- Optimization of canopy conductance models from concurrent measurements of
sap flow and stem water potential on Drooping Sheoak in South Australia
- Authors: Hailong Wang; Huade Guan, Zijuan Deng, Craig T. Simmons
Pages: n/a - n/a
Abstract: Canopy conductance (gc) is a critical component in hydrological modeling for transpiration estimate. It is often formulated as functions of environmental variables. These functions are climate and vegetation specific. Thus, it is important to determine the appropriate functions in gc models and corresponding parameter values for a specific environment. In this study, sap flow, stem water potential, and microclimatic variables were measured for three Drooping Sheoak (Allocasuarina verticillata) trees in year 2011, 2012, and 2014. Canopy conductance was calculated from the inversed Penman-Monteith (PM) equation, which was then used to examine 36 gc models that comprise different response functions. Parameters were optimized using the DiffeRential Evolution Adaptive Metropolis (DREAM) model based on a training data set in 2012. Use of proper predawn stem water potential function, vapor pressure deficit function, and temperature function improves model performance significantly, while no pronounced difference is observed between models that differ in solar radiation functions. The best model gives a correlation coefficient of 0.97, and root-mean-square error of 0.0006 m/s in comparison to the PM-calculated gc. The optimized temperature function shows different characteristics from its counterparts in other similar studies. This is likely due to strong interdependence between air temperature and vapor pressure deficit in the study area or Sheoak tree physiology. Supported by the measurements and optimization results, we suggest that the effects of air temperature and vapor pressure deficit on canopy conductance should be represented together.
- Continuous streamflow prediction in ungauged basins: The effects of
equifinality and parameter set selection on uncertainty in regionalization
- Authors: Richard Arsenault; François P. Brissette
Pages: n/a - n/a
Abstract: This paper focuses on evaluating the uncertainty of three common regionalization methods for predicting continuous streamflow in ungauged basins. A set of 268 basins covering 1.6 million km2 in the province of Quebec was used to test the regionalization strategies. The multiple linear regression, spatial proximity, and physical similarity approaches were evaluated on the catchments using a leave-one-out cross-validation scheme. The lumped conceptual HSAMI hydrological model was used throughout the study. A bootstrapping method was chosen to further estimate uncertainty due to parameter set selection for each of the parameter set/regionalization method pairs. Results show that parameter set selection can play an important role in regionalization method performance depending on the regionalization methods (and their variants) used and that equifinality does not contribute significantly to the overall uncertainty witnessed throughout the regionalization methods applications. Regression methods fail to consistently assign behavioral parameter sets to the pseudoungauged basins (i.e., the ones left out). Spatial proximity and physical similarity score better, the latter being the best. It is also shown that combining either physical similarity or spatial proximity with the multiple linear regression method can lead to an even more successful prediction rate. However, even the best methods were shown to be unreliable to an extent, as successful prediction rates never surpass 75%. Finally, this paper shows that the selection of catchment descriptors is crucial to the regionalization strategies' performance and that for the HSAMI model, the optimal number of donor catchments for transferred parameter sets lies between four and seven.
- Examining spatial and temporal variability in snow water equivalent using
a 27 year reanalysis: Kern River watershed, Sierra Nevada
- Authors: Manuela Girotto; Gonzalo Cortés, Steven A. Margulis, Michael Durand
Pages: n/a - n/a
Abstract: This paper used a data assimilation framework to estimate spatially and temporally continuous snow water equivalent (SWE) from a 27-year reanalysis (from water year 1985 to 2011) of the Landsat-5 record for the Kern River watershed in the Sierra Nevada, California. The data assimilation approach explicitly treats sources of uncertainty from model parameters, meteorological inputs, and observations. The method is comprised of two main components: (1) a coupled land surface model (LSM) and snow depletion curve (SDC) model, which is used to generate an ensemble of predictions of SWE and fractional snow cover area (FSCA) for a given set of prior (uncertain) inputs, and (2) a retrospective reanalysis step, which updates estimation variables to be consistent with the observed fractional snow cover time series. The final posterior SWE estimate is generated from the LSM-SDC using the posterior estimation variables consistently with all postulated sources of uncertainty in the model, inputs and observations. A reasonable agreement was found between the SWE reanalysis and in-situ SWE observations and streamflow data. The dataset was studied to evaluate factors controlling SWE spatial and temporal variability. Elevation was found to be the primary control on spatial patterns of peak-SWE and day-of-peak. The easting coordinate had additional explanatory power, which is hypothesized to be related to rainshadow effects due to the prevailing storm track directions. The spatial patterns were found to be interannually inconsistent. However, drier years and lower elevations were found more variable than wetter years and higher elevations respectively. Only a very small percentage of the Kern River watershed had a significant trend in peak-SWE and day-of-peak. Trends deemed to be significant were found to be positive (peak-SWE is increasing and day-of-peak occurs later) at higher elevations, but negative (peak-SWE is decreasing and day-of-peak occurs earlier) at lower elevations. The reanalysis approach proved to be useful in terms of identifying sub-watershed variability and trends, and could be extended to larger regions and areas where in-situ data are sparse or unavailable.
- Controls on groundwater flow in a semiarid folded and faulted
- Authors: Lyndsay B. Ball; Jonathan Saul Caine, Shemin Ge
Pages: n/a - n/a
Abstract: The major processes controlling groundwater flow in intermountain basins are poorly understood, particularly in basins underlain by folded and faulted bedrock and under regionally realistic hydrogeologic heterogeneity. To explore the role of hydrogeologic heterogeneity and poorly constrained mountain hydrologic conditions on regional groundwater flow in contracted intermountain basins, a series of 3D numerical groundwater flow models were developed using the South Park basin, Colorado, USA as a proxy. The models were used to identify the relative importance of different recharge processes to major aquifers, to estimate typical groundwater circulation depths, and to explore hydrogeologic communication between mountain and valley hydrogeologic landscapes. Modeling results show that mountain landscapes develop topographically controlled and predominantly local- to intermediate-scale flow systems. Permeability heterogeneity of the fold and fault belt and decreased topographic roughness led to permeability controlled flow systems in the valley. The structural position of major aquifers in the valley fold and fault belt was found to control the relative importance of different recharge mechanisms. Alternative mountain recharge model scenarios showed that higher mountain recharge rates led to higher mountain water table elevations and increasingly prominent local flow systems, primarily resulting in increased seepage within the mountain landscape and non-linear increases in mountain block recharge to the valley. Valley aquifers were found to be relatively insensitive to changing mountain water tables, particularly in structurally isolated aquifers inside the fold and fault belt.
- Improving nutrient management practices in agriculture: The role of
risk-based beliefs in understanding farmers' attitudes toward taking
- Authors: Robyn S. Wilson; Gregory Howard, Elizabeth A. Burnett
Pages: n/a - n/a
Abstract: A recent increase in the amount of dissolved reactive phosphorus (DRP) entering the western Lake Erie basin is likely due to increased spring storm events in combination with issues related to fertilizer application and timing. These factors in combination with warmer lake temperatures have amplified the spread of toxic algal blooms. We assessed the attitudes of farmers in northwest Ohio toward taking at least one additional action to reduce nutrient loss on their farm. Specifically, we 1) identified to what extent farm and farmer characteristics (e.g., age, gross farm sales) as well as risk-based beliefs (e.g., efficacy, risk perception) influenced attitudes, and 2) assessed how these characteristics and beliefs differ in their predictive ability based on unobservable latent classes of farmers. Risk perception, or a belief that negative impacts to profit and water quality from nutrient loss were likely, was the most consistent predictor of farmer attitudes. Response efficacy, or a belief that taking action on one's farm made a difference, was found to significantly influence attitudes, although this belief was particularly salient for the minority class of farmers who were older and more motivated by profit. Communication efforts should focus on the negative impacts of nutrient loss to both the farm (i.e., profit) and the natural environment (i.e., water quality) to raise individual perceived risk among the majority, while the minority need higher perceived efficacy or more specific information about the economic effectiveness of particular recommended practices.
- Local and field-scale stochastic-advective vertical solute transport in
horizontally heterogeneous unsaturated soils
- Authors: Richa Ojha; A. Prakash, Rao S. Govindaraju
Pages: n/a - n/a
Abstract: Description of field-scale solute transport in unsaturated soils is essential for assessing the degree of contamination, estimating fluxes past a control plane and for designing remedial measures. The flow field is usually described by numerical solution of the Richards equation followed by numerical solution of the advection-dispersion equation to describe contaminant movement. These numerical solutions are highly complex, and do not provide the insights that are possible from simpler analytical representations. In this study, analytical solutions at the local scale are developed to describe purely advective vertical transport of a conservative solute along the principle characteristic of the flow field. Local-scale model development is simplified by using a sharp front approximation for water movement. These local solutions are then upscaled to field-scale solute transport by adopting a log-normally distributed horizontal hydraulic conductivity field to represent the natural heterogeneity observed in field soils. Analytical expressions are developed for the mean behavior of solute transport at the field-scale. Comparisons with experimental observations find that trends of field-scale solute behavior are reasonably reproduced by the model. The accuracy of the proposed solution improves with increasing spatial variability in the hydraulic conductivity as revealed by further comparisons with numerical results of the Richards equation-based field-scale solute movement. In some cases, the sharp-front approximation may lead to anomalous field-scale behavior depending on the role of pre- and post-ponded conditions in the field, and this limitation is discussed. The proposed method shows promise for describing field-scale solute movement in loamy sand and sandy loam soils.
- Reimagining cost recovery in Pakistan's irrigation system through
willingness-to-pay estimates for irrigation water from a discrete choice
- Authors: Andrew Reid Bell; M. Azeem Ali Shah, Patrick S. Ward
Pages: n/a - n/a
Abstract: It is widely argued that farmers are unwilling to pay adequate fees for surface water irrigation to recover the costs associated with maintenance and improvement of delivery systems. In this paper, we use a discrete choice experiment to study farmer preferences for irrigation characteristics along two branch canals in Punjab Province in eastern Pakistan. We find that farmers are generally willing to pay well in excess of current surface water irrigation costs for increased surface water reliability, and that the amount that farmers are willing to pay is an increasing function of their existing surface water supply as well as location along the main canal branch. This explicit translation of implicit willingness-to-pay (WTP) for water (via expenditure on groundwater pumping) to WTP for reliable surface water demonstrates the potential for greatly enhanced cost recovery in the Indus Basin Irrigation System via appropriate setting of water user fees, driven by the higher WTP of those currently receiving reliable supplies.
- An entropy-based surface velocity method for estuarine discharge
- Authors: Adam J. Bechle; Chin H. Wu
Pages: n/a - n/a
Abstract: An entropy-based method is developed to estimate estuarine river discharge from surface velocity measurements. A two-dimensional velocity profile based on the principle of maximum entropy is employed to express the mean velocity as a function of average surface velocity. The entropy-based flow profile is parameterized by the location of maximum velocity in the channel and the shape of the velocity distribution. The entropy parameters are quantified over the tidal cycle to account for the unsteady nature of estuarine flow. The method was tested using experiments conducted at the Danshui River, the largest estuarine system in Taiwan. Surface velocities were measured using an Automated River-Estuary Discharge Imaging System (AREDIS), and full-channel velocity profiles were measured with a moving-boat ADP survey. Entropy parameters were calibrated over the tidal cycle and linearly correlated with the average surface velocity to facilitate estimation from AREDIS measurements. The discharge calculated from average surface velocity and entropy relationships exhibits a 7.7% relative error compared to the ADP velocity profiles. The error nearly doubles when the mean values for entropy parameters are used instead of the variable parameters, indicating the importance of accounting for the unsteady nature of estuarine flows. Furthermore, the effects of measurement coverage area, types of entropy distribution, and wind-induced drift current on the surface velocity-based discharge measurement are evaluated and discussed. Overall, surface velocity measurements in conjunction with the entropy profiles well represent the flow in a complex estuarine environment to provide a reliable estimate of discharge.
- Antecedent moisture conditions control mercury and dissolved organic
carbon concentration dynamics in a boreal headwater catchment
- Authors: Claire J. Oswald; Brian A. Branfireun
Pages: n/a - n/a
Abstract: The fate and transport of mercury (Hg) deposited on forested upland soils depends on the biogeochemical and hydrological processes occurring in the soil landscape. In this study, total Hg (THg) and dissolved organic carbon (DOC) concentrations were measured in streamwater from a 7.75 ha upland sub-catchment of the METAALICUS watershed in northwestern Ontario, Canada. THg and DOC concentration-discharge relationships were examined at the seasonal-scale and event-scale to assess the role of antecedent moisture conditions on the mobilization of these solutes to receiving waters. At the seasonal-scale, sub-catchment discharge poorly explained THg and DOC concentration dynamics; however, the inclusion of antecedent water storage and precipitation metrics in a multiple regression model improved the prediction of THg and DOC concentrations significantly. At the event-scale, a comparison of THg and DOC concentrations for two small summer storms with similar total discharge showed that the storm following the wet snowmelt period had a significantly lower total flux of THg and DOC than the storm following warm and dry conditions in late summer due to a distinct shift in the concentration-discharge relationship. Measurements of soilwater and groundwater THg and DOC concentrations, as well as a three-component mixing analysis, suggest that there was an accumulation of potentially-mobile DOC-bound THg in the well humified organic soil layer in the catchment during the warm and dry summer period and that as the catchment became wetter in the autumn, there was an increase in soilwater THg and DOC concentrations and these solutes were subsequently flushed during the autumn storm.
- Discontinuous Galerkin flood model formulation: luxury or necessity'
- Authors: Georges Kesserwani; Yueling Wang
Pages: n/a - n/a
Abstract: The finite volume Godunov-type flood model formulation is the most comprehensive amongst those currently employed for flood risk modelling. The local Discontinuous Galerkin method constitutes a more complex, rigorous and extended local Godunov-type formulation. However, the practical merit associated with such an increase in the level of complexity of the formulation is yet to be decided. This work makes the case for a second-order Runge-Kutta Discontinuous Galerkin (RKDG2) formulation and contrasts it with the equivalently accurate finite volume (MUSCL) formulation, both of which solve the Shallow Water Equations (SWE) in two space dimensions. The numerical complexity of both formulations are presented and their capabilities are explored for wide-ranging diagnostic and real-scale tests, incorporating all challenging features relevant to flood inundation modelling. Our findings reveal that the extra complexity associated with the RKDG2 model pays off by providing higher quality solution behaviour on very coarse meshes and improved velocity predictions. The practical implication of this is that improved accuracy for flood modelling simulations will result when terrain data are limited or of a low resolution.
