ALBERT

Description: We use dissolved silicon together with its “geochemical twin” germanium for the first time as a hydrologic tracer to study water delivery to the stream during storm events in the Rio Icacos watershed, Puerto Rico. Ge and Si were measured on base flow, stormflow, springwater, and soil water samples. Compositions of all of these waters appear to reflect varying contributions from three components, which we attribute to solutes released from bedrock weathering (groundwater), from short-term soil-water interaction (quick soil water), and longer-term soil-water interaction (matrix soil water). Base flow stream waters have high Si and moderate Ge (Ge/Si ratio ∼0.29 μmol/mol), consistent with a predominantly bedrock weathering source as indicated by their similarity with water sampled from springs emerging from the saprolite-bedrock boundary on a hillslope landslide scar. During storm events there is a shift toward more dilute compositions (but higher Ge/Si ratios) similar to those measured on water samples from temporary depression storage and overland flow (quick soil water). Geochemical mass balance shows that 80%–90% of the stream chemistry can be explained by mixing groundwater with this quick soil water composition, which we infer to reflect new water traveling as shallow throughflow. Stream water δ18O values decrease to more negative values typical of precipitation supporting rapid delivery of rainwater to the stream channel during stormflow. The third component, with a Ge-rich composition characteristic of soil matrix water sampled by tension lysimeters, is required to explain higher stream water Ge/Si ratios measured during hydrograph recession. We infer from this an additional, slower, and less dominant pathway for delivery of soil water to the stream channel.

Description: We present a predictive, multiscale modeling framework for chemotaxis in porous media. This model results from volume averaging the governing equations for bacterial transport at the microscale and is expressed in terms of effective medium coefficients that are predicted from the solution of the associated closure problems. As a result, the averaged chemotactic velocity is an explicit function of the attractant concentration field and diffusivity, rather than an empirical effective chemotactic sensitivity coefficient. The model was validated by comparing the transverse bacterial concentration profiles with experimental measurements for Escherichia coli HCB1 in a T-sensor. The averaged chemotactic velocity predicted by the model was found to be within the range of values reported in the literature. Reasonable agreement (approximately 10% mean absolute error) between theory and experiments was found for several flow rates. In order to assess the potential for decreasing the computational demands of the model, the macroscale domain was divided into subdomains for the coupling of bacterial transport to that of the attractant. Sensitivity analysis was performed regarding the number of subdomains chosen, and the results indicate that bacterial transport (as measured by concentration profiles) was not highly affected by this choice. Overall, these results suggest that the predictive, multiscale modeling framework is reliable for modeling chemotaxis in porous media when chemotactic transport is significant compared to convective transport.

Description: Water temperature determines the spatial distribution of fish species, including cold-water fish such as trout, and is driven by the balance of the heat flux across the water surface and the heat flux across the sediment surface. In this study, a modified equilibrium temperature model was developed for cold-water streams that includes the effect of groundwater inflow. The modified equilibrium temperature model gives estimates of daily average stream temperature based on climate conditions, riparian shading, stream width, and groundwater input rate and temperature. For a small tributary stream with relatively uniform riparian shading, the modified equilibrium temperature was found to be a good predictor of daily average stream temperature, with a root-mean-square errors (RMSE) of 1.2°C. The modified equilibrium temperature model also gave good estimates (1.4°C RMSE) of daily average stream temperature for a larger stream when riparian shading was averaged over sufficiently long distances. A sensitivity analysis using the modified equilibrium temperature model confirmed that water temperature in cold-water streams varies strongly with riparian shading, stream width, and both groundwater inflow rate and temperature. These groundwater parameters therefore need to be taken into account when climate change impacts on stream temperature are projected. The stream temperature model developed in this study is a useful tool to characterize temperature conditions in cold-water streams with different levels of riparian shading and groundwater inputs and to assess the impact of future land use and climate change on temperature in these streams.

Description: Perfect or even mediocre weather predictions over a long period are almost impossible because of the ultimate growth of a small initial error into a significant one. Even though the sensitivity of initial conditions limits the predictability in chaotic systems, an ensemble of prediction from different possible initial conditions and also a prediction algorithm capable of resolving the fine structure of the chaotic attractor can reduce the prediction uncertainty to some extent. All of the traditional chaotic prediction methods in hydrology are based on single optimum initial condition local models which can model the sudden divergence of the trajectories with different local functions. Conceptually, global models are ineffective in modeling the highly unstable structure of the chaotic attractor. This paper focuses on an ensemble prediction approach by reconstructing the phase space using different combinations of chaotic parameters, i.e., embedding dimension and delay time to quantify the uncertainty in initial conditions. The ensemble approach is implemented through a local learning wavelet network model with a global feed-forward neural network structure for the phase space prediction of chaotic streamflow series. Quantification of uncertainties in future predictions are done by creating an ensemble of predictions with wavelet network using a range of plausible embedding dimensions and delay times. The ensemble approach is proved to be 50% more efficient than the single prediction for both local approximation and wavelet network approaches. The wavelet network approach has proved to be 30%–50% more superior to the local approximation approach. Compared to the traditional local approximation approach with single initial condition, the total predictive uncertainty in the streamflow is reduced when modeled with ensemble wavelet networks for different lead times. Localization property of wavelets, utilizing different dilation and translation parameters, helps in capturing most of the statistical properties of the observed data. The need for taking into account all plausible initial conditions and also bringing together the characteristics of both local and global approaches to model the unstable yet ordered chaotic attractor of a hydrologic series is clearly demonstrated.

Description: The focus in the search for more reliable predictions in ungauged basins (PUB) has generally been on reducing uncertainty in watershed models (mainly their parameters). More recently, however, we seem to remember that the ultimate objective is not to define the parameters of a specific model but to understand the watershed: What behavior do we expect the ungauged watershed to exhibit? And what behavior should not occur in a particular ungauged watershed? The answers to these questions actually provide additional information that can be assimilated in watershed models for uncertainty reduction in PUB. This extension to hydrologic modeling approaches provides a quantitative link between watershed modeling and statistical hydrology as well as process hydrology that has to be explored. We witness a convergence of approaches—Bayesian, set theoretic, and optimization based—toward utilizing this link. The result is an opportunity for the (quantitative) dialog between modelers, statistical hydrologists, and experimentalists. We close our discussion of this development by presenting new and exciting research questions that we now have to address.

Description: Concept development simulation with distributed, physics-based models provides a quantitative approach for investigating runoff generation processes across environmental conditions. Disparities within data sets employed to design and parameterize boundary value problems used in heuristic simulation inevitably introduce various levels of bias. The objective was to evaluate the impact of boundary value problem complexity on process representation for different runoff generation mechanisms. The comprehensive physics-based hydrologic response model InHM has been employed to generate base case simulations for four well-characterized catchments. The C3 and CB catchments are located within steep, forested environments dominated by subsurface stormflow; the TW and R5 catchments are located in gently sloping rangeland environments dominated by Dunne and Horton overland flows. Observational details are well captured within all four of the base case simulations, but the characterization of soil depth, permeability, rainfall intensity, and evapotranspiration differs for each. These differences are investigated through the conversion of each base case into a reduced case scenario, all sharing the same level of complexity. Evaluation of how individual boundary value problem characteristics impact simulated runoff generation processes is facilitated by quantitative analysis of integrated and distributed responses at high spatial and temporal resolution. Generally, the base case reduction causes moderate changes in discharge and runoff patterns, with the dominant process remaining unchanged. Moderate differences between the base and reduced cases highlight the importance of detailed field observations for parameterizing and evaluating physics-based models. Overall, similarities between the base and reduced cases indicate that the simpler boundary value problems may be useful for concept development simulation to investigate fundamental controls on the spectrum of runoff generation mechanisms.

Description: Analytical solutions are obtained for optimization formulations that minimize energy used for groundwater pumping. The formulations choose pumping rates at groundwater wells while insuring that total pumpage meets a specified demand. Such formulations might be appropriate for an urban water supply or a large−scale agricultural irrigation system. Solutions are found by applying stationarity conditions. The solutions produce simple and physically meaningful requirements on drawdowns at each well. Under certain conditions, pumping rates are optimal when the sum of the nonpumping lift and two times the drawdown at each pumping well takes a constant value across the domain. The results are examined for steady and transient conditions. The results are based on only a few assumptions on the modeled system: the response of drawdown with head is linear, and all pumping activity occurs during the same time periods. Implications of these results for well field operation are suggested.

Description: Water pricing schedules often contain significant nonlinearities, such as the increasing block tariff (IBT) structure that is abundantly applied for residential users. The IBT is frequently supported as a good tool for achieving the goals of equity, water conservation, and revenue neutrality but seldom has been grounded on efficiency justifications. In particular, existing literature on water pricing establishes that although efficient schedules will depend on demand and supply characteristics, IBT cannot usually be recommended. In this paper, we consider whether the explicit inclusion of scarcity considerations can strengthen the appeal of IBT. Results show that when both demand and costs react to climate factors, increasing marginal prices may come about as a response to a combination of water scarcity and customer heterogeneity. We derive testable conditions and then illustrate their application through an estimation of Portuguese residential water demand. We show that the recommended tariff schedule hinges crucially on the choice of functional form for demand.

Description: In situ laser diffractometers characterize the suspended particle size distribution (PSD) by measuring laser-generated light scattered off small particles over a range of small forward angles. In environments with low particulate concentrations or high ambient light conditions the ratio of natural downwelling sunlight to scattered laser light sensed by the photodetectors is high and measurements are influenced. Here, we evaluate the effect of the ambient light field intensity on measurements made with a Laser In Situ Scattering and Transmissometry (LISST) 100X type B instrument. Paired light-dark scattering distributions are recorded over a range of underwater light intensities in high-turbidity and low-turbidity water. Light measurements displayed large erroneous concentrations of particles in the smallest size bin (1.25–1.48 μm) and showed effects over the full range of the PSD. Ambient light was found to exhibit the same constant distribution over the instrument photodetectors in both water samples, although the magnitude of the response, in laser counts per unit ambient light intensity, was PSD dependent. A technique for postprocessing data to remove the influence of light is presented for moored deployment and vertical profile data collected at Lake Tahoe, California-Nevada, United States. While measurements removed of the light effect were successfully reconstructed, the technique may not be applicable to data where the PSD or the LISST orientation relative to the sun direction change rapidly or when light intensities are high enough to quench the instrument photodetectors. Ambient light was found to have negligible effects on PSD measurements in Lake Tahoe was below intensities of ∼30 W m−2.

Description: Spatial and temporal trends in stream chemistry were investigated in a large (1600 km2) alpine watershed in the southern Rocky Mountains of Colorado to help understand mechanisms of streamflow generation. We observed linear increases of concentrations of chemical constituents in streamflow as accumulated drainage area increased along the main channel of Saguache Creek. We tested two conceptual models of streamflow generation against our stream chemistry observations. One model is essentially two-dimensional and treats streamflow generation at the large watershed scale as the aggregation of runoff responses from individual hillslopes, primarily surface and shallow subsurface flow paths. Alternatively, a fully three-dimensional conceptual model treats streamflow generation as being controlled by a distribution of large-scale groundwater flow paths as well as surface and shallow subsurface flow paths. The structure and magnitude of groundwater contributions in streamflow as a function of increasing scale provided a key distinction between these two conceptual models. End-member mixing analysis and measurements of hydraulic head gradients in streambeds were used to quantify basin-scale groundwater contributions to streamflow with increasing spatial scale in the Saguache Creek watershed. Our data show that groundwater contributions are important in streamflow generation at all scales and, more importantly, that groundwater contributions to streamflow do increase with increasing watershed scale. These results favor the three-dimensional conceptual model in which long groundwater flow paths provide a streamflow generation process at large scales that is not operative at smaller scales. This finding indicates that large watersheds may be more than simply the aggregation of hillslopes and small catchments.

Description: Microbial biodiversity in groundwater and soil presents a unique opportunity for improving characterization and monitoring at sites with multiple contaminants, yet few computational methods use or incorporate these data because of their high dimensionality and variability. We present a systematic, nonparametric decision-making methodology to help characterize a water quality gradient in leachate-contaminated groundwater using only microbiological data for input. The data-driven methodology is based on clustering a set of molecular genetic-based microbial community profiles. Microbes were sampled from groundwater monitoring wells located within and around an aquifer contaminated with landfill leachate. We modified a self-organizing map (SOM) to weight the input variables by their relative importance and provide statistical guidance for classifying sample similarities. The methodology includes the following steps: (1) preprocessing the microbial data into a smaller number of independent variables using principal component analysis, (2) clustering the resulting principal component (PC) scores using a modified SOM capable of weighting the input PC scores by the percent variance explained by each score, and (3) using a nonparametric statistic to guide selection of appropriate groupings for management purposes. In this landfill leachate application, the weighted SOM assembles the microbial community data from monitoring wells into groupings believed to represent a gradient of site contamination that could aid in characterization and long-term monitoring decisions. Groupings based solely on microbial classifications are consistent with classifications of water quality from hydrochemical information. These microbial community profile data and improved decision-making strategy compliment traditional chemical groundwater analyses for delineating spatial zones of groundwater contamination.

Description: Legal scholars and jurists have identified several criteria (e.g., hydrology, climate, population, and historical water use) to guide equitable allocation of transboundary rivers among riparian claimants. Are these criteria used in practice, such that a quantitative pattern emerges from actual water-sharing agreements regarding factors affecting allocations? To address this, we study interstate compacts, the principal mechanism for allocating the waters of transboundary rivers within the United States. We develop a georeferenced data set and construct variables representing conditions in state-based watersheds of 14 rivers at the times of compact ratification. A state's water allocation share of a compact serves as the dependent variable, and a set of explanatory variables is derived from legal and political theories. We estimate allocation shares using both ordinary least squares (OLS) and bootstrap regressions, and we apply two alternative specifications of the factors affecting compact allocations, one with and one without political variables. Estimated coefficients on variables for land area, population, prior water use, riparian position, and Congressional committee chair are statistically significant in the OLS regressions. The preferred OLS specification, which includes political variables, provides a good fit (R2 = 0.84). We also find that OLS and bootstrap regressions have a similar ability to predict state allocation shares. We discuss how the results could be used as a reference point in negotiations over new compacts or international river treaties and as a basis to identify existing compacts with statistical outliers.

