- Dating base flow in streams using dissolved gases and diurnal temperature
- Authors: Ward E. Sanford; Gerolamo Casile, Karl B. Haase
Abstract: A method is presented for using dissolved CFCs or SF6 to estimate the apparent age of stream base flow by indirectly estimating the mean concentration of the tracer in the inflowing groundwater. The mean value is estimated simultaneously with the mean residence times of the gas and water in the stream by sampling the stream for one or both age tracers, along with dissolved nitrogen and argon at a single location over a period of approximately 12‐14 hours. The data are fitted to an equation representing the temporal in‐stream gas exchange as it responds to the diurnal temperature fluctuation. The efficacy of the method is demonstrated by collecting and analyzing samples at six different stream locations across parts of northern Virginia, USA. The studied streams drain watersheds with areas of between 2 and 122 km2 during periods when the diurnal stream temperature ranged between 2 and 5°C. The method has the advantage of estimating the mean groundwater residence time of discharge from the watershed to the stream without the need for the collection of groundwater infiltrating to streambeds or local groundwater sampled from shallow observation wells near the stream. This article is protected by copyright. All rights reserved.
- Accepting managed aquifer recharge of urban stormwater reuse: The role of
- Authors: Aditi Mankad; Andrea Walton
Abstract: A between‐groups experimental design examined public acceptance for managed aquifer recharge of stormwater for indirect potable and non‐potable reuse; acceptance was based on five policy‐related variables (fairness, effectiveness, trust, importance of safety assurances, and importance of communication activities). Results showed that public acceptance (N = 408) for managed aquifer recharge of stormwater was higher for non‐potable applications, as was the importance of safety assurances. Analyses of variance also showed that perceptions of fairness and effectiveness were higher for a non‐potable scheme, but not trust. A three‐step hierarchical regression (Step 1: age, gender, education, income; Step 2: type of use; Step 3: fairness, effectiveness, trust, safety assurance, communication activities) demonstrated that type of stormwater use and the policy related factors accounted for 73% of the variance in acceptance of stormwater (R2 = .74, adjusted R2 = .74, F (10, 397) = 113.919, p
- Hydration and diffusion processes shape microbial community organization
and function in model soil aggregates
- Authors: Ali Ebrahimi; Dani Or
Abstract: The constantly changing soil hydration status affects gas and nutrient diffusion through soil pores and thus the functioning of soil microbial communities. The conditions within soil aggregates are of particular interest due to limitations to oxygen diffusion into their core, and the presence of organic carbon often acting as binding agent. We developed a model for microbial life in simulated soil aggregates comprising of 3‐D angular pore networks (APNM) that mimic soil hydraulic and transport properties. Within these APNM, we introduced individual motile (flagellated) microbial cells with different physiological traits that grow, disperse, and respond to local nutrients and oxygen concentrations. The model quantifies the dynamics and spatial extent of anoxic regions that vary with hydration conditions, and their role in shaping microbial community size and activity and the spatial (self) segregation of anaerobes and aerobes. Internal carbon source and opposing diffusion directions of oxygen and carbon within an aggregate were essential to emergence of stable coexistence of aerobic and anaerobic communities (anaerobes become extinct when carbon sources are external). The model illustrates a range of hydration conditions that promote or suppress denitrification or decomposition of organic matter and thus affect soil GHG emissions. Model predictions of CO2 and N2O production rates were in good agreement with limited experimental data. These limited tests support the dynamic modeling approach whereby microbial community size, composition and spatial arrangement emerge from internal interactions within soil aggregates. The upscaling of the results to a population of aggregates of different sizes embedded in a soil profile is underway. This article is protected by copyright. All rights reserved.
- Upwelling of deep water during thermal stratification onset – A
major mechanism of vertical transport in small temperate lakes in
- Abstract: Using airborne thermal infrared imaging and horizontally‐resolved in situ temperature monitoring at the lake surface we estimated strength and duration of regular wind‐driven upwelling of dense deep water to the lake surface in two small (in terms of Rossby radius) temperate lakes during the initial phase of summer thermal stratification. The onset and duration of the upwelling events correlated well with the balance between stratification (in terms of Schmidt Stability) and wind forcing, as expressed by Lake and Wedderburn Numbers: The period of regular upwelling appearances lasted 7‐15 days, identified by Schmidt stabilities around 30 J m−2 and Lake Numbers between 0 and 1, and resulted in persistent temperature gradients of up to 2°C across the lake surface. Our results suggest that spring upwelling should inevitably take place in all freshwater temperate lakes with mean temperatures crossing the maximum density value of freshwater on annual cycle, whereas duration and intensity of the upwelling would vary depending on lake morphometry and weather conditions. Our results suggest major contribution of upwelling in nutrient supply to the upper waters, oxygenation of the deep water column, and air‐lake gas exchange, in particular, the release of the sediment‐produced methane into the atmosphere. This article is protected by copyright. All rights reserved.
- The formation of viscous limited saturation zones behind rapid drainage
fronts in porous media
- Authors: Frouke Hoogland; Peter Lehmann, Dani Or
Abstract: Drainage characteristics of porous media are shaped by an interplay between gravitational, capillary and viscous forces that result in complex phase invasion patterns and dynamics. We propose a mechanistic model for viscous separation of temporary phase detention behind rapidly moving drainage fronts. The viscous‐limited region forming behind the front tip (tip of furthest penetrated air finger) drains at a slower rate with a characteristic time scale τ dictated by hydraulic decoupling expressed by the hydraulic properties of the medium. The region where saturation becomes viscously detained (temporarily entrapped) is determined by a critical water content θcrit that defines a viscous length LV behind the front tip. Theory developed to predict the critical water content θcrit and the characteristic secondary timescale τ was in good agreement with measured drainage characteristics using neutron radiography and direct imaging. The observed critical water content θcrit increased with higher drainage rates as predicted by theory with consideration of a percolation threshold. The observed slow drainage timescale τ as a function of mean drainage rate depended on the critical water content θcrit and the resulting counteracting effects of increased detained liquid volume and increased conductivity of the viscous limited region. The concept of drainage zonation illustrates how increasing flow rates enhances the extent of viscous limitations behind the main drainage front. The new insights could be useful for management of immiscible fluid displacement, quantification of averaging effects in experimental measurements (dynamic effects on pc‐S relationship), and explain some of the underpinnings of the field capacity phenomenon. This article is protected by copyright. All rights reserved.
- A pseudogenetic model of coarse braided‐river deposits
- Authors: Guillaume Pirot; Julien Straubhaar, Philippe Renard
Abstract: A new method is proposed to produce three‐dimensional facies models of braided‐river aquifers based on analog data. The algorithm consists of two steps. The first step involves building the main geological units. The production of the principal inner structures of the aquifer is achieved by stacking Multiple‐Point‐Statistics simulations of successive topographies, thus mimicking the major successive flooding events responsible for the erosion and deposition of sediments. The second step of the algorithm consists of generating fine scale heterogeneity within the main geological units. These smaller‐scale structures are generated by mimicking the trough‐filling process occurring in braided rivers; the imitation of the physical processes relies on the local topography and on a local approximation of the flow. This produces realistic cross‐stratified sediments, comparable to what can be observed in outcrops. The three main input parameters of the algorithm offer control over the proportions, the continuity and the dimensions of the deposits. Calibration of these parameters does not require invasive field measurements and can rely partly on analog data. This article is protected by copyright. All rights reserved.
- Issue Information
- PubDate: 2015-11-20T04:24:11.314221-05:
- Independent component analysis of local‐scale temporal variability
in sediment‐water interface temperature
- Authors: M. A. Middleton; P.H. Whitfield, D.M. Allen
Abstract: Temperature recorded at the sediment‐water interface has been identified as a valuable tracer for understanding groundwater‐surface water interactions. However, factors contributing to the variability in temperatures can be difficult to distinguish. In this study, the temporal variability in daily temperatures at the sediment‐water interface is evaluated for a 40‐m reach of a coastal stream using Independent Component Analysis (ICA). ICA separation is used to identify three independent temperature components within the reach for each of four summer periods (2008‐2011). Extracted temperature signals correlate with stream discharge, estimated streambed temperature, and groundwater level, but the strength of the correlations varies from summer to summer. Overall, variations in the temperature signals have clearer separation in summers with lower stream discharge and greater stream temperature ranges. Surface heating from solar radiation is the dominant factor influencing the sediment‐water interface temperature in most years, but there is evidence that thermal exchanges are taking place other than at the air‐water interface. These exchanges take place at the sediment‐water interface, and the correlation with groundwater levels indicates these heat exchanges are associated with groundwater inflow. This study demonstrates that ICA can be used effectively to aid in identifying component signals in environmental applications of small spatial scale. This article is protected by copyright. All rights reserved.
- Beyond annual streamflow reconstructions for the Upper Colorado River
Basin: A paleo‐water‐balance approach
- Authors: Subhrendu Gangopadhyay; Gregory J. McCabe, Connie Woodhouse
Abstract: In this paper, we present a methodology to use annual tree‐ring chronologies and a monthly water balance model to generate annual reconstructions of water balance variables (e.g. potential evapotranspiration (PET), actual evapotranspiration (AET), snow water equivalent (SWE), soil moisture storage (SMS), and runoff (R)). The method involves resampling monthly temperature and precipitation from the instrumental record directed by variability indicated by the paleoclimate record. The generated time series of monthly temperature and precipitation are subsequently used as inputs to a monthly water balance model. The methodology is applied to the Upper Colorado River basin and results indicate that the methodology reliably simulates water‐year runoff, maximum snow water equivalent, and seasonal soil moisture storage for the instrumental period. As a final application, the methodology is used to produce time series of PET, AET, SWE, SMS, and R for the 1404 through 1905 period for the Upper Colorado River basin. This article is protected by copyright. All rights reserved.
- Contribution of alluvial groundwater to the outflow of mountainous
- Abstract: Alluvial aquifers in mountainous regions cover typically a limited area. Their contribution to catchment storage and outflow is rarely isolated; alluvial groundwater discharge under gauging‐stations is generally assumed negligible; and hydrological models tend to lump alluvial storage with other units. The role of alluvial aquifers remains therefore unclear: can they contribute significantly to outflow when they cover a few percent of catchment area? Should they be considered a dynamic storage unit or merely a transmission zone? We address these issues based on the continuous monitoring of groundwater discharge, river discharge (one year), and aquifer storage (six months) in the 6 km2 alluvial system of a 194 km2 catchment. River and groundwater outflow were measured jointly through “coupled gauging stations”. The contribution of alluvial groundwater to outflow was highest at the outlet of a sub‐catchment (52 km2), where subsurface discharge amounted to 15% of mean annual outflow, and 85% of outflow during the last week of a drought. In this period, alluvial‐aquifer depletion supported 75% of the sub‐catchment outflow and 35% of catchment outflow – thus 3% of the entire catchment supported a third of the outflow. Storage fluctuations occurred predominantly in the aquifer's upstream part, where heads varied over 6 m. Not only does this section act as a significant water source, but storage recovers also rapidly at the onset of precipitation. Storage dynamics were best conceptualized along the valley‐axis, rather than across the more conventional riparian‐channel transect. Overall the contribution of alluvial aquifers to catchment outflow deserves more attention. This article is protected by copyright. All rights reserved.
- Solar radiation as a global driver of hillslope asymmetry: Insights from
an ecogeomorphic landscape evolution model
- Authors: Omer Yetemen; Erkan Istanbulluoglu, Alison R. Duvall
Abstract: Observations at the field, catchment, and continental scales across a range of arid and semiarid climates and latitudes reveal aspect‐controlled patterns in soil properties, vegetation types, ecohydrologic fluxes, and hillslope morphology. Although the global distribution of solar radiation on earth's surface and its implications on vegetation dynamics are well documented, we know little about how variation of solar radiation across latitudes influence landscape evolution and resulting geomorphic difference. Here, we used a landscape evolution model that couples the continuity equations for water, sediment, and aboveground vegetation biomass at each model element in order to explore the controls of latitude and mean annual precipitation (MAP) on the development of hillslope asymmetry (HA). In our model, asymmetric hillslopes emerged from the competition between soil creep and vegetation‐modulated fluvial transport, driven by spatial distribution of solar radiation. Latitude was a primary driver of HA because of its effects on the global distribution of solar radiation. In the Northern Hemisphere, north‐facing slopes (NFS), which support more vegetation cover and have lower transport efficiency, get steeper toward the North Pole while south‐facing slopes (SFS) get gentler. In the Southern Hemisphere, the patterns are reversed and SFS get steeper towards the South Pole. For any given latitude, MAP is found to have minor control on HA. Our results underscore the potential influence of solar radiation as a global control on the development of asymmetric hillslopes in fluvial landscapes. This article is protected by copyright. All rights reserved.
- Rapid bacteriophage MS2 transport in an oxic sandy aquifer in cold
climate: Field experiments and modeling
- Abstract: Virus removal during rapid transport in an unconfined, low‐temperature (6°C) sand and gravel aquifer was investigated at a riverbank field site, 25 km south of Trondheim in central Norway. The data from bacteriophage MS2 inactivation and transport experiments were applied in a two‐site kinetic transport model using HYDRUS‐1D, to evaluate the mechanisms of virus removal and whether these mechanisms were sufficient to protect the groundwater supplies. The results demonstrated that inactivation was negligible to the overall removal, and that irreversible MS2 attachment to aquifer grains, coated with iron precipitates, played a dominant role in the removal of MS2; 4.1 log units of MS2 were removed by attachment during 38 m travel distance and less than 2 days residence time. Although the total removal was high, pathways capable of allowing virus migration at rapid velocities were present in the aquifer. The risk of rapid transport of viable viruses should be recognized, particularly for water supplies without permanent disinfection. This article is protected by copyright. All rights reserved.
- Characterising flow behavior for gas injection: Relative permeability of
CO2‐brine and N2‐water in heterogeneous rocks
- Authors: C. A. Reynolds; S. Krevor
Abstract: We provide a comprehensive experimental study of steady state, drainage relative permeability curves with CO2‐brine and N2‐deionised water, on a single Bentheimer sandstone core with a simple two‐layer heterogeneity. We demonstrate that, if measured in the viscous limit, relative permeability is invariant with changing reservoir conditions, and is consistent with the continuum scale multiphase flow theory for water wet systems. Furthermore, we show that under capillary limited conditions, the CO2‐brine system is very sensitive to heterogeneity in capillary pressure, and by performing core floods under capillary limited conditions, we produce effective relative permeability curves that are flow rate and fluid parameter dependent. We suggest that the major uncertainty in past observations of CO2‐brine relative permeability curves is due to the interaction of CO2 flow with pore space heterogeneity under capillary limited conditions and is not due to the effects of changing reservoir conditions. We show that the appropriate conditions for measuring intrinsic or effective relative permeability curves can be selected simply by scaling the driving force for flow by a quantification of capillary heterogeneity. Measuring one or two effective curves on a core with capillary heterogeneity that is representative of the reservoir will be sufficient for reservoir simulation This article is protected by copyright. All rights reserved.
- Advective transport in heterogeneous aquifers: Are proxy models
- Authors: A. Fiori; A. Zarlenga, H. Gotovac, I. Jankovic, E. Volpi, V. Cvetkovic, G. Dagan
Abstract: We examine the prediction capability of two approximate models (Multi Rate Mass Transfer, MRMT, and Continuous Time Random Walk, CTRW) of non‐Fickian transport, by comparison with accurate 2D and 3D numerical simulations. Both non‐local in time approaches circumvent the need to solve the flow and transport equations by using proxy models to advection, providing the breakthrough curves (BTC) at control planes at any x, depending on a vector of 5 unknown parameters. Although underlain by different mechanisms, the two models have an identical structure in the Laplace Transform domain and have the Markovian property of independent transitions. We show that also the numerical BTCs enjoy the Markovian property. Following the procedure recommended in the literature, along a practitioner perspective, we first calibrate the parameters values by a best fit with the numerical BTC at a control plane at x1, close to the injection plane, and subsequently use it for prediction at further control planes for a few values of σY2≤8. Due to a similar structure and Markovian property, the two methods perform equally well in matching the numerical BTC. The identified parameters are generally not unique, making their identification somewhat arbitrary. The inverse Gaussian model and the recently developed Multi Indicator model (MIM), which does not require any fitting as it relates the BTC to the permeability structure, are also discussed. The application of the proxy models for prediction requires carrying out transport field tests of large plumes for a long duration. This article is protected by copyright. All rights reserved.
- Are all runoff processes the same? Numerical experiments comparing a
Darcy‐Richards solver to an overland flow‐based approach for
subsurface storm runoff simulation
- Authors: A. A. Ameli; J.R. Craig, J.J. McDonnell
Abstract: Hillslope runoff theory is based largely on the differentiation between infiltration excess overland flow, saturation excess overland flow and subsurface stormflow. Here we explore to what extent a 2‐D friction‐based overland flow model is useful for predicting hillslope‐scale subsurface stormflow, posited here as phenomenologically the same as infiltration excess at depth. We compare our results to a 3‐D variably saturated Darcy‐Richards subsurface solver for individual rainfall runoff events. We use field data from the well‐studied Panola Mountain Experimental hillslope in Georgia USA. Our results show that the two models are largely indistinguishable in terms of their ability to simulate the hillslope hydrograph magnitude and timing for a range of slopes and rainfall depths. Furthermore, we find that the descriptive ability of the overland flow model is comparable to the variably saturated subsurface flow model in terms of its ability to represent the spatial distribution of subsurface stormflow and infiltration across the soil‐bedrock interface. More importantly, these results imply that the physics of infiltration excess subsurface storm flow at the soil‐bedrock interface is similar to infiltration excess overland flow at the soil surface, in terms of detention storage, loss along the lower boundary and threshold‐like activation at the larger hillslope scale. Given the phenomenological similarity of overland flow and subsurface stormflow and the fact that overland flow model predictions are considerably faster to run (particularly as slope and rainfall depth increase), these findings imply that new forms of hillslope‐scale subsurface storm runoff predictions may be possible with the knowledge of bedrock permeability and limited soil information. Finally, this work suggests that the role of soil mantle vis‐à‐vis subsurface stormflow is mainly as a filter that delays the development of patches of saturation along the bedrock surface. Our model results show that simple realizations of soil based on a few soil depth measurements can possibly be enough to characterize this filtering effect. This article is protected by copyright. All rights reserved.
- Persistence of legacy soil‐P and elevated background water‐P
concentrations in Water Conservation Area 2A, a northern Everglades
- Authors: John M. Juston; Robert H. Kadlec, William F. DeBusk, Mike J. Jerauld, Thomas A. DeBusk
Abstract: Upstream source control and Stormwater Treatment Areas (STAs) have reduced phosphorus (P) loads to Water Conservation Area 2A (WCA‐2A), a northern Everglades wetland, by three‐quarters since year 2000. Nevertheless, large storages of P remain in enriched peat soils and it is unclear how legacy stores will impact spatial and temporal scales of recovery. We re‐measured soil P enrichment along a well‐studied eutrophication gradient in WCA‐2A and applied a profile modeling approach with uncertainty analysis to assess changes in longitudinal soil P gradients 13‐years after load reductions. We then analyzed existing internal water P data, using a novel data screening approach, for evidence of lowest possible water P concentrations independent from inflows. We interpret such water P limits as evidence of the strength of internal loading at a location. Results indicate that soil P enrichment persists in the ∼7.5 km long “impacted” zone, with no significant evidence of net advancement or recession, while a large pool of labile P in the flocculent layer consolidated and diminished. There is indeed evidence, both spatial and temporal, that this extensive zone of enriched soil P continues to elevate lowest achievable water P concentrations. The corresponding gradient of elevated water P limits is both receding and diminishing since load reductions, thus providing further evidence toward recovery. However, results also suggest that these “transitory P limits” due to internal loading are likely to persist for decades above water quality targets. These results advance our understanding of recovery in impacted wetlands and are relevant to Everglades restoration. This article is protected by copyright. All rights reserved.