- Using multiple donor sites for enhanced flood estimation in ungauged
- Authors: T. R. Kjeldsen; D. A. Jones, D. G. Morris
Pages: n/a - n/a
Abstract: A new generalised method is presented enabling the use of multiple donor sites when predicting an index flood variable in an ungauged catchment using a hydrological regression model. The method is developed from the premise of having an index flood prediction with minimum variance, which results in a set of optimal weights assigned to each donor site. In the model framework presented here, the weights are determined by the geographical distance between the centroids of the catchments draining to the subject site and the donor sites. The new method was applied to a case study in the United Kingdom using annual maximum series of peak flow from 602 catchments. Results show that the prediction error of the index flood is reduced by using donor sites until a minimum of six donors have been included, after which no or marginal improvements in prediction accuracy are observed. A comparison of these results is made with a variant of the method where donor sites are selected based on connectivity with the subject site through the river network. The results show that only a marginal improvement is obtained by explicitly considering the network structure over spatial proximity. The evaluation is carried out based on a new performance measure that accounts for the sampling variability of the index flood estimates at each site. Other results compare the benefits obtained by adding relevant catchment descriptors to a simple regression model with those obtained by transferring information from local donor sites.
- Soil moisture and soil properties estimation in the Community Land Model
with synthetic brightness temperature observations
- Authors: Xujun Han; Harrie-Jan Hendricks Franssen, Carsten Montzka, Harry Vereecken
Pages: n/a - n/a
Abstract: The Community Land Model (CLM) includes a large variety of parameterizations, also for flow in the unsaturated zone and soil properties. Soil properties introduce uncertainties into land surface model predictions. In this paper, soil moisture and soil properties are updated for the coupled CLM and Community Microwave Emission Model (CMEM) by the Local Ensemble Transform Kalman Filter (LETKF) and the state augmentation method. Soil properties are estimated through the update of soil textural properties and soil organic matter density. These variables are used in CLM for predicting the soil moisture retention characteristic and the unsaturated hydraulic conductivity, and the soil texture is used in CMEM to calculate the soil dielectric constant. The following scenarios were evaluated for the joint state and parameter estimation with help of synthetic L-band brightness temperature data assimilation: (i) the impact of joint state and parameter estimation; (ii) updating of soil properties in CLM alone, CMEM alone or both CLM and CMEM; (iii) updating of soil properties without soil moisture update; (iv) the observation localization of LETKF. The results show that the characterization of soil properties through the update of textural properties and soil organic matter density can strongly improve with assimilation of brightness temperature data. The optimized soil properties also improve the characterization of soil moisture, soil temperature, actual evapotranspiration, sensible heat flux, and soil heat flux. The best results are obtained if the soil properties are updated only. The coupled CLM and CMEM model is helpful for the parameter estimation. If soil properties are biased, assimilation of soil moisture data with only state updates increases the root mean square error for evapotranspiration, sensible heat flux, and soil heat flux.
- Topographic controls on shallow groundwater levels in a steep, prealpine
catchment: When are the TWI assumptions valid'
- Authors: M. Rinderer; H. J. van Meerveld, J. Seibert
Pages: n/a - n/a
Abstract: Topographic indices like the Topographic Wetness Index (TWI) have been used to predict spatial patterns of average groundwater levels and to model the dynamics of the saturated zone during events (e.g., TOPMODEL). However, the assumptions underlying the use of the TWI in hydrological models, of which the most important is that groundwater level variation can be approximated by a series of steady state situations, are rarely tested. It is also not clear how well findings from existing hillslope studies on sites with transmissive soil can be transferred to entire catchments with less permeable soils. This study, therefore, evaluated the suitability of selected topographic indices to describe spatial groundwater level variations based on time series from 51 groundwater wells in a 20 ha catchment with low-permeability soils in Switzerland. Results showed that median groundwater levels were correlated to slope, curvature, and TWI, but the strength of correlation depended on whether the indices characterized the local topography or the topography of the upslope contributing area. The correlation between TWI and groundwater levels was not constant over time but decreased at the beginning of rainfall events, indicating large spatial differences in groundwater responses, and increased after peak flow, when groundwater levels could be considered to be spatially in a steady state. Our findings indicate that topographic indices are useful to predict median groundwater levels in catchments with low-permeability soils and that the TWI assumptions are best met when groundwater levels change slowly.
- Robust, low-cost data loggers for stream temperature, flow intermittency,
and relative conductivity monitoring
- Authors: Thomas P. Chapin; Andrew S. Todd, Matthew P. Zeigler
Pages: n/a - n/a
Abstract: Water temperature and streamflow intermittency are critical parameters influencing aquatic ecosystem health. Low-cost temperature loggers have made continuous water temperature monitoring relatively simple but determining streamflow timing and intermittency using temperature data alone requires significant and subjective data interpretation. Electrical resistance (ER) sensors have recently been developed to overcome the major limitations of temperature-based methods for the assessment of streamflow intermittency. This technical note introduces the STIC (Stream Temperature, Intermittency & Conductivity logger); a robust, low-cost, simple to build instrument that provides long-duration, high-resolution monitoring of both relative conductivity (RC) and temperature. Simultaneously-collected temperature and RC data provides unambiguous water temperature and streamflow intermittency information that is crucial for monitoring aquatic ecosystem health and assessing regulatory compliance. With proper calibration, the STIC relative conductivity data can be used to monitor specific conductivity.
- A robust multimodel framework for ensemble seasonal hydroclimatic
- Authors: Pablo A. Mendoza; Balaji Rajagopalan, Martyn P. Clark, Gonzalo Cortés, James McPhee
Pages: n/a - n/a
Abstract: We provide a framework for careful analysis of the different methodological choices we make when constructing multimodel ensemble seasonal forecasts of hydroclimatic variables. Specifically, we focus on three common modeling decisions: (i) number of models, (ii) multimodel combination approach, and (iii) lead time for prediction. The analysis scheme includes a multimodel ensemble forecasting algorithm based on nonparametric regression, a set of alternatives for the options previously pointed, and a selection of probabilistic verification methods for ensemble forecast evaluation. The usefulness of this framework is tested through an example application aimed to generate spring/summer streamflow forecasts at multiple locations in Central Chile. Results demonstrate the high impact that subjectivity in decision-making may have on the quality of ensemble seasonal hydroclimatic forecasts. In particular, we note that the probabilistic verification criteria may lead to different choices regarding the number of models or the multimodel combination method. We also illustrate how this objective analysis scheme may lead to results that are extremely relevant for the case study presented here, such as skillful seasonal streamflow predictions for very dry conditions.
- Assessing the value of seasonal climate forecast information through an
end-to-end forecasting framework: Application to U.S. 2012 drought in
- Authors: Majid Shafiee-Jood; Ximing Cai, Ligang Chen, Xin-Zhong Liang, Praveen Kumar
Pages: n/a - n/a
Abstract: This study proposes an end-to-end forecasting framework to incorporate operational seasonal climate forecasts to help farmers improve their decisions prior to the crop growth season, which are vulnerable to unanticipated drought conditions. The framework couples a crop growth model with a decision-making model for rainfed agriculture and translates probabilistic seasonal forecasts into more user-related information that can be used to support farmers' decisions on crop type and some market choices (e.g., contracts with ethanol refinery). The regional Climate-Weather Research and Forecasting model (CWRF) driven by two operational general circulation models (GCMs) is used to provide the seasonal forecasts of weather parameters. To better assess the developed framework, CWRF is also driven by observational reanalysis data, which theoretically can be considered as the best seasonal forecast. The proposed framework is applied to the Salt Creek watershed in Illinois that experienced an extreme drought event during 2012 crop growth season. The results show that the forecasts cannot capture the 2012 drought condition in Salt Creek and therefore the suggested decisions can make farmers worse off if the suggestions are adopted. Alternatively, the optimal decisions based on reanalysis-based CWRF forecasts, which can capture the 2012 drought conditions, make farmers better off by suggesting “no-contract” with ethanol refineries. This study suggests that the conventional metric used for ex-ante value assessment is not capable of providing meaningful information in the case of extreme drought. Also, it is observed that institutional interventions (e.g., crop insurance) highly influences farmers' decisions and, thereby, the assessment of forecast value.
- Approximate Bayesian computation using Markov Chain Monte Carlo
- Authors: Mojtaba Sadegh; Jasper A. Vrugt
Pages: n/a - n/a
Abstract: The quest for a more powerful method for model evaluation has inspired Vrugt and Sadegh  to introduce “likelihood-free” inference as vehicle for diagnostic model evaluation. This class of methods is also referred to as Approximate Bayesian Computation (ABC) and relaxes the need for a residual-based likelihood function in favor of one or multiple different summary statistics that exhibit superior diagnostic power. Here, we propose several methodological improvements over commonly used ABC sampling methods to permit inference of complex system models. Our methodology entitled, DREAM(ABC) uses the DiffeRential Evolution Adaptive Metropolis algorithm [vrugt et al., 2008a, 2009] as its main building block and takes advantage of a continuous fitness function to efficiently explore the behavioral model space. Three case studies demonstrate that DREAM(ABC) is at least 3 - 1,000 times more eﬃcient than commonly used ABC sampling meth-ods.
- Effects of tidal fluctuations on mixing and spreading in coastal aquifers:
- Authors: María Pool; Vincent E.A. Post, Craig T. Simmons
Pages: n/a - n/a
Abstract: While the hydraulics of tidally-dominated groundwater systems have been studied extensively, tidally-induced solute spreading in the fresh-saltwater transition zone of coastal aquifers remains largely unexplored. Here, we systematically quantify tidal impacts on solute mixing and spreading in seawater intrusion problems for an idealized homogeneous system. Mixing is characterized by the spatial moments of the solute concentration distribution and quantified by an effective dispersion coefficient. Parametric analysis reveals that the key dimensionless parameter controlling the tidal mixing behavior is the tidal mixing number (ntm) which depends on the tidal amplitude, the period and the hydraulic diffusivity. We find that for ntm≤600, tides lead to a significant impact on the shape and location of the interface. The maximum effect on transverse and longitudinal dispersion occurs for large values of storativity, a hydrogeologic parameter that has been previously understated in terms of its significance. Large storativity implies a nonuniform hydraulic response to the tidal forcing, such that the resulting nonuniform time-dependent velocity field enhances mixing. As a result, the interface spreads mainly at the bottom of the aquifer, where the saline end of the mixing zone migrates seaward, whereas the spatial extent of low salt concentrations migrates landward. These insights critically underpin quantitative guidance on the inclusion and exclusion of tidal effects in the analysis of seawater intrusion.
- Spatial characterization of roughness elements in high-gradient channels
of the Fraser Experimental Forest, Colorado, USA
- Authors: Steven E. Yochum; Brian P. Bledsoe, Ellen Wohl, Gabrielle C. L. David
Pages: n/a - n/a
Abstract: We collected high-resolution LiDAR-based spatial and reach-average flow resistance data at a range of flows in headwater stream channels of the Fraser Experimental Forest, Colorado, USA. Using these data, we implemented a random field approach for assessing the variability of detrended bed elevations and flow depths for both the entire channel width and the thalweg-centered 50% of the channel width (to exclude bank effects). The spatial characteristics of these channels, due to bedforms, large clasts and instream wood, were compared with Darcy-Weisbach f and stream type through the use of the first four probability density function moments (mean, variance, skewness, kurtosis). The standard deviation of the bed elevations (σz) combined with depth (h), as relative bedform submergence (h/σz), was well correlated with f (R2 = 0.81) for the 50% of channel width. The explained variance decreased substantially (R2 = 0.69) when accounting for the entire width, indicating lesser contribution of channel edges to flow resistance. The flow depth skew also explained a substantial amount of the variance in f (R2 = 0.78). A spectrum of channel types is evident in depth plots of skew versus kurtosis, with channel types ranging from plane bed, transitional, step pool/cascade, to cascade. These results varied when bank effects were included or excluded, although definitive patterns were observed for both analyses. Random field analyses may be valuable for developing tools for predicting flow resistance, as well as for quantifying the spectrum of morphologic change in high-gradient channel types, from plane bed through cascade.
- Economic analysis of the water demand in the hotels and restaurants
sector: Shadow prices and elasticities
- Authors: Ana Angulo; Majed Atwi, Ramón Barberán, Jesús Mur
Pages: n/a - n/a
Abstract: Despite the growing economic importance of tourism, and its impact on relative water shortage, little is known about the role that water plays in the productive process of hotels and restaurants and, therefore, the possible implications of water demand management policy for this sector. This study aims to fill this gap. It is based on the microdata of 676 firms in the sector, operating in the city of Zaragoza (Spain) for a 12-year period. Based on the Translog cost function, we estimate the shadow price of water in the short-run and, from a long-run perspective, its direct price elasticity, its cross elasticities relative to labour, capital and supplies, and its elasticity with respect to the level of output. The results obtained show that water provides sector firms returns that are on average higher than its price, although in the case of hotels the margin is really narrow. This situation provides policy makers with a margin for applying price increases without affecting the sector's viability, with some caution in the case of hotels. Water demand elasticity equals -0.38 in the case of hotels, but it is not significant in the case of restaurants and bar-cafes; hence, only in hotels is there potential for influencing water use patterns, encouraging the resource's conservation through pricing policy. Moreover, capital is a substitutive factor of water, and the elasticity of water with respect to output is 0.40, all of which should also be considered by policy makers in water resource management.
- Inference of reactive transport model parameters using a Bayesian
- Authors: Luca Carniato; Gerrit Schoups, Nick van de Giesen
Pages: n/a - n/a
Abstract:  Parameter estimation of subsurface transport models from multi-species data requires the definition of an objective function that includes different types of measurements. Common approaches are weighted least squares (WLS), where weights are specified a priori for each measurement, and weighted least squares with weight estimation (WLS(we)) where weights are estimated from the data together with the parameters. In this study, we formulate the parameter estimation task as a multivariate Bayesian inference problem. The WLS and WLS(we) methods are special cases in this framework, corresponding to specific prior assumptions about the residual covariance matrix. The Bayesian perspective allows for generalizations to cases where residual correlation is important and for efficient inference by analytically integrating out the variances (weights) and selected covariances from the joint posterior. Specifically, the WLS and WLS(we) methods are compared to a multivariate (MV) approach that accounts for specific residual correlations without the need for explicit estimation of the error parameters. When applied to inference of reactive transport model parameters from column-scale data on dissolved species concentrations, the following results were obtained: (1) accounting for residual correlation between species provides more accurate parameter estimation for high residual correlation levels whereas its influence for predictive uncertainty is negligible, (2) integrating out the (co)variances leads to an efficient estimation of the full joint posterior with a reduced computational effort compared to the WLS(we) method, and (3) in the presence of model structural errors, none of the methods is able to identify the correct parameter values.