Description: Transverse mixing of solutes in steady state transport is of utmost importance for assessing mixing-controlled reactions of compounds that are continuously introduced into the subsurface. Classical spatial moments analysis fails to describe mixing because the tortuous streamlines in heterogeneous formations cause plume meandering, squeezing, and stretching, which affect transverse spatial moments even if there is no mass transfer perpendicular to the direction of flow. For transverse solute mixing, however, the decisive process is the exchange of solute mass between adjacent stream tubes. We therefore reformulate the advection-dispersion equation in streamline coordinates (i.e., in terms of the potential and the stream function values) and analyze how flux-related second central moments of plumes increase with dropping hydraulic potential. We compare the ensemble behavior of these second central moments in random two-dimensional heterogeneous flow fields with the moments in an equivalent homogeneous system, thus defining an equivalent effective transverse dispersion coefficient. Unlike transverse macrodispersion coefficients derived by traditional moment analysis, our mixing-relevant, flux-related coefficient does not increase with travel distance. We present closed-form solutions for the mean enhancement of transverse mixing by heterogeneity in two-dimensional isotropic media for linear laws of local-scale transverse dispersion. The mixing enhancement factor increases with the log conductivity variance but remains fairly low. We also evaluate the variance of our cumulative measure of transverse mixing, showing that heterogeneity causes substantial uncertainty of mixing. The analytical expressions are compared to numerical Monte Carlo simulations for various values of log conductivity variance, indicating good agreement with the analytical results at low variability. In the numerical simulations, we also consider nonlinear models of local-scale transverse dispersion.

Description: Hydrologic modelers often need to know which method of quantitative precipitation estimation (QPE) is best suited for a particular catchment. Traditionally, QPE methods are verified and benchmarked against independent rain gauge observations. However, the lack of spatial representativeness limits the value of such a procedure. Alternatively, one could drive a hydrological model with different QPE products and choose the one which best reproduces observed runoff. Unfortunately, the calibration of conceptual model parameters might conceal actual differences between the QPEs. To avoid such effects, we abandoned the idea of determining optimum parameter sets for all QPE being compared. Instead, we carry out a large number of runoff simulations, confronting each QPE with a common set of random parameters. By evaluating the goodness-of-fit of all simulations, we obtain information on whether the quality of competing QPE methods is significantly different. This knowledge is inferred exactly at the scale of interest—the catchment scale. We use synthetic data to investigate the ability of this procedure to distinguish a truly superior QPE from an inferior one. We find that the procedure is prone to failure in the case of linear systems. However, we show evidence that in realistic (nonlinear) settings, the method can provide useful results even in the presence of moderate errors in model structure and streamflow observations. In a real-world case study on a small mountainous catchment, we demonstrate the ability of the verification procedure to reveal additional insights as compared to a conventional cross validation approach.

Description: I address a range of topics that provide the sociopolitical-technological setting for my professional life. I discuss some influential features of post–World War II world geopolitics, landmark technological developments of that era, and the resulting follow-up technologies that have made it possible to approach various problems in hydrology and water resources. I next address societal needs that have driven developments in hydrology and water resources engineering and follow with a discussion of the modern foundations of our science and what I think are the principal issues in hydrology. I pose three community challenges that when accomplished should advance hydrologic science: data network needs for improving the water budgets at all scales, characterizing subsurface water flow paths, and the information archiving and mining needs from instruments that will generate substantially richer data detail than have been used for most hydrologic work to the present. I then discuss several hydrologic and water resource risk-based decision issues that matter to society to illustrate how such risks have been addressed successfully in the past. I conclude with a long-term community “grand challenge,” the coupled modeling of the ocean-atmosphere-landform hydrologic cycle for the purpose of long–lead time hydrologic prediction.

Description: The likely effects of climate change on the water resources of the eastern Mediterranean and Middle East region are investigated using a high-resolution regional climate model (PRECIS) by comparing precipitation simulations of 2040–2069 and 2070–2099 with 1961–1990. The simulations show about a 10% decline in precipitation across the region by both the middle and the end of the century, with considerable variation between countries and international river basins. Results suggest that per capita water resources will not change particularly significantly in southeastern Europe, where they are relatively plentiful and population growth is minimal. However, in much of the Middle East, climate change coupled with population growth is likely to reduce per capita water resources considerably. This will inevitably result in major social, economic, and environmental change in the region. Countries where the required adaptation is likely to be particularly challenging include Turkey and Syria because of the large agricultural workforces, Iraq because of the magnitude of the change and its downstream location, and Jordan because of its meager per capita water resources coupled with limited options for desalination. If the internal water footprint of the region declines in line with precipitation but the total water footprint of the region increases in line with population, then by midcentury, as much as half the total water needs of the region may need to be provided through desalination and imported in the form of virtual water.

Description: We review the human actions, proximal stressors and ecological responses for floodplain forests Australia's largest river system—the Murray-Darling Basin. A conceptual model for the floodplain forests was built from extensive published information and some unpublished results for the system, which should provide a basis for understanding, studying and managing the ecology of floodplains that face similar environmental stresses. Since European settlement, lowlands areas of the basin have been extensively cleared for agriculture and remnant forests heavily harvested for timber. The most significant human intervention is modification of river flows, and the reduction in frequency, duration and timing of flooding, which are compounded by climate change (higher temperatures and reduced rainfall) and deteriorating groundwater conditions (depth and salinity). This has created unfavorable conditions for all life-history stages of the dominant floodplain tree (Eucalyptus camaldulensis Dehnh.). Lack of extensive flooding has led to widespread dieback across the Murray River floodplain (currently 79% by area). Management for timber resources has altered the structure of these forests from one dominated by large, widely spreading trees to mixed-aged stands of smaller pole trees. Reductions in numbers of birds and other vertebrates followed the decline in habitat quality (hollow-bearing trees, fallen timber). Restoration of these forests is dependent on substantial increases in the frequency and extent of flooding, improvements in groundwater conditions, re-establishing a diversity of forest structures, removal of grazing and consideration of these interacting stressors.

Description: There is currently a distinct gap between what climate science can provide and information that is practically useful for (and needed by) natural resource managers. Improved understanding, and model representations, of interactions between the various climate drivers (both regional and global scale), combined with increased knowledge about the interactions between climate processes and hydrological processes at the regional scale, is necessary for improved attribution of climate change impacts, forecasting at a range of temporal scales and extreme event risk profiling (e.g., flood, drought, and bushfire). It is clear that the science has a long way to go in closing these research gaps; however, in the meantime water resource managers in the Murray-Darling Basin, and elsewhere, require hydroclimatic projections (i.e., seasonal to multidecadal future scenarios) that are regionally specific and, importantly, take into account the impacts, and associated uncertainties, of both natural climate variability and anthropogenic change. The strengths and weaknesses of various approaches for supplying this information are discussed in this paper.

Description: We use the Budyko framework to calculate catchment-scale evapotranspiration (E) and runoff (Q) as a function of two climatic factors, precipitation (P) and evaporative demand (Eo = 0.75 times the pan evaporation rate), and a third parameter that encodes the catchment properties (n) and modifies how P is partitioned between E and Q. This simple theory accurately predicted the long-term evapotranspiration (E) and runoff (Q) for the Murray-Darling Basin (MDB) in southeast Australia. We extend the theory by developing a simple and novel analytical expression for the effects on E and Q of small perturbations in P, Eo, and n. The theory predicts that a 10% change in P, with all else constant, would result in a 26% change in Q in the MDB. Future climate scenarios (2070–2099) derived using Intergovernmental Panel on Climate Change AR4 climate model output highlight the diversity of projections for P (±30%) with a correspondingly large range in projections for Q (±80%) in the MDB. We conclude with a qualitative description about the impact of changes in catchment properties on water availability and focus on the interaction between vegetation change, increasing atmospheric [CO2], and fire frequency. We conclude that the modern version of the Budyko framework is a useful tool for making simple and transparent estimates of changes in water availability.

Description: A partially penetrating well of length Lw and radius Rw starts to pump at constant discharge Qw at t = 0 from an unconfined aquifer of thickness D. The aquifer is of random and stationary conductivity characterized by KG (geometric mean), σY2 (log conductivity variance), and I and Iv (the horizontal and vertical integral scales). The flow problem is solved under a few simplifying assumptions commonly adopted in the literature for homogeneous media: Rw/Lw $\ll$ 1, linearization of the free surface condition, and constant drainable porosity n. Additionally, it is assumed that Rw/I 〈 1 and Lw/Iv $\gg$ 1 (to simplify the well boundary conditions) and that a first-order approximation in σY2 (extended to finite σY2 on a conjectural basis) is adopted. The solution is obtained for the mean head field $\langle$H(R, z, t)$\rangle$ and the associated water table equation. The main result of the analysis is that the flow domain can be divided into three zones for $\langle$H$\rangle$: (1) the neighborhood of the well R $\ll$ I, where $\langle$H$\rangle$ = (Qw/LwKA)h0(R, z, tKefuv/nD), with h0 being the zero-order solution pertaining to a homogeneous and isotropic aquifer, KA being the conductivity arithmetic mean, and Kefuv being the effective vertical conductivity in mean uniform flow, (2) an exterior zone R ⪆ I in which $\langle$H$\rangle$ = (Qw/LwKefuh)h0(R$\sqrt{K_{efuv}/K_{efuh}}$, z, tKefuv/nD), with Kefuh being the horizontal effective conductivity, and (3) an intermediate zone in which the solution requires a few numerical quadratures, not carried out here. The application to pumping tests reveals that identification of the aquifer parameters for homogeneous and anisotropic aquifers by commonly used methods can be applied for the drawdown measured in an observation well of length Low $\gg$ Iv (to ensure exchange of space and ensemble head averages) in the second zone in order to identify Kefuh, Kefuv, and n. In contrast, the use of the drawdown in the well (first zone) leads to an overestimation of Kefuh by the factor KA/Kefuh.

Description: Calcium (Ca) has declined to levels threatening aquatic biota in lakes on the eastern Canadian Shield. Predictive models for future changes in lake Ca are generally based on catchment-scale studies, but these models rarely account for unmeasured sources of Ca supply that are common in the nearshore areas of developed lakes. In this study we utilize up to 29 years of hydrological and water chemistry data for three lakes in central Ontario that differ in degree of human intervention to demonstrate that shoreline development may exert large effects on Ca mass balances. In the relative absence of shoreline development, Red Chalk Lake exhibited what we consider to be the normal response, a reduction in Ca load from the catchment over the last three decades, leading to a reduction in lake export and lake Ca concentration. Calcium load, export, and lake water Ca concentration also fell in Harp Lake, but less than in Red Chalk Lake, because Ca loads were elevated by human activities in Harp Lake's moderately developed shoreline area. By contrast, Dickie Lake experienced an exceptional change in Ca dynamics: both export and lake concentrations rose because of elevated load from the shoreline area linked to the use of dust suppressants on gravel roads. Reductions in both stream Ca concentration and flow volume have led to calcium decline in streams and lakes. Long-term soil acidification processes and climatic variability with its link to hydrology can explain the general pattern of Ca decline in lakes on the south-central Canadian Shield. However, given the widespread lakeshore development and use of dust suppressants on gravel roads, predictions of lake Ca levels need to take into account nearshore activities, especially those that augment rates of Ca supply.

Description: Accurate description of the soil water retention curve (SWRC) at low water contents is important for simulating water dynamics and biochemical vadose zone processes in arid environments. Soil water retention data corresponding to matric potentials of less than −10 MPa, where adsorptive forces dominate over capillary forces, have also been used to estimate soil specific surface area (SA). In the present study, the dry end of the SWRC was measured with a chilled-mirror dew point psychrometer for 41 Danish soils covering a wide range of clay (CL) and organic carbon (OC) contents. The 41 soils were classified into four groups on the basis of the Dexter number (n = CL/OC), and the Tuller-Or (TO) general scaling model describing water film thickness at a given matric potential ( 10. A strong correlation between the ratio of the two surface area estimates and the Dexter number was observed and applied as an additional scaling function in the TO model to rescale the soil water retention curve at low water contents. However, the TO model still overestimated water film thickness at potentials approaching ovendry condition (about −800 MPa). The semi–log linear Campbell-Shiozawa-Rossi-Nimmo (CSRN) model showed better fits for all investigated soils from −10 to −800 MPa and yielded high correlations with CL and SA. It is therefore recommended to apply the empirical CSRN model for predicting the dry part of the water retention curve (−10 to −800 MPa) from measured soil texture or surface area. Further research should aim to modify the more physically based TO model to obtain better descriptions of the SWRC in the very dry range (−300 to −800 MPa).

Description: This paper analyzes the effects of different hydrological mechanisms on the solute response in watershed stream networks. Important processes are due to the hydraulic and chemical retention of reactive solutes in transient storage zones and the cumulative consequences of these processes from a single transport pathway as well as from the network of transport pathways. Temporal moments are derived for a distributed stream network and for a compartment-in-series model. The temporal moments are evaluated and are utilized to derive formal expressions for translating the network parameters into compartmental model parameters. The analysis reveals that in addition to the hydraulic and chemical retention processes, the morphological and topological properties of a watershed have a distinct impact on the central temporal moments in terms of averaging of the solute load weighted distances as well as the transport parameters over the network. Kinetic (rate-limited) transient storage affects second-order and higher central temporal moments and thus has a secondary effect on the parameterization of compartmental models. Additional considerable contributions to all temporal moments are introduced when parameter variability along transport pathways is considered. The paper demonstrates an improved model outcome for phosphorus transport in a small Swedish watershed by accounting for the overall network effects when parameterizing a compartment-in-series model.

Description: The estimation of hydrological model parameters by calibration to field data is a critical step in the modeling process. However, calibration often fails because of parameter correlation. Here it is shown that time-lapse gravity data can be combined with hydraulic head data in a coupled hydrogeophysical inversion to decrease parameter correlation in groundwater models. This is demonstrated for a model of riverbank infiltration where combined inversion successfully constrains hydraulic conductivity and specific yield in both an analytical and a numerical groundwater model. A sensitivity study shows that time-lapse gravity data are especially useful to constrain specific yield. Furthermore, we demonstrate that evapotranspiration, and riverbed conductance are better constrained by coupled inversion to gravity and head data than to head data alone. When estimating the four parameters simultaneously, the six correlation coefficients were reduced from unity when only head data were employed to significantly lower values when gravity and head data were combined. Our analysis reveals that the estimated parameter values are not very sensitive to the choice of weighting between head and gravity data over a large interval of relative weights.

Description: Horizontal acoustic Doppler current profilers (H-ADCPs) can be employed to estimate river discharge based on water level measurements and flow velocity array data across a river transect. A new method is presented that accounts for the dip in velocity near the water surface, which is caused by sidewall effects that decrease with the width to depth ratio of a channel. A boundary layer model is introduced to convert single-depth velocity data from the H-ADCP to specific discharge. The parameters of the model include the local roughness length and a dip correction factor, which accounts for the sidewall effects. A regression model is employed to translate specific discharge to total discharge. The method was tested in the River Mahakam, representing a large river of complex bathymetry, where part of the flow is intrinsically three-dimensional and discharge rates exceed 8000 m3 s−1. Results from five moving boat ADCP campaigns covering separate semidiurnal tidal cycles are presented, three of which are used for calibration purposes, whereas the remaining two served for validation of the method. The dip correction factor showed a significant correlation with distance to the wall and bears a strong relation to secondary currents. The sidewall effects appeared to remain relatively constant throughout the tidal cycles under study. Bed roughness length is estimated at periods of maximum velocity, showing more variation at subtidal than at intratidal time scales. Intratidal variations were particularly obvious during bidirectional flow conditions, which occurred only during conditions of low river discharge. The new method was shown to outperform the widely used index velocity method by systematically reducing the relative error in the discharge estimates.

