxidation in the shale bedrock and carbonic acid derived from subsurface respiration. Sulfuric acid weathering dominates during base flow conditions when waters infiltrate below the inferred pyrite oxidation front, whereas carbonic acid weathering plays a dominant role during snowmelt as a result of shallow flow paths. Differential C-Q relationships between solutes suggest that infiltrating waters approach calcite saturation before reaching the pyrite oxidation front, after which sulfuric acid reduces carbonate alkalinity. This reduction in alkalinity results in CO2 outgassing when waters equilibrate to surface conditions, and reduces the riverine export of carbon and alkalinity by roughly 33% annually. Future changes in snowmelt dynamics that control the balance of carbonic and sulfuric acid weathering may substantially alter carbon cycling in the East River. Ultimately, we demonstrate that differential C-Q relationships between major solutes can provide unique insights into the complex subsurface flow and biogeochemical dynamics that operate at catchment scales.
- Debates—Hypothesis testing in hydrology: A view from the field: The
value of hydrologic hypotheses in designing field studies and interpreting
the results to advance hydrology
- Authors: Diane M. McKnight
Abstract: Advances in hydrology are greatly needed and approaches that employ hypotheses to guide research have the potential to contribute to future advances. In this context, hypotheses can serve a range of purposes. Overarching hypotheses can provide a common integrating framework for collaborative research and can be revised as research progresses over time. Hypotheses that attempt to explain unexpected field observations or experimental results can provide a guide for designing further field studies. Focused testable hypotheses can facilitate effective presentation of proposed research, and clarify alternative hypotheses. Finally, the value of employing a hypothesis-based approach depends upon the research environment, which can act as an “environmental filter” in determining successful research outcomes.
- Debates—Hypothesis testing in hydrology: Pursuing certainty versus
- Authors: Victor R. Baker
Abstract: Modern hydrology places nearly all its emphasis on science-as-knowledge, the hypotheses of which are increasingly expressed as physical models, whose predictions are tested by correspondence to quantitative data sets. Though arguably appropriate for applications of theory to engineering and applied science, the associated emphases on truth and degrees of certainty are not optimal for the productive and creative processes that facilitate the fundamental advancement of science as a process of discovery. The latter requires an investigative approach, where the goal is uberty, a kind of fruitfulness of inquiry, in which the abductive mode of inference adds to the much more commonly acknowledged modes of deduction and induction. The resulting world-directed approach to hydrology provides a valuable complement to the prevailing hypothesis- (theory-) directed paradigm.
- Debates—Hypothesis testing in hydrology: A subsurface perspective
- Authors: Insa Neuweiler; Rainer Helmig
Abstract: Models for flow in environmental systems are subject to uncertainty. Models can thus be interpreted as hypotheses on the validity of the underlying model assumptions. One important source of uncertainty in models for flow and transport processes in the subsurface is the model concept. While uncertain model parameters or forcing terms can be captured as random processes and random fields, this type of uncertainty cannot be included into a model in a straightforward manner. This is particularly true if established model descriptions of a given process are not known or are still being debated. In this contribution, we outline several examples of subsurface flow and transport modeling where uncertainty of the model concept plays an important role. We discuss the need for the development of methods and standards to deal with this type of uncertainty in model hypothesis testing.
- Debates—Hypothesis testing in hydrology: Introduction
- Authors: Günter Blöschl
Abstract: This paper introduces the papers in the “Debates—Hypothesis testing in hydrology” series. The four articles in the series discuss whether and how the process of testing hypotheses leads to progress in hydrology. Repeated experiments with controlled boundary conditions are rarely feasible in hydrology. Research is therefore not easily aligned with the classical scientific method of testing hypotheses. Hypotheses in hydrology are often enshrined in computer models which are tested against observed data. Testability may be limited due to model complexity and data uncertainty. All four articles suggest that hypothesis testing has contributed to progress in hydrology and is needed in the future. However, the procedure is usually not as systematic as the philosophy of science suggests. A greater emphasis on a creative reasoning process on the basis of clues and explorative analyses is therefore needed.
- Debates—Hypothesis testing in hydrology: Theory and practice
- Authors: Laurent Pfister; James W. Kirchner
Abstract: The basic structure of the scientific method—at least in its idealized form—is widely championed as a recipe for scientific progress, but the day-to-day practice may be different. Here, we explore the spectrum of current practice in hypothesis formulation and testing in hydrology, based on a random sample of recent research papers. This analysis suggests that in hydrology, as in other fields, hypothesis formulation and testing rarely correspond to the idealized model of the scientific method. Practices such as “p-hacking” or “HARKing” (Hypothesizing After the Results are Known) are major obstacles to more rigorous hypothesis testing in hydrology, along with the well-known problem of confirmation bias—the tendency to value and trust confirmations more than refutations—among both researchers and reviewers. Nonetheless, as several examples illustrate, hypothesis tests have played an essential role in spurring major advances in hydrological theory. Hypothesis testing is not the only recipe for scientific progress, however. Exploratory research, driven by innovations in measurement and observation, has also underlain many key advances. Further improvements in observation and measurement will be vital to both exploratory research and hypothesis testing, and thus to advancing the science of hydrology.
- Glacier melt buffers river runoff in the Pamir Mountains
- Authors: Eric Pohl; Richard Gloaguen, Christoff Andermann, Malte Knoche
Abstract: Newly developed approaches based on satellite altimetry and gravity measurements provide promising results on glacier dynamics in the Pamir-Himalaya but cannot resolve short-term natural variability at regional and finer scale. We contribute to the ongoing debate by upscaling a hydrological model that we calibrated for the central Pamir. The model resolves the spatiotemporal variability in runoff over the entire catchment domain with high efficiency. We provide relevant information about individual components of the hydrological cycle and quantify short-term hydrological variability. For validation, we compare the modeled total water storages (TWS) with GRACE (Gravity Recovery and Climate Experiment) data with a very good agreement where GRACE uncertainties are low. The approach exemplifies the potential of GRACE for validating even regional scale hydrological applications in remote and hard to access mountain regions. We use modeled time series of individual hydrological components to characterize the effect of climate variability on the hydrological cycle. We demonstrate that glaciers play a twofold role by providing roughly 35% of the annual runoff of the Panj River basin and by effectively buffering runoff both during very wet and very dry years. The modeled glacier mass balance (GMB) of −0.52 m w.e. yr−1 (2002–2013) for the entire catchment suggests significant reduction of most Pamiri glaciers by the end of this century. The loss of glaciers and their buffer functionality in wet and dry years could not only result in reduced water availability and increase the regional instability, but also increase flood and drought hazards.
- Reply to Comment by Añel on “Most computational hydrology is not
reproducible, so is it really science?”
- Authors: Christopher Hutton; Thorsten Wagener, Jim Freer, Dawei Han, Chris Duffy, Berit Arheimer
Abstract: In this article, we reply to a comment made on our previous commentary regarding reproducibility in computational hydrology. Software licensing and version control of code are important technical aspects of making code and workflows of scientific experiments open and reproducible. However, in our view, it is the cultural change that is the greatest challenge to overcome to achieve reproducible scientific research in computational hydrology. We believe that from changing the culture and attitude among hydrological scientists, details will evolve to cover more (technical) aspects over time.
- Comment on “Most computational hydrology is not reproducible, so is it
really science?” by Christopher Hutton et al.
- Authors: Juan A. Añel
Abstract: Nowadays, the majority of the scientific community is not aware of the risks and problems associated with an inadequate use of computer systems for research, mostly for reproducibility of scientific results. Such reproducibility can be compromised by the lack of clear standards and insufficient methodological description of the computational details involved in an experiment. In addition, the inappropriate application or ignorance of copyright laws can have undesirable effects on access to aspects of great importance of the design of experiments and therefore to the interpretation of results.
- Simulation of the cumulative hydrological response to green infrastructure
- Authors: P. M. Avellaneda; A. J. Jefferson, J. M. Grieser, S. A. Bush
Abstract: In this study, we evaluated the cumulative hydrologic performance of green infrastructure in a residential area of the city of Parma, Ohio, draining to a tributary of the Cuyahoga River. Green infrastructure included the following spatially distributed devices: 16 street side bioretention cells, 7 rain gardens, and 37 rain barrels. Data consisted of rainfall and outfall flow records for a wide range of storm events, including pre-treatment and treatment periods. The Stormwater Management Model was calibrated and validated to predict the hydrologic response of green infrastructure. The calibrated model was used to quantify annual water budget alterations and discharge frequency over a six year simulation period. For the study catchment, we observed a treatment effect with increases of 1.4% in evaporation, 7.6% in infiltration, and a 9.0% reduction in surface runoff. The hydrologic performance of green infrastructure was evaluated by comparing the flow duration curve for pre-treatment and treatment outfall flow scenarios. The flow duration curve shifted downwards for the green infrastructure scenario. Discharges with a 0.5-, 1-, 2-, and 5-year return period were reduced by an average of 29%. Parameter and predictive uncertainties were inspected by implementing a Bayesian statistical approach. This article is protected by copyright. All rights reserved.
- Approximate Bayesian Computation methods for daily spatiotemporal
precipitation occurrence simulation
- Authors: Branden Olson; William Kleiber
Abstract: Stochastic precipitation generators (SPGs) produce synthetic precipitation data and are frequently used to generate inputs for physical models throughout many scientific disciplines. Especially for large datasets, statistical parameter estimation is difficult due to the high dimensionality of the likelihood function. We propose techniques to estimate SPG parameters for spatiotemporal precipitation occurrence based on an emerging set of methods called Approximate Bayesian computation (ABC), which bypass the evaluation of a likelihood function. Our statistical model employs a thresholded Gaussian process that reduces to a probit regression at single sites. We identify appropriate ABC penalization metrics for our model parameters to produce simulations whose statistical characteristics closely resemble those of the observations. Spell length metrics are appropriate for single sites, while a variogram-based metric is proposed for spatial simulations. We present numerical case studies at sites in Colorado and Iowa where the estimated statistical model adequately reproduces local and domain statistics. This article is protected by copyright. All rights reserved.
- Network structure classification and features of water distribution
- Authors: Orazio Giustolisi; Antonietta Simone, Luca Ridolfi
Abstract: The network connectivity structure of water distribution systems (WDSs) represents the domain where hydraulic processes occur, driving the emerging behavior of such systems, for example with respect to robustness and vulnerability. In complex network theory (CNT), a common way of classifying the network structure and connectivity is the association of the nodal degree distribution to specific probability distribution models; and during the last decades, researchers classified many real networks using the Poisson or Pareto distributions. In spite of the fact that degree-based network classification could play a crucial role to assess WDS vulnerability, this task is not easy because the network structure of WDSs is strongly constrained by spatial characteristics of the environment where they are constructed. The consequence of these spatial constraints is that the nodal degree spans very small ranges in WDSs hindering a reliable classification by the standard approach based on the nodal degree distribution. This work investigates the classification of the network structure of twenty-two real WDSs, built in different environments, demonstrating that the Poisson distribution generally models the degree distributions very well. In order to overcome the problem of the reliable classification based on the standard nodal degree, we define the “neighborhood” degree, equal to the sum of the nodal degrees of the nearest topological neighbors (i.e., the adjacent nodes). This definition of “neighborhood” degree is consistent with the fact that the degree of a single node is not significant for analysis of WDSs. This article is protected by copyright. All rights reserved.
- A new process sensitivity index to identify important system processes
under process model and parametric uncertainty
- Authors: Heng Dai; Ming Ye, Anthony P. Walker, Xingyuan Chen
Abstract: A hydrological model consists of multiple process level sub-models, and each sub-model represents a process key to the operation of the simulated system. Global sensitivity analysis methods have been widely used to identify important processes for system model development and improvement. The existing methods of global sensitivity analysis only consider parametric uncertainty, and are not capable of handling model uncertainty caused by multiple process models that arise from competing hypotheses about one or more processes. To address this problem, this study develops a new method to probe model output sensitivity to competing process models by integrating model averaging methods with variance-based global sensitivity analysis. A process sensitivity index is derived as a single summary measure of relative process importance, and the index includes variance in model outputs caused by uncertainty in both process models and their parameters. For demonstration, the new index is used to assign importance to the processes of recharge and geology in a synthetic study of groundwater reactive transport modeling. The recharge process is simulated by two models that convert precipitation to recharge, and the geology process is simulated by two models of hydraulic conductivity. Each process model has its own random parameters. The new process sensitivity index is mathematically general, and can be applied to a wide range of problems in hydrology and beyond. This article is protected by copyright. All rights reserved.
- A diagnostic approach to constraining flow partitioning in hydrologic
models using a multi-objective optimization framework
- Authors: Mahyar Shafii; Nandita Basu, James R. Craig, Sherry L. Schiff, Philippe Van Cappellen
Abstract: Hydrologic models are often tasked with replicating historical hydrographs, but may do so without accurately reproducing the internal hydrological functioning of the watershed, including the flow partitioning, which is critical for predicting solute movement through the catchment. Here we propose a novel partitioning-focused calibration technique that utilizes flow partitioning coefficients developed based on the pioneering work of L'vovich . Our hypothesis is that inclusion of the L'vovich partitioning relations in calibration increases model consistency and parameter identifiability, and leads to superior model performance with respect to flow partitioning than using traditional hydrological signatures (e.g., flow duration curve indices) alone. The L'vovich approach partitions the annual precipitation into four components (quick flow, soil wetting, slow flow, and evapo-transpiration) and has been shown to work across a range of climatic and landscape settings. A new diagnostic multi-criteria model calibration methodology is proposed that first quantifies four calibration measures for watershed functions based on the L‘vovich theory, and then utilizes them as calibration criteria. The proposed approach is compared with a traditional hydrologic signature-based calibration for two conceptual bucket models. Results reveal that the proposed approach not only improves flow partitioning in the model compared to signature-based calibration, but is also capable of diagnosing flow partitioning inaccuracy and suggesting relevant model improvements. Furthermore, the proposed partitioning-based calibration approach is shown to increase parameter identifiability. This model calibration approach can be readily applied to other models. This article is protected by copyright. All rights reserved.
- Improved modeling of snow and glacier melting by a progressive two-stage
calibration strategy with GRACE and multisource data: How snow and glacier
- Authors: Xi Chen; Di Long, Yang Hong, Chao Zeng, Denghua Yan
Abstract: Snow and glacier melting and accumulation are important processes of the hydrological cycle in the cryosphere, e.g., high-mountain areas. Glaciers and snow cover respond to climate change notably over the Tibetan Plateau (TP) as the Earth's Third Pole where complex topography and lack of ground-based observations result in knowledge gaps in hydrological processes and large uncertainties in model output. This study develops a snow and glacier melt model for a distributed hydrological model (Coupled Routing and Excess Storage model, CREST) using the Upper Brahmaputra River (UBR) basin in the TP as a case study. Satellite and ground-based precipitation and land surface temperature are jointly used as model forcing. A progressive two-stage calibration strategy is developed to derive model parameters, i.e., (1) snow melting processes (stage I) and (2) glacier melting and runoff generation and routing using multisource data (stage II). Stage-I calibration is performed using the MODIS snow cover area (SCA) product and a blending snow water equivalent (SWE) product combined with partial in situ measurements. Stage-II calibration is based on Gravity Recovery and Climate Experiment (GRACE) satellite-derived total water storage (TWS) changes and streamflow observed at a gauging station of the lower reach of the UBR. Results indicate that the developed two-stage calibration method provides more reliable streamflow, snow (both SCA and SWE), and TWS change simulations against corresponding observations than commonly used methods based on streamflow and/or SCA performance. The simulated TWS time series shows high consistency with GRACE counterparts for the study period 2003–2014, and overestimated melting rates and contributions of glacier meltwater to runoff in previous studies are improved to some degree by the developed model and calibration strategy. Snow and glacier runoff contributed 10.6% and 9.9% to the total runoff, and the depletion rate of glacier mass was ∼ −10 mm/a (∼ −2.4 Gt/a, Gt/a is gigaton (km3 of water) per year) over the UBR basin during the study period. This study is valuable in examining the impacts of climate change on hydrological processes of cryospheric regions and providing an improved approach for simulating more reliable hydrological variables over the UBR basin and potentially similar regions globally.