- Numerical investigation of coupled density‐driven flow and
hydrogeochemical processes below playas
- Authors: Enrico Hamann; Vincent Post, Claus Kohfahl, Henning Prommer, Craig T. Simmons
Abstract: Numerical modeling approaches with varying complexity were explored to investigate coupled groundwater flow and geochemical processes in saline basins. Long‐term model simulations of a playa system gain insights into the complex feedback mechanisms between density driven flow and the spatio‐temporal patterns of precipitating evaporites and evolving brines. Using a reactive multicomponent transport model approach the simulations reproduced, for the first time in a numerical study, the evaporite precipitation sequences frequently observed in saline basins (“bull's eyes”). Playa‐specific flow, evapo‐concentration and chemical divides were found to be the primary controls for the location of evaporites formed, and the resulting brine chemistry. Comparative simulations with the computationally far less demanding surrogate single‐species transport models showed that these were still able to replicate the major flow patterns obtained by the more complex reactive transport simulations. However, the simulated degree of salinization was clearly lower than in reactive multicomponent transport simulations. For example, in the late stages of the simulations, when the brine becomes halite‐saturated, the non‐reactive simulation overestimated the solute mass by almost 20%. The simulations highlight the importance of the consideration of reactive transport processes for understanding and quantifying geochemical patterns, concentrations of individual dissolved solutes and evaporite evolution. This article is protected by copyright. All rights reserved.
- Depth, ice thickness, and ice‐out timing cause divergent hydrologic
responses among Arctic lakes
- Authors: Christopher D. Arp; Benjamin M. Jones, Anna K. Liljedahl, Kenneth M. Hinkel, Jeffery A. Welker
Abstract: Lakes are prevalent in the Arctic and thus play a key role in regional hydrology. Since many Arctic lakes are shallow and ice grows thick (historically 2‐m or greater), seasonal ice commonly freezes to the lake bed (bedfast ice) by winter's end. Bedfast ice fundamentally alters lake energy balance and melt‐out processes compared to deeper lakes that exceed the maximum ice thickness (floating ice) and maintain perennial liquid water below floating ice. Our analysis of lakes in northern Alaska indicated that ice‐out of bedfast ice lakes occurred on average 17 days earlier (22‐June) than ice‐out on adjacent floating ice lakes (9‐July). Earlier ice‐free conditions in bedfast ice lakes caused higher open‐water evaporation, 28% on average, relative to floating ice lakes and this divergence increased in lakes closer to the coast and in cooler summers. Water isotopes (18O and 2H) indicated similar differences in evaporation between these lake types. Our analysis suggests that ice regimes created by the combination of lake depth relative to ice thickness and associated ice‐out timing currently cause a strong hydrologic divergence among Arctic lakes. Thus understanding the distribution and dynamics of lakes by ice regime is essential for predicting regional hydrology. An observed regime shift in lakes to floating ice conditions due to thinner ice growth may initially offset lake drying because of lower evaporative loss from this lake type. This potential negative feedback caused by winter processes occurs in spite of an overall projected increase in evapotranspiration as the Arctic climate warms. This article is protected by copyright. All rights reserved.
- Temporal variability and memory in sediment transport in an experimental
- Abstract: Temporal dynamics of sediment transport in steep channels using two experiments performed in a steep flume (8%) with natural sediment composed of 12 grain sizes are studied. High‐resolution (1 sec) time series of sediment transport were measured for individual grain size classes at the outlet of the flume for different combinations of sediment input rates and flow discharges. Our aim in this paper is to quantify (a) the relation of discharge and sediment transport, and (b) the nature and strength of memory in grain‐size dependent transport. None of the simple statistical descriptors of sediment transport (mean, extreme values, quantiles) display a clear relation with water discharge, in fact a large variability between discharge and sediment transport is observed. Instantaneous transport rates have probability density functions with heavy tails and bedload bursts have a coarser grain‐size distribution than that of the entire experiment. We quantify the strength and nature of memory in sediment transport rates by estimating the Hurst exponent and the autocorrelation coefficient of the time series for different grain sizes. Our results show the presence of the Hurst phenomenon in transport rates, indicating long‐term memory which is grain‐size dependent. The short‐term memory in coarse‐grain transport increases with temporal aggregation and this reveals the importance of the sampling duration of bedload transport rates in natural streams, especially for large fractions. This article is protected by copyright. All rights reserved.
- Streambed and water profile response to in‐channel restoration
structures in a laboratory meandering stream
- Abstract: In‐channel structures are often installed in alluvial rivers during restoration to steer currents, but they also modify the streambed morphology and water surface profile, and alter hydraulic gradients driving ecologically important hyporheic exchange. Although river features before and after restoration need to be compared, few studies have collected detailed observations to facilitate this comparison. We created a laboratory mobile‐bed alluvial meandering river and collected detailed measurements in the highly sinuous meander before and after installation of in‐channel structures, which included one cross‐vane and six J‐hooks situated along one bar unit. Measurements of streambed and water surface elevation with sub‐millimeter vertical accuracy and horizontal resolution were obtained using close‐range photogrammetry. Compared to the smooth gradually varied water surface profile for control runs without structures, the structures created rapidly varied flow with sub‐ to super‐critical flow transitions, as well as backwater and forced‐morphology pools, which increased volumetric storage by 74% in the entire stream reach. The J‐hooks, located along the outer bank of the meander bend and downstream of the cross‐vane, created stepwise patterns in the streambed and water surface longitudinal profiles. The pooling of water behind the cross‐vane increased the hydraulic gradient across the meander neck by 1% and increased local groundwater gradients by 4%, with smaller increases across other transects through the intra‐meander zone. Scour pools developed downstream of the cross‐vane and around the J‐hooks situated near the meander apex. In‐channel structures significantly changed meander bend hydraulic gradients, and the detailed streambed and water surface 3D maps provide valuable data for computational modeling of changes to hyporheic exchange. This article is protected by copyright. All rights reserved.
- Analysis of groundwater flow in mountainous, headwater catchments with
- Authors: Sarah G. Evans; Shemin Ge, Sihai Liang
Abstract: Headwater catchments have a direct impact on the water resources of downstream lowland regions as they supply freshwater in the form of surface runoff and discharging groundwater. Often, these mountainous catchments contain expansive permafrost that may alter the natural topographically‐controlled groundwater flow system. As permafrost could degrade with climate change, it is imperative to understand the effect of permafrost on groundwater flow in headwater catchments. This study characterizes groundwater flow in mountainous headwater catchments and evaluates the effect of permafrost in the context of climate change on groundwater movement using a three‐dimensional, finite element, hydrogeologic model. The model is applied to a representative headwater catchment on the Qinghai‐Tibet Plateau, China. Results from the model simulations indicate that groundwater contributes significantly to streams in the form of baseflow and the majority of groundwater flow is from the shallow aquifer above the permafrost, disrupting the typical topographically‐controlled flow pattern observed in most permafrost‐free headwater catchments. Under a warming scenario where mean annual surface temperature is increased by 2 °C, reducing the areal extent of permafrost in the catchment, groundwater contribution to streamflow may increase three‐fold. These findings suggest that, in headwater catchments, permafrost has a large influence on groundwater flow and stream discharge. Increased annual air temperatures may increase groundwater discharge to streams, which has implications for ecosystem health and the long‐term availability of water resources to downstream regions. This article is protected by copyright. All rights reserved.
- A Bayesian approach to improved calibration and prediction of groundwater
models with structural error
- Authors: Tianfang Xu; Albert J. Valocchi
Abstract: Numerical groundwater flow and solute transport models are usually subject to model structural error due to simplification and/or misrepresentation of the real system, which raises questions regarding the suitability of conventional least squares regression‐based (LSR) calibration. We present a new framework that explicitly describes the model structural error statistically in an inductive, data‐driven way. We adopt a fully Bayesian approach that integrates Gaussian process error models into the calibration, prediction and uncertainty analysis of groundwater flow models. We test the usefulness of the fully Bayesian approach with a synthetic case study of the impact of pumping on surface‐ground water interaction. We illustrate through this example that the Bayesian parameter posterior distributions differ significantly from parameters estimated by conventional LSR, which does not account for model structural error. For the latter method, parameter compensation for model structural error leads to biased, overconfident prediction under changing pumping condition. In contrast, integrating Gaussian process error models significantly reduces predictive bias and leads to prediction intervals that are more consistent with validation data. Finally, we carry out a generalized LSR recalibration step to assimilate the Bayesian prediction while preserving mass conservation and other physical constraints, using a full error covariance matrix obtained from Bayesian results. It is found that the recalibrated model achieved lower predictive bias compared to the model calibrated using conventional LSR. The results highlight the importance of explicit treatment of model structural error especially in circumstances where subsequent decision‐making and risk analysis require accurate prediction and uncertainty quantification. This article is protected by copyright. All rights reserved.
- Combining regional estimation and historical floods: A multivariate
semiparametric peaks‐over‐threshold model with censored data
- Authors: Anne Sabourin; Benjamin Renard
Abstract: The estimation of extreme flood quantiles is challenging due to the relative scarcity of extreme data compared to typical target return periods. Several approaches have been developed over the years to face this challenge, including regional estimation and the use of historical flood data. This paper investigates the combination of both approaches using a multivariate peaks‐over‐threshold model, that allows estimating altogether the intersite dependence structure and the marginal distributions at each site. The joint distribution of extremes at several sites is constructed using a semi‐parametric Dirichlet Mixture model. The existence of partially missing and censored observations (historical data) is accounted for within a data augmentation scheme. This model is applied to a case study involving four catchments in Southern France, for which historical data are available since 1604. The comparison of marginal estimates from four versions of the model (with or without regionalizing the shape parameter; using or ignoring historical floods) highlights significant differences in terms of return level estimates. Moreover, the availability of historical data on several nearby catchments allows investigating the asymptotic dependence properties of extreme floods. Catchments display a a significant amount of asymptotic dependence, calling for adapted multivariate statistical models. This article is protected by copyright. All rights reserved.
- The atmospheric transport of iodine‐129 from Fukushima to British
Columbia, Canada, and its deposition and transport into groundwater
- Authors: Matt N. Herod; Martin Suchy, R. Jack Cornett, W.E. Kieser, Ian D. Clark, Gwyn Graham
Abstract: The Fukushima‐Daiichi nuclear accident (FDNA) released iodine‐129 (15.7 million year half‐life) and other fission product radionuclides into the environment in the spring and summer of 2011. 129I is recognized as a useful tracer for the short‐lived radiohazard 131I, which has a mobile geochemical behaviour with potential to contaminate water resources. To trace 129I released by the FDNA reaching Canada, pre‐ and post‐accident rain samples collected in Vancouver, on Saturna Island and from the National Atmospheric Deposition Program in Washington State were measured. Groundwater from the Abbotsford‐Sumas Aquifer was sampled to determine the fate of 129I that infiltrates below the root zone. Modelling of vadose zone transport was performed to constrain the travel time and retardation of 129I. The mean pre‐accident 129I concentration in rain was 31 x 106 atoms/L (n=4). Immediately following the FDNA, 129I values increased to 211 x 106 atoms/L and quickly returned to near‐background levels. However, pulses of elevated 129I continued for several months. The increases in 129I concentrations from both Vancouver and Saturna Island were synchronized, and occurred directly after the initial release from the FDNA. The 129I in shallow (3H/3He age
- Complementary‐relationship‐based 30 year normals
(1981–2010) of monthly latent heat fluxes across the contiguous U.S.
- Authors: Jozsef Szilagyi
Abstract: 30‐year normal (1981‐2010) monthly latent heat fluxes (ET) over the conterminous United States were estimated by a modified Advection‐Aridity model from North American Regional Reanalysis (NARR) radiation and wind as well as Parameter‐Elevation Regressions on Independent Slopes Model (PRISM) air and dew‐point temperature data. Mean annual ET values were calibrated with PRISM precipitation (P) and validated against United States Geological Survey runoff (Q) data. At the six‐digit Hydrologic Unit Code level (sample size of 334) the estimated 30‐year normal runoff (P – ET) had a bias of 18 mm yr−1, a root‐mean‐square‐error of 96 mm yr−1, and a linear correlation coefficient value of 0.95, making the estimates on par with the latest Land Surface Model results but without the need for soil and vegetation information or any soil moisture budgeting. This article is protected by copyright. All rights reserved.
- Reconstruction of missing daily streamflow data using dynamic regression
- Abstract: River discharge is one of the most important quantities in hydrology. It provides fundamental records for water resources management and climate change monitoring. Even very short data‐gaps in this information can cause extremely different analysis outputs. Therefore, reconstructing missing data of incomplete datasets is an important step regarding the performance of the environmental models, engineering and research applications, thus it presents a great challenge. The objective of this paper is to introduce an effective technique for reconstructing missing daily discharge data when one has access to only daily streamflow data. The proposed procedure uses a combination of regression and autoregressive integrated moving average models (ARIMA) called dynamic regression model. This model uses the linear relationship between neighbor and correlated stations and then adjusts the residual term by fitting an ARIMA structure. Application of the model to eight daily streamflow data for the Durance river watershed showed that the model yields reliable estimates for the missing data in the time series. Simulation studies were also conducted to evaluate the performance of the procedure. This article is protected by copyright. All rights reserved.
- Derivation of operation rules for reservoirs in parallel with joint water
- Authors: Xiang Zeng; Tiesong Hu, Lihua Xiong, Zhixian Cao, Chongyu Xu
Abstract: The purpose of this paper is to derive the general optimality conditions of the commonly used operating policies for reservoirs in parallel with joint water demand, which are defined in terms of system‐wide release rules and individual reservoir storage balancing functions. Following that, a new set of release rules for individual reservoirs are proposed in analytical forms by considering the optimality conditions for the balance of total water delivery utility and carryover storage value of individual reservoirs. Theoretical analysis indicates that the commonly used operating policies are a special case of the newly derived rules. The derived release rules are then applied to simulating the operation of a parallel reservoir system in northeastern China. Compared to the performance of the commonly used policies, some advantages of the proposed operation rules are illustrated. Most notably, less water shortage occurrence and higher water supply reliability are obtained from the proposed operation rules. This article is protected by copyright. All rights reserved.
- The role of dynamic surface water‐groundwater exchange on streambed
denitrification in a first‐order, low‐relief agricultural
- Authors: Mina Rahimi; Hedeff I. Essaid, John T. Wilson
Abstract: The role of temporally‐varying surface water ‐ groundwater (SW‐GW) exchange on nitrate removal by streambed denitrification was examined along a reach of Leary Weber Ditch (LWD), Indiana, a small, first‐order, low‐relief agricultural watershed within the Upper Mississippi River basin, using data collected in 2004 and 2005. Stream stage, GW heads (H) and temperatures (T) were continuously monitored in streambed piezometers and stream bank wells for two transects across LWD accompanied by synoptic measurements of stream stage, H, T and nitrate (NO3) concentrations along the reach. The H and T data were used to develop and calibrate vertical two‐dimensional, models of streambed water flow and heat transport across and along the axis of the stream. Model‐estimated SW‐GW exchange varied seasonally and in response to high streamflow events due to dynamic interactions between SW stage and GW H. Comparison of 2004 and 2005 conditions showed that small changes in precipitation amount and intensity, evapotranspiration, and/or nearby GW levels within a low‐relief watershed can readily impact SW‐GW interactions. The calibrated LWD flow models and observed stream and streambed NO3 concentrations were used to predict temporal variations in streambed NO3 removal in response to dynamic SW‐GW exchange. NO3 removal rates underwent slow seasonal changes, but also underwent rapid changes in response to high flow events. These findings suggest that increased temporal variability of SW‐GW exchange in low‐order, low relief watersheds may be a factor contributing their more efficient removal of NO3. This article is protected by copyright. All rights reserved.
- Optimal averaging of soil moisture predictions from ensemble land surface
- Abstract: The correct interpretation of ensemble information obtained from the parallel implementation of multiple land surface models (LSMs) requires information concerning the LSM ensemble's mutual error covariance. Here we propose a technique for obtaining such information using an instrumental variable (IV) regression approach and comparisons against a long‐term surface soil moisture dataset acquired from satellite remote sensing. Application of the approach to multi‐model ensemble soil moisture output from Phase 2 of the North American Land Data Assimilation System (NLDAS‐2) and European Space Agency (ESA) Soil Moisture (SM) Essential Climate Variable (ECV) dataset allows for the calculation of optimal weighting coefficients for individual members of the NLDAS‐2 LSM ensemble and a biased‐minimized estimate of uncertainty in a deterministic soil moisture analysis derived via optimal averaging. As such, it provides key information required to accurately condition soil moisture expectations using information gleaned from a multi‐model LSM ensemble. However, existing continuity and rescaling concerns surrounding the generation of long‐term, satellite‐based soil moisture products must likely be resolved before the proposed approach can be applied with full confidence. This article is protected by copyright. All rights reserved.
- Macroweather precipitation variability up to global and centennial scales
- Authors: M. I. P. de Lima; S. Lovejoy
Abstract: We investigate precipitation variability in the “macroweather” regime – the intermediate regime between the familiar weather and climate regimes – which is associated to time scales from about 10 days to 30–100 years. Macroweather precipitation is characterized by negative fluctuation exponents. This implies – contrary to the weather regime – that fluctuations tend to cancel each other out, they diminish with time scale, this is important for seasonal, annual and decadal forecasts. Aiming at a wide scale range space‐time statistical description of macroweather precipitation, we study the scaling of three centennial, global scale precipitation products (one gauge based, one reanalysis based, one satellite based) and systematically compare them over wide ranges of time and space scales. Although these products have very similar temporal statistics, at 5° resolution, they only agree with each other after being averaged over scales of several years. In space, there is less agreement on the statistics but since the data have low resolutions (mostly 5°×5°), the disagreement is only over a small overall range of scales: the monthly data agree fairly well at scales 20°–30° and larger. Moreover, we quantify the outer scale limit of the temporal scaling (20‐40 years, depending on the product, on the spatial scale, pixel or global). Overall, results show that precipitation can be modelled with space‐time scaling processes. The improved understanding of the space‐time macroweather precipitation variability and the limitations of precipitation products provided by this work opens new perspectives to the stochastic modelling and forecasting of macroweather fields. This article is protected by copyright. All rights reserved.
- Accelerating advances in continental domain hydrologic modeling
- Abstract: In the past, hydrologic modeling of surface water resources has mainly focused on simulating the hydrologic cycle at local to regional catchment modeling domains. There now exists a level of maturity amongst the catchment, global water security, and land surface modeling communities such that these communities are converging towards continental domain hydrologic models. This commentary, written from a catchment hydrology community perspective, provides a review of progress in each community towards this achievement, identifies common challenges the communities face, and details immediate and specific areas in which these communities can mutually benefit one another from the convergence of their research perspectives. Those include: (1) creating new incentives and infrastructure to report and share model inputs, outputs, and parameters in data services and open access, machine‐independent formats for model replication or re‐analysis; (2) ensuring that hydrologic models have: sufficient complexity to represent the dominant physical processes and adequate representation of anthropogenic impacts on the terrestrial water cycle, a process‐based approach to model parameter estimation, and appropriate parameterizations to represent large‐scale fluxes and scaling behavior; (3) maintaining a balance between model complexity and data availability as well as uncertainties and (4) quantifying and communicating significant advancements towards these modeling goals. This article is protected by copyright. All rights reserved.
