Showing papers in "Water Resources Research in 2004"

A rational function approach for estimating mean annual evapotranspiration

Abstract: [1] Mean annual evapotranspiration from a catchment is determined largely by precipitation and potential evapotranspiration; characteristics of the catchment (e.g., soil, topography, etc.) play only a secondary role. It has been shown that the ratio of mean annual potential evapotranspiration to precipitation (referred as the index of dryness) can be used to estimate mean annual evapotranspiration by using one additional parameter. This study evaluates the effects of climatic and catchment characteristics on the partitioning of mean annual precipitation into evapotranspiration using a rational function approach, which was developed based on phenomenological considerations. Over 470 catchments worldwide with long-term records of precipitation, potential evapotranspiration, and runoff were considered, and results show that model estimates of mean annual evapotranspiration agree well with observed evapotranspiration taken as the difference between precipitation and runoff. The mean absolute error between modeled and observed evapotranspiration was 54 mm, and the model was able to explain 89% of the variance with a slope of 1.00 through the origin. This indicates that the index of dryness is the most significant variable in determining mean annual evapotranspiration. Results also suggest that forested catchments tend to show higher evapotranspiration than grassed catchments and their evapotranspiration ratio (evapotranspiration divided by precipitation) is most sensitive to changes in catchment characteristics for regions with the index of dryness around 1.0. Additionally, a stepwise regression analysis was performed for over 270 Australian catchments where detailed information of vegetation cover, precipitation characteristics, catchment slopes, and plant available water capacity was available. It is shown that apart from the index of dryness, average storm depth, plant available water capacity, and storm arrival rate are also significant.

A mechanistic model for river incision into bedrock by saltating bed load

Abstract: [1] Abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock. Here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium. We assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity. Particle impact velocity and impact frequency depend on saltation trajectories, which can be predicted using empirical functions of excess shear stress. The model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity. Maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed. Maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension. Measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter, which relates rock strength to rock resistance to abrasive wear. The model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates, not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed.

Predictive pore-scale modeling of two-phase flow in mixed wet media

Abstract: [1] We show how to predict flow properties for a variety of porous media using pore-scale modeling with geologically realistic networks Starting with a network representation of Berea sandstone, the pore size distribution is adjusted to match capillary pressure for different media, keeping the rank order of pore sizes and the network topology fixed Then predictions of single and multiphase properties are made with no further adjustment of the model We successfully predict relative permeability and oil recovery for water wet, oil wet, and mixed wet data sets For water flooding we introduce a method for assigning contact angles to match measured wettability indices The aim of this work is not simply to match experiments but to use easily acquired data to predict difficult to measure properties Furthermore, the variation of these properties in the field, due to wettability trends and different pore structures, can now be predicted reliably

Multivariate hydrological frequency analysis using copulas

Abstract: [1] This article presents the modeling of multivariate extreme values using copulas. Our approach allows us to model the dependence structure independently of the marginal distributions, which is not possible with standard classical methods. The methodology has been applied on two different problems in hydrology. The first application is concerned with the combined risk in the framework of frequency analysis. Four copulas have been tested on peak flows from the watershed of Peribonka in Quebec, Canada. The second application relates to the joint modeling of peak flows and volumes. Three copulas have been applied to the watershed of the Rimouski River in Quebec, Canada. This approach using copulas is promising since it allows us to take into account a wide range of correlation which can happen in hydrology.

Frequency analysis via copulas: Theoretical aspects and applications to hydrological events

Abstract: [1] In this paper we provide a general theoretical framework exploiting copulas for studying the return periods of hydrological events; in particular, we consider events depending upon the joint behavior of two nonindependent random variables, an approach which can easily be generalized to the multivariate case. We show that using copulas may greatly simplify the calculations and may even yield analytical expressions for the isolines of the return periods, both in the unconditional and in the conditional case. In addition, we show how a new probability distribution may be associated with the return period of specific events and introduce the definitions of sub-, super-, and critical events as well as those of primary and secondary return periods. An illustration of the techniques proposed is provided by analyzing some case studies already examined in literature.