- How will increases in rainfall intensity affect semiarid ecosystems'
- Authors: Koen Siteur; Maarten B. Eppinga, Derek Karssenberg, Mara Baudena, Marc F.P. Bierkens, Max Rietkerk
Pages: n/a - n/a
Abstract: Model studies suggest that semiarid ecosystems with patterned vegetation can respond in a nonlinear way to climate change. This means that gradual changes can result in a rapid transition to a desertified state. Previous model studies focused on the response of patterned semiarid ecosystems to changes in mean annual rainfall. The intensity of rain events, however, is projected to change as well in the coming decades. In this paper, we study the effect of changes in rainfall intensity on the functioning of patterned semiarid ecosystems with a spatially explicit model that captures rainwater partitioning and runoff-runon processes with simple event-based process descriptions. Analytical and numerical analyses of the model revealed that rainfall intensity is a key parameter in explaining patterning of vegetation in semiarid ecosystems as low mean rainfall intensities do not allow for vegetation patterning to occur. Surprisingly, we found that, for a constant annual rainfall rate, both an increase and a decrease in mean rainfall intensity can trigger desertification. An increase negatively affects productivity as a greater fraction of the rainwater is lost as runoff. This can result in a shift to a bare desert state only if the mean rainfall intensity exceeds the infiltration capacity of bare soil. On the other hand, a decrease in mean rainfall intensity leads to an increased fraction of rainwater infiltrating in bare soils, remaining unavailable to plants. Our findings suggest that considering rainfall intensity as a variable may help in assessing the proximity to regime shifts in patterned semiarid ecosystems and that monitoring losses of resource through runoff and bare soil infiltration could be used to determine ecosystem resilience.
- Detection of spatially limited high porosity layers using crosshole GPR
signal analysis and full-waveform inversion
- Authors: Anja Klotzsche; Jan van der Kruk, John Bradford, Harry Vereecken
Pages: n/a - n/a
Abstract: High permittivity layers, related to high porosity layers or impermeable clay lenses, can act as low-velocity electromagnetic waveguides. Electromagnetic wave phenomena associated with these features are complicated, not well-known and not easy to interpret in borehole GPR data. Recently, a novel amplitude analysis approach was developed that is able to detect continuous low-velocity waveguides and their boundaries between boreholes by using maximum and minimum positions of the trace energy profiles in measured GPR data. By analyzing waveguide models of different thickness, dip, extent, permittivity, and conductivity parameters, we extend the amplitude analysis to detect spatially limited or terminated waveguides. Waveguides that show high amplitude elongated wave trains are most probably caused by a change in porosity rather than a change in clay content. In a crosshole GPR dataset from the Boise Hydrogeophysical Research Site, two terminated wave-guiding structures were detected using the extended amplitude analysis. Information gained from the amplitude analysis improved the starting model for full-waveform inversion which imaged the lateral extent and thickness of terminated waveguides with high resolution. Synthetic data calculated using the inverted permittivity and conductivity models show similar amplitudes and phases, as observed in the measured data, which indicates the reliability of the obtained models. Neutron-Neutron logging data from three boreholes confirm the changes in porosity and indicate that these layers were high-porosity sand units within low-porosity, poorly sorted sand and gravel units.
- Modeling spatio-temporal impacts of hydro-climatic extremes on groundwater
recharge at a Mediterranean karst aquifer
- Authors: Andreas Hartmann; Matías Mudarra, Bartolomé Andreo, Ana Marín, Thorsten Wagener, Jens Lange
Pages: n/a - n/a
Abstract: Karst aquifers provide large parts of the water supply for Mediterranean countries, though climate change is expected to have a significant negative impact on water availability. Recharge is therefore a key variable that has to be known for sustainable groundwater use. In this study, we present a new approach that combines two independent methods for karst recharge estimation. The first method derives spatially distributed information of mean annual recharge patterns through GIS analysis. The second is a process-based karst model that provides spatially lumped but temporally distributed information about recharge. By combining both methods we add a spatial reference to the lumped simulations of the process-based model. In this way we are able to provide spatio-temporal information of recharge and subsurface flow dynamics also during varying hydro-climatic conditions. We find that there is a non-linear relationship between precipitation and recharge rates resulting in strong decreases of recharge following even moderate decreases of precipitation. This is primarily due to almost constant actual evapotranspiration amounts despite varying hydro-climatic conditions. During the driest year in the record, almost the entire precipitation was consumed as actual evapotranspiration and only little diffuse recharge took place at the high altitudes of our study site. During wettest year, recharge constituted a much larger fraction of precipitation and occurred at the entire study site. Our new method and our findings are significant for decision makers in similar regions that want to prepare for possible changes of hydro-climatic conditions in the future.
- Behavioral response to contamination risk information in a spatially
explicit groundwater environment: Experimental evidence
- Authors: Jingyuan Li; Holly A. Michael, Joshua Duke, Kent Messer, Jordan Suter
Pages: n/a - n/a
Abstract: This paper assesses the effectiveness of aquifer monitoring information in achieving more sustainable use of a groundwater resource in the absence of management policy. Groundwater user behavior in the face of an irreversible contamination threat is studied by applying methods of experimental economics to scenarios that combine a physics-based, spatially explicit, numerical groundwater model with different representations of information about an aquifer and its risk of contamination. The results suggest that the threat of catastrophic contamination affects pumping decisions: pumping is significantly reduced in experiments where contamination is possible compared to those where pumping cost is the only factor discouraging groundwater use. The level of information about the state of the aquifer also affects extraction behavior. Pumping rates differ when information that synthesizes data on aquifer conditions (a “risk gauge”) is provided, despite invariant underlying economic incentives, and this result does not depend on whether the risk information is location-specific or from a whole aquifer perspective. Interestingly, users increase pumping when the risk gauge signals good aquifer status compared to a no-gauge treatment. When the gauge suggests impending contamination, however, pumping declines significantly, resulting in a lower probability of contamination. The study suggests that providing relatively simple aquifer condition guidance derived from monitoring data can lead to more sustainable use of groundwater resources.
- Comparing vertical profiles of natural tracers in the Williston Basin to
estimate the onset of deep aquifer activation
- Authors: M. Jim Hendry; Glenn A. Harrington
Pages: n/a - n/a
Abstract: Comparing high-resolution depth profiles of different naturally occurring environmental tracers in aquitards should yield consistent and perhaps complementary information about solute transport mechanisms and the timing of major hydrogeological and climatological events. This study evaluated whether deep, continuous profiles of aquitard pore water chloride concentration could provide further insight into the paleohydrology of the Williston Basin, Canada, than possible using high-resolution depth profiles of stable H/O isotopes of water (δ18O, δ2H). Pore water samples were obtained from extracts of cores take from over 346 m depth of the thick Cretaceous shale aquitard. Water samples were also collected from wells installed in the underlying regional sandy aquifer (Mannville Group; 93 m thick) and from seepage inflows into potash mine shafts (to 825 m below ground). Numerical modeling of the 1-D vertical Cl- profile supported diffusion dominated solute transport in the shales. The modeling also showed a similar timeframe for development of the Cl- profile prior to activation of the aquifer as determined from the δ18O profile (20-25 Ma); however, it provided a significantly longer and potentially better-constrained timeframe for evolution of the profile during the activation phase of the aquifer (0.5-1 Ma). The dominant paleo-event reflected in present-day profiles of both tracers is the introduction of glaciogenic meteoric water to the Mannville aquifer underlying the shale during the Pleistocene. The source area of this water remains to be determined.
- Derivation of lowland riparian wetland deposit architecture using
geophysical image analysis and interface detection
- Authors: J. E. Chambers; P. B. Wilkinson, S. Uhlemann, J. P. R. Sorensen, C. Roberts, A. J. Newell, W. O. C. Ward, A. Binley, P. J. Williams, D. C. Gooddy, G. Old, L. Bai
Pages: n/a - n/a
Abstract: For groundwater-surface water interactions to be understood in complex wetland settings, the architecture of the underlying deposits requires investigation at a spatial resolution sufficient to characterize significant hydraulic pathways. Discrete intrusive sampling using conventional approaches provides insufficient sample density and can be difficult to deploy on soft ground. Here a noninvasive geophysical imaging approach combining three-dimensional electrical resistivity tomography (ERT) and the novel application of gradient and isosurface-based edge detectors is considered as a means of illuminating wetland deposit architecture. The performance of three edge detectors were compared and evaluated against ground truth data, using a lowland riparian wetland demonstration site. Isosurface-based methods correlated well with intrusive data and were useful for defining the geometries of key geological interfaces (i.e., peat/gravels and gravels/Chalk). The use of gradient detectors approach was unsuccessful, indicating that the assumption that the steepest resistivity gradient coincides with the associated geological interface can be incorrect. These findings are relevant to the application of this approach in settings with a broadly layered geology with strata of contrasting resistivities. In addition, ERT revealed substantial structures in the gravels related to the depositional environment (i.e., braided fluvial system) and a complex distribution of low-permeability putty Chalk at the bedrock surface—with implications for preferential flow and variable exchange between river and groundwater systems. These results demonstrate that a combined approach using ERT and edge detectors can provide valuable information to support targeted monitoring and inform hydrological modeling of wetlands.
- A simple and effective method for quantifying spatial anisotropy of time
series of precipitation fields
- Authors: Tero J. Niemi; Teemu Kokkonen, Alan W. Seed
Pages: n/a - n/a
Abstract: The spatial shape of a precipitation event has an important role in determining the catchment's hydrological response to a storm. To be able to generate stochastic design storms with a realistic spatial structure, the anisotropy of the storm has to be quantified. In this paper, a method is proposed to estimate the anisotropy of precipitation fields, using the concept of linear Generalized Scale Invariance (GSI). The proposed method is based on identifying the values of GSI parameters that best describe isolines of constant power on the two-dimensional power spectrum of the fields. The method is evaluated using two sets of simulated fields with known anisotropy and a measured precipitation event with an unknown anisotropy from Brisbane, Australia. It is capable of accurately estimating the anisotropy parameters of simulated nonzero fields, whereas introducing the rain-no rain intermittency alters the power spectra of the fields and slightly reduces the accuracy of the parameter estimates. The parameters estimated for the measured event correspond well with the visual observations on the spatial structure of the fields. The method requires minimum amount of decision making and user interaction, making it suitable for analyzing anisotropy of storm events consisting of long time series of fields with a changing spatial structure.
- Modeling intersite dependence for regional frequency analysis of extreme
- Authors: Jérôme Weiss; Pietro Bernardara, Michel Benoit
Pages: n/a - n/a
Abstract: The duration of observation at a site of interest is generally too low to reliably estimate marine extremes. Regional frequency analysis (RFA), by exploiting the similarity between sites, can help to reduce uncertainties inherent to local analyses. Extreme observations in a homogeneous region are especially assumed to follow a common regional distribution, up to a local index. The regional pooling method, by gathering observations from different sites into a regional sample, can be employed to estimate the regional distribution. However, such a procedure may be highly affected by intersite dependence in the regional sample. This paper derives a theoretical model of intersite dependence, dedicated to the regional pooling method in a “peaks over threshold” framework. This model expresses the tendency of sites to display a similar behavior during a storm generating extreme observations, by describing both the storm propagation in the region and the storm intensity. The proposed model allows the assessment of (i) the regional effective duration of the regional sample and (ii) different regional hazards, e.g., return periods of storms. An application to the estimation of extreme significant wave heights from the numerical sea-state database ANEMOC-2 is provided, where different patterns of regional dependence are highlighted.
- Water resources of the Black Sea Basin at high spatial and temporal
- Authors: Elham Rouholahnejad; Karim C. Abbaspour, Raghvan Srinivasan, Victor Bacu, Anthony Lehmann
Pages: n/a - n/a
Abstract: The pressure on water resources, deteriorating water quality, and uncertainties associated with the climate change create an environment of conflict in large and complex river system. The Black Sea Basin (BSB), in particular, suffers from ecological unsustainability and inadequate resource management leading to severe environmental, social, and economical problems. To better tackle the future challenges, we used the Soil and Water Assessment Tool (SWAT) to model the hydrology of the BSB coupling water quantity, water quality, and crop yield components. The hydrological model of the BSB was calibrated and validated considering sensitivity and uncertainty analysis. River discharges, nitrate loads, and crop yields were used to calibrate the model. Employing grid technology improved calibration computation time by more than an order of magnitude. We calculated components of water resources such as river discharge, infiltration, aquifer recharge, soil moisture, and actual and potential evapotranspiration. Furthermore, available water resources were calculated at subbasin spatial and monthly temporal levels. Within this framework, a comprehensive database of the BSB was created to fill the existing gaps in water resources data in the region. In this paper, we discuss the challenges of building a large-scale model in fine spatial and temporal detail. This study provides the basis for further research on the impacts of climate and land use change on water resources in the BSB.
- Nonequilibrium water dynamics in the rhizosphere: How mucilage affects
water flow in soils
- Authors: Eva Kroener; Mohsen Zarebanadkouki, Anders Kaestner, Andrea Carminati
Pages: n/a - n/a
Abstract: The flow of water from soil to plant roots is controlled by the properties of the narrow region of soil close to the roots, the rhizosphere. In particular, the hydraulic properties of the rhizosphere are altered by mucilage, a polymeric gel exuded by the roots. In this paper we present experimental results and a conceptual model of water flow in unsaturated soils mixed with mucilage. A central hypothesis of the model is that the different drying/wetting rate of mucilage compared to the bulk soil results in non-equilibrium relations between water content and water potential in the rhizosphere. We coupled this non-equilibrium relation with the Richards equation and obtained a constitutive equation for water flow in soil and mucilage. To test the model assumptions, we measured the water retention curve and the saturated hydraulic conductivity of sandy soil mixed with mucilage from chia seeds. Additionally, we used neutron radiography to image water content in a layer of soil mixed with mucilage during drying and wetting cycles. The radiographs demonstrated the occurrence of non-equilibrium water dynamics in the soil-mucilage mixture. The experiments were simulated by numerically solving the non-equilibrium model. Our study provides conceptual and experimental evidences that mucilage has a strong impact on soil water dynamics. During drying, mucilage maintains a greater soil water content for an extended time, while during irrigation it delays the soil re-wetting. We postulate that mucilage exudation by roots attenuates plant water stress by modulating water content dynamics in the rhizosphere.