- Linking water age and solute dynamics in streamflow at the Hubbard Brook
Experimental Forest, NH, USA
- Authors: Paolo Benettin; Scott W. Bailey, John L. Campbell, Mark B. Green, Andrea Rinaldo, Gene E. Likens, Kevin J. McGuire, Gianluca Botter
Abstract: We combine experimental and modeling results from a headwater catchment at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA, to explore the link between stream solute dynamics and water age. A theoretical framework based on water age dynamics, which represents a general basis for characterizing solute transport at the catchment scale, is here applied to conservative and weathering‐derived solutes. Based on the available information about the hydrology of the site, an integrated transport model was developed and used to compute hydrochemical fluxes. The model was designed to reproduce the deuterium content of streamflow and allowed for the estimate of catchment water storage and dynamic travel time distributions (TTDs). The innovative contribution of this paper is the simulation of dissolved silicon and sodium concentration in streamflow, achieved by implementing first‐order chemical kinetics based explicitly on dynamic TTD, thus upscaling local geochemical processes to catchment scale. Our results highlight the key‐role of water stored within the sub‐soil glacial material in both the short‐ and long‐term solute circulation. The travel time analysis provided an estimate of streamflow age distributions and their evolution in time related to catchment wetness conditions. The use of age information to reproduce a 14‐year dataset of silicon and sodium stream concentration shows that, at catchment scales, the dynamics of such geogenic solutes are mostly controlled by hydrologic drivers, which determine the contact times between the water and mineral interfaces. Justifications and limitations toward a general theory of reactive solute circulation at catchment scales are discussed. This article is protected by copyright. All rights reserved.
- Hydrological response to changing climate conditions: Spatial streamflow
variability in the boreal region
- Authors: C. Teutschbein; T. Grabs, R. H. Karlsen, H. Laudon, K. Bishop
Abstract: In this paper we combined a multi‐model ensemble based on 15 regional climate models with a multi‐catchment approach to explore the hydrologic sensitivity of 14 neighboring and rather similar catchments to changing climate conditions. Current (1982‐2010) and future (2062‐2090) streamflow was simulated with the HBV model. A diagnostic approach was used, which considered major behavioral catchment functions by using hydrologically relevant signatures related to overall water balance, flow duration curves and hydrograph attributes. Projected increases in temperature and precipitation resulted in increased total available streamflow, with lower spring and summer flows, but substantially higher winter streamflow. Furthermore, significant changes in flow durations with lower chances of both high and low flows can be expected in boreal Sweden in the future. This overall trend in projected streamflow pattern changes was comparable among the analyzed catchments but the magnitude of change differed considerably. This suggests that catchments belonging to the same region can show distinctly different degrees of hydrological responses to the same external climate change signal. We reason that differences in spatially distributed physical catchment properties within catchments are not only of great importance for current streamflow behavior, but also play a major role in the sensitivity of catchments to changing climate conditions. This article is protected by copyright. All rights reserved.
- Quasi‐steady flow in sloping aquifers
- Authors: Evangelos Akylas; Elias Gravanis, Antonis D. Koussis
Abstract: Mass conservation links the storage S and the outflow Q of an aquifer. A relation between them (an S‐Q relation) provides then a model governing the evolution of these quantities. In this work we construct an analytical quasi‐steady state model which exploits the properties of the exact S‐Q relation associated with steady state solutions of the Boussinesq equation for the sloping aquifer (that is, the Henderson & Wooding 1964 solutions). The model is derived by matching the asymptotic forms of the exact S‐Q relation which arise for small and large values of the Henderson & Wooding parameter λ. These asymptotic forms provide a novel re‐derivation of well‐known semi‐empirical S‐Q relations of the form Q ∝ S and Q ∝ S2, and they lead to soluble quasi‐steady state models. The quadratic asymptotic relation turns out to hold for surprisingly low values of λ. This characteristic and its formal properties allow smooth matching with the linear relation at λ=π2/4=2.47. The obtained model holds over the entire parameter space. An important characteristic of the model, stemming from its derivation, is that it involves only the geometric and hydraulic quantities present in the exact Boussinesq equation. The model is tested by best fitting four datasets from experiments simulating aquifer drainage. The derived curves for the drained volume are in excellent agreement with the data. The estimated values for k and n are also in overall very good agreement with their reference values. This article is protected by copyright. All rights reserved.
- Robust optimization of well location to enhance hysteretical trapping of
CO2: Assessment of various uncertainty quantification methods and
utilization of mixed response surface surrogates
- Authors: Masoud Babaei; Indranil Pan, Ali Alkhatib
Abstract: The paper aims to solve a robust optimization problem (optimization in presence of uncertainty) for finding the optimal locations of a number of CO2 injection wells for geological sequestration of carbon dioxide in a saline aquifer. The parametric uncertainties are the interfacial tension between CO2 and aquifer brine, the Land's trapping coefficient and the boundary aquifer's absolute. The spatial uncertainties are due to the channelized permeability field which exhibits a binary channel‐non‐channel system. The objective function of the optimization is the amount of residually trapped CO2 due to the hysteresis of the relative permeability curves. A risk‐averse value derived from the cumulative density function of the distribution of the amount of trapped gas is chosen as the objective function value. In order to ensure that the uncertainties are effectively taken into account, Monte Carlo simulation and Polynomial Chaos Expansion (PCE)‐based methods are used and compared with each other. For different cases of parametric and spatial uncertainties, the most accurate uncertainty quantification (UQ) method is chosen to be integrated within the optimization algorithm. While for parametric uncertainty cases of up to two uncertain variables, PCE‐based methods computationally outperform Monte Carlo simulations, it is shown that for the multimodal distributions of the function of trapped gas occurring for the spatial uncertainty case, Monte Carlo simulations are more reliable than PCE‐based UQ methods. For the discrete (integer) optimization problem, various mixed response surface surrogate models are tested and the robust optimization resulted in optimal CO2 injection well locations. This article is protected by copyright. All rights reserved.
- Influence of dynamic factors on non‐wetting fluid snap‐off in
- Authors: Wen Deng; Matthew Balhoff, M. Bayani Cardenas
Abstract: Snap‐off is an important dynamic multiphase flow phenomenon which occurs in porous media. It plays a dominant role in the residual trapping and mobilization/immobilization of non‐wetting fluids such as hydrocarbons or CO2. Current studies, applications, and threshold criteria of snap‐off are mostly based on static or equilibrium conditions. Thus, the dynamics of snap‐off which is relevant for many real world applications has rarely been systematically studied. While a static criterion indicates the snap‐off potential for non‐wetting fluids, the competition between the time required for snap‐off and the local pore throat capillary number determines whether snap‐off actually occurs. Using a theoretical model to couple the wetting film thickness to the local capillary number at the pore throat, we analyzed the dynamics of the wetting/non‐wetting interface instability in sinusoidally constricted capillary tubes. The influence of dynamic factors as encapsulated by the effect of local capillary number on non‐wetting fluid snap‐off time were investigated for varying pore throat to pore body aspect ratio and pore body distances. The analysis showed that snap‐off can be inhibited by a sufficiently large local capillary number even in cases where the static snap‐off criterion has been met. This article is protected by copyright. All rights reserved.
- Introduction to a special section on ecohydrology of semiarid
environments: Confronting mathematical models with ecosystem complexity
- Authors: Tal Svoray; Shmuel Assouline, Gaby Katul
Abstract: Current literature provides large number of publications about eco‐hydrological processes and their effect on the biota in drylands. Given the limited laboratory and field experiments in such systems, many of these publications are based on mathematical models of varying complexity. The underlying implicit assumption is that the dataset used to evaluate these models covers the parameter space of conditions that characterize drylands and that the models represent the actual processes with acceptable certainty. However, a question raised is to what extent these mathematical models are valid when confronted with observed ecosystem complexity? This Introduction paper reviews the sixteen papers that comprise the Special Section on Eco‐hydrology of Semiarid Environments: Confronting Mathematical Models with Ecosystem Complexity. The subjects studied in these papers include rainfall regime, infiltration and preferential flow, evaporation and evapotranspiration, annual net primary production, dispersal and invasion and vegetation greening. The findings in the papers published in this Special Section show that innovative mathematical modeling approaches can represent actual field measurements. Hence, there are strong grounds for suggesting that mathematical models can contribute to greater understanding of ecosystem complexity through characterization of space‐time dynamics of biomass and water storage as well as their multi‐scale interactions. However, the generality of the models and their low‐dimensional representation of many processes may also be a ‘curse' that results in failures when particulars of an ecosystem are required. It is envisaged that the search for a unifying ‘general' model, while seductive, may remain elusive in the foreseeable future. It is for this reason that improving the merger between experiments and models of various degrees of complexity continues to shape the future research agenda. This article is protected by copyright. All rights reserved.
- Observations of distributed snow depth and snow duration within diverse
forest structures in a maritime mountain watershed
- Abstract: Spatially distributed snow depth and snow duration data were collected over two to four snow seasons during water years 2011‐2014 in experimental forest plots within the Cedar River Municipal Watershed, 50 km east of Seattle, Washington, USA. These 40 × 40 m forest plots, situated on the western slope of the Cascade Range, include un‐thinned second‐growth coniferous forests, variable density thinned forests, forest gaps in which a 20 m diameter (approximately equivalent to one tree height) gap was cut in the middle of each plot, and old‐growth forest.
Together, this publicly available dataset includes snow depth and density observations from manual snow surveys, distributed snow duration observations from ground temperature sensors and time‐lapse cameras, meteorological data collected at two open locations and three forested locations, and forest canopy data from airborne light detection and ranging (LiDAR) data and hemispherical photographs. These co‐located snow, meteorological, and forest data have the potential to improve understanding of forest influences on snow processes, and provide a unique model‐testing dataset for hydrological analyses in a forested, maritime watershed. We present empirical snow depletion curves within forests to illustrate an application of these data to improve sub‐grid representation of snow cover in distributed modeling. This article is protected by copyright. All rights reserved.
- Flow and scour constraints on uprooting of pioneer woody seedlings
- Abstract: Scour and uprooting during flood events is a major disturbance agent that affects plant mortality rates and subsequent vegetation composition and density, setting the trajectory of physical‐biological interactions in rivers. During flood events, riparian plants may be uprooted if they are subjected to hydraulic drag forces greater than their resisting force. We measured the resisting force of woody seedlings established on river bars with in situ lateral pull tests that simulated flood flows with and without substrate scour. We quantified the influence of seedling size, species (Populus and Tamarix), water‐table depth, and scour depth on resisting force. Seedling size and resisting force were positively related, with scour depth and water‐table depth—a proxy for root length—exerting strong and opposing controls on resisting force. Populus required less force to uproot than Tamarix, but displayed a greater increase in uprooting force with seedling size. Further, we found that calculated mean velocities required to uproot seedlings were greater than modeled flood velocities under most conditions. Only when plants were either shallowly rooted or subjected to substrate scour (≥0.3 m) did the calculated velocities required for uprooting decrease to within the range of modeled flood velocities, indicating that drag forces alone are unlikely to uproot seedlings in the absence of extreme events or bar‐scale sediment transport. Seedlings on river bars are most resilient to uprooting when they are large, deeply rooted and unlikely to experience substrate scour, which has implications for ecogeomorphic evolution and river management. This article is protected by copyright. All rights reserved.
- The impact of pore structure and surface roughness on capillary trapping
for 2‐D and 3‐D‐porous media: Comparison with
- Abstract: We study the impact of pore structure and surface roughness on capillary trapping of non‐wetting gas phase during imbibition with water for capillary numbers between 10−7 and 5 × 10−5, within glass‐beads, natural‐sands, glass‐beads monolayers, and 2D‐micromodels. The materials exhibit different roughness of the pore‐solid interface. We found that glass beads and natural sands, which exhibit nearly the same grain size distribution, pore size distribution and connectivity, showed a significant difference of the trapped gas phase of about 15%. This difference can be explained by the micro‐structure of the pore‐solid interface. Based on the visualization of the trapping dynamics within glass‐beads monolayers and 2D‐micromodels, we could show that by‐pass trapping controls the trapping process in glass‐beads monolayers, while snap‐off trapping controls the trapping process in 2D‐micromodels. We conclude that these different trapping processes are the reason for the different trapping efficiency, when comparing glass beads packs with natural sand packs.
Moreover, for small capillary numbers of 10−6 we found that the cluster size distribution of trapped gas clusters of all 2D‐ and 3D‐porous media can be described by a universal power‐law behavior predicted from percolation theory. This cannot be expected a priory for 2D‐porous media, because bicontinuity of the two bulk phases is violated. Obviously, bicontinuity holds for the thin‐film water phase and the bulk gas phase. The snap‐off trapping process leads to ordinary bond percolation in front of the advancing bulk water phase and is the reason for the observed universal power law behavior in 2D‐micromodels with rough surfaces. This article is protected by copyright. All rights reserved.
- The effect of saturation path on three‐phase relative permeability
- Authors: Amir Kianinejad; Xiongyu Chen, David A. DiCarlo
Abstract: Simulation and fluid flow prediction of many petroleum enhanced oil recovery methods as well as environmental processes such as carbon dioxide (CO2) geological storage or underground water resources remediation requires accurate modeling and determination of relative permeability under different saturation histories. Based on this critical need, several three‐phase relative permeability models were developed to predict relative permeability; however, for practical purposes most of them require a variety of parameters introducing undesired complexity to the models. In this work, we attempt to find out if there is a simpler way to express this functionality. To do so, we experimentally measure three‐phase, water/oil/gas, relative permeability in a 1‐m long water‐wet sand pack, under several saturation flow paths to cover the entire three‐phase saturation space. We obtain the in‐situ saturations along the sand pack using a CT scanner and then determine the relative permeabilities of liquid phases directly from the measured in‐situ saturations using an unsteady‐state method. The measured data shows that at a specific saturation, the oil relative permeability varies significantly (up to 2 orders of magnitude), depending on the path through saturation space. The three‐phase relative permeability data are modeled using standard relative permeability models, Corey‐type and Saturation Weighted Interpolation (SWI). Our measured data suggest that three‐phase oil relative permeability in water‐wet media is only a function of its own saturation if the residual oil saturation is treated as a function of two saturations. We determine that residual saturation is the key parameter in modeling three‐phase relative permeability (effect of saturation history). This article is protected by copyright. All rights reserved.
- Making the most of data: An information selection and assessment framework
to improve water systems operations
- Authors: M. Giuliani; F. Pianosi, A. Castelletti
Abstract: Advances in Environmental monitoring systems are making a wide range of data available at increasingly higher temporal and spatial resolution. This creates an opportunity to enhance real‐time understanding of water systems conditions and to improve prediction of their future evolution, ultimately increasing our ability to make better decisions. Yet, many water systems are still operated using very simple information systems, typically based on simple statistical analysis and the operator's experience. In this work, we propose a framework to automatically select the most valuable information to inform water systems operations supported by quantitative metrics to operationally and economically assess the value of this information. The Hoa Binh reservoir in Vietnam is used to demonstrate the proposed framework in a multi‐objective context, accounting for hydropower production and flood control. First, we quantify the expected value of perfect information, meaning the potential space for improvement under the assumption of exact knowledge of the future system conditions. Second, we automatically select the most valuable information that could be actually used to improve the Hoa Binh operations. Finally, we assess the economic value of sample information on the basis of the resulting policy performance. Results show that our framework successfully select information to enhance the performance of the operating policies with respect to both the competing objectives, attaining a 40% improvement close to the target tradeoff selected as potentially good compromise between hydropower production and flood control. This article is protected by copyright. All rights reserved.
- Multiscale hyporheic exchange through strongly heterogeneous sediments
- Authors: Timothy T. Pryshlak; Audrey H. Sawyer, Susa H. Stonedahl, Mohamad Reza Soltanian
Abstract: Heterogeneity in hydraulic conductivity (K) and channel morphology both control surface water‐groundwater exchange (hyporheic exchange), which influences stream ecosystem processes and biogeochemical cycles. Here we show that heterogeneity in K is the dominant control on exchange rates, residence times, and patterns in hyporheic zones with abrupt lithologic contrasts. We simulated hyporheic exchange in a representative low‐gradient stream with 300 different bimodal K fields composed of sand and silt. Simulations span five sets of sand‐silt ratios and two sets of low and high K contrasts (one and three orders of magnitude). Heterogeneity increases interfacial flux by an order of magnitude relative to homogeneous cases, drastically changes the shape of residence time distributions, and decreases median residence times. The positioning of highly permeable sand bodies controls patterns of interfacial flux and flow paths. These results are remarkably different from previous studies of smooth, continuous K fields that indicate only moderate effects on hyporheic exchange. Our results also show that hyporheic residence times are least predictable when sand body connectivity is low. As sand body connectivity increases, the expected residence time distribution (ensemble average for a given sand‐silt ratio) remains approximately constant, but the uncertainty around the expectation decreases. Including strong heterogeneity in hyporheic models is imperative for understanding hyporheic fluxes and solute transport. In streams with strongly heterogeneous sediments, characterizing lithologic structure is more critical for predicting hyporheic exchange metrics than characterizing channel morphology. This article is protected by copyright. All rights reserved.
- Assessing water resource system vulnerability to unprecedented
hydrological drought using copulas to characterize drought duration and
- Abstract: Global climate models suggest an increase in evapotranspiration, changing storm tracks and moisture delivery in many parts of the world which are likely to cause more prolonged and severe drought, yet the weakness of climate models in modelling persistence of hydro‐climatic variables and the uncertainties associated with regional climate projections mean that impact assessments based on climate model output may underestimate the risk of multi‐year droughts. In this paper we propose a vulnerability‐based approach to test water resource system response to drought. We generate a large number of synthetic streamflow series with different drought durations and deficits and use them as input to a water resource system model. Marginal distributions of the streamflow for each month are generated by bootstrapping the historical data, while the joint probability distributions of consecutive months are constructed using a copula‐based method. Droughts with longer durations and larger deficits than the observed record are generated by perturbing the copula parameter and by adopting an importance sampling strategy for low flows. In this way potential climate‐induced changes in monthly hydrological persistence are factored into the vulnerability analysis. The method is applied to the London water system (England) to investigate under which drought conditions severe water use restrictions would need to be imposed. Results indicate that the water system is vulnerable to drought conditions outside the range of historical events. The vulnerability assessment results were coupled with climate model information to compare alternative water management options with respect to their vulnerability to increasingly long and severe drought. This article is protected by copyright. All rights reserved.
- The structure of gravel bed flow with intermediate submergence: A
- Authors: S. H. Mohajeri; S. Grizzi, M. Righetti, G. P. Romano, V. Nikora
Abstract: The paper reports an experimental study of the flow structure over an immobile gravel bed in open channel at intermediate submergence, with particular focus on the near‐bed region. The experiments consisted of velocity measurements using three‐component (stereoscopic) Particle Image Velocimetry (PIV) in near‐bed horizontal plane and two‐component PIV in three vertical planes that covered three distinctly different hydraulic scenarios where the ratio of flow depth to roughness height (i.e., relative submergence) changes from 7.5 to 10.8. Detailed velocity measurements were supplemented with fine‐scale bed elevation data obtained with a laser scanner. The data revealed longitudinal low‐momentum and high‐momentum ‘strips' in the time‐averaged velocity field, likely induced by secondary currents. This depth‐scale pattern was superimposed with particle‐scale patches of flow heterogeneity induced by gravel particle protrusions. A similar picture emerged when considering second‐order velocity moments. The interaction between the flow field and gravel‐bed protrusions is assessed using cross‐correlations of velocity components and bed elevations in a horizontal plane just above gravel particle crests. The cross‐correlations suggest that upward and downward fluid motions are mainly associated with upstream‐facing and lee sides of particles, respectively. Results also show that the relative submergence affects the turbulence intensity profiles for vertical velocity over the whole flow depth, while only a weak effect, limited to the near‐bed region, is noticed for streamwise velocity component. The approximation of mean velocity profiles with a logarithmic formula reveals that log‐profile parameters depend on relative submergence, highlighting inapplicability of a conventional ‘universal' logarithmic law for gravel‐bed flows with intermediate submergence. This article is protected by copyright. All rights reserved.