Impact of Heterogeneity, Bed Forms, and Stream Curvature on Subchannel Hyporheic Exchange

Abstract: [1] Advection through hyporheic zones (HZ) consisting of heterogeneous channel bend streambed deposits and their equivalent homogenous medium was investigated using finite difference groundwater flow and transport simulations and forward particle tracking. The top prescribed head boundary was varied in order to mimic various stream channel head distributions resulting from the presence of bed forms and channel curvature. Flux calculations show that heterogeneity causes significant additional HZ flux compared to an equivalent homogenous medium. However, the major cause of HZ flux is a spatially periodic (sinusoidal) head distribution along the boundary, representing the effect of bed forms. The additional influence of heterogeneity on the total channel-bed exchange and the overall HZ geometry are increased when boundary head sinusoidal fluctuation is more subdued. We present dimensionless numbers that summarize these relationships. Heterogeneity's influence is further magnified by considering the effect of channel curvature on boundary heads. The simulations illustrate the dynamic influence of heterogeneity on the hyporheic zone since the various head boundaries employed in our modeling efforts are a proxy for different surface water conditions and bed form states that may occur during a single flood. Furthermore, we show that residence times (total tracking times) of particles originating from the streambed follow a lognormal distribution. In the presence of heterogeneity, residence times can decrease or they can increase compared to residence times for homogeneous conditions depending on the relative positions of the heterogeneities and the bed forms. Hence streambed heterogeneity and stream curvature, factors often neglected in previous modeling efforts, combine with bed form configuration to dynamically determine HZ geometry, fluxes, and residence time distributions.

Lattice‐Boltzmann simulation of two‐phase flow in porous media

Abstract: [1] We simulate two-fluid-phase flow at the pore scale using a lattice Boltzmann (LB) approach. Using a parallel processing version of the Shan-Chen model that we developed, we simulate a set of ideal two-fluid systems and a model two-fluid-phase porous medium system comprised of a synthetic packing with a relatively uniform distribution of spheres. We use the set of ideal two-phase systems to validate the approach and provide parameter information, which we then use to simulate a sphere-pack system. The sphere-pack system is designed to mimic laboratory experiments conducted to evaluate the hysteretic capillary pressure saturation relation for a system consisting of water, tetrachloroethylene, and a glass bead porous medium. Good agreement is achieved between the measured hysteretic capillary pressure saturation relations and the LB simulations when comparing entry pressure, displacement slopes, irreducible saturation, and residual entrapment. Our results further show that while qualitatively similar results are obtained when comparing systems consisting of 1200 spheres and 150 spheres, there is a significant difference between these two levels, suggesting a lower bound on the size of a representative elementary volume.

The limited growth of vegetated shear layers

Abstract: [1] In contrast to free shear layers, which grow continuously downstream, shear layers generated by submerged vegetation grow only to a finite thickness. Because these shear layers are characterized by coherent vortex structures and rapid vertical mixing, their thickness controls exchange between the vegetation and the overlying water. Experiments conducted in a laboratory flume show that the growth of these obstructed shear layers is arrested once the production of shear-layer-scale turbulent kinetic energy (SKE) is balanced by dissipation of SKE within the canopy. This equilibrium condition, along with a mixing length closure scheme, was used in a one-dimensional numerical model to predict the mean velocity profiles of the experimental shear layers. The agreement between model and experiment is very good, but field application of the model is limited by a lack of description of the drag coefficient in a submerged canopy.