- Impact of numerical artifact of the forward model in the inverse solution
of density-dependent flow problem
- Authors: Mohamed K. Nassar; Timothy R. Ginn
Pages: n/a - n/a
Abstract: We investigate the effect of computational error on the inversion of a density-dependent flow and transport model, using SEAWAT and UCODE-2005 in an inverse identification of hydraulic conductivity and dispersivity using head and concentration data from a 2D laboratory experiment. We investigated inversions using three different solution schemes including variation of number of particles and time step length, in terms of the three aspects: the shape and smoothness of the objective function surface, the consequent impacts to the optimization, and the resulting Pareto analyses. This study demonstrates that the inversion is very sensitive to the choice of the forward model solution scheme. In particular, standard finite difference methods provide the smoothest objective function surface; however, this is obtained at the cost of numerical artifacts that can lead to erroneous warping of the objective function surface. Total variation diminishing (TVD) schemes limit these impacts at the cost of more computation time, while the hybrid method of characteristics (HMOC) approach with increased particle numbers and/or reduced time step gives both smoothed and accurate objective function surface. Use of the most accurate methods (TVD and HMOC) did lead to successful inversion of the two parameters; however, with distinct results for Pareto analyses. These results illuminate the sensitivity of the inversion to a number of aspects of the forward solution of the density-driven flow problem, and reveal that parameter values may result that are erroneous but that counteract numerical errors in the solution.
- Issue Information
- Pages: i - vi
- Modeling water demand when households have multiple sources of water
- Authors: Lassina Coulibaly; Paul M. Jakus, John E. Keith
Pages: n/a - n/a
Abstract: A significant portion of the world's population lives in areas where public water delivery systems are unreliable and/or deliver poor quality water. In response, people have developed important alternatives to publicly supplied water. To date, most water demand research has been based on single-equation models for a single source of water, with very few studies that have examined water demand from two sources of water (where all non-public system water sources have been aggregated into a single demand). This modeling approach leads to two outcomes. First, the demand models do not capture the full range of alternatives so the true economic relationship amongst the alternatives is obscured. Second, and more seriously, economic theory predicts that demand for a good becomes more price-elastic as the number of close substitutes increases. If researchers artificially limit the number of alternatives studied to something less than the true number, the price elasticity estimate may be biased downward. This paper examines water demand in a region with near universal access to piped water, but where system reliability and quality is such that many alternative sources of water exist. In extending the demand analysis to four sources of water we are able to (i) demonstrate why households choose the water sources they do, (ii) provide a richer description of the demand relationships among sources, and (iii) calculate own-price elasticity estimates that are more elastic than those generally found in the literature.
- A multimodel regression-sampling algorithm for generating rich monthly
- Authors: Chao Li; Vijay P. Singh
Pages: n/a - n/a
Abstract: This paper presents a multi-model regression-sampling algorithm (MRS) for monthly streamflow simulation. MRS is motivated from the acknowledgment that typical nonparametric models tend to simulate sequences exhibiting too close a resemblance to historical records and parametric models have limitations in capturing complex distributional and dependence characteristics, such as multimodality and nonlinear autocorrelation. The aim of MRS is to generate streamflow sequences with rich scenarios, while properly capturing complex distributional and dependence characteristics. The basic assumptions of MRS include: 1) streamflow of a given month depends on a feature vector consisting of streamflow of the previous month and the dynamic aggregated flow of the past 12 months; and 2) streamflow can be multiplicatively decomposed into a deterministic expectation term and a random residual term. Given a current feature vector, MRS first relates the conditional expectation to the feature vector through an ensemble average of multiple regression models (a total number of 7). To infer the conditional distribution of the residual, MRS adopts the k-nearest neighbor concept. More precisely, the conditional distribution is estimated by a gamma kernel smoothed density of historical residuals inside the k-neighborhood of the given feature vector. Rather than obtaining the residuals from the averaged model only, MRS retains all residuals from all the original regression models. In other words, MRS perceives that the original residuals put together would better represent the covariance structure between streamflow and the feature vector. By doing so, the benefit is that a kernel smoothed density of the residual with reliable accuracy can be estimated, which is hardly possible in a single-model framework. It is the smoothed density that ensures the generation of sequences with rich scenarios unseen in historical record. We evaluated MRS at selected stream gauges and compared with several existing models. Results show that: 1) compared with typical nonparametric models, MRS is more apt at generating sequences with richer scenarios; and 2) in contrast to parametric models, MRS can reproduce complex distributional and dependence characteristics. Since MRS is flexible at incorporating different covariates, it can be tailored for other potential applications, such as hydrologic forecasting, downscaling, as well as post-processing deterministic forecasts into probabilistic ones.
- A novel method for estimating the onset of thermal stratification in lakes
from surface water measurements
- Authors: R. Iestyn Woolway; Stephen C. Maberly, Ian D. Jones, Heidrun Feuchtmayr
Pages: n/a - n/a
Abstract: High-frequency surface water temperature measurements were analyzed for 17 annual data series from seven lakes to assess whether the onset of thermal stratification can be determined from time series of surface water temperature measurements alone. Current methods for estimating the start of thermal stratification require depth-resolved temperature measurements, whereas many existing high-frequency measurements are often limited only to the lake surface. In this study, we show that the magnitude of the diel surface water temperature range can be used to estimate the onset of thermal stratification. We assess the accuracy of using the diel range as an estimate of the onset of thermal stratification by applying two methods based on the calculation of (1) the absolute difference in the diel surface temperature range and (2) the magnitude of the diel range from wavelet analysis. Our study shows that the onset of thermal stratification can be accurately estimated by wavelet analysis with a root mean square error of 2.1 days and by the observed diel temperature range method with a root mean square error of 11.8 days. This approach enables existing, and future, high-resolution surface water data sets to be used to estimate the onset of lake stratification. Furthermore, the continuously increasing observational powers of satellites may eventually result in surface water temperature being measured at a sufficiently high temporal resolution at the spatial scales of small lakes to allow the onset of thermal stratification to be estimated remotely.
- Finite element formulation of unilateral boundary conditions for
unsaturated flow in porous continua
- Authors: A. Abati; C. Callari
Pages: n/a - n/a
Abstract: This paper presents the numerical resolution of unilateral boundary conditions able to effectively model several problems of unsaturated flow, as those involving rainfall infiltration and seepage faces. Besides the penalty technique, we also consider the novel regularization of these conditions by means of the more effective augmented Lagrangian method. The performance of the so-obtained finite element method is carefully investigated in terms of accuracy and ill-conditioning effects, including comparisons with analytical solutions and a complete identification of the analogies with the problem of frictionless contact. In this way, we provide a priori estimates of optimal and admissible ranges for the penalty coefficient as functions of permeability and spatial discretization. The proposed method and the estimated coefficient ranges are validated in further numerical examples, involving the propagation of a wetting front due to rainfall and the partial saturation of an aged concrete dam. These applications show that the proposed regularizations do not induce any detrimental effect on solution accuracy and on convergence rate of the employed Newton-Raphson method. Hence, the present approach should be preferred to the commonly used iterative switching between the imposed-flow and the imposed-pressure conditions, which often leads to spurious oscillations and convergence failures.
- A catalog of moisture sources for continental climatic regions
- Authors: Raquel Nieto; Rodrigo Castillo, Anita Drumond, Luis Gimeno
Pages: n/a - n/a
Abstract: This technical note describes a catalog of moisture sources for two sets of continental climatic regions: one based on regions with similar late 20th century mean climate and similar projected late 21st century precipitation changes, and the other widely used in IPCC assessment reports. By illustrating with one region by classification, the European one was selected and we identify and characterize all the major sources of moisture, and analyze their interannual variability and the role of the three dominant modes of global climate variability, including the El Niño-Southern Oscillation (ENSO) and the Northern and Southern Annular Modes (NAM, SAM). We also estimate the influence of those oceanic regions that will see the greatest increases in evaporation rate in future years.
- Comment on “A blueprint for process-based modeling of uncertain
hydrological systems” by Montanari and Koutsoyiannis
- Authors: Grey Nearing
Pages: n/a - n/a
- Interplay of climate seasonality and soil moisture – Rainfall
- Authors: Jun Yin; Amilcare Porporato, John Albertson
Pages: n/a - n/a
Abstract: The soil moisture-rainfall feedback (SMRF) may significantly impact hydro-climatic dynamics, inducing persistent weather conditions that are responsible for prolonged droughts or abnormally wet states. However, externally driven seasonal variability in rainfall and potential evapotranspiration, with the associated patterns of wet and dry conditions, may both interact with such SMRF. In this study, seasonal variations in radiation and precipitation forcing are included in a stochastic SMRF model with the assumption of a soil moisture-dependent average rainfall frequency to explore their effects on the soil moisture probabilistic structure. The theoretical model results, based on a parameterization using data for soil moisture and climate in Illinois, show that average rainfall frequency peaks in late spring when both the soil condition and the SMRF strength favor convective rainfall triggering. Under such conditions, the soil moisture tends to exhibit bimodal behavior until the SMRF strength becomes weak again towards the end of the growing season. Such behavior is reminiscent of the dynamics of a system undergoing a periodic, stochastically forced pitchfork bifurcation. The presence of bimodal soil moisture behavior is also verified using nonparametric statistical tests on soil moisture data. The analysis of wet-to-wet and dry-to-dry soil moisture transitions in the joint probability distribution of soil moisture further corroborates the presence of hydro-climatic persistence in the spring-to-summer transition.
- Reply to comment by G. Nearing on “A blueprint for process-based
modeling of uncertain hydrological systems”
- Authors: Alberto Montanari; Demetris Koutsoyiannis
Pages: n/a - n/a
- A practical formulation of snow surface diffusion by wind for
- Authors: Noriaki Ohara
Pages: n/a - n/a
Abstract: Prediction of snow drift is of importance for structure design and traffic management on snowy and windy prairie landscapes. The snow redistribution by wind is also regarded as one of the largest sources of error in hydrologic snowmelt models. In this study, a snow movement equation was generalized and customized for horizontal two-dimensional watershed-scale applications by incorporating snow transport, wind snow diffusion, and snow gravitational movement. Then, the snow surface diffusion process by wind turbulence was formulated in terms of the autocorrelation functions of the measurable wind velocity field using G.I. Taylor's theorem. However, analysis of the example wind data suggested a delta correlation in wind turbulent component that resulted from subtracting their moving average values from the original wind speed data. The dynamic model based on the proposed formulation was able to effectively reproduce the observed equilibrium snow profiles affected by wind drifting. A two-dimensional model simulation using a 10 m digital elevation model in Muddy Gap, Wyoming was also presented for qualitative validation of the model in the watershed-scale applications. Additionally, the theoretical extension for preferential snow accumulation process was presented in Appendix . These modeling results together with the observations on the prairie suggested the importance of the snow surface diffusion process in addition to the snow transport.
- Scale-dependent energy conservation and its connection to flow field
instability in porous media
- Authors: M.R. Deinert
Pages: n/a - n/a
Abstract: It has been known for decades that isothermal flow fields in porous media can become unstable, resulting in the growth of preferential flow paths and non-monotonic moisture profiles. The standard approach to modeling isothermal fluid transport in a porous systems is to use Richards equation with equilibrium relationships for the driving potential and monotonic transport coefficients. However, it is well known that under these conditions, solutions to Richards' equation are unconditionally stable. This has left open the question of whether Richards' equation could predict the onset of flow field instability, and what is required to model it. Importantly, past work has shown that pore scale processes can actually cause non-equilibrium driving potentials to arise in unsaturated media. How these can lead to flow field instability can be understood using a form of spectral perturbation theory. Here the driving potential is represented using a Fourier expansion, which is then substituted into Richards equation. The results show that the evolution of perturbations to the flow field are affected by the interaction between different wavelength components in the Fourier expansion. In particular, there are situations where non-equilibrium driving potentials can set up conditions that would allow the onset of instability in solutions to Richards' equation.
- Systematic assessment of the uncertainty in integrated surface
water-groundwater modeling based on the probabilistic collocation method
- Authors: Bin Wu; Yi Zheng, Yong Tian, Xin Wu, Yingying Yao, Feng Han, Jie Liu, Chunmiao Zheng
Pages: n/a - n/a
Abstract: Systematic uncertainty analysis (UA) has rarely been conducted for integrated modeling of surface water-groundwater (SW-GW) systems, which is subject to significant uncertainty, especially at a large basin scale. The main objective of this study was to explore an innovative framework in which a systematic UA can be effectively and efficiently performed for integrated SW-GW models of large river basins, and to illuminate how process understanding, model calibration, data collection and management can benefit from such a systematic UA. The framework is based on the computationally efficient Probabilistic Collocation Method (PCM) linked with a complex simulation model. The applicability and advantages of the framework were evaluated and validated through an integrated SW-GW model for the Zhangye Basin in the middle Heihe River Basin, northwest China. The framework for systematic UA allows for a holistic assessment of the modeling uncertainty, yielding valuable insights into the hydrological processes, model structure, data deficit, and potential effectiveness of management. The study shows that, under the complex SW-GW interactions, the modeling uncertainty has great spatial and temporal variabilities, and is highly output dependent. Overall, this study confirms that a systematic UA should play a critical role in integrated SW-GW modeling of large river basins, and the PCM-based approach is a promising option to fulfill this role.
- Calibration and correction procedures for cosmic-ray neutron soil moisture
probes located across Australia
- Authors: Aaron Hawdon; David McJannet, Jim Wallace
Pages: n/a - n/a
Abstract: The cosmic-ray probe (CRP) provides continuous estimates of soil moisture over an area of ∼30 ha by counting fast neutrons produced from cosmic rays which are predominantly moderated by water molecules in the soil. This paper describes the setup, measurement correction procedures, and field calibration of CRPs at nine locations across Australia with contrasting soil type, climate, and land cover. These probes form the inaugural Australian CRP network, which is known as CosmOz. CRP measurements require neutron count rates to be corrected for effects of atmospheric pressure, water vapor pressure changes, and variations in incoming neutron intensity. We assess the magnitude and importance of these corrections and present standardized approaches for network-wide analysis. In particular, we present a new approach to correct for incoming neutron intensity variations and test its performance against existing procedures used in other studies. Our field calibration results indicate that a generalized calibration function for relating neutron counts to soil moisture is suitable for all soil types, with the possible exception of very sandy soils with low water content. Using multiple calibration data sets, we demonstrate that the generalized calibration function only applies after accounting for persistent sources of hydrogen in the soil profile. Finally, we demonstrate that by following standardized correction procedures and scaling neutron counting rates of all CRPs to a single reference location, differences in calibrations between sites are related to site biomass. This observation provides a means for estimating biomass at a given location or for deriving coefficients for the calibration function in the absence of field calibration data.