- Suspended particle capture by synthetic vegetation in a laboratory flume
- Authors: Kristen E. Fauria; Rachel E. Kerwin, Daniel Nover, S. Geoffrey Schladow
Abstract:  Vegetated floodplains and wetlands trap particles, a process that is important for water quality and wetland function and morphology. The rates of particle removal by vegetation remain poorly characterized, especially for small particles and vegetation coated with biofilm. In this study, we measured capture rates of road dust by arrays of grass‐like synthetic vegetation in a laboratory flume. We performed forty experiments in which stem density, flow velocity, the presence of biofilm, and initial particle concentration varied, and used an in situ particle size analyzer to measure the concentration of a continuous particle size distribution (1.25 – 250 µm diameter). We fit first‐order decay models to the particle concentration measurements to determine particle capture rates and found that capture rates increased with particle size, stem density, and the presence of biofilm. Capture rates decreased with increasing flow velocity, which suggests that fast flows may resuspend particles from stems. We also calculated percent particle capture efficiencies and fit a new empirical model for capture efficiency to our results. We found that particle capture efficiency was highest for low stem density treatments and propose that stem density affects capture by altering turbulent kinetic energy. This article is protected by copyright. All rights reserved.
- Assessing uncertainties in surface water security: An empirical multimodel
- Authors: Dulce B. B. Rodrigues; Hoshin V. Gupta, Eduardo M. Mendiondo, Paulo Tarso, S. Oliveira
Abstract: Various uncertainties are involved in the representation of processes that characterize interactions between societal needs, ecosystem functioning, and hydrological conditions. Here, we develop an empirical uncertainty assessment of water security indicators that characterize scarcity and vulnerability, based on a multi‐model and resampling framework. We consider several uncertainty sources including those related to: i) observed streamflow data; ii) hydrological model structure; iii) residual analysis; iv) the method for defining Environmental Flow Requirement; v) the definition of critical conditions for water provision; and vi) the critical demand imposed by human activities. We estimate the overall hydrological model uncertainty by means of a residual bootstrap resampling approach, and by uncertainty propagation through different methodological arrangements applied to a 291 km2 agricultural basin within the Cantareira water supply system in Brazil. Together, the two‐component hydrograph residual analysis and the block bootstrap resampling approach result in a more accurate and precise estimate of the uncertainty (95% confidence intervals) in the simulated time series. We then compare the uncertainty estimates associated with water security indicators using a multi‐model framework and the uncertainty estimates provided by each model uncertainty estimation approach. The range of values obtained for the water security indicators suggests that the models/methods are robust and performs well in a range of plausible situations. The method is general and can be easily extended, thereby forming the basis for meaningful support to end‐users facing water resource challenges by enabling them to incorporate a viable uncertainty analysis into a robust decision making process. This article is protected by copyright. All rights reserved.
- High‐Performance Integrated Control of Water Quality and Quantity in
Urban Water Reservoirs
- Authors: S. Galelli; A. Castelletti, A. Goedbloed
Abstract: This paper contributes a novel High‐Performance Integrated Control framework to support the real‐time operation of urban water supply storages affected by water quality problems. We use a 3D, high‐fidelity simulation model to predict the main water quality dynamics and inform a real‐time controller based on Model Predictive Control. The integration of the simulation model into the control scheme is performed by a model reduction process that identifies a low‐order, dynamic emulator running four orders of magnitude faster. The model reduction, which relies on a semi‐automatic procedural approach integrating time series clustering and variable selection algorithms, generates a compact and physically meaningful emulator that can be coupled with the controller. The framework is used to design the hourly operation of Marina Reservoir, a 3.2 Mm3 stormwater‐fed reservoir located in the centre of Singapore, operated for drinking water supply and flood control. Because of its recent formation from a former estuary, the reservoir suffers from high salinity levels, whose behavior is modelled with Delft3D‐FLOW. Results show that our control framework reduces the minimum salinity levels by nearly 40% and cuts the average annual deficit of drinking water supply by about two times the active storage of the reservoir (about 4% of the total annual demand This article is protected by copyright. All rights reserved.
- Identifying seasonal patterns of phosphorus storm dynamics with Dynamic
- Abstract: Phosphorus (P) transfer during storm events represents a significant part of annual P loads in streams and contributes to eutrophication in downstream water bodies. To improve understanding of P storm dynamics, automated or semi‐automated methods are needed to extract meaningful information from ever‐growing water quality measurement datasets. In this paper, seasonal patterns of P storm dynamics are identified in two contrasting watersheds (arable and grassland) through Dynamic Time Warping (DTW) combined with k‐means clustering. DTW was used to align discharge time series of different lengths and with differences in phase, which allowed robust application of a k‐means clustering algorithm on rescaled P time series. In the arable watershed, the main storm pattern identified from autumn to winter displayed distinct export dynamics for particulate and dissolved P, which suggests independent transport mechanisms for both P forms. Conversely, the main storm pattern identified in spring displayed synchronized export of particulate and dissolved P. In the grassland watershed, the occurrence of synchronized export of dissolved and particulate P forms was not related to the season, but rather to the amplitude of storm events. Differences between the seasonal distributions of the patterns identified for the two watersheds were interpreted in terms of P sources and transport pathways. The DTW‐based clustering algorithm used in this study proved useful for identifying common patterns in water quality time series and for isolating unusual events. It will open new possibilities for interpreting the high‐frequency and multi‐parameter water quality time series that are currently acquired worldwide. This article is protected by copyright. All rights reserved.
- Predicting glaciohydrologic change in the headwaters of the Zongo River,
Cordillera Real, Bolivia
- Authors: Chris Frans; Erkan Istanbulluoglu, Dennis P. Lettenmaier, Bibi Naz, Garry Clarke, Thomas Condom, Pat Burns, Anne Nolin
Abstract: In many partially glacierized watersheds glacier recession driven by a warming climate could lead to complex patterns of streamflow response over time, often marked with rapid increases followed by sharp declines, depending on initial glacier ice cover and rate of climate change. Capturing such “phases” of hydrologic response is critical in regions where communities rely on glacier meltwater, particularly during low flows. In this paper, we investigate glacio‐hydrologic response in the headwaters of the Zongo River, Bolivia, under climate change using a distributed glacio‐hydrological model over the period of 1987‐2100. Model predictions are evaluated through comparisons with satellite‐derived glacier extent estimates, glacier surface velocity, in‐situ glacier mass balance, surface energy flux, and stream discharge measurements. Historically (1987‐2010) modeled glacier melt accounts for 27% of annual runoff, and 61% of dry season (JJA) runoff on average. During this period the relative glacier cover was observed to decline from 35% to 21% of the watershed. In the future, annual and dry season discharge is projected to decrease by 4% and 27% by midcentury and 25% and 57% by the end of the century, respectively, following the loss of 81% of the ice in the watershed. Modeled runoff patterns evolve through the interplay of positive and negative trends in glacier melt and increased evapotranspiration as the climate warms. Sensitivity analyses demonstrate that the selection of model surface energy balance parameters greatly influences the trajectory of hydrological change projected during the first half of the 21st century. These model results underscore the importance of coupled glacio‐hydrology modeling. This article is protected by copyright. All rights reserved.
- A complete soil hydraulic model accounting for capillary and adsorptive
water retention, capillary and film conductivity, and hysteresis
- Authors: Rudiyanto Rudiyanto; Masaru Sakai, M. Th. van Genuchten, A. A. Alazba, Budi Indra Setiawan, Budiman Minasny
Abstract: A soil hydraulic model that considers capillary hysteretic and adsorptive water retention as well as capillary and film conductivity covering the complete soil moisture range is presented. The model was obtained by incorporating the capillary hysteresis model of Parker and Lenhard into the hydraulic model of Peters‐Durner‐Iden (PDI) as formulated for the van Genuchten (VG) retention equation. The formulation includes the following processes: capillary hysteresis accounting for air entrapment, closed scanning curves, non‐hysteretic sorption of water retention onto mineral surfaces, a hysteretic function for the capillary conductivity, a non‐hysteretic function for the film conductivity, and a nearly non‐hysteretic function of the conductivity as a function of water content (θ) for the entire range of water contents. The proposed model only requires two additional parameters to describe hysteresis. The model was found to accurately describe observed hysteretic water retention and conductivity data for a dune sand. Using a range of published datasets, relationships could be established between the capillary water retention and film conductivity parameters. Including vapor conductivity improved conductivity descriptions in the very dry range. The resulting model allows predictions of the hydraulic conductivity from saturation until complete dryness using water retention parameters. This article is protected by copyright. All rights reserved.
- Impact of sample geometry on the measurement of pressure‐saturation
curves: Experiments and simulations
- Abstract: In this paper we study the influence of sample geometry on the measurement of pressure‐saturation relationships, by analyzing the drainage of a two‐phase flow from a quasi‐2D random porous medium. The medium is transparent, which allows for the direct visualization of the invasion pattern during flow, and is initially saturated with a viscous liquid (a dyed glycerol‐water mix). As the pressure in the liquid is gradually reduced, air penetrates from an open inlet, displacing the liquid which leaves the system from an outlet on the opposite side. Pressure measurements and images of the flow are recorded and the pressure‐saturation relationship is computed. We show that this relationship depends on the system size and aspect ratio. The effects of the system's boundaries on this relationship are measured experimentally and compared with simulations produced using an invasion percolation algorithm. The pressure build up at the beginning and end of the invasion process are particularly affected by the boundaries of the system whereas at the central part of the model (when the air front progresses far from these boundaries), the invasion happens at a statistically constant capillary pressure. These observations have led us to propose a much simplified pressure‐saturation relationship, valid for systems that are large enough such that the invasion is not influenced by boundary effects. The properties of this relationship depend on the capillary pressure thresholds distribution, sample dimensions and average pore connectivity and its applications may be of particular interest for simulations of two‐phase flow in large porous media. This article is protected by copyright. All rights reserved.
- Channel adjustments to a succession of water pulses in gravel bed rivers
- Abstract: Gravel‐bed rivers commonly exhibit a coarse surface armor resulting from a complex history of interactions between flow and sediment supply. The evolution of the surface texture under single storm events or under steady flow conditions has been studied by a number of researchers. However, the role of successive floods on the surface texture evolution is still poorly understood. An experimental campaign in an 18 m‐long 1 m‐wide flume has been designed to study these issues. Eight consecutive runs, each one consisting of a low‐flow period of variable duration followed by a sudden flood (water pulse) lasting 1.5 h, have been conducted. The total duration of the experiment was 46 h. The initial bed surface was created during a 280 h‐long experiment focused on the influence of episodic sediment supply on channel adjustments. Our experiments represent a realistic armored and structured beds found in mountain gravel‐bed rivers. The armor surface texture persists over the duration of the experiment. The experiment exhibits downstream fining of the bed surface texture. It was found that sorting processes were affected by the duration of low‐flow between flood pulses. Since bedload transport is influenced by sediment sorting, the evolution of bedload transport is impacted by the frequency of the water pulses: short inter‐pulse durations reduce the time over which fine material (transported as bedload) can be winnowed. This, in turn, contributes to declining reduction of the bedload transport over time while the sediment storage increases. This article is protected by copyright. All rights reserved.
- Colloid filtration prediction by mapping the correlation‐equation
parameters from transport experiments in porous media
- Authors: Tamir Kamai; Mohamed K. Nassar, Kirk E. Nelson, Timothy R. Ginn
Abstract: Colloid filtration theory (CFT) is a conceptual construct for predicting the characteristic rate of colloid‐surface collisions during transport in granular porous media. A central product of this theory is the correlation equation for predicting collection‐efficiency (η), based exclusively on theoretical model development. Specifically, the η‐equation has terms combining dimensionless groups (of physicochemical properties) with unknown parameters that are usually fitted so that the predicted η matches that determined by colloid‐surface collisions simulated in idealized pore‐scale models. In this study, we replace the simulated colloid‐surface collisions in idealized models with experimental column‐scale data on apparent colloid‐surface collisions. A new correlation equation is obtained by minimizing the difference between η determined by the correlation equation and that determined experimentally, using data from a collection of experiments for favorable conditions for colloid filtration. In this way we parameterize a mechanistically‐based η‐equation with empirical evidence. The impact of different properties of colloids and porous media are studied by comparing experimental properties with different terms of the correlation equation. This comparison enables insight about the different processes that occur during colloid transport and retention in porous media, such as diffusion and interception, and provides directions for future CFT developments that will need to account for these processes differently than the current theory does. Additionally, we find that while most of the parameters of the presented η equation are only slightly different than those proposed in previous theoretical studies, the match between theory and observation is significantly improved. For the available experimental data, which provides a reasonable representation of property ranges for many applications of CFT, the proposed equation provides a closer match to the experimentally measured collection efficiencies compared to available theories to date. This article is protected by copyright. All rights reserved.
- Experimental analysis of spatial correlation effects on capillary trapping
of supercritical CO2 at the intermediate laboratory scale in heterogeneous
- Authors: Luca Trevisan; Ronny Pini, Abdullah Cihan, Jens T. Birkholzer, Quanlin Zhou, Tissa H. Illangasekare
Abstract: Several numerical studies have demonstrated that the heterogeneous nature of typical sedimentary formations can favorably dampen the accumulation of mobile CO2 phase underneath the caprock. Core flooding experiments have also shown that contrasts in capillary entry pressure can lead to buildup of non‐wetting fluid phase (NWP) at interfaces between facies. Explicit representation of geological heterogeneity at the intermediate (cm‐to‐m) scale is a powerful approach to identify the key mechanisms that control multiphase flow dynamics in porous media. The ability to carefully control flow regime and permeability contrast at a scale that is relevant to CO2 plume dynamics in saline formations offers valuable information to understand immiscible displacement processes and provides a benchmark for mathematical models. To provide insight into the impact of capillary heterogeneity on flow dynamics and trapping efficiency of supercritical CO2 under successive drainage and imbibition conditions, we present an experimental investigation conducted in a synthetic sand reservoir. By mimicking the interplay of governing forces at reservoir conditions via application of surrogate fluids, we performed three immiscible displacement experiments to observe the entrapment of NWP in heterogeneous porous media. Capillary trapping performance is evaluated for each scenario through spatial and temporal variations of NWP saturation; for this reason we adopted x‐ray attenuation to precisely measure phase saturation throughout the flow domain and apply spatial moment analysis. The sweeping performance of two different permeability fields with comparable variance but distinct spatial correlation was compared against a homogeneous base case with equivalent mean permeability by means of spatial moment analysis. This article is protected by copyright. All rights reserved.
- The implications of laser‐diffraction measurements of sediment size
distributions in a river to the potential use of acoustic backscatter for
- Authors: Y.C. Agrawal; D.M. Hanes
Abstract: We construct vertical profiles of the acoustic attenuation and back‐scattering properties of a river column from measured particle concentration and size distribution profiles. The particle size and concentration data were collected in‐situ in the Cowlitz River in Washington, USA, using a laser diffraction‐ instrument LISST‐SL. The particle size distribution was bi‐modal, comprising a vertically well‐mixed washload, and a suspended load that was similar to Rouse profiles. We then explore how well the results of converting these synthetic profiles to recover an acoustic equivalent sediment concentration and acoustic equivalent size compare with laser data in this bi‐modal environment. The acoustic scattering and attenuation properties are computed for 4 distinct frequencies: 0.5, 1, 3, and 5 MHz. It is seen that at the lowest frequency, 500 KHz, the acoustic attenuation throughout the water column is nearly constant and determined primarily by particles of size smaller than ∼30 microns, i.e. the wash load. At the next higher frequency, 1 MHz, the suspended load also contributed to attenuation, but even so, attenuation remained nearly constant over the vertical profile. Thus, at the 2 lower frequencies, attenuation was decoupled from scattering, making the inverse problem explicit for inversion. In contrast, at the 2 highest frequencies, scattering of sound by the suspended mode became the dominant contributor to attenuation, and attenuation varied by an order of magnitude over river depth. As for backscatter, the computed acoustic backscatter strength was determined by a combination of the washload and suspended sediment mode at all 4 frequencies. A fairly narrow monotonic relationship was found between total sediment concentration and locally computed backscatter, effectively providing a calibration between local backscatter signal strength and suspended sediment concentration. Such a relationship existed throughout the water column, for every frequency. The sediment concentration derived from backscatter at a pair of frequencies was within ∼50% of the value measured by laser diffraction; however, the acoustic equivalent diameter exceeded the laser volume mean diameter by up to an order of magnitude. The robustness of these results for application to other flow regimes or rivers remains to be investigated. This article is protected by copyright. All rights reserved.
- Coupled surface‐subsurface hydrologic measurements reveal
infiltration, recharge, and discharge dynamics across the swash zone of a
- Authors: James W. Heiss; Jack A. Puleo, William J. Ullman, Holly A. Michael
Abstract: Swash‐groundwater interactions affect the biogeochemistry of beach aquifers and the transport of solutes and sediment across the beachface. Improved understanding of the complex, coupled dynamics of surface and subsurface flow processes in the swash zone is required to better estimate chemical fluxes to the sea and predict the morphological evolution of beaches. Simultaneous high‐frequency measurements of saturation, water table elevation, and the cross‐shore locations of runup and the boundary between the saturated and unsaturated beachface (surface saturation boundary) were collected on a sandy beach to link groundwater flow dynamics with swash zone forcing. Saturation and lysimeter measurements showed the dynamic response of subsurface saturation to swash events and permitted estimation of infiltration rates. Surface and subsurface observations revealed a decoupling of the surface saturation boundary and the intersection between the water table and the beachface. Surface measurements alone were insufficient to delineate the infiltration and discharge zones, which moved independently of the surface saturation boundary. Results show for the first time the motion and areal extent of infiltration and recharge zones, and constrain the maximum size of the subaerial discharge zone over swash and tidal time scales. The width of the infiltration zone was controlled by swash processes and subaerial discharge was controlled primarily by tidal processes. These dynamics reveal the tightly coupled nature of surface and subsurface processes over multiple timescales, with implications for sediment transport and fluid and solute fluxes through the hydrologically and biogeochemically active intertidal zone of sandy beaches. This article is protected by copyright. All rights reserved.
- Evaluating an unconfined aquifer by analysis of age‐dating tracers
in stream water
- Authors: D.K. Solomon; T.E. Gilmore, J. Solder, B. Kimball, D.P. Genereux
Abstract: The mean transit time (MTT) is a fundamental property of a groundwater flow system that is strongly related to the ratio of recharge rate to storage volume. However, obtaining samples for estimating the MTT using environmental tracers is problematic as flow‐weighted samples over the full spectrum of transit times are needed. Samples collected from the base flow of a gaining stream in the North Carolina Coastal Plain (West Bear Creek) that were corrected for exchange with the atmosphere yielded environmental tracer concentrations (SF6 and CFC‐11) very similar to flow‐weighted values from nine or ten streambed piezometers that directly sampled groundwater during low streamflow. At higher streamflow on the falling limb of the hydrograph, stream tracer concentrations (after correction for gas exchange) were significantly higher than the flow‐weighted mean from piezometers, consistent with dominance of the stream tracer signal by transient influx of surface water and/or younger subsurface water. The apparent MTT derived from SF6 in low flow stream water samples was 26 years, suggesting a groundwater recharge rate of about 210 mm/year, that is consistent with vertical profiles obtained by sampling nested piezometers in the aquifer. When sampled under low flow conditions when streamflow consists of a high component of groundwater discharge, West Bear Creek appears to act as a flow‐weighted integrator of transit times and, streamflow samples can provide fundamental information regarding groundwater recharge rate and MTT. Our study suggests that watershed‐scale evaluation of some groundwater flow systems is possible without utilizing monitoring wells. This article is protected by copyright. All rights reserved.