Source waters and flow paths in an alpine catchment, Colorado Front Range, United States

Abstract: [1] Source waters and flow paths of streamflow draining high-elevation catchments of the Colorado Rocky Mountains were determined using isotopic and geochemical tracers during the 1996 snowmelt runoff season at two subcatchments of the Green Lakes Valley, Colorado Front Range. A two-component hydrograph separation using δ18O indicates that new water dominated (82 ± 6%) streamflow at the 8-ha Martinelli catchment and old water dominated (64 ± 2%) at the 225-ha Green Lake 4 (GL4) catchment. Snowmelt became isotopically enriched as the melt season progressed, complicating the interpretation of source water models. Thus old water may be underestimated if the temporal variation in δ18O of snowmelt is ignored or extrapolated from point measurements to the catchment. Two-component hydrograph separations for unreacted and reacted waters using a single geochemical tracer were not always meaningful. Three-component hydrograph separations using end-member mixing analysis indicated that subsurface flow contributed more than two thirds to the streamflow at both catchments. Talus fields contributed more than 40% of the total discharge during summer at the GL4 catchment. A conceptual model was established for flow generation based on these results. It is suggested that surface water and groundwater interactions are much more important to the quantity and quality of surface water in high-elevation catchments than previously thought.

Dynamics of evapotranspiration in semiarid grassland and shrubland ecosystems during the summer monsoon season, central New Mexico

Abstract: higher at the grassland than at the shrubland by 20% or 70 W m � 2 because of differences in net radiation (Rn) and soil heat flux (G). At both sites, midday evaporative fraction and daily ET are strongly correlated with surface soil moisture (q0–5cm) but poorly correlated with water content at greater depths or averaged throughout the entire root zone. The sensitivity of EF to q0–5cm is 30% lower at the grassland site. The differences in Qa and EF cancel, yielding similar time series of ET at the two sites. Decreases in q0–5cm, ET, and EF following rainfall events are rapid: exponential time constants are less than 3 days. With the exception of the largest storms, infiltration following rainfall events only wets the top 10 cm of soil. Therefore the surface soil layer is the primary reservoir for water storage and source for ET during the monsoon season, suggesting that direct evaporation is a large component of ET. Given these results, predicting ET based on root zone–averaged soil moisture is inappropriate in the semiarid environments studied here. INDEX TERMS: 1818 Hydrology: Evapotranspiration; 1833 Hydrology: Hydroclimatology; 1866 Hydrology: Soil moisture; 1878 Hydrology: Water/energy interactions; KEYWORDS: Bouteloua eriopoda, Bowen ratio, evapotranspiration, grassland, Larrea tridentata, shrubland Citation: Kurc, S. A., and E. E. Small (2004), Dynamics of evapotranspiration in semiarid grassland and shrubland ecosystems during the summer monsoon season, central New Mexico, Water Resour. Res., 40, W09305, doi:10.1029/2004WR003068.

Catchment hydrologic response with a fully distributed triangulated irregular network model

Abstract: [1] This study explores various aspects of catchment hydrology based on a mechanistic modeling of distributed watershed processes. A new physics-based, distributed-parameter hydrological model that uses an irregular spatial discretization is introduced. The model accounts, on a continuous basis, for the processes of rainfall interception, evapotranspiration, moisture dynamics in the unsaturated and saturated zones, and runoff routing. Simulations of several mid- to large-sized watersheds (∼103 km2) highlight various dynamic relationships between the vadose zone–groundwater processes and their dependence on the land surface characteristics. It is argued that the model inferences can be used for interpretation of distributed relationships in a catchment. By exploiting a multiple-resolution representation, the hydrologic features of the watershed terrain are captured with only 5–10% of the original grid nodes. This computational efficiency suggests the feasibility of the operational use of fully distributed, physics-based models for large watersheds.

An aggregate drought index: Assessing drought severity based on fluctuations in the hydrologic cycle and surface water storage

Abstract: [1] An aggregate drought index (ADI) has been developed, and evaluated within three diverse climate divisions in California The ADI comprehensively considers all physical forms of drought (meteorological, hydrological, and agricultural) through selection of variables that are related to each drought type Water stored in large surface water reservoirs was also included Hydroclimatic monthly data for each climate division underwent correlation-based principal component analysis (PCA), and the first principal component was deseasonalized to arrive at a single ADI value for each month ADI time series were compared against the Palmer Drought Severity Index (PDSI) to describe two important droughts in California, the 1976–1977 and 1987–1992 events, from a hydroclimatological perspective The ADI methodology provides a clear, objective approach for describing the intensity of drought and can be readily adapted to characterize drought on an operational basis