- Estimating information entropy for hydrological data: One-dimensional case
- Authors: Wei Gong; Dawen Yang, Hoshin V. Gupta, Grey Nearing
Pages: n/a - n/a
Abstract: There has been a recent resurgence of interest in the application of Information Theory to problems of system identification in the Earth and Environmental Sciences. While the concept of entropy has found increased application, little attention has yet been given to the practical problems of estimating entropy when dealing with the unique characteristics of two commonly used kinds of hydrologic data: rainfall and runoff. In this paper, we discuss four important issues of practical relevance that can bias the computation of entropy if not properly handled. The first (zero effect) arises when precipitation and ephemeral streamflow data must be viewed as arising from a discrete-continuous hybrid distribution due to the occurrence of many zero values (e.g., days with no rain/no runoff). Second, in the widely used bin-counting method for estimation of PDF's, significant error can be introduced if the bin width is not carefully selected. The third (measurement effect) arises due to the fact that continuously varying hydrologic variables can typically only be observed discretely to some degree of precision. The Fourth (skewness effect) arises when the distribution of a variable is significantly skewed. Here we present an approach that can deal with all four of these issues, and test them with artificially generated and real hydrological data. The results indicate that the method is accurate and robust.
- Increasing life expectancy of water resources literature
- Authors: M. Heistermann; T. Francke, C. Georgi, A. Bronstert
Pages: n/a - n/a
Abstract: In a study from 2008, Larivière and colleagues showed, for the field of natural sciences and engineering, that the median age of cited references is increasing over time. This result was considered counterintuitive: with the advent of electronic search engines, online journal issues and open access publications, one could have expected that cited literature is becoming younger. That study has motivated us to take a closer look at the changes in the age distribution of references that have been cited in water resources journals since 1965. Not only could we confirm the findings of Larivière and colleagues. We were also able to show that the aging is mainly happening in the oldest 10–25% of an average reference list. This is consistent with our analysis of top-cited papers in the field of water resources. Rankings based on total citations since 1965 consistently show the dominance of old literature, including text books and research papers in equal shares. For most top-cited old-timers, citations are still growing exponentially. There is strong evidence that most citations are attracted by publications that introduced methods which meanwhile belong to the standard toolset of researchers and practitioners in the field of water resources. Although we think that this trend should not be overinterpreted as a sign of stagnancy, there might be cause for concern regarding how authors select their references. We question the increasing citation of textbook knowledge as it holds the risk that reference lists become overcrowded, and that the readability of papers deteriorates.
- Analytical solutions for flow in porous media with multicomponent cation
- Authors: Ashwin Venkatraman; Marc Hesse, Larry W. Lake, Russell T. Johns
Pages: n/a - n/a
Abstract: Multicomponent cation exchange reactions have important applications in groundwater remediation, disposal of nuclear wastes as well as enhanced oil recovery. The hyperbolic theory of conservation laws can be used to explain the nature of displacements observed during flow with cation exchange reactions between flowing aqueous phase and stationary solid phase. Analytical solutions have been developed to predict the effluent profiles for a particular case of heterovalent cations (Na+, Ca2+ and Mg2+) and an anion (Cl-) for any combination of constant injection and constant initial composition using this theory. We assume local equilibrium, neglect dispersion and model the displacement as a Riemann problem using mass action laws, the charge conservation equation and the cation exchange capacity equation. The theoretical predictions have been compared with experimental data available at two scales - the laboratory scale and the field scale. The theory agrees well with the experimental data at both scales. Analytical theory predictions show good agreement with numerical model, developed using finite differences.
- Linking groundwater use and stress to specific crops using the groundwater
footprint in the Central Valley and High Plains aquifer systems, U.S.
- Authors: Laurent Esnault; Tom Gleeson, Yoshihide Wada, Jens Heinke, Dieter Gerten, Elizabeth Flanary, Marc F. P. Bierkens, Ludovicus P. H. van Beek
Pages: n/a - n/a
Abstract: A number of aquifers worldwide are being depleted, mainly by agricultural activities, yet groundwater stress has not been explicitly linked to specific agricultural crops. Using the newly developed concept of the groundwater footprint (the area required to sustain groundwater use and groundwater-dependent ecosystem services), we develop a methodology to derive crop-specific groundwater footprints. We illustrate this method by calculating high-resolution groundwater footprint estimates of crops in two heavily used aquifer systems: the Central Valley and High Plains, U.S. In both aquifer systems, hay and haylage, corn, and cotton have the largest groundwater footprints, which highlights that most of the groundwater stress is induced by crops meant for cattle feed. Our results are coherent with other studies in the High Plains but suggest lower groundwater stress in the Central Valley, likely due to artificial recharge from surface water diversions which were not taken into account in previous estimates. Uncertainties of recharge and irrigation application efficiency contribute the most to the total relative uncertainty of the groundwater footprint to aquifer area ratios. Our results and methodology will be useful for hydrologists, water resource managers, and policy makers concerned with which crops are causing the well-documented groundwater stress in semiarid to arid agricultural regions around the world.
- Reply to comment by J. S. Selker et al. on “Capabilities and
limitations of tracing spatial temperature patterns by fiber-optic
distributed temperature sensing”
- Authors: S. Krause; L. Rose, N. J. Cassidy
Pages: n/a - n/a
- Groundwater flow dynamics and arsenic source characterization in an
aquifer system of West Bengal, India
- Authors: A. J. Desbarats; C. E. M. Koenig, T. Pal, P. K. Mukherjee, R. D. Beckie
Pages: n/a - n/a
Abstract: Numerical groundwater flow modeling, reverse particle tracking, and environmental tracers are used to locate the source of geogenic As affecting an aquifer in West Bengal. The aquifer is hosted by point-bar sands deposited in a meandering fluvial environment. Wells tapping the aquifer exhibit As concentrations up to 531 μg/L. High-As groundwaters are recharged in ponds marking an abandoned river channel. The source of As is traced to the underlying fine-grained channel-fill sediments. Arsenic release within these sediments is accompanied by a concomitant release of Br and DOC indicating that these species may be decay products of natural organobromines codeposited along with As. Mass transfer of As to the dissolved phase and its flushing from source sediments are described using a simplified reactive solute transport model. Based on this model, a characteristic reaction time for mass transfer is estimated at 6.7 years. Average groundwater residence times in the source are estimated to have declined from 16.6 to 6.6 years with the advent of intensive irrigation pumping. The ratio of residence and reaction times, a Damköhler number, has declined correspondingly from 2.49 to 0.99, indicating a shift from transport to reaction rate limited As mobilization. Greater insight into the As problem in SE Asia may be achieved by shifting the focus of field investigations from aquifers to potential contamination sources in aquitards.
- Navigating financial and supply reliability tradeoffs in regional drought
- Authors: Harrison B. Zeff; Joseph R. Kasprzyk, Jonathan D. Herman, Patrick M. Reed, Gregory W. Characklis
Pages: n/a - n/a
Abstract: Rising development costs and growing concerns over environmental impacts have led many communities to explore more diversified water management strategies. These “portfolio”-style approaches integrate existing supply infrastructure with other options such as conservation measures or water transfers. Diversified water supply portfolios have been shown to reduce the capacity and costs required to meet demand, while also providing greater adaptability to changing hydrologic conditions. However, this additional flexibility can also cause unexpected reductions in revenue (from conservation) or increased costs (from transfers). The resulting financial instability can act as a substantial disincentive to utilities seeking to implement more innovative water management techniques. This study seeks to design portfolios that employ financial tools (e.g., contingency funds and index insurance) to reduce fluctuations in revenues and costs, allowing these strategies to achieve improved performance without sacrificing financial stability. This analysis is applied to the development of coordinated regional supply portfolios in the “Research Triangle” region of North Carolina, an area comprising four rapidly growing municipalities. The actions of each independent utility become interconnected when shared infrastructure is utilized to enable interutility transfers, requiring the evaluation of regional tradeoffs in up to five performance and financial objectives. Diversified strategies introduce significant tradeoffs between achieving reliability goals and introducing burdensome variability in annual revenues and/or costs. Financial mitigation tools can mitigate the impacts of this variability, allowing for an alternative suite of improved solutions. This analysis provides a general template for utilities seeking to navigate the tradeoffs associated with more flexible, portfolio-style management approaches.
- Correlation between groundwater flow and deformation in the fractured
carbonate Gran Sasso aquifer (INFN underground laboratories, central
- Authors: A. Amoruso; L. Crescentini, S. Martino, M. Petitta, M. Tallini
Pages: n/a - n/a
Abstract: The Gran Sasso massif is a carbonate fractured aquifer with a spring discharge of more than 18 m3 s−1. The water table has been partially drained by two motorway tunnels and an underground laboratory (UL), located into the core aquifer. Karst features have limited role below the water table, where groundwater flow is mainly regulated by the fracture network. Two paired laser extensometers (BA and BC) recorded ground deformation in the UL. Changes in deformation correlate with the seasonal recharge/discharge cycle of groundwater flow and its long-term changes. Hydrostatic conditions prevail during the recharge phases because of the low permeability of local fractures, favoring compression, and hydraulic gradient increase above the UL. Fast groundwater flow through the high-permeability fault outcropping in the UL can enhance local dilatation for short periods. Spring discharge during exhaustion periods is fed by the low-permeability fracture network, fostering hydrodynamic conditions by hydraulic gradient decrease, diminishing compression and consequently favoring dilatation. Independent support to this conceptual model comes from local tests and a numerical model which highlights the hydromechanical strain effects induced by the hydrological cycle on the jointed rock mass along BA and the role of the hydraulic gradient on the rock mass deformation.
- Improving process representation in conceptual hydrological model
calibration using climate simulations
- Authors: Marie Minville; Dominique Cartier, Catherine Guay, Louis‐Alexandre Leclaire, Charles Audet, Sébastien Le Digabel, James Merleau
Pages: n/a - n/a
Abstract: Different sets of calibrated model parameters can yield divergent hydrological simulations which in turn can lead to different operational decisions or scientific conclusions. In order to obtain reliable hydrological results, proper calibration is therefore fundamental. This article proposes a new calibration approach for conceptual hydrological models based on the paradigm that hydrological process representation, along with the reproduction of observed streamflows, need to be taken into account when assessing the performance of a hydrological model. Several studies have shown that complementary data can be used to improve hydrological process representation and make hydrological modelling more robust. In the current study, the process of interest is actual evapotranspiration (AET). In order to obtain a more realistic representation of AET, meteorological variables and the AET mean annual cycle simulated by a Regional Climate Model (RCM) driven by reanalyses are used to impose constraints during the optimization procedure. This calibration strategy is compared to a second strategy which relies on AET derived from reference data and to the classical approach based solely on the reproduction of observed discharges. The different methodologies are applied to calibrate the lumped conceptual model HSAMI, used operationally at Hydro‐Québec, for six Canadian snow‐dominated basins with various hydrometeorological and physiographical characteristics.
- A strategy for diagnosing and interpreting hydrological model
- Authors: Seth Westra; Mark Thyer, Michael Leonard, Dmitri Kavetski, Martin Lambert
Pages: n/a - n/a
Abstract: This paper presents a strategy for diagnosing and interpreting hydrological non‐stationarity, aiming to improve hydrological models and their predictive ability under changing hydroclimatic conditions. The strategy consists of four elements: (i) detecting potential systematic errors in the calibration data; (ii) hypothesising a set of “non‐stationary” parameterisations of existing hydrological model structures, where one or more parameters vary in time as functions of selected covariates; (iii) trialing alternative stationary model structures to assess whether parameter non‐stationarity can be reduced by modifying the model structure; and (iv) selecting one or more models for prediction. The Scott Creek catchment in South Australia and the hydrological model GR4J are used to illustrate the strategy. Streamflow predictions improve significantly when the GR4J parameter describing the maximum capacity of the production store is allowed to vary in time as a combined function of: (i) an annual sinusoid; (ii) the previous 365‐day rainfall and potential evapotranspiration; and (iii) a linear trend. This improvement provides strong evidence of model non‐stationarity. Based on a range of hydrologically‐oriented diagnostics such as flow‐duration curves, the GR4J model structure was modified by introducing an additional calibration parameter that controls recession behaviour and by making actual evapotranspiration dependent only on catchment storage. Model comparison using an information‐theoretic measure (the Akaike Information Criterion) and several hydrologically oriented diagnostics shows that the GR4J modifications clearly improve predictive performance in Scott Creek catchment. Based on a comparison of 22 versions of GR4J with different representations of non‐stationarity and other modifications, the model selection approach applied in the exploratory period (used for parameter estimation) correctly identifies models that perform well in a much drier independent confirmatory period.
- Estimation of spatial covariance of log‐conductivity from
- Authors: Monica Riva; Xavier Sanchez‐Vila, Alberto Guadagnini
Pages: n/a - n/a
Abstract: We derive analytical relationships between the spatial covariance of the (natural) logarithm of hydraulic conductivity (K) and that of representative soil particle sizes and porosity. The latter quantities can be directly measured during routine sedimentological analyses of soil samples and provide a way of incorporating K estimates into groundwater flow models at a relatively modest experimental cost. Here, we rely on widely used empirical formulations requiring measurements of representative particle diameters and, in some cases, of medium porosity. We derive exact formulations relating the spatial covariance of these quantities and K and present workable approximations on the basis of perturbation methods. Our formulations provide a direct link between key geostatistical descriptors of sedimentological and hydraulic parameters of heterogeneous aquifers which can be employed in classical estimation and simulation procedures. The approach and theoretical results are tested on an extensive data set comprising 411 particle‐size curves collected at 12 boreholes in a small scale alluvial aquifer.
- What does CloudSat reveal about global land precipitation detection by
other space‐borne sensors?
- Authors: Ali Behrangi; Yudong Tian, Bjorn H. Lambrigtsen, Graeme L. Stephens
Pages: n/a - n/a
Abstract: Current orbital land precipitation products have serious shortcomings in detecting light rain and snowfall, the most frequent types of global precipitation. The missed precipitation is then propagated into the merged precipitation products that are widely used. Precipitation characteristics such as frequency and intensity and their regional distribution are expected to change in a warming climate. It is important to accurately capture those characteristics to understand and model the current state of the Earth's climate and predict future changes. In this work the precipitation detection performance of a suite of precipitation sensors, commonly used in generating the merged precipitation products, are investigated. The high sensitivity of CloudSat Cloud Profiling Radar (CPR) to liquid and frozen hydrometeors enables superior estimates of light rainfall and snowfall within 80oS‐80oN. Three years (2007–2009) of CloudSat precipitation data were collected to construct a climatology reference for guiding our analysis. In addition, auxiliary data such as infrared brightness temperature, surface air temperature, and cloud types were used for a more detailed assessment. The analysis shows that no more than 50 % of the tropical (40oS‐40oN) precipitation occurrence is captured by the current suite of precipitation measuring sensors. Poleward of 50o latitude, a combination of various factors such as an abundance of light rainfall, snowfall, shallow precipitation‐bearing clouds, and frozen surfaces, reduces the space‐based precipitation detection rate to less than 20%. This shows that for a better understanding of precipitation from space, especially at higher latitudes, there is a critical need to improve current precipitation retrieval techniques and sensors.