- Experimental study on nonmonotonicity of capillary desaturation curves in
a 2‐D pore network
- Abstract: Immiscible displacement in porous media is important in many applications such as soil remediation and enhanced oil recovery. When gravitational forces are negligible, two‐phase immiscible displacement at the pore level is controlled by capillary and viscous forces whose relative importance is quantified through the dimensionless capillary number Ca and the viscosity ratio M between liquid phases. Depending on the values of Ca and M, capillary fingering, viscous fingering or stable displacement may be observed resulting in a variety of patterns affecting the phase entrapment. The Capillary Desaturation Curve (CDC), which represents the relationship between the residual oil saturation and Ca, is an important relation to describe the phase entrapment at a given Ca. In the present study, we investigated the CDC as influenced by the viscosity ratio. To do so, we have conducted a comprehensive series of experiments using a high resolution microscope and state‐of‐art micro‐models to investigate the dynamics and patterns of phase entrapment at different Ca and M. By post‐processing of the experimental high‐resolution images, we calculated the CDC and quantified the effects of the Ca and M on the phase entrapment and number of blobs trapped in the micro‐model and their size distributions during immiscible two‐phase flow. Our results show that CDCs are not necessarily monotonic for all M and the physical mechanisms causing this non‐monotonic behaviour are discussed. This article is protected by copyright. All rights reserved.
- The impact of immobile zones on the transport and retention of
nanoparticles in porous media
- Authors: Ian L. Molnar; Jason I. Gerhard, Clinton S. Willson, Denis M. O'Carroll
Abstract: Nanoparticle transport and retention within porous media is treated by conceptualizing the porous media as a series of independent collectors (e.g., Colloid Filtration Theory). This conceptualization assumes that flow phenomena near grain‐grain contacts, such as immobile zones (areas of low flow), exert a negligible influence on nanoparticle transport and assumes that retention and release of particles depends only on surface chemistry. This study investigated the impact of immobile zones on nanoparticle transport and retention by employing Synchrotron X‐ray Computed Microtomography (SXCMT) to examine pore‐scale silver nanoparticle distributions during transport through three sand columns: uniform iron oxide, uniform quartz and well graded quartz. Extended tailing was observed during the elution phase of all experiments suggesting that hydraulic retention in immobile zones, not detachment from grains, was the source of tailing. A numerical simulation of fluid flow through an SXCMT dataset predicted the presence of immobile zones near grain‐grain contacts. SXCMT‐determined silver nanoparticle concentrations observed that significantly lower nanoparticle concentrations existed near grain‐grain contacts throughout the duration of all experiments. In addition, the SXCMT‐determined pore‐scale concentration gradients were found to be independent of surface chemistry and grain size distribution, suggesting that immobile zones limit the diffusive transport of nanoparticles towards the collectors. These results suggest that the well‐known overprediction of nanoparticle retention by traditional CFT may be due to ignoring the influences of grain‐grain contacts and immobile zones. As such, accurate prediction of nanoparticle transport requires consideration of immobile zones and their influence on both hydraulic and surface retention. This article is protected by copyright. All rights reserved.
- Eulerian network modeling of longitudinal dispersion
- Authors: Yashar Mehmani; Matthew T. Balhoff
Abstract: A novel Eulerian network model that incorporates “shear dispersion”, the stretching of solute due to non‐uniform velocity profiles within pore throats is developed. The superposing transport method (STM) is non‐local in time (i.e. uses information from several previous time steps) and is equivalent to performing network‐wide time‐convolutions of elementary throat response functions. Predicted macroscopic longitudinal dispersion coefficients for disordered sphere packs are in good agreement with published experimental data. We further investigate the impact of mixing assumptions within pores on macroscopic longitudinal dispersion and find the dependence to be weak for disordered sphere packs. Limitations of Eulerian network models as a whole are also discussed, and their inappropriateness for ordered porous media concluded. This article is protected by copyright. All rights reserved.
- Power asymmetry in conflict resolution with application to a water
pollution dispute in China
- Authors: Jing Yu; D. Marc Kilgour, Keith W. Hipel, Min Zhao
Abstract: The concept of power asymmetry is incorporated into the framework of the Graph Model for Conflict Resolution (GMCR) and then applied to a water pollution dispute in China in order to show how it can provide strategic insights into courses of action. In a new definition of power asymmetry, one of the decision makers (DMs) in a conflict can influence the preferences of other DMs by taking advantage of additional options reflecting the particular DM's more powerful position. The more powerful DM may have three different kinds of power: direct positive, direct negative, or indirect. It is useful to analyze a model of a conflict without power asymmetry, and then to analyze a power‐asymmetric model. As demonstrated by analysis of the water quality controversy that took place at the border separating the Chinese provinces of Jiangsu and Zhejiang, this novel conflict resolution methodology can be readily applied to real‐world strategic conflicts to gain an enhanced understanding of the effects of asymmetric power. This article is protected by copyright. All rights reserved.
- Estimation of human‐induced changes in terrestrial water storage
through integration of GRACE satellite detection and hydrological
modeling: A case study of the Yangtze River basin
- Authors: Ying Huang; Suhyb Salama, Maarten S. Krol, Zhongbo Su, Arjen Y. Hoekstra, Yijian Zeng, Yunxuan Zhou
Abstract: Quantifying the human effects on water resources plays an important role in river basin management. In this study, we proposed a framework, which integrates the Gravity Recovery and Climate Experiment (GRACE) satellite estimation with macro‐scale hydrological model simulation, for detection and attribution of spatial terrestrial water storage (TWS) changes. In particular, it provides valuable insights for regions where ground‐based measurements are inaccessible. Moreover, this framework takes into account the feedback between land and atmosphere, and innovatively puts forward several suggestions (e.g. study period selection, hydrological model selection based on soil moisture‐climate interactions) to minimize the uncertainties brought by the interaction of human water use with terrestrial water fluxes. We demonstrate the use of the proposed framework in the Yangtze River basin of China. Our results show that, during the period 2003‐2010, the TWS was continually increasing in the middle and south eastern reaches of the basin, at a mean rate of about 3 cm yr– 1. This increment in TWS was attributed to anthropogenic modification of the hydrological cycle, rather than natural climate variability. The dominant contributor to the TWS excess was found to be intensive surface water irrigation, which recharged the water table in the middle and south eastern parts of the basin. Water impoundment in the Three Gorges Reservoir (TGR) is found to account for nearly 20% of the human‐induced TWS increment in the region where the TGR is located. The proposed framework gives water managers/researchers a useful tool to investigate the spatial human effects on TWS changes. This article is protected by copyright. All rights reserved.
- Real‐time visualization of Haines jumps in sandstone with
laboratory‐based microcomputed tomography
- Authors: Tom Bultreys; Marijn A. Boone, Matthieu N. Boone, Thomas De Schryver, Bert Masschaele, Denis Van Loo, Luc Van Hoorebeke, Veerle Cnudde
Abstract: In this work, we present a novel laboratory‐based micro‐computed tomography (micro‐CT) experiment designed to investigate the pore scale drainage behavior of natural sandstone under dynamic conditions. The fluid distribution in a Bentheimer sandstone was visualized every 4 seconds with a 12 second measurement time, allowing the investigation of single‐ and few‐pore filling events. To our knowledge, this is the first time that such measurements were performed outside of synchrotron facilities, illustrating the growing application potential of lab‐based micro‐CT with sub‐minute temporal resolutions for geological research at the pore scale. To illustrate how the workflow can lead to an improved understanding of drainage behavior, the experiment was analyzed using a decomposition of the pore space into individual geometrical pores. Preliminary results from this analysis suggest that the distribution of drainage event sizes follows a power law scaling, as expected from percolation theory. This article is protected by copyright. All rights reserved.
- Hundred years of return period: Strengths and limitations
- Authors: E. Volpi; A. Fiori, S. Grimaldi, F. Lombardo, D. Koutsoyiannis
Abstract: 100 years from its original definition by Fuller , the probabilistic concept of return period is widely used in hydrology as well as in other disciplines of geosciences to give an indication on critical event rareness. This concept gains its popularity, especially in engineering practice for design and risk assessment, due to its ease of use and understanding; however, return period relies on some basic assumptions that should be satisfied for a correct application of this statistical tool. Indeed, conventional frequency analysis in hydrology is performed by assuming as necessary conditions that extreme events arise from a stationary distribution and are independent of one another. The main objective of this paper is to investigate the properties of return period when the independence condition is omitted; hence, we explore how the different definitions of return period available in literature affect results of frequency analysis for processes correlated in time. We demonstrate that, for stationary processes, the independence condition is not necessary in order to apply the classical equation of return period (i.e. the inverse of exceedance probability). On the other hand, we show that the time‐correlation structure of hydrological processes modifies the shape of the distribution function of which the return period represents the first moment. This implies that, in the context of time‐dependent processes, the return period might not represent an exhaustive measure of the probability of failure, and that its blind application could lead to misleading results. To overcome this problem, we introduce the concept of Equivalent Return Period, which controls the probability of failure still preserving the virtue of effectively communicating the event rareness. This article is protected by copyright. All rights reserved.
- Land and atmospheric controls on initiation and intensity of moist
convection: CAPE dynamics and LCL crossings
- Authors: J. Yin; J. Albertson, J.R. Rigby, A. Porporato
Abstract: The local role that land‐atmosphere interactions play in the rainfall process has been often explored by investigating the initiation of moist convection as the top of the atmospheric boundary layer (ABL) crosses the lifting condensation level (LCL). However, this LCL crossing alone is not a sufficient indicator of the probability and intensity of subsequent convective precipitation, which is instead better characterized by the added consideration of the so‐called convective available potential energy (CAPE). In this study, both the LCL crossing and CAPE are jointly considered as the primary indicators of the occurrence and intensity of moist convection in order to analyze the land‐atmosphere interactions through a simple soil‐plant system and a zero‐dimensional mixed‐layer model. The approach is explored using the free atmospheric conditions observed at the Central Facility in the Southern Great Plains, where the ABL analysis shows both dry and wet soil can be conducive to early moist convection depending on atmospheric conditions but CAPE always tends to be larger under wetter soil conditions. The combination of the two indicators, LCL crossing and CAPE, further allows us to classify free atmosphere and soil moisture regimes into positive and negative feedback regimes for moist convection. This article is protected by copyright. All rights reserved.
- Analytical approximations of discharge recessions for steeply sloping
aquifers in alpine catchments
- Authors: Marcus Pauritsch; Steffen Birk, Thomas Wagner, Stefan Hergarten, Gerfried Winkler
Abstract: The validity and applicability of various methods to infer hydraulic properties of sloping aquifers in alpine settings using the power‐law relationship between the discharge recession and its first time derivative is explored. For this purpose a synthetic spring catchment implemented in the numerical groundwater flow model MODFLOW as well as the example of a relict rock glacier in an alpine setting is examined. The various approaches are found to differ particularly in the late time domain, whereas most of them agree fairly well in the early time domain and at the transition point between the two time domains. As the early recession may be affected by uncertainties from inappropriate initial conditions, it is proposed to use the transition point for estimating aquifer thickness and transmissivity. Using only prolonged winter recessions in the analysis of the field data from the relict rock glacier yields estimates of aquifer thickness and hydraulic conductivity consistent with results from a geophysical survey and tracer tests, respectively. In the other seasons the recession is frequently interrupted by minor recharge events, and using the lower envelope of the entire data is found to yield estimates that are too high in the given case. It is thus recommended to focus on the winter recession in the analysis of hydrograph data from alpine settings. This article is protected by copyright. All rights reserved.
- Pool‐riffle sedimentation and surface texture trends in a gravel bed
- Abstract: A 3‐year field campaign was completed to investigate spatial and temporal variability of sedimentation trends for a single pool‐riffle pair located in the Santa Cruz Mountains, California. Our measurements represent a range of hydrologic conditions over eleven sediment mobilizing events. Two different statistical methods were used to explore riffle sedimentation. Cochran's Q and McNemar's non‐parametric tests (one method) indicate that riffle sediment surface texture was spatially and temporally varied at the transect level. For McNemar's test, variation was significant at p < 0.05, with several trends evident, including strong riffle fining triggered by a 20‐year flood event. A nonlinear empirical orthogonal function method known as Self‐organizing maps (SOMs; the second method) shows that riffle sediment surface texture is well described by two characteristic temporal signals, and one transitional signal at the sampling node level. SOM mapping to each sampling node clearly shows riffle sediment surface texture change was spatially organized over the eleven sediment mobilizing events. Observations of pool sediment storage indicate that the pool‐riffle pair exhibited a coupled sedimentation response (i.e. similar texture trends between pool and riffle) following the 20‐year flood. The coupled response was characterized by a trend toward overall sedimentation conditions that were similar to those measured at the beginning of the study. The reported texture trends may be of interest to salmonid habitat studies that examine factors contributing to successful vs. unsuccessful fry emergence. This article is protected by copyright. All rights reserved.
- A GRACE‐based assessment of interannual groundwater dynamics in the
Community Land Model
- Authors: S. C. Swenson; D. M. Lawrence
Abstract: The estimation of groundwater storage variations is important for quantifying available water resources and managing storage surpluses to alleviate storage deficiencies during droughts. This is particularly true in semi‐arid regions, where multi‐year droughts can be common. To complement the local information provided by soil moisture and well level measurements, land models such as the Community Land Model (CLM) can be used to simulate regional scale water storage variations. CLM includes a bulk aquifer model to simulate saturated water storage dynamics below the model soil column. Aquifer storage increases when it receives recharge from the overlying soil column, and decreases due to lateral flow (i.e. baseflow) and capillary rise.
In this study, we examine the response of the CLM aquifer model to transitions between low and high recharge inputs, and show that the model simulates unrealistic long‐period behavior relative to total water storage (TWS) observations from the Gravity Recovery and Climate Experiment (GRACE). We attribute the model's poor response to large wetting events to the lack of a finite lower boundary in the bulk aquifer model. We show that by removing the bulk aquifer model and adding a zero‐flux boundary condition at the base of the soil column, good agreement with GRACE observations can be achieved. In addition, we examine the sensitivity of simulated total water storage to the depth at which the zero‐flux boundary is applied, i.e. the thickness of the soil column. Based on comparisons to GRACE, an optimal soil thickness map is constructed. Simulations using the modifed CLM with the derived soil thickness map are shown to perform as well or better than standard CLM simulations. The improvements in simulated, climatically induced, long‐period water storage variability will reduce the uncertainty in GRACE‐based estimates of anthropogenic groundwater depletion. This article is protected by copyright. All rights reserved.
- Flood‐type classification in mountainous catchments using crisp and
fuzzy decision trees
- Authors: Anna E. Sikorska; Daniel Viviroli, Jan Seibert
Abstract: Floods are governed by largely varying processes and thus exhibit various behaviors. Classification of flood events into flood types and the determination of their respective frequency is therefore important for a better understanding and prediction of floods. This study presents a flood classification for identifying flood patterns at a catchment scale by means of a fuzzy decision tree. Hence, events are represented as a spectrum of six main possible flood types that are attributed with their degree of acceptance. Considered types are flash, short rainfall, long rainfall, snow‐melt, rainfall on snow and, in high alpine catchments, glacier‐melt floods. The fuzzy decision tree also makes it possible to acknowledge the uncertainty present in the identification of flood processes and thus allows for more reliable flood class estimates than using a crisp decision tree, which identifies one flood type per event. Based on the data set in nine Swiss mountainous catchments, it was demonstrated that this approach is less sensitive to uncertainties in the classification attributes than the classical crisp approach. These results show that the fuzzy approach bears additional potential for analyses of flood patterns at a catchment scale and thereby it provides more realistic representation of flood processes.
- A paleoclimate rainfall reconstruction in the Murray‐Darling Basin
(MDB), Australia: 2. Assessing hydroclimatic risk using paleoclimate
records of wet and dry epochs
- Abstract: Estimates of hydrological risk are crucial to enable adequate planning and preparation for extreme events. However, the accurate estimation of hydrological risk is hampered by relatively short instrumental records in many parts of the world. Information derived from climate‐sensitive paleoclimate proxies provide an opportunity to resolve hydroclimatic variability, but many regions, such as Australia's Murray‐Darling Basin (MDB), currently lack the suitable in situ proxies necessary to do this. Here, new MDB rainfall reconstructions are presented based on a novel method using paleoclimate rainfall proxies in the Australasian region spanning from 749 BCE to 1980 CE. Our results emphasize the need to develop additional reconstructions and, with the companion paper, demonstrate how this information can be used to benefit water resource management. This study shows that prior to the 20th century both dry and wet epochs have persisted for longer periods than observed in the instrumental record – with the probability of both dry and wet periods exceeding a decade at least 10 times more likely prior to 1883 than suggested by the instrumental records. Some reconstructed rainfalls exceeded the instrumental range (i.e. drier dry epochs and wetter wet spells) despite a systematic underestimation of extremes due to a combination of proxy quality and model bias. Importantly, the results demonstrate that the instrumental record does not cover the full range of hydroclimatic variability possible in the MDB. Therefore hydroclimatic risk assessments based on the instrumental record likely underestimate, or at least misinterpret, the frequency, duration and magnitude of wet and dry epochs. This article is protected by copyright. All rights reserved.
- A paleoclimate rainfall reconstruction in the Murray‐Darling Basin
(MDB), Australia: 1. Evaluation of different paleoclimate archives,
rainfall networks, and reconstruction techniques
- Abstract: From ∼1997‐2009 the Murray‐Darling Basin (MDB), Australia's largest water catchment and reputed ‘food bowl', experienced a severe drought termed the “Millennium Drought” or “Big Dry” followed by devastating floods in the austral summers of 2010/11, 2011/12 and 2012/13. The magnitude and severity of these extreme events highlight the limitations associated with assessing hydroclimatic risk based on relatively short instrumental records (∼100 years). An option for extending hydroclimatic records is through the use of paleoclimate records. However, there are few in situ proxies of rainfall or streamflow suitable for assessing hydroclimatic risk in Australia and none are available in the MDB. In this paper available paleoclimate records are reviewed and those of suitable quality for hydroclimatic risk assessments are used to develop preinstrumental information for the MDB. Three different paleoclimate reconstruction techniques are assessed using two instrumental rainfall networks: (1) corresponding to rainfall at locations where rainfall‐sensitive Australian paleoclimate archives currently exist and (2) corresponding to rainfall at locations identified as being optimal for explaining MDB rainfall variability. It is shown that the optimised rainfall network results in a more accurate model of MDB rainfall compared to reconstructions based on rainfall at locations where paleoclimate rainfall proxies currently exist. This highlights the importance of first identifying key locations where existing and as yet unrealised paleoclimate records will be most useful in characterising variability. These results give crucial insight as to where future investment and research into developing paleoclimate proxies for Australia could be most beneficial, with respect to better understanding instrumental, pre‐instrumental and potential future variability in the MDB. This article is protected by copyright. All rights reserved.