- Comment on “Most computational hydrology is not reproducible, so is it
really science?” by Christopher Hutton et al.
- Authors: Lieke A. Melsen; Paul J. J. F. Torfs, Remko Uijlenhoet, Adriaan J. Teuling
Abstract: We discuss two definitions of reproducibility, and question if both definitions are required to be met in computational hydrological studies.
- A numerical investigation into the importance of bed permeability on
determining flow structures over river dunes
- Authors: Sumit Sinha; Richard J. Hardy, Gianluca Blois, James L. Best, Gregory H. Sambrook Smith
Abstract: Although permeable sediments dominate the majority of natural environments past work concerning bedform dynamics has considered the bed to be impermeable, and has generally neglected flow between the hyporheic zone and boundary layer. Herein, we present results detailing numerically modelled flow which allow the effects of bed permeability on bedform dynamics to be assessed.Simulation of an isolated impermeable bedform over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside, in the form of returning jets that generate horseshoe-shaped vortices. The returning flow significantly influences the leeside flow, modifying the separation zone, lifting the shear layer adjoining the separation zone away from the bed. Simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside. With two dunes placed in series the flow over the downstream dune is influenced by the developing boundary layer on the leeside of the upstream dune. For the permeable bed case the upwelling flow lifts the separated flow from the bed, modifies the shear layer through the coalescence with vortices generated, and causes the shear layer to undulate rather than be parallel to the bed.These results demonstrate the significant effect that bed permeability has on the flow over bedforms that may be critical in affecting the flux of water and nutrients. This article is protected by copyright. All rights reserved.
- Vertical groundwater storage properties and changes in confinement
determined using hydraulic head response to atmospheric tides
- Authors: R. Ian Acworth; Gabriel C. Rau, Landon J.S. Halloran, Wendy A. Timms
Abstract: Accurate determination of groundwater state of confinement and compressible storage properties at vertical resolution over depth is notoriously difficult. We use the hydraulic head response to atmospheric tides at 2 cpd frequency as a tracer to quantify barometric efficiency (BE) and specific storage (Ss) over depth. Records of synthesized Earth tides, atmospheric pressure and hydraulic heads measured in 9 piezometers completed at depths between 5-55 m into unconsolidated smectitic clay and silt, sand and gravel were examined in the frequency domain. The barometric efficiency increased over depth from ∼0.05 in silty clay to ∼0.15 in sands and gravels. BE for silty clay was confirmed by calculating the loading efficiency as 0.95 using rainfall at the surface. Specific storage was calculated using effective rather than total moisture. The differences in phase between atmospheric pressure and hydraulic heads at 2 cpd were ∼180° below 10 m indicating confined conditions despite the low BE. Heads in the sediment above a fine sand and silt layer at 12 m exhibited a time variable phase difference between 0-180° indicating varying confinement. Our results illustrate that the atmospheric tide at 2 cpd is a powerful natural tracer for quantifying groundwater state of confinement and compressible storage properties in layered formations from hydraulic heads and atmospheric pressure records without the need for externally induced hydraulic stress. This approach could significantly improve the development of conceptual hydrogeological model used for groundwater resource development and management. This article is protected by copyright. All rights reserved.
- Mixing as a driver of temporal variations in river hydrochemistry. Part 2:
Major and trace element concentration dynamics in the Andes-Amazon
- Authors: J. Jotautas Baronas; Mark A. Torres, Kathryn E. Clark, A. Joshua West
Abstract: Variations in riverine solute chemistry with changing runoff are used to interrogate catchment hydrology and to investigate chemical reactions in Earth's critical zone. This approach requires some understanding of how spatial and temporal averaging of solute-generating reactions affect the dissolved load of rivers and streams. In this study, we investigate the concentration-runoff (C-Q) dynamics of a suite of major (Na, Mg, Ca, Si, K, and SO4) and trace (Al, Ba, Cd, Co, Cr, Cu, Fe, Ge, Li, Mn, Mo, Nd, Ni, Rb, Sr, U, V, and Zn) elements in nested catchments of variable size, spanning the geomorphic gradient from the Andes mountains to the Amazon foreland-floodplain. The major elements exhibit various degrees of dilution with increasing runoff at all sites, whereas the concentrations of most trace elements either increase or show no relationship with increasing runoff in the three larger catchments (160 to 28 000 km2 area). We show that the observed mainstem C-Q dynamics are influenced by variable mixing of tributaries with distinct C-Q relationships. Trace element C-Q relationships are more variable among tributaries relative to major elements, which could be the result of variations in geomorphology, lithology, and hydrology of the sub-catchments. Certain trace metals are also lost from solution during in-channel processes (possibly related to colloidal size-partitioning), which may exert an additional control on C-Q dynamics. Overall, we suggest that aggregation effects should be assessed in heterogeneous catchments before C-Q or ratio-Q relationships can be interpreted as reflecting catchment-wide solute generation processes and their relationship to hydrology. This article is protected by copyright. All rights reserved.
- Mixing as a driver of temporal variations in river hydrochemistry. Part 1:
Insights from conservative tracers in the Andes-Amazon transition
- Authors: Mark A. Torres; J. Jotautas Baronas, Kathryn E. Clark, Sarah J. Feakins, A. Joshua West
Abstract: The response of hillslope processes to changes in precipitation may drive the observed changes in the solute geochemistry of rivers with discharge. This conjecture is most robust when variations in the key environmental factors that affect hillslope processes (e.g., lithology, erosion rate, and climate) are minimal across a river's catchment area. For rivers with heterogenous catchments, temporal variations in the relative contributions of different tributary sub-catchments may modulate variations in solute geochemistry with runoff. In the absence of a dense network of hydrologic gauging stations, alternative approaches are required to distinguish between the different drivers of temporal variability in river solute concentrations. In this contribution, we apportion the water and solute fluxes of a reach of the Madre de Dios River (Peru) between its four major tributary sub-catchments during two sampling campaigns (wet and dry seasons) using spatial variations in conservative tracers. Guided by the results of a mixing model, we identify temporal variations in solute concentrations of the mainstem Madre de Dios that are due to changes in the relative contributions of each tributary. Our results suggest that variations in tributary mixing are, in part, responsible for the observed concentration-discharge (C-Q) relationships. The implications of these results are further explored by re-analyzing previously published C-Q data from this region, developing a theoretical model of tributary mixing, and, in a companion paper, comparing the C-Q behavior of a suite of major and trace elements in the Madre de Dios River system. This article is protected by copyright. All rights reserved.
- Posterior population expansion for solving inverse problems
- Authors: C. Jäggli; J. Straubhaar, P. Renard
Abstract: Solving inverse problems in a complex, geologically realistic, and discrete model space and from a sparse set of observations is a very challenging task. Extensive exploration by Markov chain Monte Carlo (McMC) methods often results in considerable computational efforts. Most optimization methods, on the other hand, are limited to linear (continuous) model spaces and the minimization of an objective function, what often proves to be insufficient. To overcome these problems, we propose a new ensemble based exploration scheme for geostatistical prior models generated by a multiple-point statistics (MPS) tool. The principle of our method is to expand an existing set of models by using posterior facies information for conditioning new MPS realizations. The algorithm is independent of the physical parametrization. It is tested on a simple synthetic inverse problem. When compared to two existing McMC methods (Iterative Spatial Resampling (ISR) and Interrupted Markov chain Monte Carlo (IMcMC)) the required number of forward model runs was divided by a factor of 8-12. This article is protected by copyright. All rights reserved.
- The impact of sedimentary anisotropy on solute mixing in stacked
- Authors: Jeremy P. Bennett; Claus P. Haslauer, Olaf A. Cirpka
Abstract: The spatial variability of hydraulic conductivity is known to have a strong impact on solute spreading and mixing. In most investigations, its local anisotropy has been neglected. Recent studies have shown that spatially varying orientation in sedimentary anisotropy can lead to twisting flow enhancing transverse mixing, but most of these studies used geologically implausible geometries. We use an object-based approach to generate stacked scour-pool structures with either isotropic or anisotropic filling which are typically reported in glacial outwash deposits. We analyze how spatially variable isotropic conductivity and variation of internal anisotropy in these features impacts transverse plume deformation and both longitudinal and transverse spreading and mixing. In five test cases either the scalar values of conductivity or the spatial orientation of its anisotropy is varied between the scour- pool structures. Based on 100 random configurations, we compare the variability of velocity components, stretching and folding metrics, advective travel-time distributions, one- and two-particle statistics in advective-dispersive transport, and the flux-related dilution indices for steady-state advective-dispersive transport among the five test cases. Variation in the orientation of internal anisotropy causes strong variability in the lateral velocity components, which leads to deformation in transverse directions and enhances transverse mixing, whereas it hardly affects the variability of the longitudinal velocity component and thus longitudinal spreading and mixing. The latter is controlled by the spatial variability in the scalar values of hydraulic conductivity. Our results demonstrate that sedimentary anisotropy is important for transverse mixing, whereas it may be neglected when considering longitudinal spreading and mixing. This article is protected by copyright. All rights reserved.
- Modeling complex flow structures and drag around a submerged plant of
- Authors: Richard J. Boothroyd; Richard J. Hardy, Jeff Warburton, Timothy I. Marjoribanks
Abstract: Although vegetation is present in many rivers, the bulk of past work concerned with modeling the influence of vegetation on flow has considered vegetation to be morphologically simple, and has generally neglected the complexity of natural plants. Here we report on a combined flume and numerical model experiment which incorporates time-averaged plant posture, collected through Terrestrial Laser Scanning, into a Computational Fluid Dynamics model to predict flow around a submerged riparian plant. For three depth-limited flow conditions (Reynolds number = 65 000 – 110 000), plant dynamics were recorded through high-definition video imagery, and the numerical model was validated against flow velocities collected with an acoustic Doppler velocimeter. The plant morphology shows an 18% reduction in plant height and a 14% increase in plant length, compressing and reducing the volumetric canopy morphology as the Reynolds number increases. Plant shear layer turbulence is dominated by Kelvin–Helmholtz type vortices generated through shear instability, the frequency of which is estimated to be between 0.20 and 0.30 Hz, increasing with Reynolds number. These results demonstrate the significant effect that the complex morphology of natural plants has on in-stream drag, and allows a physically determined, species-dependent drag coefficient to be calculated. Given the importance of vegetation in river corridor management, the approach developed here demonstrates the necessity to account for plant motion when calculating vegetative resistance. This article is protected by copyright. All rights reserved.
- Representative point-integrated suspended sediment sampling in rivers
- Authors: A.B. Gitto; J.G. Venditti, R. Kostaschuk, M. Church
Abstract: The vast majority of continental sediment delivered to the world's oceans moves by suspension in rivers. Depth- or point-integrated bottle sampling are the traditional methods used to determine the mean concentration of suspended sediment in rivers. While there has been some investigation of the error associated with depth-integrated sampling, the representativeness of a point-integrated bottle sample has not been addressed in the literature. Here, we analyze continuous hour-long measurements of suspended sediment and grain size fractions collected using a LISST-SL in the sand-bed portion of the Fraser River, British Columbia to determine an appropriate sampling time. The 2σ uncertainty range of individual 30 s samples varied from ±3% to ±33% about the observed mean, with a systematic increase toward the streambed. Mean concentrations for suspended sediment and grain size fractions were computed over increasing time periods and compared with a long duration mean concentration to determine when a sample becomes representative. A cumulative probability distribution was generated from multiple iterations of this process. All suspended sediment load and grain size fractions bear a low probability of representing the actual mean concentration over standard bottle sample durations. A probability >90% of representing the mean concentration and grain-size of various fractions requires ∼570 seconds (9.5 minutes) of sampling. Sampling for a shorter period of 264 seconds (4.4 minutes) can yield a sample with 73% probability of representing the mean concentration. This article is protected by copyright. All rights reserved.
- Impact of eliminating fracture intersection nodes in multiphase
compositional flow simulation
- Authors: Kenneth M. Walton; Andre J. A. Unger, Marios A. Ioannidis, Beth L. Parker
Abstract: Algebraic elimination of nodes at discrete fracture intersections via the star-delta technique has proven to be a valuable tool for making multiphase numerical simulations more tractable and efficient. This study examines the assumptions of the star-delta technique and exposes its effects in a 3D, multiphase context for advective and dispersive/diffusive fluxes. Key issues of relative permeability-saturation-capillary pressure (kr-S-Pc) and capillary barriers at fracture-fracture intersections are discussed. This study uses a multiphase compositional, finite difference numerical model in discrete fracture network (DFN) and discrete fracture-matrix (DFM) modes. It verifies that the numerical model replicates analytical solutions and performs adequately in convergence exercises (conservative and decaying tracer, one- and two-phase flow, DFM and DFN domains). The study culminates in simulations of a two-phase laboratory experiment in which a fluid invades a simple fracture intersection. The experiment and simulations evoke different invading fluid flow paths by varying fracture apertures as oil invades water-filled fractures and as water invades air-filled fractures. Results indicate that the node elimination technique as implemented in numerical model correctly reproduces the long-term flow path of the invading fluid, but that short-term temporal effects of the capillary traps and barriers arising from the intersection node are lost. This article is protected by copyright. All rights reserved.
- A method for preferential selection of dates in the Schaake shuffle
approach to constructing spatiotemporal forecast fields of temperature and
- Authors: Michael Scheuerer; Thomas M. Hamill, Brett Whitin, Minxue He, Arthur Henkel
Abstract: Hydrological forecasts strongly rely on predictions of precipitation amounts and temperature as meteorological forcings for hydrological models. Ensemble weather predictions provide a number of different scenarios that reflect the uncertainty about these meteorological inputs, but these are often biased and under-dispersive, and therefore require statistical post-processing. In addition to correcting the marginal distributions of the two weather variables, post-processing methods must reconstruct their spatial, temporal, and inter-variable dependence in order to generate physically realistic forecast trajectories that can be used as forcings of hydrological streamflow forecast models.For many years, a sample reordering method referred to as “Schaake shuffle” has been used successfully to address this multivariate aspect of forecast distributions by using historical observation trajectories as multivariate “dependence templates”. This paper proposes a variant of the Schaake shuffle, in which the historical dates are selected such that the marginal distributions of the corresponding observation trajectories are similar to the forecast marginal distributions, thus making it more likely that spatial and temporal gradients are preserved during the reordering procedure. This new approach is demonstrated with temperature and precipitation forecasts over four river basins in California, and it is shown to improve upon the standard Schaake shuffle both with respect to verification metrics applied to the forcings, and verification metrics applied to the resulting streamflow predictions. This article is protected by copyright. All rights reserved.