- Hydrologic dynamics and geochemical responses within a floodplain aquifer
and hyporheic zone during Hurricane Sandy
- Authors: A.H. Sawyer; L.A. Kaplan, O. Lazareva, H.A. Michael
Pages: n/a - n/a
Abstract: Storms dominate solute export budgets from catchments and drive hydrogeochemical changes in the near‐stream environment. We captured near‐stream hydrogeochemical dynamics during an intense storm (Hurricane Sandy, October 2012), by instrumenting a riparian‐hyporheic zone transect of White Clay Creek in the Christina River Basin Critical Zone Observatory with pressure transducers, redox probes, and pore water samplers. In the floodplain aquifer, preferential vertical flow paths such as macropores facilitated rapid infiltration early in the storm. Water table rose quickly and promoted continuous groundwater discharge to the stream. Floodplain‐hillslope topography controlled post‐storm aquifer drainage rates, as the broad, western floodplain aquifer drained more slowly than the narrow, eastern floodplain aquifer adjacent to a steep hillslope. These changes in groundwater flow drove heterogeneous geochemical responses in the floodplain aquifer and hyporheic zone. Vertical infiltration in the floodplain and hyporheic exchange in the streambed increased DOC and oxygen delivery to microbially active sediments, which may have enhanced respiration. Resulting geochemical perturbations persisted from days to weeks after the storm. Our observations suggest that groundwater‐borne solute delivery to streams during storms depends on unique interactions of vertical infiltration along preferential pathways, perturbations to groundwater geochemistry, and topographically controlled drainage rates.
- Jointly deriving NMR surface relaxivity and pore size distributions by NMR
relaxation experiments on partially desaturated rocks
- Authors: O. Mohnke; B. Hughes
Pages: n/a - n/a
Abstract: Nuclear magnetic resonance (NMR) relaxometry is a geophysical method widely used in borehole and laboratory applications to non‐destructively infer transport and storage properties of rocks and soils as it is directly sensitive to the water/oil content and pore sizes. However, for inferring pore sizes NMR relaxometry data needs to be calibrated with respect to a surface interaction parameter, surface relaxivity, which depends on the type and mineral constituents of the investigated rock. This study introduces an inexpensive and quick alternative to the classical calibration methods, e.g. mercury injection, pulsed field gradient (PFG) NMR or grain size analysis, which allows for jointly estimating NMR surface relaxivity and pore size distributions using NMR relaxometry data from partially de‐saturated rocks. Hereby, NMR relaxation experiments are performed on the fully saturated sample and on a sample partially drained at a known differential pressure. Based on these data the (capillary) pore radius distribution and surface relaxivity are derived by joint optimization of the Brownstein‐Tarr and the Young‐Laplace equation assuming parallel capillaries. Moreover, the resulting pore size distributions can be used to predict water retention curves. This inverse modeling approach – tested and validated using NMR relaxometry data measured on synthetic porous borosilicate samples with known petrophysical properties (i.e. permeability, porosity, inner surfaces, pore size distributions) – yields consistent and reproducible estimates of surface relaxivity and pore radii distributions. Also, subsequently calculated water retention curves generally correlate well with measured water retention curves.
- Reactive transport controls on sandy acid sulfate soils and impacts on
shallow groundwater quality
- Authors: S.U. Salmon; Andrew W. Rate, Zed Rengel, Steven Appleyard, Henning Prommer, Christoph Hinz
Pages: n/a - n/a
Abstract: Disturbance or drainage of potential acid sulfate soils (PASS) can result in the release of acidity and degradation of infrastructure, water resources, and the environment. Soil processes affecting shallow groundwater quality have been investigated using a numerical code that integrates (bio)geochemical processes with water, solute, and gas transport. The patterns of severe and persistent acidification (pH
- Comment on “Capabilities and limitations of tracing spatial
temperature patterns by fiber‐optic distributed temperature
sensing” by L. Rose, S. Krause, and N.J. Cassidy
- Authors: J.S. Selker; Scott Tyler, Nick vandeGiesen
Pages: n/a - n/a
- Insights into the physical processes controlling correlations between snow
distribution and terrain properties
- Authors: Brian T. Anderson; James P. McNamara, Hans-Peter Marshall, Alejandro N. Flores
Pages: 4545 - 4563
Abstract: This study investigates causes behind correlations between snow and terrain properties in a 27 km2 mountain watershed. Whereas terrain correlations reveal where snow resides, the physical processes responsible for correlations can be ambiguous. We conducted biweekly snow surveys at small transect scales to provide insight into late-season correlations at the basin scale. The evolving parameters of transect variograms reveal the interplay between differential accumulation and differential ablation that is responsible for correlations between snow and terrain properties including elevation, aspect, and canopy density. Elevation-induced differential accumulation imposes a persistent source of varariabity at the basin scale, but is not sufficient to explain the elevational distribution of snow water equivalent (SWE) on the ground. Differential ablation, with earlier and more frequent ablation at lower elevations, steepens the SWE-elevation gradient through the season. Correlations with aspect are primarily controlled by differences in solar loading. Aspect related redistribution of precipitation by wind, however, is important early in the season. Forested sites hold more snow than nonforested sites at the basin scale due to differences in ablation processes, while open areas within forested sites hold more snow than covered areas due to interception. However, as the season progresses energetic differences between open and covered areas within forested sites cause differences induced by interception to diminish. Results of this study can help determine which accumulation and ablation processes must be represented explicitly and which can be parameterized in models of snow dynamics.
- Bayesian hierarchical approach and geophysical data sets for estimation of
reactive facies over plume scales
- Authors: Haruko M. Wainwright; Jinsong Chen, Douglas S. Sassen, Susan S. Hubbard
Pages: 4564 - 4584
Abstract: A stochastic model is developed to integrate multiscale geophysical and point data sets for characterizing coupled subsurface physiochemical properties over plume-relevant scales, which is desired for parameterizing reactive transport models. We utilize the concept of reactive facies, which is based on the hypothesis that subsurface units can be identified that have distinct reactive-transport-property distributions. To estimate and spatially distribute reactive facies and their associated properties over plume-relevant scales, we need to (1) document the physiochemical controls on plume behavior and the correspondence between geochemical, hydrogeological, and geophysical measurements; and (2) integrate multisource, multiscale data sets in a consistent manner. To tackle these cross-scale challenges, we develop a hierarchical Bayesian model to jointly invert various wellbore and geophysical data sets that have different resolutions and spatial coverage. We use Markov-chain Monte-Carlo sampling methods to draw many samples from the joint posterior distribution and subsequently estimate the marginal posterior distribution of reactive-facies field and their associated reactive transport properties. Synthetic studies demonstrate that our method can successfully integrate different types of data sets. We tested the framework using the data sets collected at the uranium-contaminated Savannah River Site F-Area, including wellbore lithology, cone penetrometer testing, and crosshole and surface seismic data. Results show that the method can estimate the spatial distribution of reactive facies and their associated reactive-transport properties along a 300 m plume centerline traverse with high resolution (1.2 m by 0.305 m).
- Scaling properties of tidal networks
- Authors: Mirian Jiménez; Sonia Castanedo, Zeng Zhou, Giovanni Coco, Raúl Medina, Ignacio Rodriguez-Iturbe
Pages: 4585 - 4602
Abstract: A new methodology is developed to extract tidal network from hydrodynamic conditions, and use data derived from numerical modeling or field observations to test the hypothesis that tidal networks are characterized by scale-invariant properties. Different tidal network configurations have been obtained from long-term numerical simulations in an idealized basin. These simulations show the influence of hydrodynamic conditions (tidal range, TR) and sediment (grain size sediment, D50) on the final configuration of the network. One of the signatures of scale-invariant behavior is related to the presence of a power law relationship in the probability distribution of geometrical characteristics. For each model configuration and field site, the probability distribution of drainage area and the drainage volume has been calculated, and in both cases tidal networks show scale-invariant characteristics. After assessing the sensitivity of the results, an energy expenditure analysis shows that tidal basins evolve toward a state with less morphodynamic activity, with a lower energy expenditure compare with the initial state.
- Lateral inflows, stream-groundwater exchange, and network geometry
influence stream water composition
- Authors: John Mallard; Brian McGlynn, Tim Covino
Pages: 4603 - 4623
Abstract: The role of stream networks and their hydrologic interaction with hillslopes and shallow groundwater in modifying and transporting watershed signals is an area of active research. One of the primary ways that stream networks can modify watershed signals is through spatially variable stream gains and losses, described herein as hydrologic turnover. We measured hydrologic gain and loss at the reach scale using tracer experiments throughout the Bull Trout watershed in the Sawtooth Mountains of Idaho. We extended the results of reach scale experiments to the stream network using empirical relationships between (1) watershed area and stream discharge and (2) stream discharge and percent stream water loss to the groundwater system. We thus incorporate linkages between (1) hillslopes and stream networks via lateral inflows and (2) stream networks and shallow groundwater via hydrologic exchange. We implemented these relationships within a concise analytical framework to simulate hydrologic turnover across stream networks and estimate the variable influence exerted by upstream reaches and streamflow source locations on stream water composition across stream networks. Application to six natural Sawtooth watersheds and seven synthetic watersheds with varying topographic structure and stream network geometry indicated that contributions to discharge from any upstream source depend on the magnitude of the initial input, but also on the distribution of hydrologic turnover occurring along the stream network. The evolution of stream water source compositions along stream networks was unique in each watershed due to the combination of watershed structure and stream network geometry. Our results suggest that a distributed representation of hydrologic turnover at the stream network scale can improve understanding of how the stream network can modify source water compositions along the stream.
- Application of multiobjective optimization to scheduling capacity
expansion of urban water resource systems
- Authors: Mohammad Mortazavi-Naeini; George Kuczera, Lijie Cui
Pages: 4624 - 4642
Abstract: Significant population increase in urban areas is likely to result in a deterioration of drought security and level of service provided by urban water resource systems. One way to cope with this is to optimally schedule the expansion of system resources. However, the high capital costs and environmental impacts associated with expanding or building major water infrastructure warrant the investigation of scheduling system operational options such as reservoir operating rules, demand reduction policies, and drought contingency plans, as a way of delaying or avoiding the expansion of water supply infrastructure. Traditionally, minimizing cost has been considered the primary objective in scheduling capacity expansion problems. In this paper, we consider some of the drawbacks of this approach. It is shown that there is no guarantee that the social burden of coping with drought emergencies is shared equitably across planning stages. In addition, it is shown that previous approaches do not adequately exploit the benefits of joint optimization of operational and infrastructure options and do not adequately address the need for the high level of drought security expected for urban systems. To address these shortcomings, a new multiobjective optimization approach to scheduling capacity expansion in an urban water resource system is presented and illustrated in a case study involving the bulk water supply system for Canberra. The results show that the multiobjective approach can address the temporal equity issue of sharing the burden of drought emergencies and that joint optimization of operational and infrastructure options can provide solutions superior to those just involving infrastructure options.
- Hydraulic and thermal effects of in-stream structure-induced hyporheic
exchange across a range of hydraulic conductivities
- Authors: Garrett T. Menichino; Erich T. Hester
Pages: 4643 - 4661
Abstract: In-stream structure-induced hyporheic exchange and associated thermal dynamics affect stream ecosystems. Their importance is controlled by spatial variability of sediment hydraulic conductivity (K). We calibrated a computational fluid dynamics (CFD) model of surface and groundwater hydraulics near a channel-spanning weir (represents log dams, boulder weirs) to field data and varied K from 10−7 to 10−2 m/s (silt to gravel). Surface water stopped cresting the weir for K > 10−3 m/s. Non-Darcy hyporheic flow was also prevalent for K> 10−3 m/s, and velocity errors using non-CFD models ranged up to 32.2%. We also modeled weir-induced heat transport during summer. As K increased from 10−7 to 10−3 m/s, weir-induced hyporheic heat advection steadily increased. Cooling and buffering along hyporheic flow paths decreased with increasing K, particularly above K = 10−5 and 10−4 m/s, respectively. Vertical heat conduction between surface water and groundwater near the weir decreased with increasing K, particularly for K > 10−5 m/s. Conduction between hyporheic flow paths and adjacent groundwater helped cool hyporheic flow. Downstream surface water cooling by hyporheic advection increased steadily with K as increases in hyporheic flow overwhelmed decreases in cooling along hyporheic flow paths. Yet such effects were small (0.016°C) even at K = 10−3 m/s. The largest thermal effect of weir-induced exchange was therefore spatial expansion of subsurface diel variability (particularly for K > 10−5 m/s) which affects benthic habitat and chemical reactions. The specific values of K where such trend shifts occur is likely variable among streams based on flow conditions, but we expect the presence of such trend shifts to be widespread.
- A mechanistic modeling and data assimilation framework for Mojave Desert
- Authors: Gene-Hua Crystal Ng; David R. Bedford, David M. Miller
Pages: 4662 - 4685
Abstract: This study demonstrates and addresses challenges in coupled ecohydrological modeling in deserts, which arise due to unique plant adaptations, marginal growing conditions, slow net primary production rates, and highly variable rainfall. We consider model uncertainty from both structural and parameter errors and present a mechanistic model for the shrub Larrea tridentata (creosote bush) under conditions found in the Mojave National Preserve in southeastern California (USA). Desert-specific plant and soil features are incorporated into the CLM-CN model by Oleson et al. (2010). We then develop a data assimilation framework using the ensemble Kalman filter (EnKF) to estimate model parameters based on soil moisture and leaf-area index observations. A new implementation procedure, the “multisite loop EnKF,” tackles parameter estimation difficulties found to affect desert ecohydrological applications. Specifically, the procedure iterates through data from various observation sites to alleviate adverse filter impacts from non-Gaussianity in small desert vegetation state values. It also readjusts inconsistent parameters and states through a model spin-up step that accounts for longer dynamical time scales due to infrequent rainfall in deserts. Observation error variance inflation may also be needed to help prevent divergence of estimates from true values. Synthetic test results highlight the importance of adequate observations for reducing model uncertainty, which can be achieved through data quality or quantity.