- Water table salinization due to seawater intrusion
- Authors: Sugiarto Badaruddin; Adrian D. Werner, Leanne K. Morgan
Abstract: Seawater intrusion (SWI) is a significant threat to freshwater resources in coastal aquifers around the world. Previous studies have focused on SWI impacts involving salinization of the lower domain of coastal aquifers. However, under certain conditions, SWI may cause salinization of the entire saturated zone of the aquifer, leading to watertable salinization (WTS) in unconfined aquifers by replacing freshwater within the upper region of the saturated zone with seawater, thereby posing a salinity threat to the overlying soil zone. There is presently limited guidance on the extent to which WTS may occur as a secondary impact of SWI. In this study, physical experiments and numerical modelling were used to explore WTS associated with SWI in various non‐tidal, unconfined coastal aquifer settings. Laboratory experiments and corresponding numerical simulations show that significant WTS can occur under active SWI (i.e., the freshwater hydraulic gradient slopes towards the land) because the cessation of freshwater discharge to the sea and the subsequent landward flow across the entire sea boundary eventually lead to watertable salinities approaching seawater concentration. WTS during active SWI is larger under conditions of high hydraulic conductivity, rapid SWI, high dispersivity and for deeper aquifers. Numerical modelling of four published field cases demonstrates that rates of WTS of up to 60 m/y are plausible. Under passive SWI (i.e., the hydraulic gradient slopes towards the sea), minor WTS may arise as a result of dispersive processes under certain conditions (i.e., high dispersivity and hydraulic conductivity, and low freshwater discharge). Our results show that WTS is probably widespread in coastal aquifers experiencing considerable groundwater decline sustained over several years, although further evidence is needed to identify WTS under field settings. This article is protected by copyright. All rights reserved.
- Linking age, survival, and transit time distributions
- Authors: Salvatore Calabrese; Amilcare Porporato
Abstract: Although the concepts of age, survival and transit time have been widely used in many fields, including population dynamics, chemical engineering, and hydrology, a comprehensive mathematical framework is still missing. Here we discuss several relationships among these quantities by starting from the evolution equation for the joint distribution of age and survival, from which the equations for age and survival time readily follow. It also becomes apparent how the statistical dependence between age and survival is directly related to either the age‐dependence of the loss function or the survival‐time dependence of the input function. The solution of the joint distribution equation also allows us to obtain the relationships between the age at exit (or death) and the survival time at input (or birth), as well as to stress the symmetries of the various distributions under time reversal. The transit time is then obtained as a sum of the age and survival time, and its properties are discussed along with the general relationships between their mean values. The special case of steady state case is analyzed in detail. Some examples, inspired by hydrologic applications, are presented to illustrate the theory with the specific results. This article is protected by copyright. All rights reserved.
- Basin‐scale runoff prediction: An Ensemble Kalman Filter framework
based on global hydrometeorological data sets
- Authors: Christof Lorenz; Mohammad J. Tourian, Balaji Devaraju, Nico Sneeuw, Harald Kunstmann
Abstract: In order to cope with the steady decline of the number of in situ gauges worldwide, there is a growing need for alternative methods to estimate runoff. We present an Ensemble Kalman Filter based approach that allows us to conclude on runoff for poorly or irregularly gauged basins. The approach focuses on the application of publicly available global hydrometeorological datasets for precipitation (gpcc, gpcp, cru, udel), evapotranspiration (modis, fluxnet, gleam, era interim, gldas), and water storage changes (grace, wghm, gldas, merra land). Furthermore, runoff data from the grdc and satellite altimetry derived estimates are used. We follow a least‐squares prediction that exploits the joint temporal and spatial auto‐ and cross‐covariance structures of precipitation, evapotranspiration, water storage changes and runoff. We further consider time‐dependent uncertainty estimates derived from all datasets. Our in‐depth analysis comprises of 29 large river basins of different climate regions, with which runoff is predicted for a subset of 16 basins. Six configurations are analyzed: the Ensemble Kalman Filter (Smoother) and the hard (soft) Constrained Ensemble Kalman Filter (Smoother). Comparing the predictions to observed monthly runoff shows correlations larger than 0.5, percentage biases lower than ± 20%, and nse‐values larger than 0.5. A modified nse‐metric, stressing the difference to the mean annual cycle, shows an improvement of runoff predictions for 14 of the 16 basins. The proposed method is able to provide runoff estimates for nearly 100 poorly gauged basins covering an area of more than 11,500,000 km2 with a freshwater discharge, in volume, of more than 125,000 m3/s. This article is protected by copyright. All rights reserved.
- Modeling the influence of hypsometry, vegetation, and storm energy on
snowmelt contributions to basins during rain‐on‐snow floods
- Authors: Nicholas E. Wayand; Jessica D. Lundquist, Martyn P. Clark
Abstract: Point observations and previous basin modeling efforts have suggested that snowmelt may be a significant input of water for runoff during extreme rain‐on‐snow floods within Western U.S. basins. Quantifying snowmelt input over entire basins is difficult given sparse observations of snowmelt. In order to provide a range of snowmelt contributions for water managers, a physically‐based snow model coupled with an idealized basin representation was evaluated in point simulations and used to quantify the maximum basin‐wide input from snowmelt volume during flood events. Maximum snowmelt basin contributions and uncertainty ranges were estimated as 29% (11‐47%), 29% (8‐37%), and 7% (2‐24%) of total rain plus snowmelt input, within the Snoqualmie, East North Fork Feather, and Upper San Joaquin basins, respectively, during historic flooding events between 1980 and 2008. The idealized basin representation revealed that both hypsometry and forest cover of a basin had similar magnitude of impacts on the basin‐wide snowmelt totals. However, the characteristics of a given storm (antecedent SWE and available energy for melt) controlled how much hypsometry and forest cover impacted basin‐wide snowmelt. These results indicate that for watershed managers, flood forecasting efforts should prioritize rainfall prediction first, but cannot neglect snowmelt contributions in some cases. Efforts to reduce the uncertainty in the above snowmelt simulations should focus on improving the meteorological forcing data (especially air temperature and wind speed) in complex terrain. This article is protected by copyright. All rights reserved.
- Comparing drinking water treatment costs to source water protection costs
using time series analysis
- Authors: Matthew T. Heberling; Christopher T. Nietch, Hale W. Thurston, Michael Elovitz, Kelly H. Birkenhauer, Srinivas Panguluri, Balaji Ramakrishnan, Eric Heiser, Tim Neyer
Abstract: We present a framework to compare water treatment costs to source water protection costs, an important knowledge gap for drinking water treatment plants (DWTPs). This tradeoff helps determine what incentives a DWTP has to invest in natural infrastructure or pollution reduction in the watershed rather than pay for treatment on site. To illustrate, we use daily observations from 2007‐2011 for the Bob McEwen Water Treatment Plant, Clermont County, Ohio, to understand the relationship between treatment costs and water quality and operational variables (e.g., turbidity, total organic carbon [TOC], pool elevation, and production volume). Part of our contribution to understanding drinking water treatment costs is examining both long‐run and short‐run relationships using error correction models (ECMs). Treatment costs per 1000 gallons were based on chemical, pumping, and granular activated carbon costs. Results from the ECM suggest a 1% decrease in turbidity decreases treatment costs by 0.02% immediately and an additional 0.1% over future days. Using mean values for the plant, a 1% decrease in turbidity leads to $1123/year decrease in treatment costs. To compare these costs with source water protection costs, we use a polynomial distributed lag model to link total phosphorus loads, a source water quality parameter affected by land use changes, to turbidity at the plant. We find the costs for source water protection to reduce loads much greater than the reduction in treatment costs during these years. Although we find no incentive to protect source water in our case study, this framework can help DWTPs quantify the tradeoffs. This article is protected by copyright. All rights reserved.
- A reflection on the first 50 years of Water Resources Research
- Abstract: The year 2015 marks the 50th anniversary of Water Resources Research (WRR), which was founded in 1965. More than 15,000 papers have been published in WRR since its inception, and these papers have been cited more than 430,000 times. The history of hydrology and the water sciences are also reflected in WRR, which has served as a premier publication outlet and instigator of scientific growth over the last 50 years. The legacy of WRR provides a strong scientific foundation for the hydrology community to rise to the challenges of sustainable water resources management in a future where dramatic environmental change and increasing human population are expected to stress the world's water resources from local to global scales. This article is protected by copyright. All rights reserved.
- Analysis of discontinuities across thin inhomogeneities,
groundwater/surface water interactions in river networks, and circulation
about slender bodies using slit elements in the Analytic Element Method
- Authors: David R. Steward
Abstract: Groundwater and surface water contain interfaces across which hydrologic functions are discontinuous. Thin elements with high hydraulic conductivity in a porous media focus groundwater, which flows through such inhomogeneities and causes an abrupt change in stream function across their interfaces, and elements with low conductivity retards flow with discontinuous head. Baseflow interactions at the interface between groundwater and surface water transport water between these stores and generate a discontinuous normal component of flow. Thin objects in surface water with Kutta condition generates circulation by the discontinuous tangential component of flow across their interface. These discontinuities across hydrologic interfaces are quantified and visualized using the Analytic Element Method, where slit elements are formulated using the Joukowsky transformation with Laurent series and new influence functions to represent sinks and circulation, and methods are developed for these applications expressing discontinuities as Fourier series. The specific geometries illustrate solutions for a randomly generated heterogeneous porous media with non‐intersecting inhomogeneities, for groundwater/surface water interaction in a synthetic river network, and for a slender body with geometry similar to the wings of the Wright Brothers. The mathematical details are reduced to series solutions and matrix multiplications, which are easily extensible to other geometries and applications. This article is protected by copyright. All rights reserved.
- Water storage changes as a marker for base flow generation processes in a
tropical humid basement catchment (Benin): Insights from hybrid gravimetry
- Abstract: In basement catchments of sub‐humid West Africa, baseflow is the main component of annual streamflow. However, the important heterogeneity of lithology hinders the understanding of baseflow generation processes. Since these processes are linked with water storage changes (WSCs) across the catchment, we propose the use of hybrid gravity data in addition to neutron probe‐derived water content and water levels to monitor spatiotemporal WSC of a typical crystalline basement headwater catchment (16 ha) in Benin. These behaviors are shown to provide insights into hydrological processes in terms of water redistribution toward the catchment outlet. Hybrid gravimetry produces gravity change observations from time‐lapse microgravity surveys coupled with gravity changes monitored at a base station using a superconducting gravimeter and/or an absolute gravimeter. A dense microgravity campaign (70 surveys of 14 stations) covering three contrasted years was set up with a rigorous protocol, leading to low uncertainties (< 2.5 µGal) on station gravity determinations (with respect to the network reference station). Empirical orthogonal function analyses of both gravity changes and WSCs from neutron probe data show similar spatial patterns in the seasonal signal. Areas where storage and water table show a capping behavior (when data reach a plateau during the wet season), suggesting threshold‐governed fast subsurface redistribution, are identified. This observed storage dynamics, together with geological structures investigated by electrical resistivity tomography and drill log analysis make it possible to derive a conceptual model for the catchment hydrology. This article is protected by copyright. All rights reserved.
- Hydrologic control of dissolved organic matter concentration and quality
in a semiarid artificially drained agricultural catchment
- Authors: Rebecca A. Bellmore; John A. Harrison, Joseph A. Needoba, Erin Brooks, C. Kent Keller
Abstract: Agricultural practices have altered watershed‐scale dissolved organic matter (DOM) dynamics, including in‐stream concentration, biodegradability, and total catchment export. However, mechanisms responsible for these changes are not clear, and field‐scale processes are rarely directly linked to the magnitude and quality of DOM that is transported to surface water. In a small (12 ha) agricultural catchment in eastern Washington State, we tested the hypothesis that hydrologic connectivity in a catchment is the dominant control over the concentration and quality of DOM exported to surface water via artificial subsurface drainage. Concentrations of dissolved organic carbon (DOC) and humic‐like components of DOM decreased while the Fluorescence Index and Freshness Index increased with depth through the soil profile. In drain discharge, these characteristics were significantly correlated with drain flow across seasons and years, with drain DOM resembling deep sources during low flow and shallow sources during high flow, suggesting that DOM from shallow sources bypasses removal processes when hydrologic connectivity in the catchment is greatest. Assuming changes in streamflow projected for the Palouse River (which contains the study catchment) under the A1B climate scenario (rapid growth, dependence on fossil fuel and renewable energy sources) apply to the study catchment, we project greater interannual variability in annual DOC export in the future, with significant increases in the driest years. This study highlights the variability in DOM inputs from agricultural soil to surface water on daily to interannual timescales, pointing to the need for a more nuanced understanding of agricultural impacts on DOM dynamics in surface water. This article is protected by copyright. All rights reserved.
- Dual control of flow field heterogeneity and immobile porosity on
non‐Fickian transport in Berea sandstone
- Authors: Filip Gjetvaj; Anna Russian, Philippe Gouze, Marco Dentz
Abstract: Both flow field heterogeneity and mass transfer between mobile and immobile domains have been studied separately for explaining observed anomalous transport. Here, we investigate non‐Fickian transport using high‐resolution 3D X‐ray micro‐tomographic images of Berea sandstone containing microporous cement with pore size below the setup resolution. Transport is computed for a set of representative elementary volumes and results from advection and diffusion in the resolved macroporosity (mobile domain) and diffusion in the microporous phase (immobile domain) where the effective diffusion coefficient is calculated from the measured local porosity using a phenomenological model that includes a porosity threshold (ɸϴ) below which diffusion is null and the exponent n that characterizes tortuosity‐porosity power‐law relationship. We show that both flow field heterogeneity and microporosity trigger anomalous transport. Breakthrough curve (BTC) tailing is positively correlated to microporosity volume and mobile‐immobile interface area. The sensitivity analysis showed that the BTC tailing increases with the value of ɸϴ, due to the increase of the diffusion path tortuosity until the volume of the microporosity becomes negligible. Furthermore, increasing the value of n leads to an increase in the standard deviation of the distribution of effective diffusion coefficients, which in turn results in an increase of the BTC tailing. Finally, we propose a continuous time random walk upscaled model where the transition time is the sum of independently distributed random variables characterized by specific distributions. It allows modeling a 1D equivalent macroscopic transport honoring both the control of the flow field heterogeneity and the multi‐rate mass transfer between mobile and immobile domains. This article is protected by copyright. All rights reserved.
- Global sensitivity of high‐resolution estimates of crop water
- Authors: Marta Tuninetti; Stefania Tamea, Paolo D'Odorico, Francesco Laio, Luca Ridolfi
Abstract: Most of the human appropriation of freshwater resources is for agriculture. Water availability is a major constraint to mankind's ability to produce food. The notion of virtual water content (VWC), also known as crop water footprint, provides an effective tool to investigate the linkage between food and water resources as a function of climate, soil and agricultural practices. The spatial variability in the virtual water content of crops is here explored, disentagling its dependency on climate and crop yields, and assessing the sensitivity of VWC estimates to parameter variability and uncertainty. Here we calculate the virtual water content of four staple crops (i.e., wheat, rice, maize, and soybean) for the entire world developing a high‐resolution (5 by 5 arc minute) model, and we evaluate the VWC sensitivity to input‐parameters. We find that food production almost entirely depends on green water (>90%), but, when applied, irrigation makes crop production more water efficient, thus requiring less water. The spatial variability of the VWC is mostly controlled by the spatial patterns of crop yields with an average correlation coefficient of 0.83. The results of the sensitivity analysis show that wheat is most sensitive to the length of the growing period, rice to reference evapotranspiration, maize and soybean to the crop planting date. The VWC sensitivity varies not only among crops, but also across the harvested areas of the world, even at the sub‐national scale. This article is protected by copyright. All rights reserved.
- Modeling NAPL dissolution from pendular rings in idealized porous media
- Authors: Junqi Huang; John A. Christ, Mark N. Goltz, Avery H. Demond
Abstract: The dissolution rate of non‐aqueous phase liquid (NAPL) often governs the remediation time frame at subsurface hazardous waste sites. Most formulations for estimating this rate are empirical and assume that the NAPL is the non‐wetting fluid. However, field evidence suggests that some waste sites might be organic‐wet. Thus, formulations that assume the NAPL is non‐wetting may be inappropriate for estimating the rates of NAPL dissolution. An exact solution to the Young‐Laplace equation, assuming NAPL resides as pendular rings around the contact points of porous media idealized as spherical particles in a hexagonal close packing arrangement, is presented in this work to provide a theoretical prediction for NAPL‐water interfacial area. This analytic expression for interfacial area is then coupled with an exact solution to the advection‐diffusion equation in a capillary tube assuming Hagen‐Poiseuille flow to provide a theoretical means of calculating the mass transfer rate coefficient for dissolution at the NAPL‐water interface in an organic‐wet system. A comparison of the predictions from this theoretical model with predictions from empirically‐derived formulations from the literature for water‐wet systems showed a consistent range of values for the mass transfer rate coefficient, despite the significant differences in model foundations (water‐wetting vs NAPL‐wetting, theoretical vs. empirical). This finding implies that, under these system conditions, the important parameter is interfacial area, with a lesser role played by NAPL configuration. This article is protected by copyright. All rights reserved.
- Influence of small‐scale fluvial architecture on CO2 trapping
processes in deep brine reservoirs
- Authors: Naum I. Gershenzon; Robert W. Ritzi, David F. Dominic, Mohamadreza Soltanian, Edward Mehnert, Roland T. Okwen
Abstract: A number of important candidate CO2 reservoirs exhibit sedimentary architecture reflecting fluvial deposition. Recent studies have led to new conceptual and quantitative models for sedimentary architecture in fluvial deposits over a range of scales that are relevant to CO2 injection and storage. We used a geocellular modelling approach to represent this multi‐scaled and hierarchical sedimentary architecture. With this model, we investigated the dynamics of CO2 plumes, during and after injection, in such reservoirs.
The physical mechanism of CO2 trapping by capillary trapping incorporates a number of related processes, i.e. residual trapping, trapping due to hysteresis of the relative permeability, and trapping due to hysteresis of the capillary pressure. Additionally CO2 may be trapped due to differences in capillary entry pressure for different textural sedimentary facies (e.g. coarser‐ vs. finer‐grained cross‐sets). The amount of CO2 trapped by these processes depends upon a complex system of non‐linear and hysteretic characteristic relationships including how relative permeability and capillary pressure vary with brine and CO2 saturation. The results strongly suggest that representing small‐scale features (decimeter to meter), including their organization within a hierarchy of larger‐scale features, and representing their differences in characteristic relationships, can all be critical to understanding trapping processes in some important candidate CO2 reservoirs. This article is protected by copyright. All rights reserved.
- A framework of change‐point detection for multivariate hydrological
- Abstract: Under changing environments, not only univariate but also multivariate hydrological series might become nonstationary. Nonstationarity, in forms of change‐point or trend, has been widely studied for univariate hydrological series, while it attracts attention only recently for multivariate hydrological series. For multivariate series, two types of change‐point need to be distinguished, i.e. change‐point in marginal distributions and change‐point in the dependence structure among individual variables. In this paper, a three‐step framework is proposed to separately detect two types of change‐point in multivariate hydrological series, i.e. change‐point detection for individual univariate series, estimation of marginal distributions, and change‐point detection for dependence structure. The last step is implemented using both the Cramér‐von Mises statistic (CvM) method and the copula‐based likelihood‐ratio test (CLR) method. For CLR, three kinds of copula model (symmetric, asymmetric, and pair‐copula) are employed to construct the dependence structure of multivariate series. Monte Carlo experiments indicate that CLR is far more powerful than CvM in detecting the change‐point of dependence structure. This framework is applied to the trivariate flood series composed of annual maxima daily discharge (AMDD), annual maxima 3‐day flood volume and annual maxima 15‐day flood volume of the Upper Hanjiang River, China. It is found that each individual univariate flood series has a significant change‐point; and the trivariate series presents a significant change‐point in dependence structure due to the abrupt change in the dependence structure between AMDD and annual maxima 3‐day flood volume. All these changes are caused by the construction of the Ankang Reservoir. This article is protected by copyright. All rights reserved.