- Hyphenated hydrology: Interdisciplinary evolution of water resource
- Authors: K.L. McCurley; J.W. Jawitz
Abstract: Hydrology has advanced considerably as a scientific discipline since its recognized inception in the mid-20th century. Modern water resource related questions have forced adaptation from exclusively physical or engineering science viewpoints toward a deliberate interdisciplinary context. Over the past few decades, many of the eventual manifestations of this evolution were foreseen by prominent expert hydrologists. However, their narrative descriptions have lacked substantial quantification. This study addressed that gap by measuring the prevalence of and analyzing the relationships between the terms most frequently used by hydrologists to define and describe their research. We analyzed 16,591 journal article titles from 1965-2015 in Water Resources Research, through which the scientific dialogue and its time-sensitive progression emerged. Our word frequency and term co-occurrence network results revealed the dynamic timing of the lateral movement of hydrology across multiple disciplines as well as the deepening of scientific discourse with respect to traditional hydrologic questions. The conversation among water resource scientists surrounding the hydrologic sub-disciplines of catchment-hydrology, hydro-meteorology, socio-hydrology, hydro-climatology and eco-hydrology all gained statistically significant momentum in the analyzed time period, while hydro-geology and contaminant-hydrology experienced periods of increase followed by significant decline. This study concludes that formerly exotic disciplines can potentially modify hydrology, prompting new insights and inspiring unconventional perspectives on old questions that may have otherwise become obsolete. This article is protected by copyright. All rights reserved.
- Multiscale temporal variability and regional patterns in 555 years of
conterminous U.S. streamflow
- Authors: Michelle Ho; Upmanu Lall, Xun Sun, Edward R. Cook
Abstract: The development of paleoclimate streamflow reconstructions in the conterminous United States (CONUS) has provided water resource managers with improved insights into multi-decadal and centennial scale variability that cannot be reliably detected using shorter instrumental records. Paleoclimate streamflow reconstructions have largely focused on individual catchments limiting the ability to quantify variability across the CONUS. The Living Blended Drought Atlas (LBDA), a spatially and temporally complete 555-year-long paleoclimate record of summer drought across the CONUS, provides an opportunity to reconstruct and characterize streamflow variability at a continental scale. We explore the validity of the first paleo reconstructions of streamflow that span the CONUS informed by the LBDA targeting a set of US Geological Survey streamflow sites. The reconstructions are skillful under cross validation across most of the country, but the variance explained is generally low. Spatial and temporal structures of streamflow variability are analyzed using hierarchical clustering, principal component analysis, and wavelet analyses. Nine spatially coherent clusters are identified. The reconstructions show signals of contemporary droughts such as the Dust Bowl (1930s) and 1950s droughts. Decadal-scale variability was detected in the late 1900s in the western US, however, similar modes of temporal variability were rarely present prior to the 1950s. The 20th century featured longer wet spells and shorter dry spells compared with the preceding 450 years. Streamflow in the Pacific Northwest and Northeast are negatively correlated with the central US suggesting the potential to mitigate some drought impacts by balancing economic activities and insurance pools across these regions during major droughts. This article is protected by copyright. All rights reserved.
- Theoretical analysis of non-Gaussian heterogeneity effects on subsurface
flow and transport
- Authors: Monica Riva; Alberto Guadagnini, Shlomo P. Neuman
Abstract: Much of the stochastic groundwater literature is devoted to the analysis of flow and transport in Gaussian or multi-Gaussian log hydraulic conductivity (or transmissivity) fields, Y(x) =ln K(x) (x being a position vector), characterized by one or (less frequently) a multiplicity of spatial correlation scales. Yet Y, as well as many other variables and their (spatial or temporal) increments, ΔY, are known to be generally non-Gaussian. One common manifestation of non-Gaussianity is that whereas frequency distributions of Y often exhibit mild peaks and light tails, those of increments ΔY are generally symmetric with peaks that grow sharper, and tails that become heavier, as separation scale or lag between pairs of Y values decreases. A statistical model that captures these disparate, scale-dependent distributions of Y and ΔY in a uniﬁed and consistent manner has been recently proposed by us. This new “generalized sub-Gaussian (GSG)” model has the form Y(x) = U(x) G(x) where G(x) is (generally, but not necessarily) a multi-scale Gaussian random field and U(x) is a non-negative subordinator independent of G. The purpose of this paper is to explore analytically, in an elementary manner, lead-order effects that non-Gaussian heterogeneity described by the GSG model have on the stochastic description of flow and transport. Recognizing that perturbation expansion of hydraulic conductivity K = eY diverges when Y is sub-Gaussian, we render the expansion convergent by truncating Y's domain of definition. We then demonstrate theoretically and illustrate by way of numerical examples that, as the domain of truncation expands, (a) the variance of truncated Y (denoted by Yt) approaches that of Y, (b) the pdf (and thereby moments) of Yt increments approach those of Y increments and, as a consequence, the variogram of Yt approaches that of Y. This in turn guarantees that perturbing Kt=eYt to second order in σYt (the standard deviation of Yt) yields results which approach those we obtain upon perturbing K = eY to second order in σY even as the corresponding series diverges. Our analysis is rendered mathematically tractable by considering mean uniform steady state flow in an unbounded, two-dimensional domain of mildly heterogeneous Y with a single-scale function G having an isotropic exponential covariance. Results consist of expressions for (a) lead order autocovariance and cross-covariance functions of hydraulic head, velocity and advective particle displacement and (b) analogues of preasymptotic as well as asymptotic Fickian dispersion coefficients. We compare these theoretically and graphically with corresponding expressions developed in the literature for Gaussian Y. We find the former to differ from the latter by a factor k = 〈 U2 〉/〈 U 〉2 (〈〉 denoting ensemble expectation) and the GSG covariance of longitudinal velocity to contain an additional nugget term depending on this same factor. In the limit as Y becomes Gaussian k reduces to one and the nugget term drops out. This article is protected by copyright. All rights reserved.
- Peaks over threshold (POT): A methodology for automatic threshold
estimation using goodness-of-fit p-value
- Authors: Sebastián Solari; Marta Egüen, María José Polo, Miguel A. Losada
Abstract: Threshold estimation in the Peaks Over Threshold (POT) method, and the impact of the estimation method on the calculation of high return period quantiles and their uncertainty (or confidence intervals) are issues that are still unresolved. In the past, methods based on goodness-of-fit tests and EDF-statistics have yielded satisfactory results, but their use has not yet been systematized.This paper proposes a methodology for automatic threshold estimation, based on the Anderson-Darling EDF-statistic and goodness-of-fit test. When combined with bootstrapping techniques, this methodology can be used to quantify both the uncertainty of threshold estimation and its impact on the uncertainty of high return period quantiles.This methodology was applied to several simulated series and to four precipitation/riverflow data series. The results obtained confirmed its robustness. For the measured series, the estimated thresholds corresponded to those obtained by non-automatic methods. Moreover, even though the uncertainty of the threshold estimation was high, this did not have a significant effect on the width of the confidence intervals of high return period quantiles. This article is protected by copyright. All rights reserved.
- Biofuel as an integrated farm drainage management crop: A bio-economic
- Authors: L.R. Levers; K.A. Schwabe
Abstract: Irrigated agricultural lands in arid regions often suffer from soil salinization and lack of drainage, which affect environmental quality and productivity. Integrated Farm Drainage Management (IFDM) systems, where drainage water generated from higher-valued crops grown on high quality soils are used to irrigate salt tolerant crops grown on marginal soils, is one possible strategy for managing salinity and drainage problems. If the IFDM crop were a biofuel crop, both environmental and private benefits may be generated; however, little is known about this possibility. As such, we develop a bio-economic programming model of irrigated agricultural production to examine the role salt-tolerant biofuel crops might play within an IFDM system. Our results, generated by optimizing profits over land, water, and crop choice decisions subject to resource constraints, suggest that based on the private profits alone, biofuel crops can be a competitive alternative to the common practices of land retirement and non-biofuel crop production under both low to high drainage water salinity. Yet, IFDM biofuel crop production generates 30 to 35 percent fewer GHG emissions than the other strategies. The private market competitiveness coupled with the public good benefits may justify policy changes encouraging the growth of IFDM biofuel crops in arid agricultural areas globally. This article is protected by copyright. All rights reserved.
- A regional and non-stationary model for Partial Duration Series of extreme
- Authors: Ida Bülow Gregersen; Henrik Madsen, Dan Rosbjerg, Karsten Arnbjerg-Nielsen
Abstract: Regional extreme value models for estimation of extreme rainfall intensities are widely applied, but their underlying assumption of stationarity is challenged. Many recent studies show that the rainfall extremes worldwide exhibit a non-stationary behavior. This paper presents a spatio-temporal model of extreme rainfall. The framework is built on a Partial Duration Series approach with a non-stationary, regional threshold value. The model is based on Generalized Linear Regression solved by Generalized Estimation Equations. It allows a spatial correlation between the stations in the network and accounts furthermore for variable observation periods at each station and in each year. Marginal regional and temporal regression models solved by Generalized Least Squares are used to validate and discuss the results of the full spatio-temporal model.The model is applied on data from a large Danish rain gauge network for four durations ranging from 10 minutes to 24 hours. The observation period differs between stations, and the number of stations with more than 10 years of observations has increased over the years. A spatio-temporal model for the threshold is suggested, applying the Mean Annual Precipitation and time as the explanatory variables in the regional and temporal domain, respectively. Further analysis of Partial Duration Series with non-stationary and regional thresholds shows that the mean exceedances also exhibit a significant variation in space and time for some rainfall durations, while the shape parameter is found to be constant. This article is protected by copyright. All rights reserved.
- The Future of Evapotranspiration: Global requirements for ecosystem
functioning, carbon and climate feedbacks, agricultural management, and
- Authors: Joshua B. Fisher; Forrest Melton, Elizabeth Middleton, Christopher Hain, Martha Anderson, Richard Allen, Matthew McCabe, Simon Hook, Dennis Baldocchi, Philip A. Townsend, Ayse Kilic, Kevin Tu, Diego Miralles, Johan Perret, Jean-Pierre Lagouarde, Duane Waliser, Adam J. Purdy, Andrew French, David Schimel, James S. Famiglietti, Graeme Stephens, Eric F. Wood
Abstract: The fate of the terrestrial biosphere is highly uncertain given recent and projected changes in climate. This is especially acute for impacts associated with changes in drought frequency and intensity on the distribution and timing of water availability. The development of effective adaptation strategies for these emerging threats to food and water security are compromised by limitations in our understanding of how natural and managed ecosystems are responding to changing hydrological and climatological regimes. This information gap is exacerbated by insufficient monitoring capabilities from local to global scales. Here, we describe how evapotranspiration (ET) represents the key variable in linking ecosystem functioning, carbon and climate feedbacks, agricultural management, and water resources, and highlight both the outstanding science and applications questions and the actions, especially from a space-based perspective, necessary to advance them. This article is protected by copyright. All rights reserved.
- A new model for predicting the drag exerted by vegetation canopies
- Authors: Vahid Etminan; Ryan J. Lowe, Marco Ghisalberti
Abstract: The influence of vegetation canopies on the flow structure in streams, rivers and floodplains is heavily dependent on the cumulative drag forces exerted by the vegetation. The drag coefficient of vegetation elements within a canopy has been shown to be significantly different to well-established values for a single element in isolation. This study investigates the mechanisms that determine canopy flow resistance and proposes a new model for predicting canopy drag coefficients. Large Eddy Simulations were used to investigate the fine-scale hydrodynamics within emergent canopies with solid area fractions (λ) ranging from 0.016 to 0.25. The influence of three mechanisms in modifying canopy drag, namely blockage, sheltering and delayed separation, were investigated. While the effects of sheltering and delayed separation were found to slightly reduce the drag of very sparse canopies, the blockage effect significantly increased the drag of denser canopies (λ ≳ 0.04). Furthermore, an analogy drawn between canopy flow and wall-confined flow around bluff bodies is used to propose an alternative reference velocity to the conventional spatially-averaged velocity, namely the constricted cross-section velocity (Uc), to redefine the canopy drag coefficient. Through comparison with both prior experimental data and the present numerical simulations, typical formulations for the drag coefficient of a single cylinder are shown to accurately predict the drag coefficient of staggered emergent canopies when Uc is used as the reference velocity. Finally, it is shown that this new model can be extended to predict the bulk drag coefficient of randomly-arranged vegetation canopies. This article is protected by copyright. All rights reserved.
- Validation of SMAP soil moisture for the SMAPVEX15 field campaign using a
- Authors: Xitian Cai; Ming Pan, Nathaniel W. Chaney, Andreas Colliander, Sidharth Misra, Michael H. Cosh, Wade T. Crow, Thomas J. Jackson, Eric F. Wood
Abstract: Accurate global mapping of soil moisture is the goal of the Soil Moisture Active Passive (SMAP) mission, which is expected to improve the estimation of water, energy, and carbon exchanges between the land and the atmosphere. Like other satellite products, the SMAP soil moisture retrievals need to be validated, with the validation relying heavily on in situ measurements. However, a one-to-one comparison is ill-advised due to the spatial mismatch of the large SMAP footprint (∼40 km) and the point scale in situ measurements. This study uses a recently developed hyper-resolution land surface model—HydroBlocks—as a tool to upscale in situ soil moisture measurements for the SMAPVEX15 (SMAP Validation Experiment 2015) field campaign during August 2-18, 2015. Calibrated against in situ observation, HydroBlocks shows a satisfactory Kling-Gupta efficiency (KGE) of 0.817 and RMSE of 0.019 m3/m3 for the calibration period. These results indicate that HydroBlocks can be used to upscale in situ measurements for this site. Different from previous studies, here in situ measurements are upscaled using a land surface model without bias correction. The upscaled soil moisture is then used to evaluate SMAP (passive) soil moisture products. The comparison of the upscaled network to SMAP shows that the retrievals are generally able to capture the areal-averaged soil moisture temporal variations. However, SMAP appears to be over sensitive to summer precipitation. We expect these findings can be used to improve the SMAP soil moisture product and thus facilitate its usage in studying the water, energy, and carbon cycles. This article is protected by copyright. All rights reserved.
- Development of a Water and Enthalpy Budget-based Glacier mass balance
Model (WEB-GM) and its preliminary validation
- Authors: Baohong Ding; Kun Yang, Wei Yang, Xiaobo He, Yingying Chen, Zhu La, Xiaofeng Guo, Lei Wang, Hui Wu, Tandong Yao
Abstract: This paper presents a new water and energy budget-based glacier mass balance model. Enthalpy, rather than temperature, is used in the energy balance equations to simplify the computation of the energy transfers through the water phase change and the movement of liquid water in the snow. A new parameterization for albedo estimation and state-of-the-art parameterization schemes for rainfall/snowfall type identification and surface turbulent heat flux calculations are implemented in the model. This model was driven with meteorological data and evaluated using mass balance and turbulent flux data collected during a field experiment implemented in the ablation zone of the Parlung No. 4 Glacier on the Southeast Tibetan Plateau during 2009 and 2015–2016. The evaluation shows that the model can reproduce the observed glacier ablation depth, surface albedo, surface temperature, sensible heat flux, and latent heat flux with high accuracy. Comparing with a traditional energy budget-based glacier mass balance model, this enthalpy-based model shows a superior capacity in simulation accuracy. Therefore, this model can reasonably simulate the energy budget and mass balance of glacier melting in this region and be used as a component of land surface models and hydrological models. This article is protected by copyright. All rights reserved.