Confluence effects in rivers: Interactions of basin scale, network geometry, and disturbance regimes

Abstract: [1] We reviewed 14 studies documenting the effects of tributaries on river morphology at 167 confluences along 730 km of river spanning seven orders of magnitude in drainage area in western United States and Canada. In both humid and semiarid environments the probability of observing significant confluence-related changes in channel and valley morphology due to tributary influxes of sediment (e.g., changes in gradient, particle size, and terraces, etc.) increased with the size of the tributary relative to the main stem. Effects of confluences on river morphology are conditioned by basin shape and channel network patterns, and they include the nonlinear separation of geomorphically significant confluences in river networks. Other modifying factors include local network geometry and drainage density. Confluence-related landforms (i.e., fans, bars, terraces, etc.) are predicted to be dominated by older features in headwaters and younger features downstream, a pattern driven by the frequency and magnitude of floods and punctuated sediment supply that scale with watershed size. INDEX TERMS: 1824 Hydrology: Geomorphology (1625); 1815 Hydrology: Erosion and sedimentation; 1821 Hydrology: Floods; 1848 Hydrology: Networks; KEYWORDS: confluences, fluvial geomorphology, river networks

Hyporheic exchange with heterogeneous streambeds: Laboratory experiments and modeling

Abstract: [1] Hyporheic exchange is generally analyzed with the assumption of a homogeneous hyporheic zone. In reality, streambed sediments have a heterogeneous structure, and this natural heterogeneity produces spatially variable interfacial fluxes and complex hyporheic exchange patterns. To assess the basic effects of sediment structure on hyporheic exchange, we performed salt and dye injection experiments in a recirculating laboratory flume with two heterogeneous sediment beds characterized by negative-exponential correlated random hydraulic conductivity fields. Dye injections showed that the hyporheic flow structure was controlled by the spatial relationship of bed forms to high- and low-permeability regions of the streambed. As no existing model could represent these effects, we developed a new finite element model to calculate the pore water flow field resulting from the interaction of the bed form-induced boundary head distribution and the heterogeneous sediment structure. A numerical particle-tracking approach was then used to assess the resulting hyporheic exchange. The combined flow-transport model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange up to t ≈ 30 hours. The heterogeneous hydraulic conductivity field caused both greater spatial variability in the water flux through the bed surface and a greater average interfacial flux than would have occurred with a homogeneous bed. The layered correlation structure of the streambed produced an effective anisotropy that favored longitudinal pore water flow and caused a relatively rapid decrease of the mean pore water velocity with depth. As a result, solute penetration into the bed was confined to a more shallow region than would have occurred with a homogeneous bed. The combination of faster near-surface transport and shallower solute penetration produced a shorter mean hyporheic residence time. On the basis of the combination of experimental results and model simulations we conclude that the structural heterogeneity of streambed sediments produces more spatially limited hyporheic exchange that occurs with greater spatial variability and at a higher overall rate.

Analysis of water resources variability in the Yellow River of China during the last half century using historical data

Abstract: [1] The river discharges have decreased continuously during the last half century in the Yellow River, the second-largest river basin in China. In particular, a drying up of the main river along the lower reach has occurred since 1972, and the situation has become more and more serious during the 1990s. Using 50 years of meteorological data from 108 stations together with a collection of irrigation data, the long-term changes in the river discharge have been investigated with a view to identifying the reason for the drying up of the Yellow River. It was found that the annual precipitation generally decreased (−45.3 mm/50 yr) while the air temperature generally increased (+1.28°C/50 yr). From the 1960s to the 1970s the precipitation decreased by 29.6 mm/10 yr, the evaporation increased by 7 mm/10 yr (for pan evaporation), and the irrigation water usage increased by 10.5 mm/10 yr. As a consequence the drying up of the Yellow River has occurred since 1972. Irrigation was developed continuously in the 1980s, but the drying-up situation maintained at the same level as during the 1970s. The reason for this was the increase in precipitation (by 10.3 mm/10 yr) and the sharp decrease in the evaporation (by 133 mm/10 yr for pan evaporation). During the 1990s the irrigation was maintained at a level similar to that during the 1980s, but the drying-up situation was greatly aggravated. The reason for this was found to be a result of the decrease in precipitation (by 38.2 mm/10 yr) and the increase in evaporation (by 52 mm/10 yr for pan evaporation).