Description: Seven high-resolution (0.3–0.6 m depth intervals), 1-D vertical profiles of the δ²H of pore water were collected across a 300 km2 study area in southern Saskatchewan, Canada, to define the vertical controls on solute transport in a 〉120 m thick, two-layered aquitard system. The 1-D profiles were augmented with an existing δ²H profile collected from a previous study. The surficial aquitard in the area consists of Quaternary deposits (either glacial till or lacustrine deposits; 13 to 128 m thick) underlain by an upper Cretaceous claystone aquitard (80–110 m thick). The shape of the individual δ²H profiles and associated 1-D transport modeling suggest diffusion is the regionally dominant vertical transport mechanism across the aquitards. The profile shape is controlled by the thickness of the Quaternary deposit and the δ²H value at the upper boundary, which coincides with the depth of the water table. The upper boundary δ²H value varies considerably across the area (−149‰ to −101‰), perhaps due to differences in local hydrological conditions (e.g., slope, aspect, infiltration) across the landscape. Modeling of all profiles shows the timing for till deposition and the timing of climate change during the Holocene are consistent across the area (∼30 ka and 7–10 ka before the present, respectively), corroborating other studies. This study provides insights into the hydrogeologic controls on solute transport in an aquitard system and associated geologic and climatic changes for a prairie region over the past 30 ka, and improves our understanding of initial and time-dependent transport boundary conditions for the study of aquitards.

Description: We develop a novel method of parameterization for spatial hydraulic property characterization to mitigate the challenges associated with the nonlinear inverse problem of subsurface flow model calibration. The parameterization is performed by the projection of the estimable hydraulic property field onto an orthonormal basis derived from the grid connectivity structure. The basis functions represent the modal shapes or harmonics of the grid, are defined by a modal frequency, and converge to special cases of the discrete Fourier series under certain grid geometries and boundary assumptions; therefore, hydraulic property updates are performed in the spectral domain and merge with Fourier analysis in ideal cases. Dependence on the grid alone implies that the basis may characterize any grid geometry, including corner point and unstructured, is model independent, and is constructed off-line and only once prior to flow data assimilation. We apply the parameterization in an adaptive multiscale model calibration workflow for three subsurface flow models. Several different grid geometries are considered. In each case the prior hydraulic property model is updated using a parameterized multiplier field that is superimposed onto the grid and assigned an initial value of unity at each cell. The special case corresponding to a constant multiplier is always applied through the constant basis function. Higher modes are adaptively employed during minimization of data misfit to resolve multiscale heterogeneity in the geomodel. The parameterization demonstrates selective updating of heterogeneity at locations and spatial scales sensitive to the available data, otherwise leaving the prior model unchanged as desired.

Description: We present a new technique for identifying and quantifying the discharge of long residence time, regional groundwater to rivers using naturally occurring tracers measured within the river. Terrigenic 4He and 222Rn, synoptically sampled along a 100 km reach in the Fitzroy River in northern Western Australia, are used to identify areas of groundwater inflow to the river and to distinguish shallow, local and deep, regional groundwater. Models of tracer transport in the river can be numerically optimized to calculate total groundwater discharge and to separate regional and local discharge fractions. Discharge of regional groundwater composes close to 15% of the total groundwater discharge along the entire reach, varying spatially along the reach from 0% to 100% of total groundwater discharge. This method should be applicable in river systems where groundwater with elevated terrigenic helium could be discharging to the river. The ability to separate locally from regionally derived groundwater discharge has significant implications for calculating catchment water budgets, for predicting catchment response to changes in precipitation, and for sustainable management of the catchment.

Description: Mean May–September Potomac River streamflow was reconstructed from 950–2001 using a network of tree ring chronologies (n = 27) representing multiple species. We chose a nested principal components reconstruction method to maximize use of available chronologies backward in time. Explained variance during the period of calibration ranged from 20% to 53% depending on the number and species of chronologies available in each 25 year time step. The model was verified by two goodness of fit tests, the coefficient of efficiency (CE) and the reduction of error statistic (RE). The RE and CE never fell below zero, suggesting the model had explanatory power over the entire period of reconstruction. Beta weights indicated a loss of explained variance during the 1550–1700 period that we hypothesize was caused by the reduction in total number of predictor chronologies and loss of important predictor species. Thus, the reconstruction is strongest from 1700–2001. Frequency, intensity, and duration of drought and pluvial events were examined to aid water resource managers. We found that the instrumental period did not represent adequately the full range of annual to multidecadal variability present in the reconstruction. Our reconstruction of mean May–September Potomac River streamflow was a significant improvement over the Cook and Jacoby (1983) reconstruction because it expanded the seasonal window, lengthened the record by 780 years, and better replicated the mean and variance of the instrumental record. By capitalizing on variable phenologies and tree growth responses to climate, multispecies reconstructions may provide significantly more information about past hydroclimate, especially in regions with low aridity and high tree species diversity.

Description: Well-validated rainfall-runoff models are able to capture the relationships between rainfall and streamflow and to reliably estimate initial catchment states. While future streamflows are mainly dependent on initial catchment states and future rainfall, use of the rainfall-runoff models together with estimated future rainfall can produce skilful forecasts of future streamflows. This is the basis for the ensemble streamflow prediction system, but this approach has not been explored in Australia. In this paper, two conceptual rainfall-runoff models, together with rainfall ensembles or analogues based on historical rainfall and the Southern Oscillation index (SOI), were used to forecast streamflows at monthly and 3-monthly scales at two catchments in east Australia. The results showed that both models forecast monthly streamflow well when forecasts for all months were evaluated together, but their performance varied significantly from month to month. Best forecasting skills were obtained (both monthly and 3 monthly) when the models were coupled with ensemble forcings on the basis of long-term historical rainfall. SOI-based resampling of forcings from historical data led to improved forecasting skills only in the period from September to December at the catchment in Queensland. For 3 month streamflow forecasts, best skills were in the period from April to June at the catchment in Queensland and in the period from October to January for the catchment in New South Wales, both of which were the periods after the rainy season. The forecasting skills are indicatively comparable to the statistical forecasting skills using a Bayesian joint probability approach. The potential approaches for improved hydrologic modeling through conditional parameterization and for improved forecasting skills through advanced model updating and bias corrections are also discussed.

Description: Sediments are a pervasive source of fecal indicator bacteria (FIB) in rivers, lakes, estuaries, and oceans and may constitute a long-term reservoir of human disease. Previous attempts to quantify the flux of FIB across the sediment-water interface (SWI) are limited to extreme flow events, for which the primary mechanism of bacterial release is disruption and/or erosion of the sediment substrate. Here we report measurements of FIB flux across the SWI in a turbulent stream that is not undergoing significant erosion. The stream is formed by the steady discharge of bacteria-free disinfected and highly treated wastewater effluent to an earthen channel harboring high concentrations of FIB in the sediment from in situ growth. The flux j″ of FIB across the SWI, estimated from mass balance on FIB measurements in the water column, scales linearly with the concentration of bacteria in sediment pore fluids Cpore over a 3 decade change in both variables: $j^{\,\prime\prime}\; = \;k_m^{\rm obs} C_{\rm pore}.$ The magnitude of the observed mass transfer velocity ($\[k_m^{\rm obs}\, = \; 5\times{10^{ - 5}}\]$ m s−1) is significantly larger than values predicted for either the diffusion of bacteria across a concentration boundary layer ($k_m^{\rm diff}\, = \;8\; \times \;{10^{ - 6}}$ m s−1) or sweep and eject fluid motions at the SWI ($\[k_m^{\rm sweep}\, = \; {10^{ - 6}}\]$ m s−1) but is similar to the flux of water between the stream and its hyporheic zone estimated from dye injection experiments. These results support the hypothesis that hyporheic exchange controls the trafficking of bacteria, and perhaps other types of particulate organic matter, across the SWI in turbulent streams.

Description: Reactive transport modeling is a critical element in assessing the potential of natural attenuation of groundwater pollutants. In the present study, we developed a comprehensive quantitative model that incorporates the key processes affecting the long-term fate of complex organic compound mixtures released from coal tar–type dense nonaqueous phase liquid sources. The model framework addresses the simulation of the long-term dynamics of source zone depletion, the fate of the released compounds during reactive transport in the groundwater, the evolution of the aquifer's biogeochemical response, in particular its redox conditions, and the redox-dependent carbon isotope fractionation of selected organic compounds. The modeling framework was applied for the interpretation of observed biogeochemical and isotopic data from a well-characterized coal tar–contaminated site in northern Germany. The simulations highlight the diversity of fates of the individual compounds, which result from their widely varying physicochemical characteristics, and also how complex interactions develop over the lifetime of the contamination. The highly transient release of contaminants from the coal tar as pool and as heterogeneously distributed blobs in the source zone triggers continuously changing biogeochemical conditions and isotope signatures. The modeling results illustrate how difficult and uncertain the assessment of contaminant fate can be if the collected data cover only a small time window relative to the transport time scale. This emphasizes the need for a holistic understanding of the governing processes that control the effectiveness of monitored natural attenuation before it is implemented as a passive remediation strategy at nonaqueous phase liquid–contaminated sites.

Description: In recent years local, national, and international authorities have showed an increasing awareness of flood and inundation hazard, likely due to the large floods which occurred in the past years in many regions of the world. In this context, the estimation of the design flood values to be adopted for flood risk assessment or floodplain management represents a crucial factor. In the case of ungauged or scarcely gauged catchments where a sufficiently long discharge time series is missing, a relevant uncertainty is involved in the flood frequency analysis and a possible solution to reduce this uncertainty is the application of continuous simulation (CS) approaches. Because of the complex structure of this type of approaches and pursuing the parameters parsimony criteria, in the hydrological practice the approaches based on the design storm (DS) estimation are more widely known and applied, mainly for their simplicity. However, one major limit of the DS method is the choice of the “design soil moisture” conditions, representing a critical parameter for assessing the initial wetness of the basin. To that end, this study of investigating six subcatchments of the upper Tiber River basin (Central Italy), with drainage area ranging from 13 to 284 km2, proposes a procedure based on the application of the CS approach as a tool to define the design soil moisture to be afterwards incorporated into the more simple DS method. For each catchment, the procedure consists of (1) stochastic generation of long synthetic rainfall and temperature series starting from observed hourly data; (2) application of a lumped continuous rainfall-runoff model to generate synthetic discharge series and, hence, to obtain the corresponding flood frequency curves; (3) estimation of the design soil moisture, for each return period, by varying in the DS approach the initial wetness conditions of the catchment so that the peak discharge estimated by the DS method matches the one given by the synthetic flood frequency curve. Moreover, in order to apply the more simple DS approach avoiding the use of the CS one, a preliminary analysis to regionalize the design soil moisture as a function of the geo-morphological characteristics and the return period is also shown.

Description: Hydrologists routinely analyze pumping test data using conventional interpretation methods that are based on the assumption of homogeneity and that, consequently, yield single estimates of representative flow parameters. However, natural subsurface formations are intrinsically heterogeneous, and hence, the flow parameters influencing the drawdown vary as the cone of depression expands in time. In this paper a novel procedure for the analysis of pumping tests in heterogeneous confined aquifers is developed. We assume that a given heterogeneous aquifer can be represented by a homogeneous system whose flow parameters evolve in time as the pumping test progresses. At any point in time, the interpreted flow parameters are estimated using the ratio of the drawdown and its derivative observed at that particular time. The procedure is repeated for all times, yielding time-dependent estimates of transmissivity Ti(t) and storativity, Si(t). Based on the analysis of the sensitivity of drawdown to inhomogeneities in the T field, the time-dependent interpreted transmissivity values are found to be a good estimate of Tg(r), the geometric mean of the transmissivity values encompassed within a progressively increasing radius r from the well. The procedure is illustrated for Gaussian heterogeneous fields with ln(T) variances up to a value of 2. The impact of the separation distance between the pumping well and observation point on data interpretation is discussed. The results show that information about the spatial variability of the transmissivity field can be inferred from time-drawdown data collected at a single observation point.

Description: Flooding affects ecosystems by transporting water and solutes across aquatic-terrestrial interfaces, removing nutrient and organic substrate limitations, and spurring biogeochemical activity. Few studies have considered the influence of flooding on surface water–groundwater interactions. This research examines the temporally variable water storage and exchange in a stream in the McMurdo Dry Valleys (MDV) of Antarctica, where diel flood pulses occur due to glacial melt. Several MDV streams display truncated discharge peaks, suggesting water storage between the source glacier and the gauging station. We tested the hypothesis that stream braids and subsurface water storage contribute to the difference between glacial melt and stream outflow hydrographs by constructing a coupled surface water routing and subsurface water flow model. This model routes water into stream braids at high flows and allows this water to infiltrate and return to the stream via subsurface flow paths as flows recede. Our simulation demonstrates the importance of surface–subsurface water interactions in controlling the hydrograph shape. Maximum simulated discharge was sensitive to storage parameters including aquifer depth and the flooding threshold, while minimum discharge was sensitive to hydraulic conductivity. Subsurface storage volume varied by 38% over a diel cycle and stream-subsurface exchange rates varied from 0 to 0.19 m3 h−1 m−1, with exchange from the stream to the subsurface during high flows, and vice versa at low flows. These results underscore how unsteady flow can increase hyporheic interactions and ecosystem productivity, and provide support for maintaining natural stream morphology and flow regimes.

Description: Bayesian theory of model calibration provides a coherent framework for distinguishing and encoding multiple sources of uncertainty in probabilistic predictions of flooding. This paper demonstrates the use of a Bayesian approach to computer model calibration, where the calibration data are in the form of spatial observations of flood extent. The Bayesian procedure involves generating posterior distributions of the flood model calibration parameters and observation error, as well as a Gaussian model inadequacy function, which represents the discrepancy between the best model predictions and reality. The approach is first illustrated with a simple didactic example and is then applied to a flood model of a reach of the river Thames in the UK. A predictive spatial distribution of flooding is generated for a flood of given severity.

Description: Evapotranspiration rates at the catchment scale are very difficult to quantify. One possible manner to continuously observe this variable could be the estimation of sensible heat fluxes (H) across large distances (in the order of kilometers) using a large aperture scintillometer (LAS), and inverting these observations into evapotranspiration rates, under the assumption that the LAS observations are representative for the entire catchment. The objective of this paper is to assess whether measured sensible heat fluxes from a LAS over a long distance (9.5 km) can be assumed to be valid for a 102.3 km2 heterogeneous catchment. Therefore, a fully process-based water and energy balance model with a spatial resolution of 50 m has been thoroughly calibrated and validated for the Bellebeek catchment in Belgium. A footprint analysis has been performed. In general, the sensible heat fluxes from the LAS compared well with the modeled sensible heat fluxes within the footprint. Moreover, as the modeled H within the footprint has been found to be almost equal to the modeled catchment averaged H, it can be concluded that the scintillometer measurements over a distance of 9.5 km and an effective height of 68 m are representative for the entire catchment.