- Potential effects of landscape change on water supplies in the presence of
- Authors: Andrew J. Guswa; Perrine Hamel, P. James Dennedy-Frank
Abstract: This work presents a set of methods to evaluate the potential effects of landscape changes on water supplies. Potential impacts are a function of the seasonality of precipitation, losses of water to evapotranspiration and deep recharge, the flow-regulating ability of watersheds, and the availability of reservoir storage. For a given reservoir capacity, simple reservoir simulations with daily precipitation and streamflow enable the determination of the maximum steady supply of water for both the existing watershed and a hypothetical counter-factual that has neither flow-regulating benefits nor any losses. These two supply values, representing land-use end-members, create an envelope that defines the water supply-service and bounds the effect of landscape change on water supply. These bounds can be used to discriminate between water supplies that may be vulnerable to landscape change and those that are unlikely to be affected. Two indices of the water-supply service exhibit substantial variability across 593 watersheds in the continental United States. Rcross, the reservoir capacity at which landscape change is unlikely to have any detrimental effect on water supply has an interquartile range of 0.14% to 4% of mean-annual-streamflow. Steep, forested watersheds with seasonal climates tend to have greater service values, and the indices of water-supply service are positively correlated with runoff ratios during the months with lowest flows. This article is protected by copyright. All rights reserved.
- The mechanistic basis for storage-dependent age distributions of water
discharged from an experimental hillslope
- Authors: Luke A. Pangle; Minseok Kim, Charlene Cardoso, Marco Lora, Antonio A. Meira Neto, Till H.M. Volkmann, Yadi Wang, Peter A Troch, Ciaran J. Harman
Abstract: Distributions of water transit times (TTDs), and related storage-selection (SAS) distributions, are spatially-integrated metrics of hydrological transport within landscapes. Recent works confirm that the form of TTDs and SAS distributions should be considered time-variant—possibly depending, in predictable ways, on the dynamic storage of water within the landscape. We report on a 28-day periodic-steady-state-tracer experiment performed on a model hillslope contained within a 1-m3 sloping lysimeter. Using experimental data, we calibrate physically-based, spatially-distributed flow and transport models, and use the calibrated models to generate time-variable SAS distributions, which are subsequently compared to those directly observed from the actual experiment. The objective is to use the spatially-distributed estimates of storage and flux from the model to characterize how temporal variation in water storage influences temporal variation in flow-path configurations, and resulting SAS distributions. The simulated SAS distributions mimicked well the shape of observed distributions, once the model domain reflected the spatial heterogeneity of the lysimeter soil. The spatially-distributed flux vectors illustrate how the magnitude and directionality of water flux changes as the water-table surface rises and falls, yielding greater contributions of younger water when the water-table surface rises nearer to the soil surface. The illustrated mechanism is compliant with conclusions drawn from other recent studies, and supports the notion of an inverse-storage effect, whereby the probability of younger water exiting the system increases with storage. This mechanism may be prevalent in hillslopes and headwater catchments where discharge dynamics are controlled by vertical fluctuations in the water-table surface of an unconfined aquifer. This article is protected by copyright. All rights reserved.
- Incorporating geologic information into hydraulic tomography: A general
framework based on geostatistical approach
- Authors: Yuanyuan Zha; Tian-Chyi J. Yeh, Walter A. Illman, Hironori Onoe, Chin Man, W. Mok, Jet-Chau Wen, Shao-Yang Huang, Wenke Wang
Abstract: Hydraulic tomography (HT) has become a mature aquifer test technology over the last two decades. It collects non-redundant information of aquifer heterogeneity by sequentially stressing the aquifer at different wells and collecting aquifer responses at other wells during each stress. The collected information is then interpreted by inverse models. Among these models, the geostatistical approaches, built upon the Bayesian framework, first conceptualize hydraulic properties to be estimated as random fields, which are characterized by means and covariance functions. They then use the spatial statistics as prior information with the aquifer response data to estimate the spatial distribution of the hydraulic properties at a site. Since the spatial statistics describe the generic spatial structures of the geologic media at the site rather than site-specific ones (e.g., known spatial distributions of facies, faults, or paleochannels), the estimates are often not optimal. To improve the estimates, we introduce a general statistical framework, which allows the inclusion of site-specific spatial patterns of geologic features. Subsequently, we test this approach with synthetic numerical experiments. Results show that this approach, using conditional mean and covariance that reflect site-specific large-scale geologic features, indeed improves the HT estimates. Afterward, this approach is applied to HT surveys at a kilometer-scale fractured granite field site with a distinct fault zone. We find that by including fault information from outcrops and boreholes for HT analysis, the estimated hydraulic properties are improved. The improved estimates subsequently lead to better prediction of flow during a different pumping test at the site. This article is protected by copyright. All rights reserved.
- Gas bubble size estimation in peat soils from EM wave scattering observed
with ground penetrating radar
- Authors: Neil Terry; Lee Slater
Abstract: The size of biogenic gas bubbles in peatlands is believed to regulate ebullition of carbon gases to the atmosphere. The measurement of electromagnetic (EM) wave travel times using ground penetrating radar (GPR) is a proven field-scale method for indirect estimation of volumetric gas content. However, there is also the possibility that information on the size of the gas bubbles can be determined from the analysis of the spectral content of GPR signals as scattering attenuation possesses a frequency dependence for bubbles smaller than the EM wavelength (Rayleigh type scattering). Theoretical modeling shows that GPR data acquired with typical antenna frequencies are likely to be affected by bubble size in peat soils. Analysis of GPR data from two recent studies on peat monoliths where biogenic gas production was documented produced results consistent with the model predictions. Using the approach, zero offset cross borehole GPR data in a northern peatland suggest that large bubble clusters (i.e., 0.05 m radius) occur in peat. These findings broaden the utility of GPR for providing information on biogenic gas dynamics in peatlands. This article is protected by copyright. All rights reserved.
- Developing reservoir monthly inflow forecasts using Artificial
Intelligence and Climate Phenomenon Information
- Authors: Tiantian Yang; Ata Akbari Asanjan, Edwin Welles, Xiaogang Gao, Soroosh Sorooshian, Xiaomang Liu
Abstract: Reservoirs are fundamental human-built infrastructures that collect, store, and deliver fresh surface water in a timely manner for many purposes. Efficient reservoir operation requires policy makers and operators to understand how reservoir inflows are changing under different hydrological and climatic conditions to enable forecast-informed operations. Over the last decade, the uses of Artificial Intelligence and Data Mining (AI & DM) techniques in assisting reservoir streamflow sub-seasonal to seasonal forecasts have been increasing. In this study, Random Forest (RF), Artificial Neural Network (ANN) and Support Vector Regression (SVR), are employed and compared with respect to their capabilities for predicting one-month-ahead reservoir inflows for two headwater reservoirs in USA and China, respectively. Both current and lagged hydrological information and 17 known climate phenomenon indices, i.e. PDO and ENSO, etc., are selected as predictors for simulating reservoir inflows. Results show (1) three methods are capable of providing monthly reservoir inflows with satisfactory statistics; (2) the results obtained by Random Forest have the best statistical performances compared with the other two methods; (3) another advantage of Random Forest algorithm is its capability of interpreting raw model inputs; (4) climate phenomenon indices are useful in assisting monthly or seasonal forecasts of reservoir inflow; and (5) different climate conditions are auto-correlated with up to several months, and the climatic information and their lags are cross-correlated with local hydrological conditions in our case studies. This article is protected by copyright. All rights reserved.
- Tropical river suspended sediment and solute dynamics in storms during an
- Authors: Kathryn E. Clark; James B. Shanley, Martha A. Scholl, Nicolas Perdrial, Julia N. Perdrial, Alain F. Plante, William H. McDowell
Abstract: Droughts, which can strongly affect both hydrologic and biogeochemical systems, are projected to become more prevalent in the tropics in the future. We assessed the effects of an extreme drought during 2015 on stream water composition in the Luquillo Mountains of Puerto Rico. We demonstrated that drought baseflow in the months leading up to the study was sourced from trade-wind orographic rainfall, suggesting a resistance to the effects of an otherwise extreme drought. In two catchments (Mameyes and Icacos), we sampled a series of four rewetting events that partially alleviated the drought. We collected and analyzed dissolved constituents (major cations and anions, organic carbon and nitrogen) and suspended sediment [inorganic and organic matter (particulate organic carbon and particulate nitrogen)]. The rivers appeared to be resistant to extreme drought, recovering quickly upon rewetting, as 1) the concentration - discharge (C-Q) relationships deviated little from the long-term patterns; 2) “new water” dominated streamflow during the latter events; 3) suspended sediment sources had accumulated in the channel during the drought flushed out during the initial events; and 4) the severity of the drought, as measured by the US drought monitor, was reduced dramatically after the rewetting events. Through this interdisciplinary study we were able to investigate the impact of extreme drought through rewetting events on the river biogeochemistry. This article is protected by copyright. All rights reserved.
- Space-time duality for the fractional advection dispersion equation
- Authors: James F. Kelly; Mark M. Meerschaert
Abstract: The fractional advection dispersion equation replaces the second spatial derivative in the usual advection dispersion equation with a fractional derivative in the spatial variable. It was first applied to tracer tests in underground aquifers, and later to tracer tests in rivers. An alternative model replaces the first time derivative with a fractional derivative in time. Previous work has shown that both models provide a reasonable fit to breakthrough curves in rivers, which has led to a controversy regarding the physically appropriate fractional model. This paper shows that the relevant space fractional model is mathematically equivalent to the corresponding time-fractional model, thus resolving the controversy. This article is protected by copyright. All rights reserved.
- Quantifying local rainfall dynamics and uncertain boundary conditions into
a nested regional-local flood modelling system
- Authors: Maria Bermudez; Jeffrey C. Neal, Paul D. Bates, Gemma Coxon, Jim E. Freer, Luis Cea, Jeronimo Puertas
Abstract: Inflow discharge and outflow stage estimates for hydraulic flood models are generally derived from river gauge data. Uncertainties in the measured inflow data and the neglect of rainfall-runoff contributions to the modelled domain downstream of the gauging locations can have a significant impact on these estimated ‘whole reach' inflows and consequently on flood predictions. In this study, a method to incorporate rating curve uncertainty and local rainfall-runoff dynamics into the predictions of a reach-scale flood model is proposed. The methodology is applied to the July 2007 floods of the River Severn in the UK. Discharge uncertainty bounds are generated applying a non-parametric local weighted regression approach to stage-discharge measurements for two gauging stations. Measured rainfall downstream from these locations is used as input to a series of sub-catchment regional hydrological model to quantify additional local inflows along the main channel. A regional simplified-physics hydraulic model is then applied to combine these contributions and generate an ensemble of discharge and water elevation time series at the boundaries of a local-scale high complexity hydraulic model. Finally, the effect of these rainfall dynamics and uncertain boundary conditions are evaluated on the local-scale model. Accurate prediction of the flood peak was obtained with the proposed method, which was only possible by resolving the additional complexity of the extreme rainfall contributions over the modelled area. The findings highlight the importance of estimating boundary condition uncertainty and local rainfall contributions for accurate prediction of river flows and inundation at regional scales. This article is protected by copyright. All rights reserved.
- A new drought index that considers the joint effects of climate and land
- Authors: Meixian Liu; Xianli Xu, Chaohao Xu, Alexander Y. Sun, Kelin Wang, Bridget R. Scanlon, Lu Zhang
Abstract: This study proposes a hydrological drought index, the standardized wetness index (SWI), by combining the structure of the Standardized Precipitation-Evapotranspiration Index and actual-evaporation-based residual water-energy ratio, in which actual evaporation is estimated using the Budyko hypothesis. The SWI requires three parameters, precipitation, potential evaporation, and parameter n of a Budyko-type formulae. Based on different types of n (fixed or dynamic), SWI can be used to estimate the dryness/wetness resulting from climate change (variability) solely, and from the joint effects of climate and land surface change (variability). Performance of SWI is evaluated using historical droughts and by comparing to the self-calibrated Palmer Drought Severity Index. Results show that SWI effectively captures global droughts. Furthermore, a case study in two catchments with significant land surface modification, indicates that the joint effects of climate and land surface have greater impacts on dryness/wetness in the water-limited Wuding catchment than in the energy-limited Poyang catchment. This article is protected by copyright. All rights reserved.
- Field estimates of groundwater circulation depths in two mountainous
watersheds in the western U.S. and the effect of deep circulation on
solute concentrations in streamflow
- Authors: Marty D. Frisbee; Douglas G. Tolley, John L. Wilson
Abstract: Estimates of groundwater circulation depths based on field data are lacking. These data are critical to inform and refine hydrogeologic models of mountainous watersheds, and to quantify depth- and time-dependencies of weathering processes in watersheds. Here we test two competing hypotheses on the role of geology and geologic setting in deep groundwater circulation and the role of deep groundwater in the geochemical evolution of streams and springs. We test these hypotheses in two mountainous watersheds that have distinctly different geologic settings (one crystalline, metamorphic bedrock and the other volcanic bedrock). Estimated circulation depths for springs in both watersheds range from 0.6 to 1.6 km and may be as great as 2.5 km. These estimated groundwater circulation depths are much deeper than commonly modeled depths suggesting that we may be forcing groundwater flowpaths too shallow in models. In addition, the spatial relationships of groundwater circulation depths are different between the two watersheds. Groundwater circulation depths in the crystalline bedrock watershed increase with decreasing elevation indicative of topography-driven groundwater flow. This relationship is not present in the volcanic bedrock watershed suggesting that both the source of fracturing (tectonic versus volcanic) and increased primary porosity in the volcanic bedrock play a role in deep groundwater circulation. The results from the crystalline bedrock watershed also indicate that relatively deep groundwater circulation can occur at local-scales in headwater drainages less than 9.0 km2 and at larger fractions than commonly perceived. Deep groundwater is a primary control on streamflow processes and solute concentrations in both watersheds. This article is protected by copyright. All rights reserved.
- Steady-state fractionation of heavy noble gas isotopes in a deep
- Authors: Alan M. Seltzer; Jeffrey P. Severinghaus, Brian J. Andraski, David A. Stonestrom
Abstract: To explore steady-state fractionation processes in the unsaturated zone (UZ), we measured argon, krypton and xenon isotope ratios throughout a ∼110-m deep UZ at the United States Geological Survey (USGS) Amargosa Desert Research Site (ADRS) in Nevada, USA. Prior work has suggested that gravitational settling should create a nearly linear increase in heavy-to-light isotope ratios toward the bottom of stagnant air columns in porous media. Our high-precision measurements revealed a binary mixture between 1) expected steady-state isotopic compositions, and 2) unfractionated atmospheric air. We hypothesize that the presence of an unsealed pipe connecting the surface to the water table allowed for direct inflow of surface air in response to extensive UZ gas sampling prior to our first (2015) measurements. Observed isotopic resettling in deep UZ samples collected a year later, after sealing the pipe, supports this interpretation. Data and modeling each suggest that the strong influence of gravitational settling and weaker influences of thermal diffusion and fluxes of CO2 and water vapor accurately describe steady-state isotopic fractionation of argon, krypton and xenon within the UZ. The data confirm that heavy noble gas isotopes are sensitive indicators of UZ depth. Based on this finding, we outline a potential inverse approach to quantify past water-table depths from noble gas isotope measurements in paleogroundwater, after accounting for fractionation during dissolution of UZ air and bubbles. This article is protected by copyright. All rights reserved.