- Individual and coupled influences of AMO and ENSO on regional
precipitation characteristics and extremes
- Authors: Aneesh Goly; Ramesh S. V. Teegavarapu
Pages: 4686 - 4709
Abstract: Understanding the influences of Atlantic multidecadal oscillation (AMO) and El Niño southern oscillation (ENSO) on regional precipitation extremes and characteristics in the state of Florida is the focus of this study. Exhaustive evaluations of individual and combined influences of these oscillations using, descriptive indices-based assessment of statistically significant changes in rainfall characteristics, identification of spatially varying influences of oscillations on dry and wet spell transition states, antecedent precipitation prior to extreme events, intraevent temporal distribution of precipitation and changes in temporal occurrences of extremes including dry/wet cycles are carried out. Rain gage and gridded precipitation data analysis using parametric hypothesis tests confirm statistically significant changes in the precipitation characteristics from one phase to another of each oscillation and also in coupled phases. Spatially nonuniform and uniform influences of AMO and ENSO, respectively, on precipitation are evident. AMO influences vary in peninsular and continental parts of Florida and the warm (cool) phase of AMO contributes to increased precipitation extremes during wet (dry) season. The influence of ENSO is confined to dry season with El Niño (La Niña) contributing to increase (decrease) in extremes and total precipitation. Wetter antecedent conditions preceding daily extremes are dominant in AMO warm phase compared to the cool and are likely to impact design floods in the region. AMO influence on dry season precipitation extremes is noted for ENSO neutral years. The two oscillations in different phases modulate each other with seasonal and spatially varying impacts and implications on flood control and water supply in the region.
- Inference of permeability heterogeneity from joint inversion of transient
flow and temperature data
- Authors: Zhishuai Zhang; Behnam Jafarpour, Lianlin Li
Pages: 4710 - 4725
Abstract: Characterization of the rock permeability distribution in compartmentalized deep aquifers, enhanced geothermal systems, and hydrocarbon reservoirs is important for predicting the flow and transport behavior in these formations. Reliable prediction of the fluid flow and transport processes can, in turn, lead to effective development of the subsurface energy and environmental resources. In deep formations where thermal gradients are significant, the transient temperature data can provide valuable information about the permeability distribution with depth and about the vertical fluid displacement. This paper examines the importance of temperature data in resolving the distribution of permeability with depth by jointly, and individually, integrating the transient temperature and flow data. We demonstrate that when estimating permeability distributions in deep geothermal reservoirs, incorporating temperature data can increase the resolution of the permeability distribution profile with depth. To illustrate the importance of temperature measurements, we adopt a coupled transient heat and fluid flow as a forward model to predict the heat and fluid transport in a geothermal reservoir and develop an adjoint model for efficient computation of the gradient information for model calibration. We perform a series of numerical experiments for integration of flow and pressure data alone, temperature data alone, and flow and pressure jointly with temperature data. In each case, we apply the maximum A-posteriori (MAP) method and the randomized maximum likelihood (RML) method for inversion and uncertainty quantification. Analysis of the sensitivity of temperature and production data to heterogeneous permeability distributions reveals that the temperature of fluid, even when measured at the surface, is sensitive to the permeability distribution in the vertical extent of the reservoir. Hence, temperature measurements can be augmented with flow-related data to enhance the resolution of the estimated permeability field with depth.
- Simulating water markets with transaction costs
- Authors: Tohid Erfani; Olga Binions, Julien J. Harou
Pages: 4726 - 4745
Abstract: This paper presents an optimization model to simulate short-term pair-wise spot-market trading of surface water abstraction licenses (water rights). The approach uses a node-arc multicommodity formulation that tracks individual supplier-receiver transactions in a water resource network. This enables accounting for transaction costs between individual buyer-seller pairs and abstractor-specific rules and behaviors using constraints. Trades are driven by economic demand curves that represent each abstractor's time-varying water demand. The purpose of the proposed model is to assess potential hydrologic and economic outcomes of water markets and aid policy makers in designing water market regulations. The model is applied to the Great Ouse River basin in Eastern England. The model assesses the potential weekly water trades and abstractions that could occur in a normal and a dry year. Four sectors (public water supply, energy, agriculture, and industrial) are included in the 94 active licensed water diversions. Each license's unique environmental restrictions are represented and weekly economic water demand curves are estimated. Rules encoded as constraints represent current water management realities and plausible stakeholder-informed water market behaviors. Results show buyers favor sellers who can supply large volumes to minimize transactions. The energy plant cooling and agricultural licenses, often restricted from obtaining water at times when it generates benefits, benefit most from trades. Assumptions and model limitations are discussed.
- Revealed and stated preference valuation and transfer: A within-sample
comparison of water quality improvement values
- Authors: Silvia Ferrini; Marije Schaafsma, Ian Bateman
Pages: 4746 - 4759
Abstract: Benefit transfer (BT) methods are becoming increasingly important for environmental policy, but the empirical findings regarding transfer validity are mixed. A novel valuation survey was designed to obtain both stated preference (SP) and revealed preference (RP) data concerning river water quality values from a large sample of households. Both dichotomous choice and payment card contingent valuation (CV) and travel cost (TC) data were collected. Resulting valuations were directly compared and used for BT analyses using both unit value and function transfer approaches. WTP estimates are found to pass the convergence validity test. BT results show that the CV data produce lower transfer errors, below 20% for both unit value and function transfer, than TC data especially when using function transfer. Further, comparison of WTP estimates suggests that in all cases, differences between methods are larger than differences between study areas. Results show that when multiple studies are available, using welfare estimates from the same area but based on a different method consistently results in larger errors than transfers across space keeping the method constant.
- Hydrology and pore water chemistry in a permafrost wetland, Ilulissat,
- Authors: Søren Jessen; Hanne D. Holmslykke, Kristine Rasmussen, Niels Richardt, Peter E. Holm
Pages: 4760 - 4774
Abstract: Hydrological and geochemical processes controlling the pore water chemistry in a permafrost wetland, with loam overlain by sphagnum peat, were investigated. The vertical distributions of dissolved Cl, and of pore water δ18O, appeared unrelated to ion freeze-out and isotope ice-water fractionation processes, respectively, dismissing solute freeze-out as a main control on the water chemistry. However, concentrations of major ions, others than Cl, generally increased with depth into the active layer. A conceptual model for water and solute movement in the active layer was derived. The model indicates upward diffusive transport of elements, released in the loam layer by mineral weathering, to the peat layer, in which lateral advective transport dominates. Active layer pore water and water of melted core sections of permafrost were of Ca-Mg-HCO3 type (1:1:4 stoichiometry) and were subsaturated for calcite and dolomite. The results are consistent with an annual cycling of inorganic carbon species, Ca and Mg, via cryogenic carbonate precipitation during fall freeze-up and their redissolution following spring thaw. Similarly, elevated Fe2+ concentrations appear to be related to cryogenic siderite formation. Pore water in the active layer showed high partial pressures of CO2, indicating the feasibility of bubble ebullition as a greenhouse gas emission pathway from permafrost wetlands. Elevated concentrations of geogenic trace elements (Ni, Al, and As) were observed, and the controlling geochemical processes are discussed. The conceptual model for water and solute movement was applied to quantify the contribution of released trace elements to a downstream lake in the permafrost catchment.
- Transient pore pressure response to confining stress excursions in Berea
sandstone flooded with an aqueous solution of CO2
- Authors: Jackson B. Crews; Clay A. Cooper
Pages: 4775 - 4786
Abstract: We measured the pore pressure response due to carbon dioxide (CO2) gas bubble nucleation and growth in a Berea sandstone core flooded with an initially subsaturated aqueous solution of CO2, in response to a rapid drop in confining stress, under conditions representative of a confined aquifer. A portion of the CO2 in the Earth's crust, derived from volcanic, magmatic, and biogenic sources, dissolves in groundwater. Sudden reductions in confining stress in the Earth's crust occur due to dilational strain generated by the propagation of seismic Rayleigh and P waves, or aseismic slip in the near field of earthquakes. A drop in confining stress produces a proportional drop in pore fluid pressure. When the pore fluid contains dissolved CO2, the pore pressure responds to a drop in confining stress like it does in the dissolved gas-free case, until the pore pressure falls below the bubble pressure. Gas bubble nucleation and diffusive growth in the pore space trigger spontaneous, transient buildup of the pore fluid pressure, and reduction of effective stress. We measured the rate of pore fluid pressure buildup in the 100 s immediately following the confining stress drop, as a function of the saturation with respect to CO2 at the lowest pore pressure realized during the confining stress drop, using five different CO2 partial pressures. The rate scales with the saturation with respect to dissolved CO2, from 10 kPa/min at 1.25 to 166 kPa/min at 1.8. The net pore pressure rise was as large as 0.7 MPa (100 psi) over 5 h.
- The concept of field capacity revisited: Defining intrinsic static and
dynamic criteria for soil internal drainage dynamics
- Authors: Shmuel Assouline; Dani Or
Pages: 4787 - 4802
Abstract: Across many soil types and conditions, post wetting soil internal drainage exhibits predictable dynamics that lead to a stable and repeatable hydration state termed “field capacity” (FC). Soil regulation of internal drainage toward FC has long been recognized as producing a useful hydrologic benchmark for modeling and for estimation of plant available soil water. To overcome ambiguities and inconsistencies in various ad hoc definitions of FC, we propose using a soil intrinsic characteristic length (a matric potential value derived from drainable soil pore size distribution) to characterize the loss of hydraulic continuity associated with the attainment of FC. The resulting static criterion for FC was extended to formulate a self-consistent dynamic criterion based on soil internal drainage dynamics. A systematic evaluation of the proposed definitions of FC using numerical simulations and experimental data reveals remarkable consistency and predictability across a wide range of soil types. The new metrics add definitiveness and robustness of this widely used concept with potential expansion to additional agronomic, hydrologic, ecological, and climatic applications.
- Controls on sediment production from an unpaved resource road in a Pacific
- Authors: H. J. van Meerveld; E.J. Baird, W. C. Floyd
Pages: 4803 - 4820
Abstract: Unpaved resource roads have the potential to produce large amounts of sediment and can negatively impact water quality and aquatic ecology. In order to better understand the dominant controls on sediment generation from unpaved resource roads, we did 23 large-scale rainfall simulation experiments on a road section in the Honna Watershed, Haida Gwaii, British Columbia, Canada. The experiments were performed with different rainfall intensities (4–52 mm/h), with and without traffic. Precipitation intensity was the dominant control on the amount of sediment generated from the road surface; the total mass of sediment increased linearly with precipitation intensity. The number of passages of loaded logging trucks during an experiment was the second most dominant control on the total amount of sediment generated from the road surface. Elevated sediment concentrations in road surface runoff persisted for 30 min following the passage of loaded logging trucks during low intensity (
- Deep conduit flow in karst aquifers revisited
- Authors: Georg Kaufmann; Franci Gabrovšek, Douchko Romanov
Pages: 4821 - 4836
Abstract: Caves formed in soluble rocks such as limestone, anhydrite, or gypsum are efficient drainage paths for water moving through the aquifer from the surface of the host rock toward a resurgence. The formation of caves is controlled by the physical solution through dissociation of the host rock by water or by the chemical solution through reactions of the host rock with water enriched with carbon dioxide. Caves as large underground voids are simply the end-member of secondary porosity and conductivity characterizing the aquifer. Caves and their relation to a present or past base level are found both close to a past or present water table (water table caves) and extending far below a past or present water table (bathy-phreatic caves). One explanation for this different speleogenetic evolution is the structural control: fractures and bedding partings are preferentially enlarged around more prominent faults, thus the fracture density in the host rock controls the speleogenetic evolution. This widely accepted explanation can be extended by adding other controls, e.g., a hydraulic control: as temperature generally increases with depth, density and viscosity of water change, and particularly the reduction of viscosity due to the increase in temperature enhances flow. This hypothesis was proposed by Worthington (2001, 2004) as a major controlling factor for the evolution of deep bathy-phreatic caves. We compare the efficiency of structural and hydraulic control on the evolution of a cave passage by numerical means, adding a third control, the chemical control to address the change in solubility of the circulating water with depth. Our results show that the increase in flow through deep bathy-phreatic passages due to the decrease in viscosity is by far outweighted by effects such as the decrease in fracture width with depth due to lithostatic stress and the decrease in solubility with depth. Hence, the existence of deep bathy-phreatic cave passages is more likely to be controlled by the structural effect of prominent faults.
- Spatiotemporal relations between water budget components and soil water
content in a forested tributary catchment
- Authors: Alexander Graf; Heye R. Bogena, Clemens Drüe, Horst Hardelauf, Thomas Pütz, Günther Heinemann, Harry Vereecken
Pages: 4837 - 4857
Abstract: We examined 3 years of measured daily values of all major water budget components (precipitation P, potential evapotranspiration PET, actual evapotranspiration ET, and runoff R) and volumetric soil water content θ of a small, forested catchment located in the west of Germany. The spatial distribution of θ was determined from a wireless sensor network of 109 points with 3 measurement depths each; ET was calculated from eddy-covariance tower measurements. The water budget was dominantly energy limited, with ET amounting to approximately 90% of PET, and a runoff ratio R/P of 56%. P, ET, and R closed the long-term water budget with a residual of 2% of precipitation. On the daily time scale, the residual of the water budget was larger than on the annual time scale, and explained to a moderate extent by θ (R2 = 0.40). Wavelet analysis revealed subweekly time scales, presumably dominated by unaccounted fast-turnover storage terms such as interception, as a major source of uncertainty in water balance closure. At weekly resolution, soil water content explained more than half (R2 = 0.62) of the residual. By means of combined empirical orthogonal function and cluster analysis, two slightly different spatial patterns of θ could be identified that were associated with mean θ values below and above 0.35 cm3/cm3, respectively. The timing of these patterns as well as the varying coherence between PET, ET, and soil water content responded to changes in water availability, including a moderate response to the European drought in spring 2011.
- Managing flow, sediment, and hydropower regimes in the Sre Pok, Se San,
and Se Kong Rivers of the Mekong basin
- Authors: Thomas B. Wild; Daniel P. Loucks
Pages: 5141 - 5157
Abstract: The Lancang/Mekong River Basin is presently undergoing a period of rapid hydropower development. In its natural undeveloped state, the river transports about 160 million metric tons of sediment per year, maintaining the geomorphologic features of the basin, sustaining habitats, and transporting the nutrients that support ecosystem productivity. Despite the importance of sediment in the river, currently little attention is being paid to reservoir sediment trapping. This study is devoted to assessing the potential for managing sediment and its impact on energy production in the Se San, Sre Pok, and Se Kong tributaries of the Mekong River. These tributaries drain a set of adjacent watersheds that are important with respect to biodiversity and ecological productivity, and serve as a significant source of flow and sediment to the mainstream Mekong River. A daily sediment transport model is used to assess tradeoffs among energy production and sediment and flow regime alteration in multiple reservoir systems. This study finds that eventually about 40%–80% of the annual suspended sediment load may be trapped in reservoirs. Clearly, these reservoirs will affect the rivers' sediment regimes. However, even after 100 years of simulated sedimentation, reservoir storage capacities and hydropower production at most reservoir sites are not significantly reduced. This suggests that the strongest motivation for implementing measures to reduce trapped sediment is their impact not on hydropower production but on fish migration and survival and on sediment-dependent ecosystems such as the Vietnam Delta and Cambodia's Tonle Sap Lake.