Description: The primary aim of this paper is to present a fuzzy probabilistic approach for optimal design and rehabilitation of water distribution systems, combining aleatoric and epistemic uncertainties in a unified framework. The randomness and imprecision in future water consumption are characterized using fuzzy random variables whose realizations are not real but fuzzy numbers, and the nodal head requirements are represented by fuzzy sets, reflecting the imprecision in customers' requirements. The optimal design problem is formulated as a two-objective optimization problem, with minimization of total design cost and maximization of system performance as objectives. The system performance is measured by the fuzzy random reliability, defined as the probability that the fuzzy head requirements are satisfied across all network nodes. The satisfactory degree is represented by necessity measure or belief measure in the sense of the Dempster-Shafer theory of evidence. An efficient algorithm is proposed, within a Monte Carlo procedure, to calculate the fuzzy random system reliability and is effectively combined with the nondominated sorting genetic algorithm II (NSGAII) to derive the Pareto optimal design solutions. The newly proposed methodology is demonstrated with two case studies: the New York tunnels network and Hanoi network. The results from both cases indicate that the new methodology can effectively accommodate and handle various aleatoric and epistemic uncertainty sources arising from the design process and can provide optimal design solutions that are not only cost-effective but also have higher reliability to cope with severe future uncertainties.

Description: This paper discusses issues arising when Tropical Rainfall Measuring Mission (TRMM) estimates of rainfall characteristics are calculated for the sparsely gauged Brazilian Amazon. The particular issue considered is the calculation of statistical uncertainty in (1) comparisons between TRMM estimates of rainfall characteristics and their corresponding rain gauge estimates and (2) interpolated estimates of rainfall characteristics, which are conditional on local TRMM estimates near ungauged sites. Three characteristics selected for particular comparison are (1) mean annual rainfall, 1998–2005, (2) mean value, over these years, of the 95% quantile of within-year daily rainfall, and (3) mean number of days in the year with rainfall of 2 mm or more. The paper demonstrates how positive spatial correlation leads to underestimation of the uncertainty in the comparisons (i.e., the standard errors of differences), so that the significance of differences, when observed, is likely to be overestimated if spatial correlation is ignored. For the three characteristics mentioned, the Matérn spatial correlation function with shape parameter $\kappa$ = 1/2, giving an exponential correlation function, was found to be adequate for modeling spatial correlation. It is also argued that the statistic 1 − b, where b is the slope of the regression of a rain gauge estimate of a rainfall characteristic on its counterpart derived from satellite instrumentation, is a measure of the errors in the satellite (spatially averaged) estimates. Finally, although issues of spatial correlation are discussed with particular reference to rainfall, it is argued that the same issues arise whenever satellite-derived estimates of a component of the water cycle must be combined with ground-based measurements.

Description: The ensemble Kalman filter (EnKF) has recently been proposed as a promising parameter estimation approach for constraining the description of rock flow properties, such as permeability and porosity, to reproduce flow measurements that are modeled as nonlinear functions of these properties. One of the key factors that strongly affect the performance of the EnKF is the quality or representativeness of the prior ensemble of property fields used to initialize the EnKF assimilation procedure. The initial ensemble is commonly constructed by assuming a known geological continuity model such as a variogram. However, geologic continuity models are derived from incomplete information and imperfect modeling assumptions, which can introduce a significant level of uncertainty into the produced models. Neglecting this important source of uncertainty can lead to systematic errors and questionable estimation results. In this paper, we investigate the performance of the EnKF under varying levels of uncertainty in the variogram model parameters. We first attempt to directly estimate variogram model parameters from flow data and show that the complex and nonunique relation they have with the flow data provides little sensitivity for an effective inversion with the EnKF. We then assess the performance of the EnKF for estimation of permeability values under uncertain and incorrect initial variogram parameters and show that any bias in specifying variogram parameters tends to persist throughout the EnKF analysis even though locally reasonable permeability updates may be obtained near observation points. More importantly, we show that when variogram parameters are specified probabilistically to account for the full range of structural variability in the initial permeability ensemble, the EnKF update results are quite promising. The results suggest that under uncertain geologic continuity, the EnKF tends to perform better if a very diverse set of property fields is used to form the initial ensemble than when a deterministic and potentially erroneous variogram model is used. Therefore, in applying the EnKF to model calibration problems, it is preferable to overestimate the uncertainty in geologic continuity and to initialize the EnKF procedure with a wide range of variability in property description than to overlook the variogram uncertainty at the risk of introducing systematic bias that cannot be corrected by the EnKF updates. The practical implications of the results in this paper are significant for designing the EnKF for realistic ensemble model calibration problems where the level of uncertainty in the initial ensemble is usually not known a priori.

Description: This paper presents a theoretical investigation of the effects of spatial heterogeneity of runoff generation on the scaling behavior of runoff timing responses. A previous modeling study on the Illinois River Basin in Oklahoma had revealed a systematic spatial trend in the relative dominance of different runoff generation mechanisms, attributable to corresponding systematic trends in landscape properties. Considering the differences in the timing of hillslope responses between the different runoff mechanisms, this paper explores their impacts on the catchment-scale runoff routing responses, including how they change with spatial scale. For this purpose we utilize a distributed, physically based hydrological model, with a fully hydraulic stream network routing component. The model is used to generate instantaneous response functions (IRF) for nested catchments of a range of sizes along the river network and quantitative measures of their shape, e.g., peak and time to peak. In order to separate the effects of soil heterogeneity from those due to basin geomorphology, the model simulations are carried out for three hypothetical cases that make assumptions regarding landscape properties (uniform, a systematic trend, and heterogeneity plus the trend), repeating these simulations under wet and dry antecedent conditions. The simulations produced expected and also surprising results. The power law relationship between the peak of the IRF and drainage area is shown to be flatter under wet conditions than under dry conditions, even though the (faster) saturation excess mechanism is more dominant under wet conditions. This result appears to be caused by partial area runoff generation: under wet conditions, the fraction of saturation area is about 30%, while under dry conditions it is less than 10% for the same input of rainfall. This means travel times associated with overland flow (which mostly contributes to the peak and time to peak) are, in fact, longer during wet conditions than during dry conditions. The power law relationship between peak and drainage area also exhibits a scaling break at around 1000 km2, which can be shown to be related to the peculiar geomorphology of the catchment.

Description: This study is focused on Pseudomonas putida bacteria transport in porous media in the presence of suspended kaolinite clay particles. Experiments were performed with bacteria and kaolinite particles separately to determine their individual transport characteristics in water-saturated columns packed with glass beads. The results indicated that the mass recovery of bacteria and clay particles decreased as the pore water velocity decreased. Batch experiments were carried out to investigate the attachment of Pseudomonas putida onto kaolinite particles. The attachment process was adequately described by a Langmuir isotherm. Finally, bacteria and kaolinite particles were injected simultaneously into a packed column in order to investigate their cotransport behavior. The experimental data suggested that the presence of clay particles significantly inhibited the transport of bacteria in water-saturated porous media. The observed reduction of Pseudomonas putida recovery in the column outflow was attributed to bacteria attachment onto kaolinite particles, which were retained onto the solid matrix of the column. A mathematical model was developed to describe the transport of bacteria in the presence of suspended clay particles in one-dimensional water-saturated porous media. Model simulations were in good agreement with the experimental results.

Description: A recently introduced measurement approach allows in situ determination of subsurface soil water evaporation by means of heat-pulse probes (HPP). The latent heat component of subsurface evaporation is estimated from the residual of the sensible heat balance. This heat balance method requires measurement of vertical soil temperature and estimates of thermal properties for soil water evaporation determination. Our objective was to employ numerically simulated thermal and hydraulic processes using constant or diurnally cycled surface boundary conditions to evaluate and understand this technique. Three observation grid spacings, namely, 6 mm (tri-needle HPP), 3 mm (penta-needle HPP) and 1 mm, along with three soil textures (sand, silt, and silty clay) were used to test the heat balance method. The comparison of heat balance–based evaporation rate estimates with an independent soil profile water balance revealed substantial errors when thermal conductivity $(\lambda)$ was averaged spatially across the evaporation front. Since the conduction component of heat flux is the dominant process at the evaporation front, the estimation of evaporation rate was significantly improved using depth-dependent $\lambda$ instead of a space-averaged $\lambda$. A near-surface “undetectable zone” exists, where the heat balance calculation is irreconcilable, resulting in underestimation of total subsurface evaporation. The method performs better for medium- and coarse-textured soils than for fine-textured soils, where portions of the drying front may be maintained longer within the undetectable zone. Using smaller temperature sensor spacing near the soil surface minimized underestimation from the undetectable zone and improved accuracy of total subsurface evaporation rate estimates.

Description: Skillful and reliable forecasts of seasonal streamflows are highly valuable to water management. In a previous study, we developed a Bayesian joint probability (BJP) modeling approach for seasonal forecasting of streamflows at multiple sites. The approach has been adopted by the Australian Bureau of Meteorology for seasonal streamflow forecasting in Australia. This study extends the applicability of the BJP modeling approach to streams with zero flow occurrences. The aim is to produce forecasts for these streams in the form of probabilities of zero seasonal flows and probability distributions of above zero seasonal flows. We turn a difficult mathematical problem of mixed discrete-continuous multivariate probability distribution modeling into one of continuous multivariate probability distribution modeling by treating zero flow occurrences as censored data. This paper presents the mathematical formulation and implementation of the modeling approach, methods for forecast verification, and results of a test application to the Burdekin river catchment in northern Queensland, Australia.

Description: We report observed short-term (3 years) channel adjustment in an incised, semiarid stream to the removal of invasive plants, tamarisk (Tamarix spp.) and Russian olive (Elaeagnus angustifolia) by (1) removing the above-ground portion of the plant (cut-stump method) and (2) removing the entire plant (whole-plant method). The stream flows through Canyon de Chelly National Monument in Arizona, USA., draining an ∼1500 km2 catchment. Average channel width is 13 m; average thalweg depth is 2–3 m, although channel banks exceed 8 m locally. Channels adjusted primarily through widening, with significantly larger changes occurring in whole-plant removal reaches; however, neither plant removal method elicited large-scale bank destabilization, and the channels remained entrenched. Particular site conditions limiting large-scale destabilization include the absence of sufficient streamflow magnitudes, the presence of clay layers at the bank toe, the remaining presence of native vegetation, and the entrenched morphology. Our findings serve as a cautionary note regarding the temporal and spatial variability in channel response to invasive plant removal and underscore the importance of considering site-specific conditions in future restoration projects that include invasive plant removal.

Description: Major changes in the morphology of the Trinity River in California, such as narrowing of the cross section and sedimentation of fine sediment in pools, occurred after the closure of a system of dams. These changes caused a dramatic reduction in the salmonid population and a resulting decline of the fishery. Gravel augmentation, regulated flood releases, and mechanical channel rehabilitation are currently being implemented to help restore the aquatic habitat of the river. The present paper describes a tool, named the Spawning Gravel Refresher, for designing and predicting the effects of gravel augmentation in gravel bed rivers. The tool assumes an imposed, cycled hydrograph. The model is calibrated and applied to the regulated reach of the Trinity River in four steps: (1) zeroing runs to reproduce conditions of mobile bed equilibrium as best can be estimated for the predam Trinity River, (2) runs to compare the predictions with the results of previous studies, (3) runs at an engineering time scale to reproduce the effects of the dams, and (4) runs to design gravel augmentation schemes. In the fourth group of runs, the combined effects of engineered flood flow releases and gravel augmentation are predicted. At an engineering time scale, the model indicates that the fraction of fine sediment in the surface layer and in the topmost part of the substrate should decrease when subjected to these two restoration measures, with a consequent improvement of the quality of the spawning gravel.

Description: Monitoring Earth's terrestrial water conditions is critically important to many hydrological applications such as global food production; assessing water resources sustainability; and flood, drought, and climate change prediction. These needs have motivated the development of pilot monitoring and prediction systems for terrestrial hydrologic and vegetative states, but to date only at the rather coarse spatial resolutions (∼10–100 km) over continental to global domains. Adequately addressing critical water cycle science questions and applications requires systems that are implemented globally at much higher resolutions, on the order of 1 km, resolutions referred to as hyperresolution in the context of global land surface models. This opinion paper sets forth the needs and benefits for a system that would monitor and predict the Earth's terrestrial water, energy, and biogeochemical cycles. We discuss six major challenges in developing a system: improved representation of surface-subsurface interactions due to fine-scale topography and vegetation; improved representation of land-atmospheric interactions and resulting spatial information on soil moisture and evapotranspiration; inclusion of water quality as part of the biogeochemical cycle; representation of human impacts from water management; utilizing massively parallel computer systems and recent computational advances in solving hyperresolution models that will have up to 109 unknowns; and developing the required in situ and remote sensing global data sets. We deem the development of a global hyperresolution model for monitoring the terrestrial water, energy, and biogeochemical cycles a “grand challenge” to the community, and we call upon the international hydrologic community and the hydrological science support infrastructure to endorse the effort.

Description: The link between river network structure and hydrologic response for natural watersheds has been the subject of ongoing research for the past 30 years. In this paper we investigate the link between sewer network structure and hydrologic response in urban catchments. It has been shown in natural watersheds that there are dispersion mechanisms that contribute to the impulse response function of the catchment: hydrodynamic dispersion, geomorphologic dispersion, and hydrodynamic dispersion. We introduce a fourth dispersion mechanism, intrastate dispersion, which accounts for the variance in conduit (e.g., slope, length, diameter, etc.) and overland region input parameters (e.g., slope, area, imperviousness, etc.) within an order. This dispersion mechanism is found to be the second largest contributor to the total dispersion in the urban catchments analyzed, contributing less than hydrodynamic dispersion but more than kinematic and geomorphologic dispersion. This is primarily a result of the shorter network travel times observed in urban catchment. The dispersion mechanisms are incorporated in the Illinois Urban Hydrologic Model, which is a recently developed probabilistic approach for predicting the hydrologic response in highly urbanized catchments. Furthermore, an analysis is performed to help better understand the uncertainty in the predicted hydrologic response that is introduced by spatial variation in conduit and overland input parameters. It is identified that conduit slope and length are the greatest sources of uncertainty in the predicted direct runoff hydrograph for the CDS-51 catchment in the village of Dolton, Illinois, and the CDS-36 catchment in the city of Chicago, Illinois.