- Groundwater similarity across a watershed derived from time-warped and
flow-corrected time series
- Authors: M. Rinderer; B. L. McGlynn, H.J. van Meerveld
Abstract: Information about catchment-scale groundwater dynamics is necessary to understand how catchments store and release water and why water quantity and quality varies in streams. However, groundwater level monitoring is often restricted to a limited number of sites. Knowledge of the factors that determine similarity between monitoring sites can be used to predict catchment-scale groundwater storage and connectivity of different runoff source areas. We used distance-based and correlation-based similarity measures to quantify the spatial and temporal differences in shallow groundwater similarity for 51 monitoring sites in a Swiss pre-alpine catchment. The 41 months long time series were pre-processed using dynamic time-warping and a flow-corrected time transformation to account for small timing differences and bias towards low-flow periods. The mean distance-based groundwater similarity was correlated to topographic indices, such as upslope contributing area, Topographic Wetness Index and local slope. Correlation-based similarity was less related to landscape position but instead revealed differences between seasons. Analysis of Variance and Partial Mantel tests showed that landscape position, represented by the Topographic Wetness Index, explained 52% of the variability in mean distance-based groundwater similarity, while spatial distance, represented by the Euclidean distance, explained only 5%. The variability in distance-based similarity and correlation-based similarity between groundwater and streamflow time series was significantly larger for midslope locations than for other landscape positions. This suggests, that groundwater dynamics at these midslope sites, which are important in order to understand runoff source areas and hydrological connectivity at the catchment-scale, are most difficult to predict. This article is protected by copyright. All rights reserved.
- Preferences for policy attributes and willingness to pay for water quality
improvements under uncertainty
- Authors: Jeffrey D. Mullen; Kayla C. Calhoun, Gregory J. Colson
Abstract: When exploring environmental policy options, sometimes neither the current state of the environmental good being analyzed nor the effectiveness of the proposed policy is known with certainty. This is the case with privately-owned, residential, onsite wastewater treatment systems (septic systems) – there is ample evidence that they can contribute to water quality impairment, but their contribution is generally stochastic in nature and the efficacy of technological solutions is uncertain. Furthermore, the benefits of ameliorating water quality impairments are public in nature. Septic system owners are legally responsible for maintaining their systems, but requiring them to upgrade otherwise properly functioning tanks is outside the scope of water quality regulations. An incentive structure is necessary to induce private homeowners to invest in septic upgrades that deliver both private benefits in addition to the positive externality for the wider public and environment. The question for policy makers is how these private incentives should be financed, and whether public support can be garnered. Results of a choice experiment in Gwinnett County, Georgia, accounting for both sources of uncertainty – the current state of water quality and the efficacy of the intervention – in the design of water quality policy are presented. We find baseline water quality conditions and policy efficacy significantly affect public support for a policy transferring public funds to private homeowners, in terms of both sentiment and willingness to pay. The manner in which costs are shared across stakeholders also affects the selection of a policy option, but not willingness to pay for it. This article is protected by copyright. All rights reserved.
- Changes in cold region flood regimes inferred from long record reference
- Authors: Donald H. Burn; Paul H. Whitfield
Abstract: Variability and nonstationarity in flood regimes of cold regions are examined using data from hydrometric reference streamflow gauging stations from 27 natural watersheds in Canada and adjacent areas of the United States. Choosing stations from reference networks with nearly 100 years of data allows for the investigation of changes that span several phases of some of the atmospheric drivers that may influence flood behaviour. The reference hydrologic networks include only stations considered to have good quality data and were screened to avoid the influences of regulation, diversions, or land use change. Changes and variations in flood regimes are complex and require a multifaceted approach to properly characterize the types of changes that have occurred and are likely to occur in the future. Peaks over threshold (POT) data are extracted from daily flow data for each watershed and changes to the magnitude, timing, frequency, volume and duration of threshold exceedences are investigated. Seasonal statistics are used to explore changes in the nature of the flood regime based on changes in the timing of flood threshold exceedences. A variety of measures are developed to infer flood regime shifts including from a nival regime to a mixed regime and a mixed regime to a more pluvial-dominated regime. The flood regime at many of the watersheds demonstrates increased prominence of rainfall floods and decreased prevalence of snowmelt contributions to flood responses. While some individual stations show a relationship between flood variables and climate indices, these relationships are generally weak. This article is protected by copyright. All rights reserved.
- The effects of spatial resolution and dimensionality on modeling
regional-scale hydraulics in a multichannel river
- Authors: Elizabeth H. Altenau; Tamlin M. Pavelsky, Paul D. Bates, Jeffrey C. Neal
Abstract: As modeling capabilities at regional and global scales improve, questions remain regarding the appropriate process representation required to accurately simulate multichannel river hydraulics. This study uses the hydrodynamic model LISFLOOD-FP to simulate patterns of water surface elevation (WSE), depth, and inundation extent across a ∼90 km, anabranching reach of the Tanana River, Alaska. To provide boundary conditions, we collected field observations of bathymetry and WSE during a two-week field campaign in summer 2013. For the first time at this scale, we test a simple, raster-based model's capabilities to simulate 2D, in-channel patterns of WSE and inundation extent. Additionally, we compare finer resolution (≤ 25 m) 2D models to four other models of lower dimensionality and coarser resolution (100–500 m) to determine the effects of simplifying process representation. Results indicate that simple, raster-based models can accurately simulate 2D, in-channel hydraulics in the Tanana. Also, the fine-resolution, 2D models produce lower errors in spatiotemporal outputs of WSE and inundation extent compared to coarse-resolution, 1D models: 22.6 cm vs. 56.4 cm RMSE for WSE, and 90% vs. 41% Critical Success Index values for simulating inundation extent. Incorporating the anabranching channel network using subgrid representations for smaller channels is important for simulating accurate hydraulics and lowers RMSE in spatially distributed WSE by at least 16%. As a result, better representation of the converging and diverging multichannel network by using subgrid solvers or downscaling techniques in multichannel rivers is needed to improve errors in regional to global scale models. This article is protected by copyright. All rights reserved.
- Hydro-geomorphic perturbations on the soil-atmosphere CO2 exchange: How
(un)certain are our balances?
- Authors: Yannis G. Dialynas; Rafael L. Bras, Daniel deB. Richter
Abstract: Attempts to estimate the influence of erosion on the carbon (C) cycle are limited by difficulties in accounting for the fate of mobilized organic material and for the uncertainty associated with land management practices. This study proposes a method to quantify the uncertainty introduced by the influence of land management on soil organic C (SOC) generation and decomposition at eroding soils. The framework is implemented in tRIBS-ECO (Triangulated Irregular Network-based Real-time Integrated Basin Simulator-Erosion and Carbon Oxidation). tRIBS-ECO is a spatially- and depth-explicit model of C dynamics coupled with a process-based hydro-geomorphic model. We assess the impact of soil erosion on the net soil-atmosphere CO2 exchange at the Calhoun Critical Zone Observatory, one of the most severely agriculturally eroded regions in the U.S. Measurements of SOC storage are used from different catena positions. We demonstrate that the spatio-temporal variations of land management practices introduce significant uncertainty in estimates of the erosion-induced CO2 exchange with the atmosphere. Observations and simulations suggest that a substantial portion of eroded organic material is buried in alluvial sediments at the study site. According to results, recent reforestation led to a partial decline in soil and SOC erosion rates. It is suggested that the representation of the fine spatio-temporal variability of the dynamics of eroded C is important in the computation of C budgets in regional and global scales. This article is protected by copyright. All rights reserved.
- An estimation of the main wetting branch of the soil water retention curve
based on its main drying branch using the machine learning method
- Authors: Krzysztof Lamorski; Jiří Šimůnek, Cezary Sławiński, Joanna Lamorska
Abstract: In this paper, we estimated using the machine learning methodology the main wetting branch of the soil water retention curve based on the knowledge of the main drying branch and other, optional, basic soil characteristics (particle size distribution, bulk density, organic matter content, or soil specific surface). The support vector machine algorithm, was used for the models' development. The data needed by this algorithm for model training and validation consisted of 104 different undisturbed soil core samples collected from the topsoil layer (A horizon) of different soil profiles in Poland. The main wetting and drying branches of SWRC, as well as other basic soil physical characteristics, were determined for all soil samples. Models relying on different sets of input parameters were developed and validated. The analysis showed that taking into account other input parameters (i.e., particle size distribution, bulk density, organic matter content, or soil specific surface) than information about the drying branch of the SWRC has essentially no impact on the models' estimations. Developed models are validated and compared with well-known models that can be used for the same purpose, such as the Mualem  (M77) and Kool and Parker  (KP87) models. The developed models estimate the main wetting SWRC branch with estimation errors (RMSE=0.018 m3/m3) that are significantly lower than those for the M77 (RMSE=0.025 m3/m3) or KP87 (RMSE=0. 047 m3/m3) models. This article is protected by copyright. All rights reserved.
- An Eulerian equation for snow accumulation downstream of an object
- Authors: Noriaki Ohara
Abstract: This study investigated the form of the governing equation for the particle distribution by focusing on the particle motion processes rather than flow regime and particle characteristics. A linear erosion term for a fetch-eddy effect was introduced to the advection dispersion equation. The equation formulated in this paper described most of the particle deposit patterns behind an object including porous and solid snow fences, and a tree. This theory may enable us to estimate particle motion parameters, such as diffusion, drift, and erosion coefficients, from field observed particle distributions. Snow stratigraphy observed by ground penetrating radar (GPR) was used verify to result of the modeled theoretical snow redistribution. These analyses confirmed the effectiveness of the linear erosion term at explaining the particle deposition patterns due to eddys around a porous snow fence. This article is protected by copyright. All rights reserved.
- Assessment of uncertainties in global land cover products for hydroclimate
modeling in India
- Authors: C. G. Madhusoodhanan; K. G. Sreeja, T. I. Eldho
Abstract: Earth's land cover (LC) has significant influence on land-atmospheric processes and affects the climate at multiple scales. There are multiple Global LC (GLC) datasets which are yet to be evaluated for uncertainties and their propagation into the simulation of land surface fluxes (LSFs) in land surface/climate modeling. The present study assesses the uncertainties in seven GLC products with reference to a regional dataset for the simulation of LSFs in India using a macro-scale land surface model. There is considerable overestimation of the extent of croplands in most of the GLCs. The uncertainties in LCs exert significant bias in the simulation of the LSFs of actual evapotranspiration (ETa), latent heat (LE) and sensible heat (H) fluxes. Error propagation in LSFs is proportional to the bias in cropping intensity under rainfed condition. The high under-representation of cropland area in the UMd dataset results in highest bias in LSFs whereas the least cropland bias in Globland30 leads to least bias. Irrigation has higher potential to alter the LSFs than uncertainties related to LC especially in regions with large area under irrigation like India. The changes in LSFs are higher in arid/semi-arid regions with medium irrigation intensity than in sub-humid regions with high irrigation intensity. This has significant implications for the country's future irrigation expansion plans in the arid/semi-arid regions. The study also emphasizes the need for focused efforts to quantify the uncertainties from varying irrigation intensities in the next generation CMIP6 experiments. This article is protected by copyright. All rights reserved.
- An index for drought induced financial risk in the mining industry
- Authors: L. Bonnafous; U. Lall, J. Siegel
Abstract: Water scarcity has emerged as a potential risk for mining operations. High capital spending for desalination and water conflicts leading to asset stranding have recently occurred. Investors in mining companies are interested in the exposure to such risks across portfolios of mining assets (whether the practical at-site consequences are foregone production, higher OPEX and CAPEX and ensuing lost revenues, or asset-stranding). In this paper, an index of the potential financial exposure of a portfolio is developed and its application is illustrated. Since the likely loss at each mine is hard to estimate a priori, one needs a proxy for potential loss. The index considers drought duration, severity and frequency (defined by a return-level in years) at each mining asset, and provides a measure of financial exposure through weighing of production or Net Asset Value. Changes in human needs are not considered, but are relevant, and could be incorporated if global data on mine and other water use were available at the appropriate resolution. Potential for contemporaneous drought incidence across sites in a portfolio is considered specifically. Through an appropriate choice of drought thresholds, an analyst can customize a scenario to assess potential losses in production value or profits, or whether conflicts could emerge that would lead to stranded assets or capital expenditure to secure alternate water supplies. Global climate data sets that allow a customized development of such an index are identified, and selected mining company portfolios are scored as to the risk associated with one publicly available drought index. This article is protected by copyright. All rights reserved.
- Tracer-based characterization of hyporheic exchange and benthic biolayers
- Authors: Julia L.A. Knapp; Ricardo González-Pinzón, Jennifer D. Drummond, Laurel G. Larsen, Olaf A. Cirpka, Judson W. Harvey
Abstract: Shallow benthic biolayers at the top of the streambed are believed to be places of enhanced biogeochemical turnover within the hyporheic zone. They can be investigated by reactive stream tracer tests with tracer recordings in the streambed and in the stream channel. Common in-stream measurements of such reactive tracers cannot localize where the processing primarily takes place, whereas isolated vertical depth profiles of solutes within the hyporheic zone are usually not representative of the entire stream. We present results of a tracer test where we injected the conservative tracer bromide together with the reactive tracer resazurin into a third-order stream and combined the recording of in-stream breakthrough curves with multi-depth sampling of the hyporheic zone at several locations. The transformation of resazurin was used as an indicator of metabolism, and high-reactivity zones were identified from depth profiles. The results from our subsurface analysis indicate that the potential for tracer transformation (i.e., the reaction rate constant) varied with depth in the hyporheic zone. This highlights the importance of the benthic biolayer, which we found to be on average 2 cm thick in this study, which ranged from one third to one half of the full depth of the hyporheic zone. The reach-scale approach integrated the effects of processes along the reach length, isolating hyporheic processes relevant for whole-stream chemistry and estimating effective reaction rates. This article is protected by copyright. All rights reserved.
- The influence of NaCl concentration on salt precipitation in heterogeneous
- Authors: Mina Bergstad; Dani Or, P.J. Withers, Nima Shokri
Abstract: Evaporation of saline solutions from porous media is governed by the complex interactions between the transport properties of the porous media, the evaporating solution and the external boundary conditions. In the present study, we have investigated the effects of salt concentration on the evaporation process from porous media in the presence of a sharp textural discontinuity; a common heterogeneity in natural porous media formed due to the weathering or formation of soil horizons, wind deposition and erosion. We have conducted a comprehensive series of macro- and micro-scale experiments to delineate how the precipitation pattern is modified as salt concentration varies from relatively low values to a concentration close to the solubility limit. For concentrations much less than the solubility limit, the precipitation begins at the coarse-textured part of the heterogeneous porous media (which is a counter-intuitive result considering the preferential evaporation of water from the fine-textured part of the heterogeneous surface). However, when the concentration is close to the solubility limit, precipitation initiates preferentially at the fine-textured part of the heterogeneous porous surface. This behaviour results from the interaction between the transport properties of the porous media and the properties of the evaporating solution which must be considered. Additionally, using pore-scale images obtained by X-ray micro-computed tomography (CT), we have visualized the dynamics of precipitation in the presence of heterogeneity at high spatial and temporal resolution. The pore-scale results corroborate the mechanisms controlling the precipitation patterns in the presence of textural discontinuities inferred from the macro-scale experiments. This article is protected by copyright. All rights reserved.