- Dams on the Mekong: Cumulative sediment starvation
- Authors: G. M. Kondolf; Z. K. Rubin, J. T. Minear
Pages: 5158 - 5169
Abstract: The Mekong River, largely undeveloped prior to 1990, is undergoing rapid dam construction. Seven dams are under construction on the mainstem in China and 133 proposed for the Lower Mekong River and tributaries. We delineated nine distinct geomorphic regions, for which we estimated sediment yields based on geomorphic characteristics, tectonic history, and the limited sediment transport data available. We then applied the 3W model to calculate cumulative sediment trapping by these dams, accounting for changing trap efficiency over time and multiple dams on a single river system. Under a “definite future” scenario of 38 dams (built or under construction), cumulative sediment reduction to the Delta would be 51%. Under full build-out of all planned dams, cumulative sediment trapping will be 96%. That is, once in-channel stored sediment is exhausted, only 4% of the predam sediment load would be expected to reach the Delta. This scenario would have profound consequences on productivity of the river and persistence of the Delta landform itself, and suggests that strategies to pass sediment through/around dams should be explored to prevent the consequences of downstream sediment starvation.
- A dynamical system perspective on plant hydraulic failure
- Authors: Stefano Manzoni; Gabriel Katul, Amilcare Porporato
Pages: 5170 - 5183
Abstract: Photosynthesis is governed by leaf water status that depends on the difference between the rates of transpiration and water supply from the soil and through the plant xylem. When transpiration increases compared to water supply, the leaf water potential reaches a more negative equilibrium, leading to water stress. Both high atmospheric vapor pressure deficit and low soil moisture increase the water demand while decreasing the supply due to lowered soil-to-root conductance and xylem cavitation. Therefore, dry conditions may eventually reduce the leaf water potential to the point of collapsing the plant hydraulic system. This “hydraulic failure” is shown to correspond to a fold bifurcation where the environmental parameters (vapor pressure deficit and soil moisture) trigger the loss of a physiologically sustainable equilibrium. Using a minimal plant hydraulic model, coordination among plant hydraulic traits is shown to result in increased resilience to environmental stresses, thereby impeding hydraulic failure unless hydraulic traits deteriorate due to prolonged water shortage or other damages.
- Moisture profiles of the upper soil layer during evaporation monitored by
- Authors: Steffen Merz; Andreas Pohlmeier, Jan Vanderborght, Dagmar van Dusschoten, Harry Vereecken
Pages: 5184 - 5195
Abstract: Near-surface soil moisture profiles contain important information about the evaporation process from a bare soil. In this study, we demonstrated that such profiles could be monitored noninvasively and with high spatial resolution using Nuclear Magnetic Resonance (NMR). Soil moisture profiles were measured in a column exposed to evaporation for a period of 67 days using a stationary Magnetic Resonance Imaging (MRI) high field scanner and a unilateral NMR sensor. The column was packed with medium sand and initially saturated. Two distinct shapes of soil moisture profiles that are characteristic for stage I (evaporation rate is controlled by atmospheric demand) and stage II (evaporation rate is controlled by the porous medium) of the evaporation process were followed by both MRI and unilateral NMR. During stage I, an approximately uniform decrease of soil moisture over time was monitored, whereas during stage II, S-shaped moisture profiles developed which receded progressively into the soil column. These promising results and the specific design of the unilateral NMR system make it very well suited for determining soil moisture profiles in the field.
- Effect of low-permeability layers on spatial patterns of hyporheic
exchange and groundwater upwelling
- Authors: Jesus D. Gomez-Velez; Stefan Krause, John L. Wilson
Pages: 5196 - 5215
Abstract: Bed form-induced hyporheic interactions are characterized by a nested system of flow paths that continuously exchange water, solutes, momentum, and energy. At the local scale, sediment heterogeneity plays a key role in the hydrodynamics and potential for biogeochemical transformations within the hyporheic zone. This manuscript explores the role of low-permeability sedimentary layers on the interplay between bed form-induced hyporheic exchange and groundwater upwelling. A hydrodynamic conceptualization that sequentially couples fully-turbulent flow in the water column and Darcian flow in the sediment is used. Low-permeability layers are characterized by long residence times and solute accumulation. Furthermore, these layers induce hydrodynamic sequestration due to the relocation and, in some cases, emergence of new stagnation zones. Spatial patterns of residence time distributions and flushing intensities indicate that the interface of the low-permeability layers has the potential to be a hot spot for biogeochemical transformations and flow acceleration near such interface can increase the mobilization capacity for the products of redox chemical and microbial processes. A discussion about the possible implications that hydrodynamic changes have on the biogeochemistry of hyporheic zones is presented; however, further biogeochemical experimentation and modeling are needed to validate these arguments.
- Seasonal snowpack characteristics influence soil temperature and water
content at multiple scales in interior western U.S. mountain ecosystems
- Authors: Gregory E. Maurer; David R. Bowling
Pages: 5216 - 5234
Abstract: Mountain snowpacks directly and indirectly influence soil temperature (Tsoil) and soil water content (θ). Vegetation, soil organisms, and associated biogeochemical processes certainly respond to snowpack-related variability in the soil biophysical environment, but there is currently a poor understanding of how snow-soil interactions vary in time and across the mountain landscape. Using data from a network of automated snowpack monitoring stations in the interior western U.S., we quantified seasonal and landscape patterns in Tsoil and θ, and their dependence on snowpack characteristics over an eleven year period. Elevation gradients in Tsoil were absent beneath winter snowpacks, despite large gradients in air temperature (Tair). Winter Tsoil was warmer and less variable than Tair, but interannual and across-site variations in Tsoil were likely large enough to impact biogeochemical processes. Winter θ varied between years and across sites, but during a given winter at a site it changed little between the start of snowpack accumulation and the initiation of spring snowmelt. Winter Tsoil and θ were both higher when early winter snow accumulation was greater. Summer θ was lower when summer Tair was high. Depending on the site and the year examined, summer θ was higher when there was greater summer precipitation, a larger snowpack, later snowpack melt, or a combination of these factors. We found that snowpack-related variability in the soil environment was of sufficient magnitude to influence biogeochemical processes in snow-dominated ecosystems.
- Field testing of the universal calibration function for determination of
soil moisture with cosmic-ray neutrons
- Authors: David McJannet; Trenton Franz, Aaron Hawdon, Dave Boadle, Brett Baker, Auro Almeida, Richard Silberstein, Trish Lambert, Darin Desilets
Pages: 5235 - 5248
Abstract: The semitheoretical universal calibration function (UCF) for estimating soil moisture using cosmic-ray neutron sensors was tested by comparing to field measurements made with the same neutron detector across a range of climates, soil, latitude, altitude, and biomass. There was a strong correlation between neutron intensity and the total amount of hydrogen at each site; however, the relationship differed from that predicted by the UCF. A linear fit to field measurements explained 99% of the observed variation and provides a robust empirical means to estimate soil moisture at other sites. It was concluded that measurement errors, neutron count corrections, and scaling to remove altitudinal and geomagnetic differences were unlikely to explain differences between observations and the UCF. The differences may be attributable to the representation of organic carbon, biomass or detector geometry in the neutron particle code, or to differences in the neutron energy levels being measured by the cosmic-ray sensor and modeled using the particle code. The UCF was derived using simulations of epithermal neutrons; however, lower energy thermal neutrons may also be important. Using neutron transport code, we show the differences in response of thermal and epithermal neutrons to the relative size of the hydrogen pool. Including a thermal neutron component in addition to epithermal neutrons in a modified UCF provided a better match to field measurements; however, thermal neutron measurements are needed to confirm these results. A simpler generalized relationship for estimating soil moisture from neutron counts was also tested with encouraging results for low biomass sites.
- A 2000 year natural record of magnitudes and frequencies for the largest
Upper Colorado River floods near Moab, Utah
- Authors: Noam Greenbaum; Tessa M. Harden, Victor R. Baker, John Weisheit, Michael L. Cline, Naomi Porat, Rafi Halevi, John Dohrenwend
Pages: 5249 - 5269
Abstract: Using well-established procedures for paleoflood hydrology and employing optically stimulated luminescence (OSL) geochronology, we analyzed a very well-preserved natural record of 44 Upper Colorado River extreme floods with discharges ranging from 1800 to 9200 m3s−1. These are the largest floods occurring during the last 2140 ± 220 years, and this natural record indicates that large floods are much more frequent than can be estimated by extrapolation from the stream gaging record that extends back to 1914. Most of these large floods occurred during the last 500 years, and the two largest floods in the record both exceeded the probable maximum flood (PMF) estimated at 8500 m3s−1 (300,000 cfs) for nearby Moab, Utah. Another four floods, with discharges greater than 7000 m3s−1, occurred during the last two millennia. Flood frequency analyses using the FLDFRQ3 model yields the following values, depending on the Manning n roughness coefficients: 100 yr flood—4670–4990 m3s−1; 500 yr flood—6675–7270 m3s−1; 1000 yr flood—7680–8440 m3s−1. The presumed PMF discharge (8500 m3s−1) gets assigned a recurrence interval of about 1000 years, and the largest historical 1884 flood (3540 m3s−1)—a recurrence interval of
- Seasonal soil moisture patterns: Controlling transit time distributions in
a forested headwater catchment
- Authors: Michael Paul Stockinger; Heye Reemt Bogena, Andreas Lücke, Bernd Diekkrüger, Markus Weiler, Harry Vereecken
Pages: 5270 - 5289
Abstract: The Transit Time Distribution (TTD) of a catchment is frequently used for understanding flow paths, storage characteristics, and runoff sources. Despite previous studies, the connections between catchment characteristics and TTDs are still not fully understood. We present results from a 2 year stable isotope tracer investigation in the forested Wüstebach headwater catchment (38.5 ha), including precipitation, stream, and tributary locations. We used the gauged outlet to determine effective precipitation (peff), subdivided for wet and dry catchment state, and assumed it to be spatially uniform. We then calculated TTDs of 14 ungauged stream and tributary locations where stable isotope tracer information was available and compared them to respective subcatchment areas and the proportion of riparian zone within the subcatchments. Our approach gave insight into the spatial heterogeneity of TTDs along the Wüstebach River. We found that hydrological hillslope-riparian zone disconnection was an important factor, as the catchment shifted between two distinct, time-variant hydrological responses that were governed by seasonal changes of overall catchment wetness. The difference in hydrological behavior of the riparian zone and hillslopes could explain the often encountered “old water phenomenon,” where considerable amounts of old water quickly appear as runoff. TTD results showed a negative correlation between riparian zone proportion and Mean Transit Time (MTT), corroborated by the dense network of soil water content measurements. No correlation between subcatchment size and MTT was found.
- Capillary pressure overshoot for unstable wetting fronts is explained by
Hoffman's velocity-dependent contact-angle relationship
- Authors: Christine E. Baver; J.-Yves Parlange, Cathelijne R. Stoof, David A. DiCarlo, Rony Wallach, Deanna S. Durnford, Tammo S. Steenhuis
Pages: 5290 - 5297
Abstract: Pore velocity-dependent dynamic contact angles provide a mechanism for explaining the formation of fingers/columns in porous media. To study those dynamic contact angles when gravity is present, rectangular capillary tubes were used to facilitate observation of the complete interface without geometric distortion. Results show that the Hoffman (1975) relationship between dynamic contact angle and water velocity applies to gravity-affected flow fields, and that it (when adjusted for nonzero static contact angles) can be used to model dynamic capillary pressures for unstable wettings fronts in porous media by assuming that (1) pressure at the wetting front is discontinuous, (2) the flow field behind the fingertip is highly heterogeneous, and (3) the front line advances one or a few pores at the time. We demonstrate the utility of the Hoffman relationship for porous media with a published infiltration experiment by calculating the capillary pressure successfully at the unstable wetting front as a function of the flux of water in the finger and the grain size diameter.
- Correcting artifacts in transition to a wound optic fiber: Example from
high-resolution temperature profiling in the Dead Sea
- Authors: Ali Arnon; John Selker, Nadav Lensky
Pages: 5329 - 5333
Abstract: Spatial resolution fiber-optic cables allow for detailed observation of thermally complex heterogeneous hydrologic systems. A commercially produced high spatial resolution helically wound optic fiber sensing cable is employed in the Dead Sea, in order to study the dynamics of thermal stratification of the hypersaline lake. Structured spatial artifacts were found in the data from the first 10 m of cable (110 m of fiber length) following the transition from straight fiber optic. The Stokes and Anti-Stokes signals indicate that this is the result of differential attenuation, thought to be due to cladding losses. Though the overall spatial form of the loss was consistent, the fine structure of the loss changed significantly in time, and was strongly asymmetrical, and thus was not amenable to standard calibration methods. Employing the fact that the cable was built with a duplex construction, and using high-precision sensors mounted along the cable, it was possible to correct the artifact in space and time, while retaining the high-quality of data obtained in the early part of the cable (prior to significant optical attenuation). The defect could easily be overlooked; however, reanalyzing earlier experiments, we have observed the same issue with installations employing similar cables in Oregon and France, so with this note we both alert the community to this persistent concern and provide an approach to correct the data in case of similar problems.
- Debates—The future of hydrological sciences: A (common) path
- Authors: Alberto Montanari
Pages: 5334 - 5334
- Debates—The future of hydrological sciences: A (common) path
forward' One water. One world. Many climes. Many souls
- Authors: Upmanu Lall
Pages: 5335 - 5341
- Debates—The future of hydrological sciences: A (common) path
forward' A call to action aimed at understanding velocities,
celerities and residence time distributions of the headwater hydrograph
- Authors: Jeffrey J. McDonnell; Keith Beven
Pages: 5342 - 5350
Abstract: Celerity is faster than velocity
Watershed models are evaluated only against celerity
Velocity must be addressed in future watershed models
- Debates—the future of hydrological sciences: A (common) path
forward' Using models and data to learn: A systems theoretic
perspective on the future of hydrological science
- Authors: Hoshin V. Gupta; Grey S. Nearing
Pages: 5351 - 5359
Abstract: Discovery can be advanced by taking a perspective based in Information Theory
Much can be gained by focusing on the a priori role of Process Modeling
System Parameterization can result in information loss
- Comment on “Spatial and temporal phosphorus distribution changes in
a large wetland ecosystem” by X. Zapata-Rios et al.
- Authors: John M. Juston; William F. DeBusk, Michael J. Jerauld, Thomas A. DeBusk
Pages: 5360 - 5366
- Reply to comment by John M. Juston et al. on “Spatial and temporal
phosphorus distribution changes in a large wetland ecosystem”
- Authors: Xavier Zapata-Rios; Rosanna G. Rivero, Ghinwa M. Naja, Pierre Goovaerts
Pages: 5367 - 5371