Description: In this study, the S-shaped log-log drawdown-time curve typical of pumping tests in unconfined aquifers is reinvestigated via numerical experiments. Like previous investigations, this study attributes the departure of the S shape from the drawdown-time behavior of the confined aquifer to the presence of an “additional” source of water. Unlike previous studies, this source of water is reinvestigated by examining the temporal and spatial evolution of the rate of change in storage in an unconfined aquifer during pumping. This evolution is then related to the transition of water release mechanisms from the expansion of water and compaction of the porous medium to the drainage of water from the unsaturated zone above the initial water table and initially saturated pores as the water table falls during the pumping of the aquifer. Afterward, the 1-D vertical drainage process in a soil column is simulated. Results of the simulation show that the transition of the water release mechanisms in the 1-D vertical flow without an initial unsaturated zone can also yield the S-shaped drawdown-time curve as in an unconfined aquifer. We therefore conclude that the transition of the water release mechanisms and vertical flow in the aquifer are the cause of the S-shaped drawdown-time curve observed during pumping in an unconfined aquifer. We also find that the moisture retention characteristics of the aquifer material have greater impact than its relative permeability characteristics on the drawdown-time curve. Furthermore, influences of the spatial variability of saturated hydraulic conductivity, specific storage, and saturated moisture content on the drawdown curve in the saturated zone are found to be more significant than those of other unsaturated properties. Finally, a cross-correlation analysis reveals that the drawdown at a location in a heterogeneous unconfined aquifer is mainly affected by local heterogeneity near the pumping and observation wells. Applications of a model assuming homogeneity to the estimation of aquifer parameters as such may require a large number of observation wells to obtain representative parameter values. In conclusion, we advocate that the governing equation for variably saturated flow through heterogeneous media is a more appropriate and realistic model that explains the S-shaped drawdown-time curves observed in the field.

Description: We assess hydroclimatic projections for the Murray-Darling Basin (MDB) using an ensemble of 39 Intergovernmental Panel on Climate Change AR4 climate model runs based on the A1B emissions scenario. The raw model output for precipitation, P, was adjusted using a quantile-based bias correction approach. We found that the projected change, ΔP, between two 30 year periods (2070–2099 less 1970–1999) was little affected by bias correction. The range for ΔP among models was large (∼±150 mm yr−1) with all–model run and all-model ensemble averages (4.9 and −8.1 mm yr−1) near zero, against a background climatological P of ∼500 mm yr−1. We found that the time series of actually observed annual P over the MDB was indistinguishable from that generated by a purely random process. Importantly, nearly all the model runs showed similar behavior. We used these facts to develop a new approach to understanding variability in projections of ΔP. By plotting ΔP versus the variance of the time series, we could easily identify model runs with projections for ΔP that were beyond the bounds expected from purely random variations. For the MDB, we anticipate that a purely random process could lead to differences of ±57 mm yr−1 (95% confidence) between successive 30 year periods. This is equivalent to ±11% of the climatological P and translates into variations in runoff of around ±29%. This sets a baseline for gauging modeled and/or observed changes.

Description: Current global river routing models do not represent floodplain inundation dynamics realistically because the storage and movement of surface waters are regulated by small-scale topography rather than the commonly used spatial resolution of global models. In this study, we propose a new global river routing model, CaMa-Flood, which explicitly parameterizes the subgrid-scale topography of a floodplain, thus describing floodplain inundation dynamics. The relationship between water storage, water level, and flooded area in the model is decided on the basis of the subgrid-scale topographic parameters based on 1 km resolution digital elevation model. Horizontal water transport is calculated with a diffusive wave equation, which realizes the backwater effect in flat river basins. A set of global-scale river flow simulations demonstrated an improved predictability of daily-scale river discharge in many major world rivers by incorporating the floodplain inundation dynamics. Detailed analysis of the simulated results for the Amazon River suggested that introduction of the diffusive wave equation is essential for simulating water surface elevation realistically. The simulated spatiotemporal variation of the flooded area in the Amazon basin showed a good correlation with satellite observations, especially when the backwater effect was considered. The improved predictability for daily river discharge, water surface elevation, and inundated areas by the proposed model will promote climate system studies and water resource assessments.

Description: In the 300 Area of a U(VI)-contaminated aquifer at Hanford, Washington, USA, inorganic carbon and major cations, which have large impacts on U(VI) transport, change on an hourly and seasonal basis near the Columbia River. Batch and column experiments were conducted to investigate the factors controlling U(VI) adsorption/desorption by changing chemical conditions over time. Low alkalinity and low Ca concentrations (Columbia River water) enhanced adsorption and reduced aqueous concentrations. Conversely, high alkalinity and high Ca concentrations (Hanford groundwater) reduced adsorption and increased aqueous concentrations of U(VI). An equilibrium surface complexation model calibrated using laboratory batch experiments accounted for the decrease in U(VI) adsorption observed with increasing (bi)carbonate concentrations and other aqueous chemical conditions. In the column experiment, alternating pulses of river and groundwater caused swings in aqueous U(VI) concentration. A multispecies multirate surface complexation reactive transport model simulated most of the major U(VI) changes in two column experiments. The modeling results also indicated that U(VI) transport in the studied sediment could be simulated by using a single kinetic rate without loss of accuracy in the simulations. Moreover, the capability of the model to predict U(VI) transport in Hanford groundwater under transient chemical conditions depends significantly on the knowledge of real-time change of local groundwater chemistry.

Description: Because of the extensive computational burden and perhaps a lack of awareness of existing methods, rigorous uncertainty analyses are rarely conducted for variable-density flow and transport models. For this reason, a recently developed null-space Monte Carlo (NSMC) method for quantifying prediction uncertainty was tested for a synthetic saltwater intrusion model patterned after the Henry problem. Saltwater intrusion caused by a reduction in fresh groundwater discharge was simulated for 1000 randomly generated hydraulic conductivity distributions, representing a mildly heterogeneous aquifer. From these 1000 simulations, the hydraulic conductivity distribution giving rise to the most extreme case of saltwater intrusion was selected and was assumed to represent the “true” system. Head and salinity values from this true model were then extracted and used as observations for subsequent model calibration. Random noise was added to the observations to approximate realistic field conditions. The NSMC method was used to calculate 1000 calibration-constrained parameter fields. If the dimensionality of the solution space was set appropriately, the estimated uncertainty range from the NSMC analysis encompassed the truth. Several variants of the method were implemented to investigate their effect on the efficiency of the NSMC method. Reducing the dimensionality of the null-space for the processing of the random parameter sets did not result in any significant gains in efficiency and compromised the ability of the NSMC method to encompass the true prediction value. The addition of intrapilot point heterogeneity to the NSMC process was also tested. According to a variogram comparison, this provided the same scale of heterogeneity that was used to generate the truth. However, incorporation of intrapilot point variability did not make a noticeable difference to the uncertainty of the prediction. With this higher level of heterogeneity, however, the computational burden of generating calibration-constrained parameter fields approximately doubled. Predictive uncertainty variance computed through the NSMC method was compared with that computed through linear analysis. The results were in good agreement, with the NSMC method estimate showing a slightly smaller range of prediction uncertainty than was calculated by the linear method.

Description: River confluences are complex hydrodynamic environments where convergence of incoming flows produces complicated patterns of fluid motion, including the development of large-scale turbulence structures. Accurately simulating confluence hydrodynamics represents a considerable challenge for numerical modeling of river flows. This study uses an eddy-resolving numerical model to simulate the mean flow and large-scale turbulence structure at an asymmetrical river confluence with a concordant bed when the momentum ratio between the two incoming streams is close to 1. Results of the simulation are compared with field data on mean flow and turbulence structure. The simulation shows that the mixing interface is populated by quasi-two-dimensional eddies. Successive eddies have opposing senses of rotation. The mixing layer structure resembles that of a wake behind a bluff body (wake mode). Strong streamwise-oriented vortical (SOV) cells form on both sides of the mixing layer, a finding consistent with patterns inferred from the field data. The predicted mean flow fields show that flow curvature has an important influence on streamwise variation of circulation within the cores of the two primary SOV cells. These SOV cells, along with vortices generated by flow over a submerged block of sediment at one of the banks, strongly influence distributions of the streamwise velocity and turbulent kinetic energy downstream of the junction. Comparison of the eddy-resolving simulation results with predictions from the steady Spalart-Allmaras RANS model shows that the latter fails to predict important features of the measured distributions of streamwise velocity and turbulent kinetic energy because the RANS model underpredicts the strength of the SOV cells. Analysis of instantaneous and mean bed shear stress distributions indicates that the SOV cells enhance bed shear stresses to a greater degree than the quasi-two-dimensional eddies in the mixing interface.

Description: The classic Ghyben-Herzberg estimate of the depth of the freshwater-saltwater interface together with the Dupuit approximation is a useful tool for developing analytical solutions to many seawater intrusion problems. On the basis of these assumptions, Strack (1976) developed a single-potential theory to calculate critical pumping rates in a coastal pumping scenario. The sharp interface assumption and, in particular, this analytical solution are widely used to study seawater intrusion and the sustainable management of groundwater resources in coastal aquifers. The sharp interface assumption neglects mixing and implicitly assumes that salt water remains static. Consequently, this approximation overestimates the penetration of the saltwater front and underestimates the critical pumping rates that ensure a freshwater supply. We investigate the error introduced by adopting the sharp interface approximation, and we include the effects of dispersion on the formulation of Strack (1976). To this end, we perform numerical three-dimensional variable density flow simulations. We find that Strack's equations can be extended to the case of mixing zone if the density factor is multiplied by an empirically derived dispersion factor [1 − (αT/b′)1/6], where αT is transverse dispersivity and b′ is aquifer thickness. We find that this factor can be used not only to estimate the critical pumping rate but also to correct the Ghyben-Herzberg estimate of the interface depth. Its simplicity facilitates the generalization of sharp interface analytical solutions and good predictions of seawater penetration for a broad range of conditions.

Description: Lithologic transitions and glaciations create complex longitudinal profiles that control contemporary erosion and deposition processes. In areas with these characteristics, traditional morphometric approaches for predicting process domains, such as area-slope plots, can be augmented by considering other predictors measured from high resolution lidar-derived digital elevation models (DEMs). Ordinal logistic regression was used to model the distribution of hillslope, swale, colluvial channel, and fluvial channel domains, as identified during field surveys. The study area was a glaciated region of the Rocky Mountain foothills with a complex lithostructural setting. Relationships between domains and a suite of geographic information system–derived descriptors were explored. Predictors included profile anomalies measured at the reach and basin scale using a normalized stream length–gradient (SL/k) index. Drainage area was the dominant factor controlling domains. A model with area as the only predictor was 82% accurate. Reach slope relations were not consistent. A model that also included lithology and basin-scale SL/k index variation was 87% accurate. Domain transitions had larger area thresholds in basins with resistant conglomerate versus sandstone or shale formations and where SL/k index was more variable along a profile. In a restricted model of hillslope, swale, and colluvial channel domains, profile curvature measured over 100 m was also related to domain occurrence. A model for regional-scale mapping applications with six additional predictors was 95% accurate. The results showed that ordinal logistic regression can be used to predict and map process domains in regions with complex physiography using descriptors measured from high -resolution DEMs.

Description: We propose a novel ecohydrological model for the invasion of inland waters by the zebra mussel Dreissena polymorpha and test it against field data gathered within the Mississippi-Missouri river system in North America. This biological invasion poses major ecological and economic threats, especially due to the huge population densities reached by local zebra mussel colonies and the species' unparalleled dispersal abilities within fluvial systems. We focus on a quantitative evaluation, attempted here for the first time, of the individual roles and the mutual interactions of drivers and controls of the Mississippi-Missouri invasion. To this end, we use a multilayer network model accounting explicitly for zebra mussel demographic dynamics, hydrologic transport, and dispersal due to anthropic activities. By testing our results against observations, we show that hydrologic transport alone is not sufficient to explain the spread of the species at the basin scale. We also quantify the role played by commercial navigation in promoting the initial, selective colonization of the river system, and show how recreational boating may have determined the capillary penetration of the species into the water system. The role of post-establishment dispersal mechanisms and the effectiveness of possible prevention measures are also discussed in the context of model sensitivity and robustness to reparametrization.

Description: Sustainable water resource developments require reservoir operations that provide environmental flows (e-flows) to support the downstream riverine ecosystem by minimizing the degree of hydrologic alteration under given water supply constraints. The effective management of e-flows below dams was explored by combining three e-flow management strategies for “normal,” wet, and dry year situations, delivering a range of e-flow regimes for different reservoir storage levels and different natural flow (reservoir inflow) conditions. We combined these into a single combined e-flow strategy and coupled this with reservoir operating rule curves to form a reservoir operating approach that optimized e-flow provision under given water supply constraints. We also explored constraints imposed by the dam's valve release capacity. To investigate the capability of the approach to minimize the degree of hydrologic alteration downstream from a reservoir, we presented a case study of the Tanghe Reservoir in China's Tang river basin. The approach triggered a preferred e-flow strategy when the reservoir water level was high to better maintain the ecological functions within downstream riverine ecosystems, a basic e-flow strategy when the water level was low to reduce the water supply risk, and an acceptable strategy under intermediate conditions. The approach effectively balanced human and ecosystem needs and demonstrated the minimum levels of flow regime alteration achievable within the regulated river.

Description: High altitude basins in the Sierra Nevada, California, have negligible summer precipitation and very little groundwater storage, making them ideal laboratories for indirectly monitoring changes in evaporative losses between wet and dry years. Dry years typically have greater potential evapotranspiration (ET) due to warmer June and July air temperatures, warmer summer water/soil temperatures, greater solar radiation exposure due to less frequent cloud cover, greater vapor pressure deficit, and longer growing seasons. However, dry years also have limited moisture availability compared to wetter years, and thus actual evapotranspiration is much less than the potential in dry years. The balance of these factors varies with elevation. Here, we use gridded temperature, precipitation, and snow data, along with historic streamflow records in two nested basins of the Merced River, California, and a simple model to determine the following: Annual ET increases in wetter years at midelevations (2100–2600 m), but this pattern can only be represented in model simulations that include some representation of water transfer between higher and lower elevation soil reservoirs. At higher elevations (〉2600 m), greater water availability in wet years is offset by shorter growing seasons due to longer snow cover duration. These results suggest that models seeking to represent changes in ET in mountainous terrain must, at a minimum, include both hillslope processes (water transfer down steep slopes) and snow processes (timing of water and energy supply).

Description: In this paper, we use a theoretical framework of coupled human and natural systems to review the methodological advances in urban water demand modeling over the past 3 decades. The goal of this review is to quantify the capacity of increasingly complex modeling techniques to account for complex human and natural processes, uncertainty, and resilience across spatial and temporal scales. This review begins with coupled human and natural systems theory and situates urban water demand within this framework. The second section reviews urban water demand literature and summarizes methodological advances in relation to four central themes: (1) interactions within and across multiple spatial and temporal scales, (2) acknowledgment and quantification of uncertainty, (3) identification of thresholds, nonlinear system response, and the consequences for resilience, and (4) the transition from simple statistical modeling to fully integrated dynamic modeling. This review will show that increasingly effective models have resulted from technological advances in spatial science and innovations in statistical methods. These models provide unbiased, accurate estimates of the determinants of urban water demand at increasingly fine spatial and temporal resolution. Dynamic models capable of incorporating alternative future scenarios and local stochastic analysis are leading a trend away from deterministic prediction.