- Characterizing the spatial correlation of daily streamflows
- Authors: A. Betterle; M. Schirmer, G. Botter
Abstract: In this study we propose an analytical framework to estimate the spatial correlation of daily flows in two arbitrary locations within a given hydrologic district or river basin. The method builds on the description of the coupled streamflow dynamics at the outlet of two catchments, which are represented as correlated shot noises forced by Poisson rainfall. Novel analytical expressions for the spatial correlation of discharge are derived using a limited number of parameters that encapsulate effective precipitation regime and catchment drainage rates. The method is suited to describe how heterogeneity of climate and landscape features impact the spatial and temporal variability of flow regimes along river systems. The analysis suggests that frequency and intensity of synchronous effective rainfall events in the relevant contributing catchments are the main driver of the spatial correlation of daily discharge, unless the drainage rates of the two catchments differ by almost one order of magnitude. The method also portrays how the topological arrangement of the two outlets along the river network influences the underlying streamflow correlation, and shows how nested catchments tend to maximize the spatial correlation of flow regimes. To demonstrate the potential of the tool, the model is tested on a set of sixteen catchments belonging to a 120,000 km2 region of the United States. The application evidences satisfactory performance (RMSE
- Controls on solute concentration-discharge relationships revealed by
simultaneous hydrochemistry observations of hillslope runoff and stream
flow: The importance of critical zone structure
- Authors: Hyojin Kim; William E. Dietrich, Benjamin M. Thurnhoffer, Jim K. B. Bishop, Inez Y. Fung
Abstract: We investigated controls on concentration-discharge relationships of a catchment underlain by argillite by monitoring both groundwater along a hillslope transect and stream chemistry. Samples were collected at 1-3-day intervals over four years (2009-2013) in Elder Creek in the Eel River Critical Zone Observatory in California. Runoff at our study hillslope is driven by vadose zone flux through deeply weathered argillite (5-25 m thick) to a perched, seasonally dynamic groundwater that then drains to Elder Creek. Low flow derives from the slowly draining deepest perched groundwater that reaches equilibrium between primary and secondary minerals and saturation with calcite under high subsurface pCO2. Arriving winter rains pass through the thick vadose zone, where they rapidly acquire solutes via cation exchange reactions (driven by high pCO2), and then recharge the groundwater that delivers runoff to the stream. These new waters displayed lower solute concentrations than the deep groundwater by less than a factor of 5 (except for Ca). Up to 74% of the total annual solute flux is derived from the vadose zone. The deep groundwater's Ca concentration decreased as it exfiltrates to the stream due to CO2 degassing and this Ca loss is equivalent of 30% of the total chemical weathering flux of Elder Creek. The thick vadose zone in weathered bedrock and the perched groundwater on underlying fresh bedrock result in two distinct processes that lead to the relatively invariant (chemostatic) concentration-discharge behavior. The processes controlling solute chemistry are not evident from stream chemistry and runoff analysis alone. This article is protected by copyright. All rights reserved.
- Impacts of precipitation and potential evapotranspiration patterns on
downscaling soil moisture in regions with large topographic relief
- Authors: Garret S. Cowley; Jeffrey D. Niemann, Timothy R. Green, Mark S. Seyfried, Andrew S. Jones, Peter J. Grazaitis
Abstract: Soil moisture can be estimated at coarse resolutions (>1 km) using satellite remote sensing, but that resolution is poorly suited for many applications. The Equilibrium Moisture from Topography, Vegetation, and Soil (EMT+VS) model downscales coarse-resolution soil moisture using fine-resolution topographic, vegetation, and soil data to produce fine-resolution (10-30 m) estimates of soil moisture. The EMT+VS model performs well at catchments with low topographic relief (≤124 m), but it has not been applied to regions with larger ranges of elevation. Large relief can produce substantial variations in precipitation and potential evapotranspiration (PET), which might affect the fine-resolution patterns of soil moisture. In this research, simple methods to downscale temporal average precipitation and PET are developed and included in the EMT+VS model, and the effects of spatial variations in these variables on the surface soil moisture estimates are investigated. The methods are tested against ground truth data at the 239 km2 Reynolds Creek watershed in southern Idaho, which has 1145 m of relief. The precipitation and PET downscaling methods are able to capture the main features in the spatial patterns of both variables. The space-time Nash-Sutcliffe coefficients of efficiency of the fine-resolution soil moisture estimates improve from 0.33 to 0.36 and 0.41 when the precipitation and PET downscaling methods are included, respectively. PET downscaling provides a larger improvement in the soil moisture estimates than precipitation downscaling likely because the PET pattern is more persistent through time, and thus more predictable, than the precipitation pattern. This article is protected by copyright. All rights reserved.
- Quantifying streambed advection and conduction heat fluxes
- Authors: Daniel Caissie; Charles H. Luce
Abstract: Groundwater and accompanying heat fluxes are particularly relevant for aquatic habitats as they influence living conditions both within the river and streambed. This study focuses on the theory and the development of new equations to estimate conduction and advection heat fluxes into and out of the bed, correcting some earlier misunderstandings and adding parameterizations that extend our understanding of timing of heat fluxes. The new heat flux equations are illustrated using Catamaran Brook (New Brunswick, Canada) stream/streambed temperature data. We show important relationships between fluxes when the surface water temperature 1) follows a sinusoidal function superimposed on a steady-state condition (constant deep streambed temperature) and 2) when sinusoidal variations in stream temperature at two frequencies (annual and diel) are superimposed. When the stream temperature is used as a prescribed boundary condition, the contribution of bed fluid fluxes to stream temperature occurs through the effects of conductive thermal gradients, not through direct contribution/mixing of cold/warm water. Boundary conditions can be modified however to account for direct contribution of cold/warm water (e.g., localised upwelling) and consequences for the conduction heat flux. Equations developed allow for prediction of conductive fluxes to the bed during summer driven by diel and annual temperature fluctuations of the stream water and good agreement was observed between analytic solutions and field data. Results from this study provide a better insight into groundwater and heat fluxes which will ultimately result in better stream temperature models and a better management of fisheries resources. This article is protected by copyright. All rights reserved.
- An interdisciplinary framework for participatory modeling design and
evaluation What makes models effective participatory decision tools?
- Authors: Stefanie M. Falconi; Richard N. Palmer
Abstract: Increased requirements for public involvement in water resources management (WRM) over the past century have stimulated the development of more collaborative decision-making methods. Participatory modeling (PM) uses computer models to inform and engage stakeholders in the planning process in order to influence collaborative decisions in WRM. Past evaluations of participatory models focused on process and final outcomes, yet, were hindered by diversity of purpose and inconsistent documentation. This paper presents a two-stage framework for evaluating PM based on mechanisms for improving model effectiveness as participatory tools. The five dimensions characterize the ‘who, when, how, and why' of each participatory effort (stage 1). Models are evaluated as “boundary objects,” a concept used to describe tools that bridge understanding and translate different bodies of knowledge to improve credibility, salience, and legitimacy (stage 2). This evaluation framework is applied to five existing case studies from the literature. Though the goals of participation can be diverse, the novel contribution of the two-stage proposed framework is the flexibility it has to evaluate a wide range of cases that differ in scope, modeling approach, and participatory context. Also, the evaluation criteria provide a structured vocabulary based on clear mechanisms that extend beyond previous process- and outcome-based evaluations. Effective models are those that take advantage of mechanisms that facilitate dialogue and resolution and improve the accessibility and applicability of technical knowledge. Furthermore, the framework can help build more complete records and systematic documentation of evidence to help standardize the field of PM. This article is protected by copyright. All rights reserved.
- Nonmonotonic and spatial-temporal dynamic slope effects on soil erosion
during rainfall-runoff processes
- Authors: Songbai Wu; Minghui Yu, Li Chen
Abstract: The slope effect on flow erosivity and soil erosion still remains a controversial issue. This theoretical framework explained and quantified the direct slope effect by coupling the modified Green-Ampt equation accounting for slope effect on infiltration, 1D kinematic wave overland flow routing model, and WEPP soil erosion model. The flow velocity, runoff rate, shear stress, interrill, and rill erosion were calculated on 0°-60° isotropic slopes with equal horizontal projective length. The results show that, for short duration rainfall events, the flow erosivity and erosion amounts exhibit a bell-shaped trend which first increase with slope gradient, and then decrease after a critical slope angle. The critical slope angles increase significantly or even vanish with increasing rainfall duration but are nearly independent of the slope projective length. The soil critical shear stress, rainfall intensity and temporal patterns have great influences on the slope effect trend, while the other soil erosion parameters, soil type, hydraulic conductivity, and antecedent soil moisture have minor impacts. Neglecting the slope effect on infiltration would generate smaller erosion and reduce critical slope angles. The relative slope effect on soil erosion in physically-based model WEPP was compared to those in the empirical models USLE and RUSLE. The trends of relative slope effect were found quite different, but the difference may diminish with increasing rainfall duration. Finally, relatively smaller critical slope angles could be obtained with the equal slope length and the range of variation provides a possible explanation for the different critical slope angles reported in previous studies. This article is protected by copyright. All rights reserved.
- Subgrid parameterization for snow depth over mountainous terrain from flat
field snow depth
- Authors: N. Helbig; A. van Herwijnen
Abstract: Snow depth is an important variable for a variety of models including land-surface, meteorological and climate models. Various measurement networks were therefore developed to measure snow depth on the ground. Measurement stations are generally located in gentle terrain (flat field measurements) most often at lower or mid elevation. While these sites have provided a wealth of information, various studies have questioned the representativity of such flat field measurements for the surrounding topography, especially in alpine regions. Using highly-resolved snow depth maps at the peak of winter from two distinct climatic regions in Switzerland and in the Spanish Pyrenees, we developed two parameterizations to estimate domain-averaged snow depth in coarse-scale model applications over complex topography using easy to derive topographic parameters. The first parameterization uses a commonly applied linear lapse rate. Removing the dominant elevation gradient in mean snow depth revealed remaining underlying correlations with other topographic parameters, in particular the sky view factor. The second parameterization combines a power law elevation trend scaled with the subgrid parameterized sky view factor. Using a variety of statistic measures showed that the more complex parameterization performs better when using mean high-resolution flat field snow depth. The performances slightly decreased when formulating the parameterizations for a single flat field snow depth measurement. Nevertheless, the more complex parameterization still outperformed the linear lapse rate model. As the parameterization was developed independently of a specific geographic region we suggest it could be used to assimilate flat field snow depth or snowfall into coarse-scale snow model frameworks. This article is protected by copyright. All rights reserved.
- A new geophone device for understanding environmental impacts caused by
gravel bedload during artificial floods
- Authors: Ryota Tsubaki; Yoshihisa Kawahara, Xin-Hua Zhang, Kentaro Tsuboshita
Abstract: Here, to assess the contribution of gravel bedload on the removal of attached-algae and aquatic plants from a cobble-bed river during small floods, we propose a geophone type method for measuring the local bedload of non-uniform sized gravel. Due to limited peak discharge for focused events during our study, a large fraction of bed material (here cobbles) was immobile and only a small fraction of bed material (sand and gravel) was expected to be transported during the flushing flows we analyzed. The device we developed has a size equivalent to immobile bed material and a shape similar to bed material (rounded cobbles) at the site. The instrument's design allows avoidance of disturbances in river bed micro-topography during installation and local bedload transport during floods. A flume experiment was conducted in order to establish an empirical algorithm for estimating the diameter of impacted gravel and, here, discuss uncertainty related to diameter estimations. The proposed method was utilized to quantify gravel bedload in a cobble-bed river during flushing flows. In the text, we also discuss the contribution of measured gravel bedload during flushing flows on the removal of attached-algae (up to a 37% reduction in chlorophyll-a density) and aquatic plants (a reduction of 38% in dry mass per area). Based on time variation for the measured gravel bedload, we also suggest the propagation of a bed-form composed of the fine sediment fraction migrating on immobile larger sediment and implications for the propagation of the fine sediment wave for attached-algae removal. This article is protected by copyright. All rights reserved.
- Impact of saturation on dispersion and mixing in porous media:
Photo-bleaching pulse injection experiments and shear-enhanced mixing
- Authors: Joaquín Jiménez-Martínez; Tanguy Le Borgne, Hervé Tabuteau, Yves Méheust
Abstract: The dynamics of solute dispersion and mixing in unsaturated flows is analyzed from photo-bleaching experiments in two-dimensional porous micro-models. This technique allows producing pulse line (delta-Dirac) injections of a conservative tracer by bleaching a finite volume of fluorescent without disturbing the flow field. The temporal evolution of the concentration field and the spatial distribution of the air and water phases can be monitored at pore scale. We study the dispersion and mixing of a line of tracer under different water saturations. While dispersion in saturated porous media follows an approximately Fickian scaling, a shift to ballistic scaling is observed as soon as saturation is lowered. Hence, at the time scale of observation, dispersion in our unsaturated flows is dominated by the ballistic separation of tracer blobs within the water phase, between trapped clusters and preferential flow paths. While diffusion plays a minor role in the longitudinal dispersion during the time scale of the experiments, its interplay with fluid deformation is apparent in the dynamics of mixing. The scalar dissipation rates show an initial stretching regime, during which mixing is enhanced by fluid deformation, followed by a dissipation regime, during which diffusion overcomes compression induced by stretching. The transition between these two regimes occurs at the mixing time, when concentration gradients are maximum. We propose a predictive analytical model, based on shear-enhanced diffusion, that captures the dynamics of mixing from basic unsaturated porous media parameters, suggesting that this type of model may be a useful framework at larger scales. This article is protected by copyright. All rights reserved.
- Water quality data for national-scale aquatic research: The Water Quality
- Authors: Emily K. Read; Lindsay Carr, Laura De Cicco, Hilary A. Dugan, Paul C. Hanson, Julia A. Hart, James Kreft, Jordan S. Read, Luke A. Winslow
Abstract: Aquatic systems are critical to food, security, and society. But, water data are collected by hundreds of research groups and organizations, many of which use non-standard or inconsistent data description and dissemination, and disparities across different types of water observation systems represent a major challenge for freshwater research. In response to this, the Water Quality Portal (WQP) was developed by the U.S. Environmental Protection Agency, the U.S. Geological Survey, and the National Water Quality Monitoring Council to be a single point of access for water quality data dating back more than a century. The WQP is the largest standardized water quality data set available at the time of this writing, with more than 290 million records from more than 2.7 million sites in groundwater, inland, and coastal waters. The number of data contributors, data consumers, and third-party application developers making use of the WQP are rapidly growing. Here, we introduce the WQP, including an overview of data, the standardized data model, and data access and services; and we describe challenges and opportunities associated with using WQP data. We also demonstrate the value of the WQP data by characterizing seasonal variation in lake water clarity for regions of the continental U.S. The code used to access, download, analyze, and display this WQP data as shown in the figures is included as Supplemental Materials. This article is protected by copyright. All rights reserved.
- The future role of dams in the United States of America
- Authors: Michelle Ho; Upmanu Lall, Maura Allaire, Naresh Devineni, Hyun Han Kwon, Indrani Pal, David Raff, Dave Wegner
Abstract: Storage and controlled distribution of water have been key elements of a human strategy to overcome the space and time variability of water, which have been marked by catastrophic droughts and floods throughout the course of civilization. In the United States the peak of dam building occurred in the mid-20th century with knowledge limited to the scientific understanding and hydrologic records of the time. Ecological impacts were considered differently than current legislative and regulatory controls would potentially dictate. Additionally, future costs such as maintenance or removal beyond the economic design life were not fully considered. The converging risks associated with aging water storage infrastructure and uncertainty in climate in addition to the continuing need for water storage, flood protection, and hydropower result in a pressing need to address the state of dam infrastructure across the nation. Decisions regarding the future of dams in the United States may, in turn, influence regional water futures through groundwater outcomes, economic productivity, migration, and urban growth. We advocate for a comprehensive national water assessment and a formal analysis of the role dams play in our water future. We emphasize the urgent need for environmentally and economically sound strategies to integrate surface and groundwater storage infrastructure in local, regional, and national water planning considerations. A research agenda is proposed to assess dam failure impacts and the design, operation, and need for dams considering both paleo and future climate, utilization of groundwater resources, and the changing societal values towards the environment. This article is protected by copyright. All rights reserved.