Multidecadal variability of rainfall and streamflow: Eastern Australia

Abstract: [1] This study investigates the influence of the El Nino–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO) on rainfall and streamflow regimes of eastern Australia. An analysis of historical rainfall and streamflow data for Queensland (QLD), New South Wales (NSW), and Victoria (VIC) reveals strong relationships between these indices and seasonal rainfall and streamflow totals. Rainfall and streamflow in NSW and QLD are shown to be significantly enhanced during the La Nina phase of ENSO, with La Nina impacts diminishing as one moves south into VIC. In addition, the study shows that on a multidecadal timescale the negative phase of the IPO is associated with “wetter” conditions than the positive phase. Importantly, the already enhanced La Nina rainfall and streamflow is demonstrated to be even further magnified during La Nina events that occur in the IPO negative phase. This result is of particular importance as the influence of ENSO in VIC appears to be weak; however, the results indicate that some useful predictability of ENSO impacts can be achieved during the negative phase of the IPO for VIC.

Use of hydraulic tests at different scales to characterize fracture network properties in the weathered‐fractured layer of a hard rock aquifer

Abstract: The hydrodynamic properties of the weathered-fractured layer of a hard-rock pilot watershed in a granitic terrain are characterized using hydraulic tests at different scales. The interpretation of numerous slug tests leads to characterize the statistical distribution of local permeabilities in the wells. The application of flowmeter profiles during injection tests determines the vertical distribution of conductive fracture zones and their permeabilities. It appears that the extension of the most conductive part of the weathered-fractured layer is limited down to 35 meters depth. The partition of drainage porosity between blocks (90%) and fractures (10%) is determined thanks to the interpretation of pumping tests using a double porosity model. The application of anisotropic and single fracture analytical solutions on pumping test data allows to determine, respectively, the degree of anisotropy of permeability ( ) and the radius (4 to 16 meters) of the horizontal conductive fractures crossed by the wells. Two different scales of fractures networks are identified: the primary fracture network (PFN), which affects the matrix on a decimeter scale by contributing to an increase in the permeability and storage capacity of the blocks, and the secondary fracture network (SFN), which affects the blocks at the borehole scale. SFN is composed of two sets of fractures. The main set of horizontal fractures is responsible for the sub-horizontal permeability of the weathered-fractured layer. A second set of less permeable sub-vertical fractures insures the connectivity of the aquifer at the borehole scale. The good connectivity of fractures networks is shown by fractional dimension flow solutions. The absence of scale effect in the study area suggests that the hydraulic conductivity at the borehole scale is laterally homogeneous. Finally, the analysis and synthesis of the hydrodynamic properties allow to propose a comprehensive hydrodynamic model of the fractured-weathered layer. Many geological and hydrogeological indicators suggest that a continuous and laterally homogeneous weathering process is responsible for the origin of the fractures and permeability encountered in the aquifer. These results confirm the major role played by weathering in the origin of fractures and on resulting hydrodynamic parameters in the shallow part of hard-rock aquifers.