Description: Growing season soil CO2 efflux is known to vary laterally by as much as seven fold within small subalpine watersheds (

Description: The use of scintillometers to determine sensible and latent heat flux is becoming increasingly common because of their ability to quantify convective fluxes over distances of hundreds of meters to several kilometers. The majority of investigations using scintillometry have focused on processes above land surfaces, but here we propose a new methodology for obtaining sensible and latent heat fluxes from a scintillometer deployed over open water. This methodology has been tested by comparison with eddy covariance measurements and through comparison with alternative scintillometer calculation approaches that are commonly used in the literature. The methodology is based on linearization of the Bowen ratio, which is a common assumption in models such as Penman's model and its derivatives. Comparison of latent heat flux estimates from the eddy covariance system and the scintillometer showed excellent agreement across a range of weather conditions and flux rates, giving a high level of confidence in scintillometry-derived latent heat fluxes. The proposed approach produced better estimates than other scintillometry calculation methods because of the reliance of alternative methods on measurements of water temperature or water body heat storage, which are both notoriously hard to quantify. The proposed methodology requires less instrumentation than alternative scintillometer calculation approaches, and the spatial scales of required measurements are arguably more compatible. In addition to scintillometer measurements of the structure parameter of the refractive index of air, the only measurements required are atmospheric pressure, air temperature, humidity, and wind speed at one height over the water body.

Description: Phosphorus (P) fractionation and distribution in sediments are of great concern in the Florida Everglades ecosystem because potential eutrophication of surface waters usually results from P external loading and stability. Intact core sediment samples were collected to a depth of 35 cm from wetlands and canals across Water Conservation Area 3 (WCA-3) of the Florida Everglades. These sediment cores were sliced into 5 cm increments and analyzed for P contents in different fractions by sequential extraction. These fractions mainly included total P (TP), readily available P (Pi-KCl), Fe/Al-bound P (Pi-NaOH), Ca/Mg-bound P (Pi-HCl), organic P (Po-NaOH), and residual P (PoResidue). Results showed that the canal sediments had the highest concentrations of TP, with about 87% in the form of Ca/Mg-bound fraction, and the concentrations of TP in these sediments increased with depth. In contrast, the wetland sediments contained the lowest concentrations of TP (predominantly in the organic fraction), with 43% residual P and 27% Po-NaOH, and the concentrations of TP in these sediments decreased with depth. In addition, a large amount of the readily available P (up to 1500 mg kg−1) in the canal sediments was accumulated at the top layer of 0–5 cm. This study suggests that any disturbance and/or environmental alterations, such as high canal flow and dredging in canal sediments, could pose a potential risk of a P increase in the water column and, consequently, in the wetlands because of the release of readily available P despite the relatively stable nature of such P fractions in these sediments.

Description: Land water storage plays a fundamental role in the West African water cycle and has an important impact on climate and on the natural resources of this region. However, measurements of land water storage are scarce at regional and global scales and especially in poorly instrumented endorheic regions, such as most of the Sahel, where little useful information can be derived from river flow measurements and basin water budgets. The Gravity Recovery and Climate Experiment (GRACE) satellite mission provides an accurate measurement of the terrestrial gravity field variations from which land water storage variations can be derived. However, their retrieval is not straightforward, and different methods are employed, which results in different water storage GRACE products. On the other hand, water storage can be estimated by land surface modeling forced with observed or satellite-based boundary conditions, but such estimates can be highly model dependent. In this study, land water storage by six GRACE products and soil moisture estimations by nine land surface models (run within the framework of the African Monsoon Multidisciplinary Analysis Land Surface Intercomparison Project (ALMIP)) are evaluated over West Africa, with a particular focus on the Sahelian area. The water storage spatial distribution, including zonal transects, its seasonal cycle, and its and interannual variability, are analyzed for the years 2003–2007. Despite the nonnegligible differences among the various GRACE products and among the different models, a generally good agreement between satellite and model estimates is found over the West Africa study region. In particular, GRACE data are shown to reproduce well the water storage interannual variability over the Sahel for the 5 year study period. The comparison between satellite estimates and ALMIP results leads to the identification of processes needing improvement in the land surface models. In particular, our results point out the importance of correctly simulating slow water reservoirs as well as evapotranspiration during the dry season for accurate soil moisture modeling over West Africa.

Description: Mishra and Neuman (2010) developed an analytical solution for flow to a partially penetrating well of zero radius in a compressible unconfined aquifer that allows inferring its saturated and unsaturated hydraulic properties from responses recorded in the saturated and/or unsaturated zones. Their solution accounts for horizontal as well as vertical flows in each zone. It represents unsaturated zone constitutive properties in a manner that is at once mathematically tractable and sufficiently flexible to provide much improved fits to standard constitutive models. In this paper we extend the solution of Mishra and Neuman [2010] to the case of a finite diameter pumping well with storage; investigate the effects of storage in the pumping well and delayed piezometer response on drawdowns in the saturated and unsaturated zones as functions of position and time; validate our solution against numerical simulations of drawdown in a synthetic aquifer having unsaturated properties described by the van Genuchten [1980]–Mualem [1976] model; use our solution to analyze 11 transducer-measured drawdown records from a seven-day pumping test conducted by University of Waterloo researchers at the Canadian Forces Base Borden in Ontario, Canada; validate our parameter estimates against manually-measured drawdown records in 14 other piezometers at Borden; and compare (a) our estimates of aquifer parameters with those obtained on the basis of all these records by Moench [2008], (b) on the basis of 11 transducer-measured drawdown records by Endres et al. [2007], (c) our estimates of van Genuchten–Mualem parameters with those obtained on the basis of laboratory drainage data from the site by Akindunni and Gillham [1992], and (d) our corresponding prediction of how effective saturation varies with elevation above the initial water table under static conditions with a profile based on water contents measured in a neutron access tube at a radial distance of about 5 m from the center of the pumping well. We also use our solution to analyze 11 transducer-measured drawdown records from a 7 day pumping test conducted by University of Waterloo researchers at the Canadian Forces Base Borden in Ontario, Canada. We validate our parameter estimates against manually measured drawdown records in 14 other piezometers at Borden. We compare our estimates of aquifer parameters with those obtained on the basis of all these records by Moench (2008) and on the basis of 11 transducer-measured drawdown records by Endres et al. (2007), and we compare our estimates of van Genuchten–Mualem parameters with those obtained on the basis of laboratory drainage data from the site by Akindunni and Gillham (1992); finally, we compare our corresponding prediction of how effective saturation varies with elevation above the initial water table under static conditions with a profile based on water contents measured in a neutron access tube at a radial distance of about 5 m from the center of the pumping well.

Description: Dryland rivers function as strongly linked ecologic-hydrologic systems, including both extended periods of drought and episodic flooding events. However, few studies have combined hydrologic and biogeochemical measurements to better understand the ecology of pools within dryland rivers. We used δ2H and δ18O values of pool water, rainfall, and groundwater combined with pool water measurements of C, N, and P and dissolved organic matter (DOM) fluorescence characteristics to determine (1) the concentration and chemical composition of DOM and (2) the origin of surface water in 16 pools of a dryland river in northern Western Australia. Parallel factor analysis of excitation-emission matrices showed that humic-like components derived mainly from terrestrial plant material dominated total DOM fluorescence for all pools. Evaporation models using δ2H and δ18O showed a variety of pool hydrologic regimes, including pools with moderate to high evaporative water loss that were largely isolated from shallow alluvium water inputs and pools with consistent alluvium water throughflow and low evaporation. Concentrations of C, N, and P as well as total DOM fluorescence were generally greater in pools with high evaporative loss and lower in pools with alluvium water inputs. Pool δ2H and δ18O values were also significantly correlated with DOM fluorescence characteristics and C, N, and P concentrations, providing quantitative evidence of the hydrologic influence on DOM biogeochemistry. Taken together, our findings suggest that individual pools function as distinct ecosystems within the riverine environment.

Description: Large-scale groundwater pumping or deep fluid injection in a multilayered subsurface system may generate pressure perturbation not only in the target formation(s), but also in over- and underlying units. Hydraulic communication in the vertical direction may occur via diffuse leakage through aquitards and/or via focused leakage through leaky wells. Existing analytical solutions for pressure perturbation and fluid flow in such systems consider either diffuse leakage or focused leakage, but never in combination with each other. In this study, we developed generalized analytical solutions that account for the combined effect of diffuse and focused leakage. The new solutions solve for pressure changes in a system of N aquifers with alternating leaky aquitards in response to fluid injection/extraction with any number, NI, of injection/pumping (active) wells, and passive leakage/recharge in any number, NL, of leaky wells. The equations of horizontal groundwater flow in the aquifers are coupled by the vertical flow equations in the aquitards and by the flow continuity equations in the leaky wells. The solution methodology, described in detail in this paper, involves transforming the transient flow equations into the Laplace domain; decoupling the resulting ordinary differential equations (ODEs) coupled by diffuse leakage via eigenvalue analysis; solving a system of NL × N linear algebraic equations for the unknown rates of flow through leakage wells; and superposing the solution of pressure buildup/drawdown in aquifers and aquitards resulting from flow in the NI active and NL leaky wells. Verification of the new methodology was achieved by comparison with existing analytical solutions for diffuse leakage and for focused leakage, and against a numerical solution for combined diffuse and focused leakage. Application to an eight-aquifer system with leaky aquitards and one leaky well demonstrates the usefulness and efficiency of the approach, and illustrates the pressure behavior over a spectrum of leakage scenarios and parameters.

Description: Precipitation in complex terrain is subject to orographic enhancement which leads to identifiable signatures in the space-time precipitation distribution over the long term, for instance the presence of long-term persistence. Here we quantify long-term persistence by the scaling exponent H of the increase in the spatial variance of accumulated precipitation with time from high resolution radar data over a 2 year period in a section of the Swiss-Italian Alps with relief greater than 4000 m. We find that the long-term persistence signal is strong and seasonal with mean H = 0.87 – 0.99 in the cold season (autumn-winter), when stratiform precipitation is dominant, and H = 0.73 – 0.81 in the warm season (spring-summer), when convective events are dominant. H is also significantly correlated with mean altitude and surface variance, suggesting that singularities in the deposition process are partly driven by topography. The correlation is negative in the warm season and positive in the cold season, indicating the different physical origin of precipitation formation, atmospheric stability, and orographic effects in those seasons. By analyzing the binary precipitation occurrence process, we find that a large part of the long-term persistence signal comes from the space-time distribution of precipitation occurrence and clustering, regardless of precipitation intensity.

Description: Hydraulic tomography has been proposed as an alternative site characterization method, however, relatively few field scale studies have been attempted. In this paper, we characterize the highly heterogeneous glaciofluvial aquifer-aquitard system at the North Campus Research Site, located at the University of Waterloo, Waterloo, Ontario, Canada using transient hydraulic tomography (THT). In particular, we performed 9 pumping tests in a network of wells to image the hydraulic conductivity (K) and specific storage (Ss) distributions (or tomograms) as well as their uncertainties in three-dimensions using the THT code of J. Zhu and T.-C. J. Yeh (2005). We first performed stochastic inverse modeling of the 9 pumping tests individually to gain insight into the level of detail that can be imaged. Then, we sequentially included 4 of the pumping tests in a THT analysis. The resulting K and Ss tomograms were then validated visually by comparing them to stratigraphy and permeameter K estimates. The K and Ss tomograms were also rigorously assessed through the simulation of all 9 pumping tests and comparing the simulated and observed drawdowns. We find that performing the inversion with multiple pumping tests (i.e., hydraulic tomography) yields improved results when compared to the analysis of individual pumping tests.

Description: A state estimation method for two-dimensional shallow water equations (SWE) in rivers using Lagrangian drifter positions as measurements is proposed. Lagrangian drifters are sensors moving with the flow and reporting their location. The aim of this method is to compensate for the lack of accurate information about boundary conditions. The drifters move with the local flow and report their positions. Thus, they provide additional information about the state of the river compared to a case in which, for example, only a computational model is used to describe the river flow. In this work, the measurement information is incorporated into shallow water equations using ensemble Kalman filtering. Special attention is paid to the handling of modeling errors that arise from the use of simple and computationally lightweight models as evolution models for the flow. The proposed approach is tested with simulated and experimental data collected for this study in the Sacramento–San Joaquin Delta in California. It is shown that when modeling errors are taken into account, better estimates for the state of the river are obtained.

Description: Seawater intrusion in coastal aquifers is a 3-D phenomenon. However, 3-D regional aquifer models are often limited by insufficient geological and hydrological data, the large horizontal to vertical scales ratio, and by numerical constraints. We present an effective formulation for modeling seawater intrusion that relies on a dimensional reduction of the original density-dependent flow and transport problem. We carry out a vertical integration of the 3-D problem and arrive at a coupled set of 2-D equations for the mean flux and salt concentration, which are essentially identical to those of 2-D groundwater flow. However, two new terms emerge from the integration: (1) Darcy's law needs not only the buoyancy term reflecting aquifer bottom slope, but also another one reflecting variability of aquifer thickness; and (2) transport requires a new term reflecting vertical variations of groundwater flux, which are essential for density-dependent flow and we approximate by means of a Fickian dispersion term. The proposed equations are verified by direct steady state numerical simulations of confined aquifers. The results show that the effective formulation correctly reflects the effective dynamics in the 3-D system.

Description: Recent studies have found that the El Niño–Southern Oscillation (ENSO) has statistically significant influences on extreme precipitation. A limitation of most existing work is that a separate generalized extreme value (GEV) distribution is fitted for each individual site. Such models cannot address important questions that involve events jointly defined across multiple sites; for instance, what is the probability that the 50 year return levels of three sites in the vicinity of a city occur in the same season? With the latest statistical methodology for spatial extremes, we fit max-stable process models to winter maximum daily precipitation of 192 sites in California over 55 years. A composite likelihood approach is used since the full likelihood is unavailable either analytically or numerically. In addition to latitude, longitude, and elevation, the Southern Oscillation Index (SOI) is incorporated into the parameters of the marginal GEV models. We find that, in a spatial context, the ENSO has a significant influence on the extreme precipitation in California by shifting the location parameter of the GEV distributions, with higher values of the SOI corresponding to lower maximum winter daily precipitation. The joint spatial model is used to assess risks concerning joint extremal events at network of sites with spatial dependence properly accounted for. The probability of extremal events occurring at multiple sites in the same season is found to be much higher than what would be expected under the independence assumption.