- Analytical solutions for aquifer thermal energy storage
- Authors: Jan Martin Nordbotten
Abstract: The concept of aquifer thermal energy storage involves injection of water at elevated temperature, and possibly non-ambient salinity, into a host aquifer. We consider axisymmetric injection, wherein both the composition and temperature of the injected fluid differs from the fluid in the target aquifer. In this setting, we derive the governing equations within a vertically integrated framework, and show their self-similar structure. We subsequently derive explicit approximate solutions to the self-similar equations for parameter ranges of relevance to thermal energy storage (small density and viscosity differences), we derive explicit approximate solutions to the self-similar equations.The analysis is supported by numerical validation, covering the relevant parameter regime. The resulting comparisons demonstrate the mathematical qualities of the analytical approximations. A study based on field data from analogue sites, justifies the assertions regarding the magnitude of the dimensionless parameters used in the analysis. This article is protected by copyright. All rights reserved.
- Incorporating institutions and collective action into a socio-hydrological
model of flood resilience
- Authors: David J. Yu; Nikhil Sangwan, Kyungmin Sung, Xi Chen, Venkatesh Merwade
Abstract: Stylized socio-hydrological models have mainly used social memory aspects such as community awareness or sensitivity to connect hydrologic change and social response. However, social memory alone does not satisfactorily capture the details of how human behavior is translated into collective action for water resources governance. Nor is it the only social mechanism by which the two-way feedbacks of socio-hydrology can be operationalized. This study contributes towards bridging of this gap by developing a socio-hydrological model of a flood resilience that includes two additional components: (1) institutions for collective action, and (2) connections to an external economic system. Motivated by the case of community-managed flood protection systems (polders) in coastal Bangladesh, we use the model to understand critical general features that affect long-term resilience of human-flood systems. Our findings suggest that occasional adversity can enhance long-term resilience. Allowing some hydrological variability to enter into the polder can increase its adaptive capacity for resilience through the preservation of social norm for collective action. Further, there are potential trade-offs associated with optimization of flood resistance through structural measures. By reducing sensitivity to floods, the system may become more fragile under the double impact of floods and economic change. This article is protected by copyright. All rights reserved.
- Soil moisture background error covariance and data assimilation in a
coupled land-atmosphere model
- Authors: Liao-Fan Lin; Ardeshir M. Ebtehaj, Jingfeng Wang, Rafael L. Bras
Abstract: This study characterizes the space-time structure of soil moisture background error covariance and paves the way for the development of a soil moisture variational data assimilation system for the Noah land surface model coupled to the Weather Research and Forecasting (WRF) model. The soil moisture background error covariance over the contiguous United States exhibits strong seasonal and regional variability with the largest values occurring in the uppermost soil layer during the summer. Large background error biases were identified, particularly over the Southeastern United States, caused mainly by the discrepancy between the WRF-Noah simulations and the initial conditions derived from the used operational global analysis dataset. The assimilation of the Soil Moisture and Ocean Salinity (SMOS) soil moisture data notably reduces the error of soil moisture simulations. On average, data assimilation with space-time varying background error covariance results in 33% and 35% reduction in the root-mean-square error and the mean absolute error, respectively, in the simulation of hourly top 10-cm soil moisture, mainly due to implicit reductions in soil moisture biases. In terms of correlation, the improvement in soil moisture simulations is also observed but less notable, indicating the limitation of coarse-scale soil moisture data assimilation in capturing fine-scale soil moisture variation. In addition, soil moisture data assimilation improves the simulations of latent heat fluxes but shows a marginal impact on the simulations of sensible latent heat fluxes and precipitation. This article is protected by copyright. All rights reserved.
- Evaluating waterpoint sustainability and access implications of revenue
collection approaches in rural Kenya
- Authors: T. Foster; R. Hope
Abstract: Water policies in many sub-Saharan African countries stipulate that rural communities are responsible for self-financing their waterpoint's operation and maintenance. In the absence of policy consensus or evidence on optimal payment models, rural communities adopt a diversity of approaches. This study empirically assesses waterpoint sustainability and access outcomes associated with different revenue collection approaches on the south coast of Kenya. The analysis draws on a unique data set comprising financial records spanning 27 years and 100 communities, operational performance indicators for 200 waterpoints, and water source choices for more than 2,000 households. Results suggest communities collecting pay-as-you-fetch fees on a volumetric basis generate higher levels of income and experience improved operational performance compared with communities charging flat fees. In both cases, financial flows mirror seasonal rainfall peaks and troughs. These outcomes are tempered by evidence that households are more likely to opt for an unimproved drinking water source when a pay-as-you-fetch system is in place. The findings illuminate a possible tension between financial sustainability and universal access. If the Sustainable Development Goal of 'safe water for all' is to become a reality, policymakers and practitioners will need to address this issue and ensure rural water services are both sustainable and inclusive. This article is protected by copyright. All rights reserved.
- Issue Information
- Pages: 979 - 981
- Statistical distributions for monthly aggregations of precipitation and
streamflow in drought indicator applications
- Authors: Cecilia Svensson; Jamie Hannaford, Ilaria Prosdocimi
Pages: 999 - 1018
Abstract: Drought indicators are used as triggers for action and so are the foundation of drought monitoring and early warning. The computation of drought indicators like the standardized precipitation index (SPI) and standardized streamflow index (SSI) require a statistical probability distribution to be fitted to the observed data. Both precipitation and streamflow have a lower bound at zero, and their empirical distributions tend to have positive skewness. For deriving the SPI, the Gamma distribution has therefore often been a natural choice. The concept of the SSI is newer and there is no consensus regarding distribution. In the present study, twelve different probability distributions are fitted to streamflow and catchment average precipitation for four durations (1, 3, 6, and 12 months), for 121 catchments throughout the UK. The more flexible three- and four-parameter distributions generally do not have a lower bound at zero, and hence may attach some probability to values below zero. As a result, there is a censoring of the possible values of the calculated SPIs and SSIs. This can be avoided by using one of the bounded distributions, such as the reasonably flexible three-parameter Tweedie distribution, which has a lower bound (and potentially mass) at zero. The Tweedie distribution has only recently been applied to precipitation data, and only for a few sites. We find it fits both precipitation and streamflow data nearly as well as the best of the traditionally used three-parameter distributions, and should improve the accuracy of drought indices used for monitoring and early warning.
- Generation of 3-D hydrostratigraphic zones from dense airborne
electromagnetic data to assess groundwater model prediction error
- Authors: N. K. Christensen; B. J. Minsley, S. Christensen
Pages: 1019 - 1038
Abstract: We present a new methodology to combine spatially dense high-resolution airborne electromagnetic (AEM) data and sparse borehole information to construct multiple plausible geological structures using a stochastic approach. The method developed allows for quantification of the performance of groundwater models built from different geological realizations of structure. Multiple structural realizations are generated using geostatistical Monte Carlo simulations that treat sparse borehole lithological observations as hard data and dense geophysically derived structural probabilities as soft data. Each structural model is used to define 3-D hydrostratigraphical zones of a groundwater model, and the hydraulic parameter values of the zones are estimated by using nonlinear regression to fit hydrological data (hydraulic head and river discharge measurements). Use of the methodology is demonstrated for a synthetic domain having structures of categorical deposits consisting of sand, silt, or clay. It is shown that using dense AEM data with the methodology can significantly improve the estimated accuracy of the sediment distribution as compared to when borehole data are used alone. It is also shown that this use of AEM data can improve the predictive capability of a calibrated groundwater model that uses the geological structures as zones. However, such structural models will always contain errors because even with dense AEM data it is not possible to perfectly resolve the structures of a groundwater system. It is shown that when using such erroneous structures in a groundwater model, they can lead to biased parameter estimates and biased model predictions, therefore impairing the model's predictive capability.
- Identifying modern and historic recharge events from tracer-derived
groundwater age distributions
- Authors: James L. McCallum; Peter. G. Cook, Shawan Dogramaci, Roland Purtschert, Craig T. Simmons, Lawrence Burk
Pages: 1039 - 1056
Abstract: Understanding groundwater ages offers insight into the time scales of recharge, aquifer storage turnover times, and contaminant protection time frames. The ability to quantify groundwater age distributions heavily depends on the choice of the interpretive model, and often important features of the age distribution cannot be identified with the subset of available models. In this paper, we implemented a multiple tracer method using a technique that assumes limited details regarding the shape of the age distribution and applied it to dewatering wells at a mine site in the Pilbara region of north-western Australia. Using our method, we were able to identify distinct age components in the groundwater. We calculated the presence of four distinct age groups in the samples. All wells contained water aged between zero and 20 years. However, the rest of the samples were composed of water between 50 and 100 years, 100 and 600 years, or water approximately 1000 years old. These were consistent with local recharge sources (50–100 years) and knowledge of paleoclimate from lake sediment records. We found that although the age components were well constrained, the relative proportions of each component were highly sensitive to errors of environmental tracer data. Our results show that our method can identify distinct age groups in groundwater samples without prior knowledge of the age distribution. The presence of distinct recharge times gives insight into groundwater flow conditions over long periods of time.
- Heat and water transport in soils and across the soil-atmosphere
interface: 1. Theory and different model concepts
- Authors: Jan Vanderborght; Thomas Fetzer, Klaus Mosthaf, Kathleen M. Smits, Rainer Helmig
Pages: 1057 - 1079
Abstract: Evaporation is an important component of the soil water balance. It is composed of water flow and transport processes in a porous medium that are coupled with heat fluxes and free air flow. This work provides a comprehensive review of model concepts used in different research fields to describe evaporation. Concepts range from nonisothermal two-phase flow, two-component transport in the porous medium that is coupled with one-phase flow, two-component transport in the free air flow to isothermal liquid water flow in the porous medium with upper boundary conditions defined by a potential evaporation flux when available energy and transfer to the free airflow are limiting or by a critical threshold water pressure when soil water availability is limiting. The latter approach corresponds with the classical Richards equation with mixed boundary conditions. We compare the different approaches on a theoretical level by identifying the underlying simplifications that are made for the different compartments of the system: porous medium, free flow and their interface, and by discussing how processes not explicitly considered are parameterized. Simplifications can be grouped into three sets depending on whether lateral variations in vertical fluxes are considered, whether flow and transport in the air phase in the porous medium are considered, and depending on how the interaction at the interface between the free flow and the porous medium is represented. The consequences of the simplifications are illustrated by numerical simulations in an accompanying paper.
- Heat and water transport in soils and across the soil-atmosphere
interface: 2. Numerical analysis
- Authors: Thomas Fetzer; Jan Vanderborght, Klaus Mosthaf, Kathleen M. Smits, Rainer Helmig
Pages: 1080 - 1100
Abstract: In an accompanying paper, we presented an overview of a wide variety of modeling concepts, varying in complexity, used to describe evaporation from soil. Using theoretical analyses, we explained the simplifications and parameterizations in the different approaches. In this paper, we numerically evaluate the consequences of these simplifications and parameterizations. Two sets of simulations were performed. The first set investigates lateral variations in vertical fluxes, which emerge from both homogeneous and heterogeneous porous media, and their importance to capturing evaporation behavior. When evaporation decreases from parts of the heterogeneous soil surface, lateral flow and transport processes in the free flow and in the porous medium generate feedbacks that enhance evaporation from wet surface areas. In the second set of simulations, we assume that the vertical fluxes do not vary considerably in the simulation domain and represent the system using one-dimensional models which also consider dynamic forcing of the evaporation process, for example, due to diurnal variations in net radiation. Simulated evaporation fluxes subjected to dynamic forcing differed considerably between model concepts depending on how vapor transport in the air phase and the interaction at the interface between the free flow and porous medium were represented or parameterized. However, simulated cumulative evaporation losses from initially wet soil profiles were very similar between model concepts and mainly controlled by the desorptivity, Sevap, of the porous medium, which depends mainly on the liquid flow properties of the porous medium.
- The influence of mixing on stable isotope ratios in porous media: A
revised Rayleigh model
- Authors: Jennifer L. Druhan; Kate Maher
Pages: 1101 - 1124
Abstract: For an irreversible reaction, the Rayleigh or distillation-type relationship between stable isotope enrichment and reactant concentration is compromised if fluid samples are characterized by a range of water ages or different extents of reaction progress. Such mixed samples are rarely avoided in the standard methods of sampling fluid from natural porous media. As a result, application of a Rayleigh model to stable isotope ratios measured in aquifers commonly requires a diminished or effective fractionation factor relative to the intrinsic value obtained in the absence of transport effects. Thus, quantitative application of intrinsic parameter values to a fractionating reaction occurring in porous media flow requires revision to the functional form of the relationship between reactant concentration and isotope fractionation. Here we derive a series of analytical solutions for the relationship between fractionation and flow subject to nonuniform fluid travel time distributions. These solutions are unique from previous approaches in that they avoid the use of a dispersion coefficient. The results are demonstrated against multicomponent reactive transport simulations of stable isotope fractionation in homogeneous and spatially correlated heterogeneous flow fields, and applied to a data set of stable chromium (Cr) isotope enrichment obtained from a contaminated aquifer. We show that the flux-weighted isotope ratio of a solute is more sensitive to the effects of physical heterogeneity than solute concentrations. Our results support an updated functional form of the traditional Rayleigh model that describes the relationship between reactant concentration and isotope fractionation and is valid for a mixed-fluid sample.
- Improving physically based snow simulations by assimilating snow depths
using the particle filter
- Authors: Jan Magnusson; Adam Winstral, Andreas S. Stordal, Richard Essery, Tobias Jonas
Pages: 1125 - 1143
Abstract: Data assimilation can help to ensure that model results remain close to observations despite potential errors in the model, parameters, and inputs. In this study, we test whether assimilation of snow depth observations using the particle filter, a generic data assimilation method, improves the results of a multilayer energy-balance snow model, and compare the results against a direct insertion method. At the field site Col de Porte in France, the particle filter reduces errors in SWE, snowpack runoff, and soil temperature when forcing the model with coarse resolution reanalysis data, which is a typical input scenario for operational simulations. For those variables, the model performance after assimilation of snow depths is similar to model performance when forcing with high-quality, locally observed input data. Using the particle filter, we could also estimate a snowfall correction factor accurately at Col de Porte. The assimilation of snow depths also improves forecasts with lead-times of, at least, 7 days. At further 40 sites in Switzerland, the assimilation of snow depths in a model forced with numerical weather prediction data reduces the root-mean-squared-error for SWE by 64% compared to the model without assimilation. The direct insertion method shows similar performance as the particle filter, but is likely to produce inconsistencies between modeled variables. The particle filter, on the other hand, avoids such limitations without loss of performance. The methods proposed in this study efficiently reduces errors in snow simulations, seems applicable for different climatic and geographic regions, and are easy to deploy.
- A long-term perspective of the hydroclimatological impacts of atmospheric
rivers over the central United States
- Authors: Munir Ahmad Nayak; Gabriele Villarini
Pages: 1144 - 1166
Abstract: The focus of this study is on the climatology of atmospheric rivers (ARs) over the central United States using six atmospheric reanalysis products. This climatology is used to understand the long-term impacts of ARs on annual precipitation, precipitation extremes, and flooding over the central United States. The relationship between the frequency of ARs and three prominent large-scale atmospheric modes [Pacific-North American (PNA) teleconnection, Artic Oscillation (AO), and North Atlantic Oscillation (NAO)] is investigated, and the results are used to statistically model the frequency of ARs at the seasonal scale. AR characteristics (e.g., frequency, duration) are generally robust across the different reanalysis products. ARs exhibit a marked seasonality, with the largest activity in winter (more than 10 ARs per season on average), and the lowest in summer (less than two ARs per season on average). Overall, the duration of most ARs is less than 3 days, but exceptionally persistent ARs (more than 6 days) are also observed. The year-to-year variations in the total annual precipitation over the central United States are largely explained by the variations in AR-related precipitation. Moreover, 40% of the top 1% daily precipitation extremes are associated with ARs, and more than 70% of the annual instantaneous peak discharges and peaks-overthreshold floods are associated with these storms, in particular during winter and spring. The seasonal frequency of ARs can be described in terms of large-scale atmospheric modes, with PNA playing a major role in particular in winter and spring.