- Output‐feedback control of combined sewer networks through receding
horizon control with moving horizon estimation
- Abstract: An output‐feedback control strategy for pollution mitigation in combined sewer networks is presented. The proposed strategy provides means to apply model‐based predictive control to large‐scale sewer networks, in‐spite of the lack of measurements at most of the network sewers. In previous works, the authors presented a hybrid linear control‐oriented model for sewer networks together with the formulation of Optimal Control Problems (OCP) and State Estimation Problems (SEP). By iteratively solving these problems, preliminary Receding Horizon Control with Moving Horizon Estimation (RHC/MHE) results, based on flow measurements, were also obtained. In this work, the RHC/MHE algorithm has been extended to take into account both flow and water level measurements and the resulting control loop has been extensively simulated to assess the system performance according different measurement availability scenarios and rain events. All simulations have been carried out using a detailed physically‐based model of a real case‐study network as virtual reality. This article is protected by copyright. All rights reserved.
- Control of tree water networks: A geometric programming approach
- Authors: L. Sela Perelman; S. Amin
Abstract: This paper presents a modeling and operation approach for tree water supply systems. The network control problem is approximated as a geometric programming (GP) problem. The original nonlinear nonconvex network control problem is transformed into a convex optimization problem. The optimization model can be efficiently solved to optimality using state‐of‐the‐art solvers. Two control schemes are presented: (1) operation of network actuators (pumps and valves) and (2) controlled demand shedding allocation between network consumers with limited resources. The dual of the network control problem is formulated and is used to perform sensitivity analysis with respect to hydraulic constraints. The approach is demonstrated on a small branched‐topology network and later extended to a medium‐size irrigation network. The results demonstrate an intrinsic trade‐off between energy costs and demand shedding policy, providing an efficient decision support tool for active management of water systems. This article is protected by copyright. All rights reserved.
- Nonpayment of water bills in Guatemala: Dissatisfaction or inability to
- Abstract: This paper investigates nonpayment behavior in Guatemala. Determinants of nonpayment behavior are identified through zero‐inflated negative binomial regression models in order to take into account particular distributional characteristics of the amount of outstanding payments. Findings indicate that nonpayment behavior is a demonstration of consumer dissatisfaction with current water services. The amount of outstanding bill payments also responds to system unreliability. Results also suggest that nonpayment behaviors are more prominent in community‐managed systems than in municipal systems. No evidence was found on a potential relationship between nonpayment behavior and household income. Policy implications are discussed. This article is protected by copyright. All rights reserved.
- An analytical study on artesian flow conditions in
unconfined‐aquifer drainage basins
- Abstract: Although it has been reported that flowing artesian wells could be topographically‐controlled, there is no quantitative research on artesian flow conditions in unconfined aquifers. In this study, the water table, which has a lower amplitude than the land surface, is damped from the topography and used as the boundary condition to obtain the analytical solution of hydraulic head of a unit basin with a single flow system. The term artesian head is defined to characterize the condition of flowing artesian wells. The zone with positive artesian head is called artesian zone while with negative artesian head is non‐artesian zone. The maximum artesian head and the size of artesian zones are found to increase with the damping factor and the anisotropy ratio, and decrease with the ratio of basin width to depth and the depth‐decay exponent of hydraulic conductivity. Moreover, the artesian head increases with depth nearby the valley and decreases with depth near by the divide, and the variation rates are influenced by the decay exponent and the anisotropy ratio. Finally, the distribution of flowing artesian wells and the artesian head measurements in different depths of a borehole in a small catchment in the Ordos Plateau, Northwestern China is used to illustrate the theoretical findings. The change in artesian head with depth was used to estimate the anisotropy ratio and the decay exponent. This study opens up a new door to analyze basin‐scale groundwater flow. This article is protected by copyright. All rights reserved.
- The stationarity paradigm revisited: Hypothesis testing using diagnostics,
summary metrics, and DREAM(ABC)
- Authors: Mojtaba Sadegh; Jasper A. Vrugt, Chonggang Xu, Elena Volpi
Abstract: Many watershed models used within the hydrologic research community assume (by default) stationary conditions ‐ that is ‐ the key watershed properties that control water flow are considered to be time‐invariant. This assumption is rather convenient and pragmatic and opens up the wide arsenal of (multivariate) statistical and nonlinear optimization methods for inference of the (temporally‐fixed) model parameters. Several contributions to the hydrologic literature have brought into question the continued usefulness of this stationary paradigm for hydrologic modeling. This paper builds on the likelihood‐free diagnostics approach of Vrugt and Sadegh  and uses a diverse set of hydrologic summary metrics to test the stationary hypothesis and detect changes in the watersheds response to hydro‐climatic forcing. Models with fixed parameter values cannot simulate adequately temporal variations in the summary statistics of the observed catchment data, and consequently the DREAM(ABC) algorithm cannot find solutions that sufficiently honor the observed metrics. We demonstrate that the presented methodology is able to differentiate successfully between watersheds that are classified as stationary and those that have undergone significant changes in land‐use, urbanization and/or hydro‐climatic conditions, and thus are deemed nonstationary. This article is protected by copyright. All rights reserved.
- Evidence of an emerging levee failure mechanism causing disastrous floods
- Authors: Stefano Orlandini; Giovanni Moretti, John D. Albertson
Abstract: A levee failure occurred along the Secchia River, Northern Italy, on January 19, 2014, resulting in flood damage in excess of $500 Million. In response to this failure, immediate surveillance of other levees in the region led to the identification of a second breach developing on the neighboring Panaro River, where rapid mitigation efforts were successful in averting a full levee failure. The paired breach events that occurred along the Secchia and Panaro Rivers provided an excellent window on an emerging levee failure mechanism. In the Secchia River, by combining the information content of photographs taken from helicopters in the early stage of breach development and 10‐cm resolution aerial photographs taken in 2010 and 2012, animal burrows were found to exist in the precise levee location where the breach originated. In the Panaro River, internal erosion was observed to occur at a location where a crested porcupine den was known to exist and this erosion led to the collapse of the levee top. This paper uses detailed numerical modeling of rainfall, river flow, and variably saturated flow in the levee to explore the hydraulic and geotechnical mechanisms that were triggered along the Secchia and Panaro Rivers by activities of burrowing animals leading to levee failures. As habitats become more fragmented and constrained along river corridors it is possible that this failure mechanism could become more prevalent and, therefore, will demand greater attention in both the design and maintenance of earthen hydraulic structures as well as in wildlife management. This article is protected by copyright. All rights reserved.
- Water consumption patterns as a basis for water demand modeling
- Authors: Noa Avni; Barak Fishbain, Uri Shamir
Abstract: Future water demand is a main consideration in water system management. Consequently, water demand models (WDMs) have evolved in past decades, identifying principal demand‐generating factors and modeling their influence on water demand. Regional water systems serve consumers of various types (e.g., municipalities, farmers, industrial regions) and consumption patterns. Thus, one of the challenges in regional water demand modeling is the heterogeneity of the consumers served by the water system. When a high‐resolution, regional WDM is desired, accounting for this heterogeneity becomes all the more important. This paper presents a novel approach to regional water demand modeling. The two‐step approach includes aggregating the dataset into groups of consumers having similar consumption characteristics, and developing a WDM for each homogeneous group. The development of WDMs is widely applied in the literature and thus, the focus of this paper is to discuss the first step of data aggregation. The research hypothesis is that water consumption records in their original or transformed form can provide a basis for aggregating the dataset into groups of consumers with similar consumption characteristics. This paper presents a methodology for water consumption data clustering by comparing several data representation methods (termed Feature Vectors): monthly normalized average, monthly consumption coefficient of variation, a combination of the monthly average and monthly variation, and the autocorrelation coefficients of the consumption time‐series. Clustering using solely normalized monthly average provided homogeneous and distinct clusters with respect to monthly consumption, which succeed in capturing different consumer characteristics (water use, geographical location) that were not specified a‐priori. Clustering using the monthly coefficient of variation provided different, yet homogeneous clusters, clustering consumers characterized by similar variation trends that were closely related to consumer water use type. The concatenation of these two Feature Vectors provided further insight into the relationship between consumption patterns and variability of consumers. An autocorrelation Feature Vector provided results that can form a basis for constructing a time‐series model that is based on a group of resembling time‐series. The approaches presented here are steps towards utilizing the increasing amount of available water consumption data and data analysis techniques to facilitate the modeling of water demands in larger and heterogeneous regions with sufficient resolution. This article is protected by copyright. All rights reserved.
- Potential of hydraulically induced fractures to communicate with existing
- Authors: James A. Montague; George F. Pinder
Abstract: The probability that new hydraulically fractured wells drilled within the area of New York underlain by the Marcellus Shale will intersect existing an wellbore is calculated using a statistical model, which incorporates: the depth of a new fracturing well, the vertical growth of induced fractures, and the depths and locations of existing nearby wells. The model first calculates the probability of encountering an existing well in plan view and combines this with the probability of an existing well being at sufficient depth to intersect the fractured region. Average probability estimates for the entire region of New York underlain by the Marcellus Shale range from 0.00% to 3.45% based upon the input parameters used. The largest contributing parameter on the probability value calculated is the nearby density of wells meaning that due diligence by oil and gas companies during construction in identifying all nearby wells will have the greatest effect in reducing the probability of interwellbore communication. This article is protected by copyright. All rights reserved.
- A generalized complementary principle with physical constraints for
- Authors: Wilfried Brutsaert
Abstract: The idea of complementary evaporative fluxes, first advanced by Bouchet in 1963 is reformulated as a general polynomial, satisfying boundary conditions based on strictly physical considerations. Experimental evidence supports the validity of the imposed constraints. Earlier complementary relationships are shown to be special cases which satisfy only one of the necessary conditions. The new formulation provides a more rigorous base for the complementary principle. This article is protected by copyright. All rights reserved.
- Capillarity and wetting of carbon dioxide and brine during drainage in
Berea sandstone at reservoir conditions
- Abstract: The wettability of CO2‐brine‐rock systems will have a major impact on the management of carbon sequestration in subsurface geological formations. Recent contact angle measurement studies have reported sensitivity in wetting behaviour of this system to pressure, temperature and brine salinity. We report observations of the impact of reservoir conditions on the capillary pressure characteristic curve and and relative permeability of a single Berea sandstone during drainage ‐ CO2 displacing brine ‐ through effects on the wetting state. Eight reservoir condition drainage capillary pressure characteristic curves were measured using CO2 and brine in a single fired Berea sandstone at pressures (5 to 20 MPa), temperatures (25 to 50°C) and ionic strengths (0 to 5 mol kg−1 NaCl). A ninth measurement using a N2‐water system provided a benchmark for capillarity with a strongly water wet system. The capillary pressure curves from each of the tests were found to be similar to the N2‐water curve when scaled by the interfacial tension. Reservoir conditions were not found to have a significant impact on the capillary strength of the CO2‐brine system during drainage through a variation in the wetting state. Two steady‐state relative permeability measurements with CO2 and brine and one with N2 and brine similarly show little variation between conditions, consistent with the observation that the CO2‐brine‐sandstone system is water wetting and multiphase flow properties invariant across a wide range of reservoir conditions. This article is protected by copyright. All rights reserved.
- Interactive design of experiments: A priori global versus sequential
optimization, revised under changing states of knowledge
- Authors: A. Geiges; Y. Rubin, W. Nowak
Abstract: Predicting hydro(geo)logical or environmental systems is subject to high levels of uncertainties, especially if appropriate data for model calibration are lacking. For subsurface systems, where data acquisition is cost intensive and time demanding, it is especially important to collect only those data that provide the largest amount of relevant information. The high expenses call for optimal experimental design, which is widely recognized for maximizing the efficiency of experiments. In model‐based design of experiments, the analysis of the design efficiency and the resulting optimal design are based on the initial state of knowledge about the modeled system. Joint optimization of multi‐measurement designs is a well known challenge and the usefulness of global optimization approaches is widely recognized in this context. However, we will show that the benefit for such global optimization becomes questionable when measurement data become available sequentially. Instead, the optimization effort should be invested within an interactive design approach. Today's fast telecommunication, global connectivity and high‐performance computing allow to consider such interactive coupling. This study will use a synthetic case study to compare the standard en‐bloc global optimization approach to two interactive design approaches. The approaches are implemented in a Bayesian framework and are compared based on their complexity and overall performance. The key conclusion confirms a previously untested presumption: for models that trigger nonlinear parameter inference problems, interaction (which may come at a loss of global optimization) is more beneficial than global optimization based on the initial state of knowledge (which typically implies the impossibility of interactivity). This article is protected by copyright. All rights reserved.
- Using noble gas tracers to constrain a groundwater flow model with
recharge elevations: A novel approach for mountainous terrain
- Authors: Jessica. M. Doyle; Tom Gleeson, Andrew H. Manning, Ulrich Mayer
Abstract: Environmental tracers provide information on groundwater age, recharge conditions, and flow processes which can be helpful for evaluating groundwater sustainability and vulnerability. Dissolved noble gas data have proven particularly useful in mountainous terrain because they can be used to determine recharge elevation. However, tracer‐derived recharge elevations have not been utilized as calibration targets for numerical groundwater flow models. Herein we constrain and calibrate a regional groundwater flow model with noble‐gas‐derived recharge elevations for the first time. Tritium and noble gas tracer results improved the site conceptual model by identifying a previously uncertain contribution of mountain‐block recharge from the Coast Mountains to an alluvial coastal aquifer in humid southwestern British Columbia. The revised conceptual model was integrated into a three‐dimensional numerical groundwater flow model and calibrated to hydraulic head data in addition to recharge elevations estimated from noble gas recharge temperatures. Recharge elevations proved to be imperative for constraining hydraulic conductivity, recharge location and bedrock geometry, and thus minimizing model non‐uniqueness. Results indicate that 45% of recharge to the aquifer is mountain‐block recharge. A similar match between measured and modeled heads was achieved in a second numerical model that excludes the mountain block (no mountain block recharge), demonstrating that hydraulic head data alone are incapable of quantifying mountain block recharge. This result has significant implications for understanding and managing source water protection in recharge areas, potential effects of climate change, the overall water budget, and ultimately ensuring groundwater sustainability. This article is protected by copyright. All rights reserved.
- Flushing of distal hillslopes as an alternative source of stream dissolved
organic carbon in a headwater catchment
- Authors: John P. Gannon; Scott W. Bailey, Kevin J. McGuire, James B. Shanley
Abstract: We investigated potential source areas of dissolved organic carbon (DOC) in headwater streams by examining DOC concentrations in lysimeter, shallow well, and streamwater samples from a reference catchment at the Hubbard Brook Experimental Forest. These observations were then compared to high frequency temporal variations in fluorescent dissolved organic matter (FDOM) at the catchment outlet and the predicted spatial extent of shallow groundwater in soils throughout the catchment. While near‐stream soils are generally considered a DOC source in forested catchments, DOC concentrations in near‐stream groundwater were low (mean = 2.4 mg/L, standard error = 0.6 mg/L), less than hillslope groundwater farther from the channel (mean = 5.7 mg/L, standard error = 0.4 mg/L). Furthermore, water tables in near‐stream soils did not rise into the carbon rich upper B or O horizons even during events. In contrast, soils below bedrock outcrops near channel heads where lateral soil formation processes dominate had much higher DOC concentrations. Soils immediately downslope of bedrock areas had thick eluvial horizons indicative of leaching of organic materials, Fe, and Al and had similarly high DOC concentrations in groundwater (mean = 14.5 mg/L, standard error = 0.8 mg/L). Flow from bedrock outcrops partially covered by organic soil horizons produced the highest groundwater DOC concentrations (mean = 20.0 mg/L, standard error = 4.6 mg/L) measured in the catchment. Correspondingly, streamwater in channel heads sourced in part by shallow soils and bedrock outcrops had the highest stream DOC concentrations measured in the catchment. Variation in FDOM concentrations at the catchment outlet followed water table fluctuations in shallow to bedrock soils near channel heads. We show that shallow hillslope soils receiving runoff from organic matter‐covered bedrock outcrops may be a major source of DOC in headwater catchments in forested mountainous regions where catchments have exposed or shallow bedrock near channel heads. This article is protected by copyright. All rights reserved.
- A hybrid‐3‐D hillslope hydrological model for use in Earth
- Abstract: Hillslope‐scale rainfall‐runoff processes leading to a fast catchment response are not explicitly included in land surface models (LSMs) for use in earth system models (ESMs) due to computational constraints.
This study presents a hybrid‐3D hillslope hydrological model (h3D) that couples a 1D vertical soil column model with a lateral pseudo‐2D saturated zone and overland flow model for use in ESMs. By representing vertical and lateral responses separately at different spatial resolutions, h3D is computationally efficient.
The h3D model was first tested for three different hillslope planforms (uniform, convergent and divergent). We then compared h3D (with single and multiple soil columns) with a complex physically‐based 3D model and a simple 1D soil moisture model coupled with an unconfined aquifer (as typically used in LSMs).
It is found that simulations obtained by the simple 1D model vary considerably from the complex 3D model and are not able to represent hillslope‐scale variations in the lateral flow response. In contrast, the single soil column h3D model shows a much better performance and saves computational time by 2‐3 orders of magnitude compared with the complex 3D model. When multiple vertical soil columns are implemented, the resulting hydrological responses (soil moisture, water table depth, and baseflow along the hillslope) from h3D are nearly identical to those predicted by the complex 3D model, but still saves computational time. As such, the computational efficiency of the h3D model provides a valuable and promising approach to incorporating hillslope‐scale hydrological processes into continental and global‐scale ESMs. This article is protected by copyright. All rights reserved.
- Exploring the water storage changes in the largest lake (Selin Co) over
the Tibetan Plateau during 2003–2012 from a basin‐wide
- Authors: Jing Zhou; Lei Wang, Yinsheng Zhang, Yanhong Guo, Xiuping Li, Wenbin Liu
Abstract: Lake water storage change (ΔSw) is an important indicator of the hydrologic cycle and greatly influences lake expansion/shrinkage over the Tibetan Plateau (TP). Accurate estimation of ΔSw will contribute to improved understanding of lake variations in the TP. Based on a water balance, this study explored the variations of ΔSw for the Lake Selin Co (the largest closed lake on the TP) during 2003‐2012 using the Water and Energy Budget‐based Distributed Hydrological Model (WEB‐DHM) together with two different evapotranspiration (ET) algorithms (the Penman‐Monteith method and a simple sublimation estimation approach for water area in unfrozen and frozen period). The contributions of basin discharge and climate causes to the ΔSw are also quantitatively analyzed. The results showed that WEB‐DHM could well reproduce daily discharge, the spatial pattern and basin‐averaged values of MODIS land surface temperature (LST) during nighttime and daytime. Compared with the ET reference values estimated from the basin‐wide water balance, our ET estimates showed better performance than three global ET products in reproducing basin‐averaged ET. The modeled ET at point scale matches well with short‐term in situ daily measurements (RMSE = 0.82 mm/day). Lake inflows and precipitation over the water area had stronger relationships with ΔSw in the warm season and monthly scale, whereas evaporation from the water area had remarkable effects on ΔSw in the cold season. The total contribution of the three factors to ΔSw was about 90%, and accounting for 49.5%, 22.1% and 18.3%, respectively. This article is protected by copyright. All rights reserved.