Description: We summarize the results of flume experiments examining the transport behavior of dilute suspensions of silt-sized particles carried in an open channel flow over three separate bed types including porous open mesh, glass beads, and small cobbles. In these experiments the time-varying concentration of particles in suspension transport was measured as a function of suspended-particle size and bed shear stress, and analyzed using a 1-D model incorporating simultaneous deposition and entrainment. Rates of fine-particle deposition to the bed are found to approach Stoke's settling velocity in slow flows, but diminish systematically with increasing bed shear stress and mean flow speed. No discernible re-entrainment from the porous beds was observed, indicating that such beds act as an effective sink for fine particles. When our new results are compared to those of related, previously reported experiments examining fine-particle transport over smooth impermeable beds, silt-sized particles display similar behavior with regard to systematic reduction in deposition velocity independent of suspended-silt-particle size or bed porosity. This behavior is tentatively interpreted to reflect the effects of lift in a linear shear flow in excess of submerged weight of individual particles. Our findings compare favorably with values of effective settling velocity of fine particulate organic matter in natural channel flows reported elsewhere.

Description: The estimation of evapotranspiration (ET) from observed diurnal surface water level fluctuations may be highly sensitive to measurement errors. Available total pressure transducers (TPT) can provide requisite accuracy and precision but require atmospheric pressure compensation with equally accurate and precise barometric pressure transducers (BPTs). The BPT installation location determines the thermal setting, and sensor sensitivity to temperature could affect compensated water levels and any analyses requiring fine-scale data. We investigated the effects of BPT installation location on compensated surface water levels and estimates of ET and net infiltration using the White method in four isolated forested wetlands in northern Florida. Water levels compensated with two differently positioned BPTs, one in buffered thermal conditions (BPTbuf; dry well at the TPT depth) and one in ambient temperatures (BPTamb; screened wellhead space), resulted in markedly different diurnal signatures. Both displayed the expected diurnal pattern, but compensation with BPTamb amplified diurnal variation by as much as 1.5 cm, greatly overestimated ET, and suggested net exfiltration, while BPTbuf compensation resulted in ET estimates similar to potential ET and suggested net infiltration. BPT temperature sensitivity followed diurnal temperature variation and generated systematic water level bias with the same signature as the expected trend, making these errors difficult to detect and underscoring the importance of proper BPT positioning.

Description: Sediment resuspension in lake littoral zones (nearshore region) is strongly related to the properties of the surface wavefield. The occurrence and characteristics of surface waves, near-bottom current velocities, and related suspended sediment concentrations and properties were measured simultaneously in Lake Constance over 1 year. Wind and ship waves are distinguished on the basis of their typical properties, enabling a detailed investigation of their respective importance for sediment resuspension. In the littoral zone of Lake Constance, resuspension occurred during 25% of the entire observation period. Of those observations, 54% were caused by ship waves, which therefore were as important for resuspension as wind waves. Resuspension induced by wind waves occurs rather sporadically throughout the year, whereas ship wave-induced resuspension occurs regularly during the daytime in summer and is hence associated with pronounced diurnal and seasonal patterns in the suspended sediment concentration.

Description: Effective governance that contributes to the integration of water management and land use planning is essential for successful protection of drinking water sources and, ultimately, provision of safe drinking water. In many jurisdictions, land use planning and watershed management occur on separate tracks. This article examines the prospects for integration of these two critical processes. A multicase study approach is used, focusing on the specific objective of protection of drinking water sources. Experiences in three case study watershed regions in Ontario, Canada (Grand River, Upper Thames, and Lake Simcoe), were analyzed. The goal was to identify the extent to which source water protection components and indicators are expressed in land use and watershed-based planning documents. Similarities and differences among the watershed regions are distinguished through a cross-case analysis. The results suggest that a shifting governance regime for drinking water safety in Ontario is contributing to integration between land use and water management. However, proactive and ongoing efforts are required to ensure that integration occurs and that barriers to integration are addressed. Timely guidelines, incentive-based tools, up-to-date and accurate information, and adequate financial resources are essential.

Description: Time compression approximation (TCA) is a practical and often quite accurate tool to predict postponding infiltration for field applications. A modified approximation (MTCA) can be used just as easily and, in general, will reduce the error by about 50%. This is based on two results: (1) After ponding, TCA and MTCA predict very close infiltration rates; and (2) MTCA, but not TCA, uses the actual cumulative infiltration up to the ponding time. Thus, TCA has an additional error in its prediction of postponding infiltration. Previously, those results, including the 50% reduction in error, were observed numerically for linear and Burger's soils. They are illustrated here numerically with an actual soil (a Grenoble sand). More importantly, we developed a general analytical approximation for this problem and showed that it can provide a very convenient predictive tool which can then be used for arbitrary soil properties.

Description: The estimation of rainfall using commercial microwave links is a new and promising measurement technique. Commercial link networks cover large parts of the land surface of the earth and have a high density, particularly in urban areas. Rainfall attenuates the electromagnetic signals transmitted between antennas within this network. This attenuation can be calculated from the difference between the received powers with and without rain and is a measure of the path-averaged rainfall intensity. This study uses a 17-day data set of, on average, 57 single-frequency links from 2009 to estimate rainfall in the Rotterdam region, a densely populated delta city in Netherlands (≈1250 km2, 〉1 million inhabitants). A methodology is proposed where nearby links are used to remove signal fluctuations that are not related to rainfall in order to be able to reliably identify wet and dry weather spells. Subsequently, received signal powers are converted to path-averaged rainfall intensities, taking into account the temporal sampling protocol and attenuation due to wet antennas. Link-based rainfall depths are compared with those based on gauge-adjusted radar data. In addition, the rainfall retrieval algorithm is applied to an independent data set of 21 rainy days in 2010 with on average 16 single-frequency links in the same region. Rainfall retrievals are compared against gauge-adjusted radar rainfall estimates over the link path. Moreover, the retrieval algorithm is also tested using high-resolution research link data to investigate the algorithm's sensitivity to temporal rainfall variations. All presented comparisons confirm the quality of commercial microwave link data for quantitative precipitation estimation over urban areas.

Description: The literature contains contradictory conclusions regarding the relative effects of urbanization on peak flood flows due to increases in impervious area, drainage density and width function, and the addition of subsurface storm drains. We used data from an urbanized catchment, the 14.3 km2 Dead Run watershed near Baltimore, Maryland, USA, and the physics-based gridded surface/subsurface hydrologic analysis (GSSHA) model to examine the relative effect of each of these factors on flood peaks, runoff volumes, and runoff production efficiencies. GSSHA was used because the model explicitly includes the spatial variability of land-surface and hydrodynamic parameters, including subsurface storm drains. Results indicate that increases in drainage density, particularly increases in density from low values, produce significant increases in the flood peaks. For a fixed land-use and rainfall input, the flood magnitude approaches an upper limit regardless of the increase in the channel drainage density. Changes in imperviousness can have a significant effect on flood peaks for both moderately extreme and extreme storms. For an extreme rainfall event with a recurrence interval in excess of 100 years, imperviousness is relatively unimportant in terms of runoff efficiency and volume, but can affect the peak flow depending on rainfall rate. Changes to the width function affect flood peaks much more than runoff efficiency, primarily in the case of lower density drainage networks with less impermeable area. Storm drains increase flood peaks, but are overwhelmed during extreme rainfall events when they have a negligible effect. Runoff in urbanized watersheds with considerable impervious area shows a marked sensitivity to rainfall rate. This sensitivity explains some of the contradictory findings in the literature.

Description: Freezing of unsaturated soils is associated with the formation of a moving freezing zone and liquid water flow toward the zone. An equilibrium thermodynamic formulation of coupled flow and heat transport in variably saturated partially frozen porous media is developed and a self-similar solution is derived for the case of a semi-infinite horizontal porous media column with a constant freezing temperature on one boundary. Solutions to the self-similar equations are derived using a Runge-Kutta solution procedure. The solution is found to yield two possible modes distinguished by zones composed of different combinations of ice, liquid water, and air. One of the modes contains three zones: a frozen zone (WI) with just ice and liquid water; a transition zone (AWI) with ice, liquid water, and air; and an unsaturated zone (AW) with liquid water and air. The second mode contains only the WI zone and the AW zone. It is found that the WI zone is a quintessential part of the solution. The AWI zone is found to exist when the advancement of the freezing zone is relatively fast, while it is absent when the zone advances slowly. Predictions of ice saturation and liquid water saturation with the self-similar solution are compared to published experimental data. Pore pressure is calculated as a linear combination of ice pressure and liquid water pressure, and the calculated figures are used to provide a condition for model limitation in the case of incipient ice lens formation. The developed similarity solution provides insight into the mechanics of liquid water movement and pore filling with ice and the conditions for incipient heaving.

Description: In this article's companion paper, flexible approaches for conceptual hydrological modeling at the catchment scale were motivated, and the SUPERFLEX framework, based on generic model components, was introduced. In this article, the SUPERFLEX framework and the “fixed structure” GR4H model (an hourly version of the popular GR4J model) are applied to four hydrologically distinct experimental catchments in Europe and New Zealand. The estimated models are scrutinized using several diagnostic measures, ranging from statistical metrics, such as the statistical reliability and precision of the predictive distribution of streamflow, to more process-oriented diagnostics based on flow-duration curves and the correspondence between model states and groundwater piezometers. Model performance was clearly catchment specific, with a single fixed structure unable to accommodate intercatchment differences in hydrological behavior, including seasonality and thresholds. This highlights an important limitation of any “fixed” model structure. In the experimental catchments, the ability of competing model hypotheses to reproduce hydrological signatures of interest could be interpreted on the basis of independent fieldwork insights. The potential of flexible frameworks such as SUPERFLEX is then examined with respect to systematic and stringent hypothesis-testing in hydrological modeling, for characterizing catchment diversity, and, more generally, for aiding progress toward a more unified formulation of hydrological theory at the catchment scale. When interpreted in physical process-oriented terms, the flexible approach can also serve as a language for dialogue between modeler and experimentalist, facilitating the understanding, representation, and interpretation of catchment behavior.

Description: Spatial variability of hydraulic aquifer parameters causes meandering, squeezing, stretching, and enhanced mixing of steady state plumes in concentrated hot-spots of mixing. Because the exact spatial distribution of hydraulic parameters is uncertain, the spatial distribution of enhanced mixing rates is also uncertain. We discuss how relevant the resulting uncertainty of mixing rates is for predicting concentrations. We develop analytical solutions for the full statistical distribution of steady state concentration in two-dimensional, statistically uniform domains with log-hydraulic conductivity following an isotropic exponential model. In particular, we analyze concentration statistics at the fringes of wide plumes, conceptually represented by a solute introduced over half the width of the domain. Our framework explicitly accounts for uncertainty of streamline meandering and uncertainty of effective transverse mixing (defined at the Darcy scale). We make use of existing low-order closed-form expressions that lead to analytical expressions for the statistical distribution of local concentration values. Along the expected position of the plume fringe, the concentration distribution strongly clusters at its extreme values. This behavior extends over travel distances of up to tens of integral scales for the parameters tested in our study. In this regime, the uncertainty of effective transverse mixing is substantial enough to have noticeable effects on the concentration probability density function. At significantly larger travel distances, intermediate concentration values become most likely, and uncertainty of effective transverse mixing becomes negligible. A comparison to numerical Monte Carlo simulations of flow and solute transport show excellent agreement with the theoretically derived expressions.

Description: A regionalization method based on a cluster probability model (a mixed multivariate Gaussian model) is proposed for grouping sites into nonoverlapping contiguous homogeneous regions defined by a Voronoi tessellation (Theisson polygons). The cluster probability model is applied to second-order standardized annual sample properties (mean, coefficient of variation, and autocorrelation) evaluated at the daily level of aggregation taken from each of 234 daily rainfall records with positions located in the Basque Country, Spain. Using the Bayesian information criterion, four clusters of sites are identified (which do not fall into contiguous regions). The distances between all neighboring pairs of sites connected by edges from the Delaunay planar graph are found. The probability that a site belongs to each of the four clusters is extracted from the fitted Gaussian model and multiplied into the probability that the neighboring site belongs to the same cluster. These products are divided by the squared distance between the sites and are summed to give an overall measure of a site belonging to a cluster that takes into account the classification of neighboring sites. Regions from the Voronoi tessellation of the points are classed on the basis of this measure and according to whether they are spatially isolated from other regions of the same class. Points that have the least influence on the variance of residual errors of the fitted model are found using a criterion based on Wilks' lambda for multivariate analysis of variance, and the classes of the least influential points are adjusted to ensure the overall regions are contiguous.

Description: A new model for Mediterranean forest fire regime assessment is presented and discussed. The model is based on the experimental evidence that fire is due to both hydrological and ecological processes and the relative role of fuel load versus fuel moisture is an important driver in fire ecology. Diverse scenarios are analyzed where either the hydrological forcing or the feedback between fire and hydrological characterization of the site is changed. The model outcome demonstrates that the two-way interaction between hydrological processes, biology, and fire regime drives the ecosystem toward a typical fire regime that may be altered either by an evolution of the biological characterization of the site or by a change of the hydrological forcing. This tenet implies that not every fire regime is compatible with the ecohydrological characterization of the site under study. This means that natural (nonantropogenic) fire cannot be modeled as an arbitrary external forcing because the coupled hydrological and biological processes determines its statistical characterization, and conversely, the fire regime affects the soil moisture availability and the outcome of different species competition under possible water stress. The new modeling approach presented here, when provided by a proper model parameterization, can advance the capability in predicting and managing fires in ecosystems influenced by climate and land use changes.

Description: This study explores the decomposition of predictive uncertainty in hydrological modeling into its contributing sources. This is pursued by developing data-based probability models describing uncertainties in rainfall and runoff data and incorporating them into the Bayesian total error analysis methodology (BATEA). A case study based on the Yzeron catchment (France) and the conceptual rainfall-runoff model GR4J is presented. It exploits a calibration period where dense rain gauge data are available to characterize the uncertainty in the catchment average rainfall using geostatistical conditional simulation. The inclusion of information about rainfall and runoff data uncertainties overcomes ill-posedness problems and enables simultaneous estimation of forcing and structural errors as part of the Bayesian inference. This yields more reliable predictions than approaches that ignore or lump different sources of uncertainty in a simplistic way (e.g., standard least squares). It is shown that independently derived data quality estimates are needed to decompose the total uncertainty in the runoff predictions into the individual contributions of rainfall, runoff, and structural errors. In this case study, the total predictive uncertainty appears dominated by structural errors. Although further research is needed to interpret and verify this decomposition, it can provide strategic guidance for investments in environmental data collection and/or modeling improvement. More generally, this study demonstrates the power of the Bayesian paradigm to improve the reliability of environmental modeling using independent estimates of sampling and instrumental data uncertainties.