- Differences in behavior and distribution of permafrost-related lakes in
Central Yakutia and their response to climatic drivers
- Authors: M. Ulrich; H. Matthes, L. Schirrmeister, J. Schütze, H. Park, Y. Iijima, A. N. Fedorov
Pages: 1167 - 1188
Abstract: The Central Yakutian permafrost landscape is rapidly being modified by land use and global warming, but small-scale thermokarst process variability and hydrological conditions are poorly understood. We analyze lake-area changes and thaw subsidence of young thermokarst lakes on ice-complex deposits (yedoma lakes) in comparison to residual lakes in alas basins during the last 70 years for a local study site and we record regional lake size and distribution on different ice-rich permafrost terraces using satellite and historical airborne imagery. Statistical analysis of climatic and ground-temperature data identified driving factors of yedoma- and alas-lake changes. Overall, lake area is larger today than in 1944 but alas-lake levels have oscillated greatly over 70 years, with a mean alas-lake-radius change rate of 1.6 ± 3.0 m/yr. Anthropogenic disturbance and forest degradation initiated, and climate forced rapid, continuous yedoma-lake growth. The mean yedoma lake-radius change rate equals 1.2 ± 1.0 m/yr over the whole observation period. Mean thaw subsidence below yedoma lakes is 6.2 ± 1.4 cm/yr. Multiple regression analysis suggests that winter precipitation, winter temperature, and active-layer properties are primary controllers of area changes in both lake types; summer weather and permafrost conditions additionally influence yedoma-lake growth rates. The main controlling factors of alas-lake changes are unclear due to larger catchment areas and subsurface hydrological conditions. Increasing thermokarst activity is currently linked to older terraces with higher ground-ice contents, but thermokarst activity will likely stay high and wet conditions will persist within the near future in Central Yakutian alas basins.
- Experimental investigation of the impact of compound-specific dispersion
and electrostatic interactions on transient transport and solute
- Authors: Muhammad Muniruzzaman; Massimo Rolle
Pages: 1189 - 1209
Abstract: This study investigates the effects of compound-specific diffusion/dispersion and electrochemical migration on transient solute transport in saturated porous media. We conducted laboratory bench-scale experiments, under advection-dominated regimes (seepage velocity: 0.5, 5, 25 m/d), in a quasi two-dimensional flow-through setup using pulse injection of multiple tracers (both uncharged and ionic species). Extensive sampling and measurement of solutes' concentrations (∼1500 samples; >3000 measurements) were performed at the outlet of the flow-through setup, at high spatial and temporal resolution. The experimental results show that compound-specific effects and charge-induced Coulombic interactions are important not only at low velocities and/or for steady state plumes but also for transient transport under high flow velocities. Such effects can lead to a remarkably different behavior of measured breakthrough curves also at very high Péclet numbers. To quantitatively interpret the experimental results, we used four modeling approaches: classical advection-dispersion equation (ADE), continuous time random walk (CTRW), dual-domain mass transfer model (DDMT), and a multicomponent ionic dispersion model. The latter is based on the multicomponent formulation of coupled diffusive/dispersive fluxes and was used to describe and explain the electrostatic effects of charged species. Furthermore, we determined experimentally the temporal profiles of the flux-related dilution index. This metric of mixing, used in connection with the traditional solute breakthrough curves, proved to be useful to correctly distinguish between plume spreading and mixing, particularly for the cases in which the sole analysis of integrated concentration breakthrough curves may lead to erroneous interpretation of plume dilution.
- Measurement and simulation of heat exchange in fractured bedrock using
inert and thermally degrading tracers
- Authors: Adam J. Hawkins; Don B. Fox, Matthew W. Becker, Jefferson W. Tester
Pages: 1210 - 1230
Abstract: Multicomponent groundwater tracer tests were conducted in a well-characterized field site in Altona, NY using inert carbon-cored nanoparticles and a thermally degrading phenolic compound. Experiments were conducted in a mesoscale reservoir consisting of a single subhorizontal bedding plane fracture located 7.6 m below ground surface contained between two wells separated by 14.1 m. The reservoir rock, initially at 11.7°C, was heated using 74°C water. During the heating process, a series of tracer tests using thermally degrading tracers were used to characterize the progressive in situ heating of the fracture. Fiber-Optic Distributed Temperature Sensing (FODTS) was used to measure temperature rise orthogonal to the fracture surface at 10 locations. Recovery of the thermally degrading tracer's product was increased as the reservoir was progressively heated indicating that the advancement of the thermal front was proportional to the mass fraction of the thermally degrading tracer recovered. Both GPR imaging and FODTS measurements reveal that flow was reduced to a narrow channel which directly connected the two wells and led to rapid thermal breakthrough. Computational modeling of inert tracer and heat transport in a two-dimensional discrete fracture demonstrate that subsurface characterization using inert tracers alone could not uniquely characterize the Altona field site. However, the inclusion of a thermally degrading tracer may permit accurate subsurface temperature monitoring. At the Altona field site, however, fluid-rock interactions appear to have increased reaction rates relative to laboratory-based measurements made in the absence of rock surfaces.
- Probabilistic inversion with graph cuts: Application to the Boise
Hydrogeophysical Research Site
- Authors: Guillaume Pirot; Niklas Linde, Grégoire Mariethoz, John H. Bradford
Pages: 1231 - 1250
Abstract: Inversion methods that build on multiple-point statistics tools offer the possibility to obtain model realizations that are not only in agreement with field data, but also with conceptual geological models that are represented by training images. A recent inversion approach based on patch-based geostatistical resimulation using graph cuts outperforms state-of-the-art multiple-point statistics methods when applied to synthetic inversion examples featuring continuous and discontinuous property fields. Applications of multiple-point statistics tools to field data are challenging due to inevitable discrepancies between actual subsurface structure and the assumptions made in deriving the training image. We introduce several amendments to the original graph cut inversion algorithm and present a first-ever field application by addressing porosity estimation at the Boise Hydrogeophysical Research Site, Boise, Idaho. We consider both a classical multi-Gaussian and an outcrop-based prior model (training image) that are in agreement with available porosity data. When conditioning to available crosshole ground-penetrating radar data using Markov chain Monte Carlo, we find that the posterior realizations honor overall both the characteristics of the prior models and the geophysical data. The porosity field is inverted jointly with the measurement error and the petrophysical parameters that link dielectric permittivity to porosity. Even though the multi-Gaussian prior model leads to posterior realizations with higher likelihoods, the outcrop-based prior model shows better convergence. In addition, it offers geologically more realistic posterior realizations and it better preserves the full porosity range of the prior.
- Modeling multidecadal surface water inundation dynamics and key drivers on
large river basin scale using multiple time series of Earth-observation
and river flow data
- Authors: V. Heimhuber; M. G. Tulbure, M. Broich
Pages: 1251 - 1269
Abstract: Periodically inundated floodplain areas are hot spots of biodiversity and provide a broad range of ecosystem services but have suffered alarming declines in recent history. Despite their importance, their long-term surface water (SW) dynamics and hydroclimatic drivers remain poorly quantified on continental scales. In this study, we used a 26 year time series of Landsat-derived SW maps in combination with river flow data from 68 gauges and spatial time series of rainfall, evapotranspiration and soil moisture to statistically model SW dynamics as a function of key drivers across Australia's Murray-Darling Basin (∼1 million km2). We fitted generalized additive models for 18,521 individual modeling units made up of 10 × 10 km grid cells, each split into floodplain, floodplain-lake, and nonfloodplain area. Average goodness of fit of models was high across floodplains and floodplain-lakes (r2 > 0.65), which were primarily driven by river flow, and was lower for nonfloodplain areas (r2 > 0.24), which were primarily driven by rainfall. Local climate conditions were more relevant for SW dynamics in the northern compared to the southern basin and had the highest influence in the least regulated and most extended floodplains. We further applied the models of two contrasting floodplain areas to predict SW extents of cloud-affected time steps in the Landsat series during the large 2010 floods with high validated accuracy (r2 > 0.97). Our framework is applicable to other complex river basins across the world and enables a more detailed quantification of large floods and drivers of SW dynamics compared to existing methods.
- Elemental properties, hydrology, and biology interact to shape
concentration-discharge curves for carbon, nutrients, sediment, and major
- Authors: F. Moatar; B. W. Abbott, C. Minaudo, F. Curie, G. Pinay
Pages: 1270 - 1287
Abstract: To investigate the prevalence and cause of concentration-discharge (C-Q) relationships for carbon, nutrients, major ions, and particulates, we analyzed 40 years of water quality data from 293 monitoring stations in France. Catchments drained diverse landscapes and ranged from 50 to 110,000 km2, together covering nearly half of France. To test for differences during low and high flows, we calculated independent C-Q slopes above and below the median discharge. We found that 84% of all catchment-element combinations were chemodynamic for at least half of the hydrograph and 60% of combinations showed nonlinear C-Q curves. Only two or three of the nine possible C-Q modalities were manifest for each parameter, and these modalities were stable through time, suggesting that intrinsic and extrinsic elemental properties (e.g., solubility, reactivity, and source dynamics) set basic C-Q templates for each parameter, which are secondarily influenced by biological activity during low flows, and the interaction between hydrology and catchment characteristics at high flows. Several patterns challenged current C-Q views, including low-flow chemostasis for TSS in 66% of catchments, low-flow biological mediation of NO3− in 71% of catchments, and positive C-Q for dissolved organic carbon independent of catchment size in 80% of catchments. Efforts to reduce nutrient loading decreased phosphorus concentration and altered C-Q curves, but NO3− continued to increase. While C-Q segmentation requires more data than a single analysis, the prevalence of nonlinear C-Q slopes demonstrates the potential information loss associated with linear or monotonic analysis of C-Q relationships, and conversely, the value of long-term monitoring.
- Impact of mountain permafrost on flow path and runoff response in a high
- Authors: M. Rogger; G. B. Chirico, H. Hausmann, K. Krainer, E. Brückl, P. Stadler, G. Blöschl
Pages: 1288 - 1308
Abstract: Permafrost in high alpine catchments is expected to disappear in future warmer climates, but the hydrological impact of such changes is poorly understood. This paper investigates the flow paths and the hydrological response in a 5 km2 high alpine catchment in the Ötztal Alps, Austria, and their changes resulting from a loss of permafrost. Spatial permafrost distribution, depth to the permafrost table, and depth to the bedrock were mapped by geophysical methods. Catchment runoff and meteorological variables were monitored from June 2008 to December 2011. These data were used along with field experience to infer conceptual schemes of the dominant flow paths in four types of hillslopes that differ in terms of their unconsolidated sediment characteristics and the presence of permafrost. The four types are: talus fans, rock glaciers, Little Ice Age (LIA) till, and pre-LIA till. Permafrost tends to occur in the first three types, but is absent from pre-LIA till. Based on these flow path concepts, runoff was simulated for present conditions and for future conditions when permafrost has completely disappeared. The simulations indicate that complete disappearance of permafrost will reduce flood peaks by up to 17% and increase runoff during recession by up to 19%. It is argued that change modeling needs to account for flow path types and their changes based on geophysical surveys and field investigations.
- Approximate solutions for diffusive fracture-matrix transfer: Application
to storage of dissolved CO2 in fractured rocks
- Authors: Quanlin Zhou; Curtis M. Oldenburg, Lee H. Spangler, Jens T. Birkholzer
Pages: 1746 - 1762
Abstract: Analytical solutions with infinite exponential series are available to calculate the rate of diffusive transfer between low-permeability blocks and high-permeability zones in the subsurface. Truncation of these series is often employed by neglecting the early-time regime. In this paper, we present unified-form approximate solutions in which the early-time and the late-time solutions are continuous at a switchover time. The early-time solutions are based on three-term polynomial functions in terms of square root of dimensionless time, with the first coefficient dependent only on the dimensionless area-to-volume ratio. The last two coefficients are either determined analytically for isotropic blocks (e.g., spheres and slabs) or obtained by fitting the exact solutions, and they solely depend on the aspect ratios for rectangular columns and parallelepipeds. For the late-time solutions, only the leading exponential term is needed for isotropic blocks, while a few additional exponential terms are needed for highly anisotropic rectangular blocks. The optimal switchover time is between 0.157 and 0.229, with highest relative approximation error less than 0.2%. The solutions are used to demonstrate the storage of dissolved CO2 in fractured reservoirs with low-permeability matrix blocks of single and multiple shapes and sizes. These approximate solutions are building blocks for development of analytical and numerical tools for hydraulic, solute, and thermal diffusion processes in low-permeability matrix blocks.
- Improving operating policies of large-scale surface-groundwater systems
through stochastic programming
- Authors: H. Macian-Sorribes; A. Tilmant, M. Pulido-Velazquez
Abstract: The management of large-scale water resource systems with surface and groundwater resources requires considering stream-aquifer interactions. Optimization models applied of large-scale systems have either employed deterministic optimization (with perfect foreknowledge of future inflows, which hinders their applicability to real-life operations) or stochastic programming (in which stream-aquifer interaction is often neglected due to the computational burden associated with these methods). In this paper, stream-aquifer interaction is integrated in a stochastic programming framework by combining the Stochastic Dual Dynamic Programming (SDDP) optimization algorithm with the Embedded Multireservoir Model (EMM). The resulting extension of the SDDP algorithm, named Combined Surface-Groundwater SDDP (CSG-SDDP), is able to properly represent the stream-aquifer interaction within stochastic optimization models of large-scale surface-groundwater resources systems. The algorithm is applied to build a hydroeconomic model for the Jucar River Basin (Spain), in which stream-aquifer interactions are essential to the characterization of water resources in the system. Besides the uncertainties regarding the economic characterization of the demand functions, the results show that the economic efficiency of the operating policies under the current system can be improved by better management of groundwater and surface resources. This article is protected by copyright. All rights reserved.
- Flood type-specific construction of synthetic design hydrographs
- Authors: Manuela I. Brunner; Daniel Viviroli, Anna E. Sikorska, Olivier Vannier, Anne-Catherine Favre, Jan Seibert
Abstract: Accurate estimates of flood peaks, corresponding volumes and hydrographs are required to design safe and cost-effective hydraulic structures. In this paper, we propose a statistical approach for the estimation of the design variables peak and volume by constructing synthetic design hydrographs for different flood types such as flash-floods, short-rain floods, long-rain floods, and rain-on-snow floods. Our approach relies on the fitting of probability density functions to observed flood hydrographs of a certain flood type and accounts for the dependence between peak discharge and flood volume. It makes use of the statistical information contained in the data and retains the process information of the flood type. The method was tested based on data from 39 meso-scale catchments in Switzerland and provides catchment specific and flood type specific synthetic design hydrographs for all of these catchments. We demonstrate that flood type specific synthetic design hydrographs are meaningful in flood risk management when combined with knowledge on the seasonality and the frequency of different flood types. This article is protected by copyright. All rights reserved.