- Migration behavior of supercritical and liquid CO2 in a stratified system:
Experiments and numerical simulations
- Abstract: Multiple scenarios of upward CO2 migration driven by both injection‐induced pressure and buoyancy force were investigated in a horizontally and vertically stratified core utilizing a core‐flooding system with a 2D X‐ray scanner. Two reservoir type scenarios were considered: (1) the terrestrial reservoir scenario (10 MPa and 50°C), where CO2 exists in a supercritical state and (2) the deep‐sea sediment reservoir scenario (28 MPa and 25°C), where CO2 is stored in the liquid phase. The core‐flooding experiments showed a 36% increase in migration rate in the vertical core setting compared with the horizontal setting, indicating the significance of the buoyancy force under the terrestrial reservoir scenario. Under both reservoir conditions, the injected CO2 tended to find a preferential flow path (low capillary entry pressure and high‐permeability (high‐k) path) and bypass the unfavorable pathways, leaving low CO2 saturation in the low‐permeability (low‐k) layers. No distinctive fingering was observed as the CO2 moved upward, and the CO2 movement was primarily controlled by media heterogeneity. The CO2 saturation in the low‐k layers exhibited a more sensitive response to injection rates, implying that the increase in CO2 injection rates could be more effective in terms of storage capacity in the low‐k layers in a stratified reservoir. Under the deep‐sea sediment condition, the storage potential of liquid CO2 was more than twice as high as that of supercritical CO2 under the terrestrial reservoir scenario. In the end, multiphase transport simulations were conducted to assess the effects of heterogeneity on the spatial variation of pressure build‐up, CO2 saturation and CO2 flux. Finally, we showed that a high gravity number () tended to be more influenced by the heterogeneity of the porous media. This article is protected by copyright. All rights reserved.
- Effect of advective flow in fractures and matrix diffusion on natural gas
- Authors: Satish Karra; Nataliia Makedonska, Hari S. Viswanathan, Scott L. Painter, Jeffrey D. Hyman
Abstract: Although hydraulic fracturing has been used for natural gas production for the past couple of decades, there are significant uncertainties about the underlying mechanisms behind the production curves that are seen in the field. A discrete fracture network based reservoir‐scale work flow is used to identify the relative effect of flow of gas in fractures and matrix diffusion on the production curve. With realistic three dimensional representations of fracture network geometry and aperture variability, simulated production decline curves qualitatively resemble observed production decline curves. The high initial peak of the production curve is controlled by advective fracture flow of free gas within the network and is sensitive to the fracture aperture variability. Matrix diffusion does not significantly affect the production decline curve in the first few years, but contributes to production after approximately 10 years. These results suggest that the initial flushing of gas‐filled background fractures combined with highly heterogeneous flow paths to the production well are sufficient to explain observed initial production decline. These results also suggest that matrix diffusion may support reduced production over longer time frames. This article is protected by copyright. All rights reserved.
- Steady state analytical solutions for pumping in a fully bounded
- Authors: Chunhui Lu; Pei Xin, Ling Li, Jian Luo
Abstract: Using the Schwartz‐Christoffel conformal mapping method together with the complex variable techniques, we derive steady‐state analytical solutions for pumping in a rectangular aquifer with four different combinations of impermeable and constant‐head boundaries. These four scenarios include: (1) one constant‐head boundary and three impermeable boundaries, (2) two pairs of orthogonal impermeable and constant‐head boundaries, (3) three constant‐head boundaries and one impermeable boundary, and (4) four constant‐head boundaries. For these scenarios, the impermeable and constant‐head boundaries can be combined after applying the mapping functions, and hence only three image wells exist in the transformed plane, despite an infinite number of image wells in the real plane. The closed‐form solutions reflect the advantage of the conformal mapping method, though the method is applicable for the aspect ratio of the rectangle between 1/10.9 and 10.9/1 due to the limitation in the numerical computation of the conformal transformation from a half plane onto an elongated region (i.e., so‐called “crowding” phenomenon). By contrast, for an additional scenario with two parallel constant‐head boundaries and two parallel impermeable boundaries, an infinite series of image wells is necessary to express the solution, since it is impossible to combine these two kinds of boundaries through the conformal transformation. The usefulness of the results derived is demonstrated by an application to pumping in a finite coastal aquifer. This article is protected by copyright. All rights reserved.
- The role of stratification on lakes' thermal response: The case of Lake
- Authors: Sebastiano Piccolroaz; Marco Toffolon, Bruno Majone
Abstract: During the last several decades, the Great Lakes region has been experiencing a significant rise in temperatures, with the extraordinary summer warming that affected Lake Superior in 1998 as an example of the marked response of the lake to increasingly warmer atmospheric conditions. In this work we combine the analysis of this exceptional event with some synthetic scenarios, to achieve a deeper understanding of the main processes driving the thermal dynamics of surface water temperature in Lake Superior. The analysis is performed by means of the lumped model air2water, which simulates lake surface temperature as a function of air temperature alone. The model provides information about the seasonal stratification dynamics, suggesting that unusual warming events can result from two factors: anomalously high summer air temperatures, and increased strength of stratification resulting from a warm spring. The relative contribution of the two factors is quantified using the model by means of synthetic scenarios, which provide a simple but effective description of the positive feedback between the thermal behavior and the stratification dynamics of the lake. This article is protected by copyright. All rights reserved.
- Understanding handpump sustainability: Determinants of rural water source
functionality in the Greater Afram Plains region of Ghana
- Authors: Michael B. Fisher; Katherine F. Shields, Terence U. Chan, Elizabeth Christenson, Ryan D. Cronk, Hannah Leker, Destina Samani, Patrick Apoya, Alexandra Lutz, Jamie Bartram
Abstract: Safe drinking water is critical to human health and development. In rural sub‐Saharan Africa, most improved water sources are boreholes with handpumps; studies suggest that up to one third of these handpumps are non‐functional at any given time. This work presents findings from a secondary analysis of cross‐sectional data from 1509 water sources in 570 communities in the rural Greater Afram Plains (GAP) region of Ghana; one of the largest studies of its kind.
79.4% of enumerated water sources were functional when visited; in multivariable regressions, functionality depended on source age, management, the number of other sources in the community, and the district. A Bayesian network (BN) model developed using the same dataset found strong dependencies of functionality on implementer, pump type, management, and the availability of tools, with synergistic effects from management determinants on functionality, increasing the likelihood of a source being functional from a baseline of 72% to more than 97% with optimal management and available tools.
We suggest that functionality may be a dynamic equilibrium between regular breakdowns and repairs, with management a key determinant of repair rate. Management variables may interact synergistically in ways better captured by BN analysis than by logistic regressions. These qualitative findings may prove generalizable beyond the study area, and may offer new approaches to understanding and increasing handpump functionality and safe water access. This article is protected by copyright. All rights reserved.
- Correlation equations for average deposition rate coefficients of
nanoparticles in a cylindrical pore
- Authors: N. Seetha; S. Majid Hassanizadeh, M. S. Mohan Kumar, Amir Raoof
Abstract: Nanoparticle deposition behavior observed at the Darcy scale represents an average of the processes occurring at the pore scale. Hence, the effect of various pore‐scale parameters on nanoparticle deposition can be understood by studying nanoparticle transport at pore scale and upscaling the results to the Darcy scale. In this work, correlation equations for the deposition rate coefficients of nanoparticles in a cylindrical pore are developed as a function of nine pore‐scale parameters: the pore radius, nanoparticle radius, mean flow velocity, solution ionic strength, viscosity, temperature, solution dielectric constant, and nanoparticle and collector surface potentials. Based on dominant processes, the pore space is divided into three different regions, namely, bulk, diffusion, and potential regions. Advection‐diffusion equations for nanoparticle transport are prescribed for the bulk and diffusion regions, while the interaction between the diffusion and potential regions is included as a boundary condition. This interaction is modeled as a first‐order reversible kinetic adsorption. The expressions for the mass transfer rate coefficients between the diffusion and the potential regions are derived in terms of the interaction energy profile. Among other effects, we account for nanoparticle‐collector interaction forces on nanoparticle deposition. The resulting equations are solved numerically for a range of values of pore‐scale parameters. The nanoparticle concentration profile obtained for the cylindrical pore is averaged over a moving averaging volume within the pore in order to get the 1D concentration field. The latter is fitted to the 1D advection‐dispersion equation with an equilibrium or kinetic adsorption model to determine the values of the average deposition rate coefficients. In this study, pore‐scale simulations are performed for three values of Péclet number, Pe = 0.05, 5 and 50. We find that under unfavorable conditions, the nanoparticle deposition at pore scale is best described by an equilibrium model at low Péclet numbers (Pe = 0.05), and by a kinetic model at high Péclet numbers (Pe = 50). But, at an intermediate Pe (e.g., near Pe = 5), both equilibrium and kinetic models fit the 1D concentration field. Correlation equations for the pore‐averaged nanoparticle deposition rate coefficients under unfavorable conditions are derived by performing a multiple‐linear regression analysis between the estimated deposition rate coefficients for a single pore and various pore‐scale parameters. The correlation equations, which follow a power law relation with nine pore‐scale parameters, are found to be consistent with the column‐scale and pore‐scale experimental results, and qualitatively agree with the colloid filtration theory. These equations can be incorporated into pore network models to study the effect of pore‐scale parameters on nanoparticle deposition at larger length scales such as Darcy scale. This article is protected by copyright. All rights reserved.
- Annual water, sediment, nutrient, and organic carbon fluxes in river
basins: A global meta‐analysis as a function of scale
- Abstract: Process controls on water, sediment, nutrient and organic carbon exports from the landscape through runoff are not fully understood. This paper provides analyses from 446 sites worldwide to evaluate the impact of environmental factors (MAP and MAT: mean annual precipitation and temperature; CLAY and BD: soil clay content and bulk density; S: slope gradient and LU: land use) on annual exports (RC: runoff coefficients; SL: sediment loads; TOCL: organic carbon losses; TNL: nitrogen losses and TPL: phosphorus losses) from different spatial scales. RC was found to increase, on average, from 18% at local scale (in headwaters), 25% at micro and subcatchment scale (mid‐reaches) to 41% at catchment scale (lower reaches of river basins) in response to multiple factors. SL increased from microplots (468 g m−2 yr−1) to plots (901 g m−2 yr−1), accompanied by decreasing TOCL and TNL. Climate was a major control masking the effects of other factors. For example, RC, SL, TOCL, TNL and TPL tended to increase with MAP at all spatial scales. These variables, however, decreased with MAT. The impact of CLAY, BD, LU and S on erosion variables was largely confined to the hillslope scale, where RC, SL and TOCL decreased with CLAY, while TNL and TPL increased. The results contribute to better understanding of water, nutrient and carbon cycles in terrestrial ecosystems, and should inform river basin modelling and ecosystem management. The important role of spatial climate variability points to a need for comparative research in specific environments at nested spatio‐temporal scales. This article is protected by copyright. All rights reserved.
- Estimating mountain basin‐mean precipitation from streamflow using
- Authors: Brian Henn; Martyn P. Clark, Dmitri Kavetski, Jessica D. Lundquist
Abstract: Estimating basin‐mean precipitation in complex terrain is difficult due to uncertainty in the topographical representativeness of precipitation gauges relative to the basin. To address this issue, we use Bayesian methodology coupled with a multi‐model framework to infer basin‐mean precipitation from streamflow observations, and we apply this approach to snow‐dominated basins in the Sierra Nevada of California. Using streamflow observations, forcing data from lower‐elevations stations, the Bayesian Total Error Analysis (BATEA) methodology and the Framework for Understanding Structural Errors (FUSE), we infer basin‐mean precipitation, and compare it to basin‐mean precipitation estimated using topographically‐informed interpolation from gauges (PRISM, the Parameter‐elevation Regression on Independent Slopes Model). The BATEA‐inferred spatial patterns of precipitation show agreement with PRISM in terms of the rank of basins from wet to dry, but differ in absolute values. In some of the basins, these differences may reflect biases in PRISM, because some implied PRISM runoff ratios may be inconsistent with the regional climate. We also infer annual time series of basin precipitation using a two‐step calibration approach. Assessment of the precision and robustness of the BATEA approach suggests that uncertainty in the BATEA‐inferred precipitation is primarily related to uncertainties in hydrologic model structure. Despite these limitations, time series of inferred annual precipitation under different model and parameter assumptions are strongly correlated with one another, suggesting that this approach is capable of resolving year‐to‐year variability in basin‐mean precipitation. This article is protected by copyright. All rights reserved.
- Numerical simulations of hydraulic redistribution across climates: The
role of the root hydraulic conductivities
- Authors: Juan C. Quijano; Praveen Kumar
Abstract: Hydraulic redistribution, a process by which vegetation roots redistribute soil moisture, has been recognized as an important mechanism impacting several processes that regulate plant water uptake, energy and water partitioning, and biogeochemical cycling. We analyze how the magnitude of hydraulic redistribution varies across ecosystems that are exposed to different climates and seasonal patterns of incoming shortwave radiation and precipitation. Numerical simulation studies are performed over ten Ameriflux sites, which show that hydraulic redistribution predictions are significantly influenced by the specified root hydraulic conductivities. We performed sensitivity analyses by considering expected ranges of root conductivities based on previous experimental studies, and found contrasting patterns in energy‐limited and water‐limited ecosystems. In energy‐limited ecosystems, there is a threshold above which high root conductivities enhance hydraulic redistribution with no increase in transpiration, while in water‐limited ecosystems increase in root conductivities was always associated with enhancements in both transpiration and hydraulic redistribution. Further we found differences in the magnitude and seasonality of hydraulic redistribution and transpiration across different climates, regulated by interplay between precipitation and transpiration. The annual hydraulic redistribution to transpiration flux ratio (HR/Tr) was significant in Mediterranean climates (HR/Tr ≈ 30%), and in the tropical humid climates (HR/Tr ≈ 15%). However, in the continental climates hydraulic redistribution occurs only during sporadic precipitation events throughout the summer resulting in lower annual magnitudes (HR/Tr
- Resolving two‐dimensional flow structure in rivers using
large‐scale particle image velocimetry: An example from a stream
- Authors: Quinn W. Lewis; Bruce L. Rhoads
Abstract: Large scale particle image velocimetry (LSPIV) has emerged as a valuable tool for measuring surface velocity in a variety of fluvial systems. LSPIV has typically been used in the field to obtain velocity or discharge measurements in relatively simple one‐dimensional flow. Detailed two‐dimensional or three‐dimensional characterization of flow structure has been relegated to laboratory settings because of the difficulty in controlling PIV limiting factors such as poor particle seeding, the need for camera rectification, and challenging field conditions. In this study we implement a low‐cost LSPIV setup using a high‐resolution action camera mounted above a stream confluence and water seeded with recycled landscape mulch. Time‐averaged 2D velocities derived from LSPIV are compared with those measured with an acoustic Doppler velocimeter (ADV) in the camera's field of view. We also assess the capabilities of this setup to resolve both turbulent and time‐averaged flow structures at a stream confluence. Our results reveal that even in challenging field conditions a basic LSPIV setup can yield accurate data on velocity and resolve in detail the temporal evolution of flow structures on the surface of rivers. The resulting dataset contains velocity information at high spatial and temporal resolution, a significant advance in understanding flow processes at stream confluences. Our LSPIV analysis provides support for previous numerical modeling studies that have distinguished between Kelvin‐Helmholtz and wake modes of turbulent behavior within the mixing interface at confluences. This study shows that LSPIV should be considered as both an alternative for traditional methods and a tool that can provide unprecedented levels of resolution of surface velocity patterns on rivers that can be used to evaluate numerical predictions of flow structure in complex fluvial environments. This article is protected by copyright. All rights reserved.
- Self‐adjustment of stream bed roughness and flow velocity in a steep
- Authors: Johannes M. Schneider; Dieter Rickenmann, Jens M. Turowski, James W. Kirchner
Abstract: Understanding how channel bed morphology affects flow conditions (and vice versa) is important for a wide range of fluvial processes and practical applications. We investigated interactions between bed roughness and flow velocity in a steep, glacier‐fed mountain stream (Riedbach, Ct. Valais, Switzerland) with almost flume‐like boundary conditions. Bed gradient increases along the 1‐km study reach by roughly one order of magnitude (S=3‐41%), with a corresponding increase in streambed roughness, while flow discharge and width remain approximately constant due to the glacial runoff regime. Streambed roughness was characterized by semi‐variograms and standard deviations of point clouds derived from terrestrial laser scanning. Reach‐averaged flow velocity was derived from dye tracer breakthrough curves measured by 10 fluorometers installed along the channel. Commonly used flow resistance approaches (Darcy‐Weisbach equation and dimensionless hydraulic geometry) were used to relate the measured bulk velocity to bed characteristics. As a roughness measure, D84 yielded comparable results to more laborious measures derived from point clouds. Flow resistance behavior across this large range of steep slopes agreed with patterns established in previous studies for both lower‐gradient and steep reaches, regardless of which roughness measures were used. We linked empirical critical shear stress approaches to the variable power equation for flow resistance to investigate the change of bed roughness with channel slope. The predicted increase in D84 with increasing channel slope was in good agreement with field observations. This article is protected by copyright. All rights reserved.
- Group‐sparsity regularization for ill‐posed subsurface flow
- Abstract: Sparse representations provide a flexible and parsimonious description of high‐dimensional model parameters for reconstructing subsurface flow property distributions from limited data. To further constrain ill‐posed inverse problems, group‐sparsity regularization can take advantage of possible relations among the entries of unknown sparse parameters when: (i) groups of sparse elements are either collectively active or inactive; and (ii) only a small subset of the groups is needed to approximate the parameters of interest. Since subsurface properties exhibit strong spatial connectivity patterns, they may lead to sparse descriptions that satisfy the above conditions. When these conditions are established a group‐sparsity regularization can be invoked to facilitate the solution of the resulting inverse problem by promoting sparsity across the groups (and not within each group). The proposed regularization penalizes the number of groups that are active without promoting sparsity within each group. Two implementations are presented in this paper: one based on the multi‐resolution tree structure of Wavelet decomposition, without a need for explicit prior models, and another learned from explicit prior model realizations using sparse principal component analysis (SPCA). In each case, the approach first classifies the parameters of the inverse problem into groups with specific connectivity features, and then takes advantage of the grouped structure to recover the relevant patterns in the solution from the flow data. Several numerical experiments are presented to demonstrate the advantages of additional constraining due to group‐sparsity in solving ill‐posed subsurface model calibration problems. This article is protected by copyright. All rights reserved.
- Dry‐season length and runoff control annual variability in stream
DOC dynamics in a small, shallow groundwater‐dominated agricultural
- Authors: G. Humbert; A. Jaffrezic, O. Fovet, G. Gruau, P. Durand
Pages: 7860 - 7877
Abstract: As a phenomenon integrating climate conditions and hydrological control of the connection between streams and terrestrial dissolved organic carbon (DOC) sources, groundwater dynamics control patterns of stream DOC characteristics (concentrations and fluxes). Influence of intra‐annual variations in groundwater level, discharge and climatic factors on DOC concentrations and fluxes were assessed over 13 years at the headwater watershed of Kervidy‐Naizin (5 km2) in western France. Four seasonal periods were delineated within each year according to groundwater fluctuations (A: rewetting, B: high flow, C: recession, and D: drought). Annual and seasonal base flow versus stormflow DOC concentrations were defined based on daily hydrograph readings. High interannual variability of annual DOC fluxes (5.4–39.5 kg ha−1 yr−1) indicates that several years of data are required to encompass variations in water flux to evaluate the actual DOC export capacity of a watershed. Interannual variability of mean annual DOC concentrations was much lower (4.9–7.5 mg C L−1), with concentrations decreasing within each year from ca. 9.2 mg C L−1 in A to ca. 3.0 mg C L−1 in C. This indicates an intra‐annual pattern of stream DOC concentrations controlled by DOC source characteristics and groundwater dynamics very similar across years. Partial least squares regressions combined with multiple linear regressions showed that the dry season characteristics (length and drawdown) determine the mean annual DOC concentration while annual runoff determines the annual flux. Antagonistic mechanisms of production‐accumulation and dilution‐depletion combined with an unlimited DOC supply from riparian wetland soils can mitigate the response of stream concentrations to global changes and climatic variations.