Description: In many least developed countries, inadequate user willingness to pay (WTP) to achieve cost recovery for improvements to substandard rural water services is a major barrier to reaching targets such as the Millennium Development Goals. A meta-analysis of 21 contingent valuation studies conducted in least developed countries reveals that cost recovery from user demand is infeasible in most cases, and that rural areas are especially unwilling to pay enough to finance water service improvements. We argue that this is largely due to inability to pay cash rather than an absence of demand and propose two alternative financing approaches that may enable capital deficient communities to afford improvements. A discrete choice experiment, conducted in a rural catchment of Zambia, compares conventional cash-based WTP for different water service attributes with two alternative measures. (1) Willingness to borrow: Monthly payments in cash, with a no-interest loan given to the user. (2) Willingness to work: Instead of cash, payment in the form of contributing time devoted to unskilled labor. To different degrees, these alternatives elicit higher demand and enable cost recovery, providing evidence that demand-driven, economically sustainable water development efforts, as described here for Simango, Zambia, may be implemented for rural, resource-poor communities.

Description: Widespread pollution of groundwater by nutrients due to 20th century agricultural intensification has been of major concern in the developed world for several decades. This paper considers the River Thames catchment (UK), where water-quality monitoring at Hampton (just upstream of London) has produced continuous records for nitrate for the last 140 years, the longest continuous record of water chemistry anywhere in the world. For the same period, data are available to characterize changes in both land use and land management at an annual scale. A modeling approach is used that combines two elements: an estimate of nitrate available for leaching due to land use and land management; and, an algorithm to route this leachable nitrate through to surface or groundwaters. Prior to agricultural intensification at the start of World War II, annual average inputs were around 50 kg ha−1, and river concentrations were stable at 1 to 2 mg l−1, suggesting in-stream denitrification capable of removing 35 (±15) kt N yr−1. Postintensification data suggest an accumulation of 100 (±40) kt N yr−1 in the catchment, most of which is stored in the aquifer. This build up of reactive N species within the catchments means that restoration of surface nitrate concentrations typical of the preintensification period would require massive basin-wide changes in land use and management that would compromise food security and take decades to be effective. Policy solutions need to embrace long-term management strategies as an urgent priority.

Description: Shenzhen, as the first special economic zone in the world, has been in the process of rapid urbanization for 30 years. Many special economic zones have been established in China and other nations following Shenzhen's experience. However, Shenzhen has attained significant economic development with an attendant cost of environmental degradation, and similar results may be seen in other zones in the future. Here we use a pollution index method to evaluate the effect of such rapid urban development on the surface water quality in Shenzhen from 1991 to 2008. Rapid urbanization has affected surface water quality, but environmental policies can mitigate some of these effects, although such policy-induced improvements required some time before showing efficacy. As their use of special economic zones proliferates worldwide, greater consideration of the potential effects on water quality, and their overall sustainability, must receive greater attention.

Description: Regional frequency analysis (RFA) has a long history in hydrology, and numerous distinct approaches have been proposed over the years to perform the estimation of some hydrologic quantity at a regional level. However, most of these approaches still rely on strong hypotheses that may limit their application and complicate the quantification of predictive uncertainty. The objective of this paper is to propose a general Bayesian hierarchical framework to implement RFA schemes that avoid these difficulties. The proposed framework is based on a two-level hierarchical model. The first level of the hierarchy describes the joint distribution of observations. An arbitrary marginal distribution, whose parameters may vary in space, is assumed for at-site series. The joint distribution is then derived by means of an elliptical copula, therefore providing an explicit description of the spatial dependence between data. The second level of the hierarchy describes the spatial variability of parameters using a regression model that links the parameter values with covariates describing site characteristics. Regression errors are modeled with a Gaussian spatial field, which may exhibit spatial dependence. This framework enables performing prediction at both gaged and ungaged sites and, importantly, rigorously quantifying the associated predictive uncertainty. A case study based on the annual maxima of daily rainfall demonstrates the applicability of this hierarchical approach. Although numerous avenues for improvement can already be identified (among which is the inclusion of temporal covariates to model time variability), the proposed model constitutes a general framework for implementing flexible RFA schemes and quantifying the associated predictive uncertainty.

Description: The vadose zone plays an important role in surface water–groundwater interaction and exerts strong influences on biogeochemical, ecological, and hyporheic processes. It is also the presence of an unsaturated zone that controls the state of connection between surface water and groundwater. Despite recent advances on how hydrogeological variables affect surface water–groundwater interactions, there is limited understanding of the hydroclimatic effects of precipitation and evapotranspiration. More specifically, there is a need for a physically based understanding on the changes that may occur in response to changes in vegetation. While it may seem qualitatively obvious that the presence of vegetation can cause an unsaturated zone to develop underneath a riverbed and alter the state of connection, it has so far not been demonstrated quantitatively. Also, the influence of variables such as root extinction depth, topography, and the influence of land clearance has so far not been explored. In this study, fully coupled, physically based 2-D transient homogeneous models were used to simulate the impact of land clearance and revegetation on the state of connection of a perennial river system. The simulations showed that the presence of vegetation can create an unsaturated zone between a river and an aquifer and affect the state of connection and that the removal of deep-rooted vegetation from a catchment may have a significant impact on the state of connection as well as the condition of the water resource.

Description: Steady, Darcian, one-phase, phreatic surface flow of groundwater into a horizontal well with a pancake lens of light nonaqueous phase liquid (LNAPL) accumulated in the water table trough is studied by the method of complex analysis. A sharp interface model assumes groundwater capped by two isobaric limbs (groundwater–vadose zone interfaces) of a free surface with an in-between cambered segment of an immiscible LNAPL-water interface, along which pressure is hydrostatically increasing with the depth of the LNAPL “channel.” The complex potential polygon is mapped onto an auxiliary half plane where the complex physical coordinate of the flow domain is represented in terms of singular integrals as a solution of the Keldysh-Sedov problem. The shapes of semi-infinite “wings” of the water table contacting the vadose zone gas and of a finite length LNAPL-groundwater interface are found from parametric equations that involve the sink strength and location with respect to the pancake surface, the ordinate of the lowest trough point, and the volume of LNAPL accreted in the lens. Critical conditions, corresponding to the lens contour cusping toward the sink, are found. The Riesenkampf solution contains a free parameter, which is fixed by specifying either a point on the free surface or the volume of the trough-intercepted LNAPL.

Description: Water impoundment in the Three Gorges Reservoir (TGR) of China caused a large mass redistribution from the oceans to a concentrated land area in a short time period. We show that this mass shift is captured by the Gravity Recovery and Climate Experiment (GRACE) unconstrained global solutions at a 400 km spatial resolution after removing correlated errors. The WaterGAP Global Hydrology Model (WGHM) is selected to isolate the TGR contribution from regional water storage changes. For the first time, this study compares the GRACE (minus WGHM) estimated TGR volume changes with in situ measurements from April 2002 to May 2010 at a monthly time scale. During the 8 year study period, GRACE-WGHM estimated TGR volume changes show an increasing trend consistent with the TGR in situ measurements and lead to similar estimates of impounded water volume. GRACE-WGHM estimated total volume increase agrees to within 14% (3.2 km3) of the in situ measurements. This indicates that GRACE can retrieve the true amplitudes of large surface water storage changes in a concentrated area that is much smaller than the spatial resolution of its global harmonic solutions. The GRACE-WGHM estimated TGR monthly volume changes explain 76% (r2 = 0.76) of in situ measurement monthly variability and have an uncertainty of 4.62 km3. Our results also indicate reservoir leakage and groundwater recharge due to TGR filling and contamination from neighboring lakes are nonnegligible in the GRACE total water storage changes. Moreover, GRACE observations could provide a relatively accurate estimate of global water volume withheld by newly constructed large reservoirs and their impacts on global sea level rise since 2002.

Description: Fluvial sediment transport studies have long underscored the difficulty in reliably estimating transport model parameters, collecting accurate observations, and making predictions because of measurement error, natural variability, and conceptual model uncertainty. Thus, there is a need to identify modeling frameworks that accommodate these realities while incorporating functional relationships, providing probability-based predictions, and accommodating for conceptual model discrimination. Bayesian statistical approaches have been widely used in a number of disciplines to accomplish just this, yet applications in sediment transport are few. In this paper we propose and demonstrate a Bayesian statistical approach to a simple sediment transport problem as a means to overcome some of these challenges. This approach provides a means to rigorously estimate model parameter distributions, such as critical shear, given observations of sediment transport; provides probabilistically based predictions that are robust and easily interpretable; facilitates conceptual model discrimination; and incorporates expert judgment into model inference and predictions. We demonstrate a simple unisize sediment transport model and test it against simulated observations for which the “true” model parameters are known. Experimental flume observations were also used to assess the proposed model's robustness. Results indicate that such a modeling approach is valid and presents an opportunity for more complex models to be built in the Bayesian framework.

Description: Terrestrial water storage (TWS) estimates retrieved from the Gravity Recovery and Climate Experiment (GRACE) satellite mission were compared to TWS modeled by the Australian Water Resources Assessment (AWRA) system. The aim was to test whether differences could be attributed and used to identify model deficiencies. Data for 2003–2010 were decomposed into the seasonal cycle, linear trends and the remaining de-trended anomalies before comparing. AWRA tended to have smaller seasonal amplitude than GRACE. GRACE showed a strong (〉15 mm yr−1) drying trend in northwest Australia that was associated with a preceding period of unusually wet conditions, whereas weaker drying trends in the southern Murray Basin and southwest Western Australia were associated with relatively dry conditions. AWRA estimated trends were less negative for these regions, while a more positive trend was estimated for areas affected by cyclone Charlotte in 2009. For 2003–2009, a decrease of 7–8 mm yr−1 (50–60 km3 yr−1) was estimated from GRACE, enough to explain 6%–7% of the contemporary rate of global sea level rise. This trend was not reproduced by the model. Agreement between model and data suggested that the GRACE retrieval error estimates are biased high. A scaling coefficient applied to GRACE TWS to reduce the effect of signal leakage appeared to degrade quantitative agreement for some regions. Model aspects identified for improvement included a need for better estimation of rainfall in northwest Australia, and more sophisticated treatment of diffuse groundwater discharge processes and surface-groundwater connectivity for some regions.

Description: Understanding the relative influence of catchment structure (topography and topology), underlying geology, and vegetation on runoff response is key to interpreting catchment hydrology. Hillslope-riparian-stream (HRS) water table connectivity serves as the hydrologic linkage between a catchment's uplands and the channel network and facilitates the transmission of water and solutes to streams. While there has been tremendous interest in the concept of hydrological connectivity to characterize catchments, few studies have quantified hydrologic connectivity at the stream network and catchment scales with observational data. Here we examine how catchment topography, vegetation, and geology influenced patterns of stream network HRS connectivity and runoff dynamics across 11 nested headwater catchments in the Tenderfoot Creek Experimental Forest (TCEF), MT. This study builds on the empirical findings of Jencso et al. (2009) who found a strong linear relationship (r2 = 0.91) between the upslope accumulated area (UAA) and the annual duration of shallow groundwater table connectivity observed across 24 HRS transects (146 groundwater recording wells). We applied this relationship to the entire stream network across 11 nested catchments to quantify the frequency distribution of stream network connectivity through time, and quantify its relationship to catchment-scale runoff dynamics. Each catchment's hydrologic connectivity duration curve (CDC) was highly related to its flow duration curve (FDC) and the slope of the relationship varied across catchments. The slope represents the streamflow yield per unit connectivity (Conyield). We analyzed the slope of each catchment's CDC-FDC relationship or Conyield (annual, peak, transition, and base flow periods) in multiple linear regression models with common terrain, land cover vegetation, and geology explanatory variables. Significant predictors (p 〈 0.05) across 11 catchments included the ratio of flow path distances and gradients to the creek (DFC/GTC), geology, and a vegetation index. The order and strength of these predictors changed seasonally and highlight the hierarchical controls on headwater catchment runoff generation. Our results highlight direct and quantifiable linkages between catchment topography, vegetation, geology, their topology, and hydrologic dynamics.

Description: Water flow and solute transport in the vadose zone and groundwater during flood events were investigated in the lower reach of the Kuiseb River, Namibia, and in controlled column experiments. Simultaneous measurements of water level and electrical conductivity of the flood water in the stream channel and in the groundwater together with variations in the vadose zone water content, temperature, and pressure profiles allowed a detailed analysis of the various mechanisms governing solute transport in the subsurface during flash floods. The results indicated that on the land surface, flash floods emit, at their wetting fronts, instantaneous compression waves that propagate downward through the unsaturated zone to the water table. These compression waves generate abrupt solute-displacement events in the groundwater immediately after the arrival of the flood on the land surface, long before deep percolation and groundwater recharge begin. Each flood event launches into the vadose zone a wetting front that propagates down through the vadose zone and recharges the groundwater upon arrival at the water table. The first wetting front of each flood season leaches out soluble salts that have accumulated in the vadose zone during the dry season. However, water percolation through the unsaturated zone does not leach out the entire soluble salt capacity of the sediment, even if percolation takes place under high water-head flooding conditions for long periods. The incomplete leaching of the unsaturated zone by the percolating water releases soluble salts into the groundwater during every recharge event as a result of the rise of the water table into the vadose zone; this process results in a temporal increase of the groundwater electrical conductivity (EC).

Description: Previous studies, motivated by understanding water quality, have explored the mechanisms for heat transport and heat exchange in surface streams. In karst aquifers, temperature signals play an additional important role since they carry information about internal aquifer structures. Models for heat transport in karst conduits have previously been developed; however, these models make different, sometimes contradictory, assumptions. Additionally, previous models of heat transport in karst conduits have not been validated using field data from conduits with known geometries. Here we use analytical solutions of heat transfer to examine the relative importance of heat exchange mechanisms and the validity of the assumptions made by previous models. The relative importance of convection, conduction, and radiation is a function of time. Using a characteristic timescale, we show that models neglecting rock conduction produce spurious results in realistic cases. In contrast to the behavior of surface streams, where conduction is often negligible, conduction through the rock surrounding a conduit determines heat flux at timescales of weeks and longer. In open channel conduits, radiative heat flux can be significant. In contrast, convective heat exchange through the conduit air is often negligible. Using the rules derived from our analytical analysis, we develop a numerical model for heat transport in a karst conduit. Our model compares favorably to thermal responses observed in two different karst settings: a cave stream fed via autogenic recharge during a snowmelt event, and an allogenically recharged cave stream that experiences continuous temperature fluctuations on many timescales.

Description: In this paper, the robust counterpart (RC) approach (Ben-Tal et al., 2009) is applied to optimize management of a water supply system (WSS) fed from aquifers and desalination plants. The water is conveyed through a network to meet desired consumptions, where the aquifers recharges are uncertain. The objective is to minimize the net present value cost of multiyear operation, satisfying operational and physical constraints. The RC is a min-max guided approach, which converts the original problem into a deterministic equivalent problem, requiring only that the uncertain parameters resides within a user-defined uncertainty set. The robust policy obtained by the RC approach is compared with polices obtained by other decision-making approaches including stochastic approaches.