Influence of hydraulic property correlation on predicted dense nonaqueous phase liquid source zone architecture, mass recovery and contaminant flux

Abstract: [1] Organic liquid saturation distributions resulting from a simulated tetrachloroethene (PCE) spill were generated with alternative models of spatially varying aquifer properties for a statistically homogeneous, nonuniform sand aquifer. The distributions were analyzed to quantify DNAPL source zone characteristics and then incorporated as initial conditions for simulated PCE recovery using surfactant-enhanced aquifer remediation (SEAR). The predicted evolution of the spatial distribution of DNAPL saturations or source zone ‘‘architectures’’ and associated remediation efficiencies are strongly influenced by the spatial correlation of aquifer parameters and multiphase flow constitutive relationships. Model predictions suggest that removal of 60 to 99% of entrapped PCE can reduce dissolved contaminant concentration and mass flux under natural gradient conditions by approximately two orders of magnitude. Aqueous phase contaminant flux, however, does not vary consistently as a function of the percentage of DNAPL removed, and notable differences in flux evolution were observed for models incorporating correlated versus uncorrelated capillary entry pressure and permeability fields. Simulation results demonstrate that the application of alternative models of aquifer parameter spatial variability can influence predicted DNAPL infiltration, entrapment, and recovery, even for relatively homogeneous aquifers of the type investigated here. Results also demonstrate potential benefits, in the form of reduced mass flux, accruing from partial mass removal that may not be readily predicted from analyses relying on simplified conceptual models for DNAPL source zone architecture or aquifer flow fields. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 1832 Hydrology: Groundwater transport; 1831 Hydrology: Groundwater quality; KEYWORDS: contamination mass flux, DNAPL remediation, DNAPL source zone, heterogeneity, nonuniformity

A stochastic approach for assessing the uncertainty of rainfall‐runoff simulations

Abstract: [1] Rainfall-runoff models have received a great deal of attention by researchers in the last decades. However, the analysis of their reliability and uncertainty has not been treated as thoroughly. In the present study, a technique for assessing the uncertainty of rainfall-runoff simulations is presented that makes use of a meta-Gaussian approach in order to estimate the probability distribution of the model error conditioned by the simulated river flow. The proposed technique is applied to the case study of an Italian river basin, for which the confidence limits of simulated river flows are derived and compared with the respective actual observations.

A general power equation for predicting bed load transport rates in gravel bed rivers

Abstract: [1] A variety of formulae has been developed to predict bed load transport in gravel bed rivers, ranging from simple regressions to complex multiparameter formulations. The ability to test these formulae across numerous field sites has, until recently, been hampered by a paucity of bed load transport data for gravel bed rivers. We use 2104 bed load transport observations in 24 gravel bed rivers in Idaho to assess the performance of eight different formulations of four bed load transport equations. Results show substantial differences in performance but no consistent relationship between formula performance and degree of calibration or complexity. However, formulae containing a transport threshold typically exhibit poor performance. Furthermore, we find that the transport data are best described by a simple power function of discharge. From this we propose a new bed load transport equation and identify channel and watershed characteristics that control the exponent and coefficient of the proposed power function. We find that the exponent is principally a factor of supply-related channel armoring (transport capacity in excess of sediment supply), whereas the coefficient is related to drainage area (a surrogate for absolute sediment supply). We evaluate the accuracy of the proposed power function at 17 independent test sites.

Wavelet analysis of variability in annual Canadian streamflows

Abstract: [1] Wavelet analysis is used to identify and describe variability in annual Canadian streamflows and to gain insights into the dynamical link between the streamflows and the dominant modes of climate variability in the Northern Hemisphere. Results from applying continuous wavelet transform to mean annual streamflows from 79 rivers selected from the Canadian Reference Hydrometric Basin Network (RHBN) reveal striking climate-related features before the 1950s and after the 1970s. The span of available observations, 1911–1999, allows for depicting variance for periods up to 12 years. Scale-averaged wavelet power spectra are used to simultaneously assess the interannual and spatial variability in 79 annual streamflow time series. The most striking feature, in the 2–3 year band and in the 3–6 year band (the 6–12 year band is dominated by white noise (since 1950) and is not considered further) is a net distinction between the timing and intensity of the interannual variability in western, central, and eastern streamflows, which is shown to be linked to the regional climatology. It is found that for the 2–3 year band, the Canadian streamflows are depicted mainly by the Pacific North America (PNA) during 1950–1999, and the Northern Hemisphere Annular Mode (NAM) only prior to 1950, and the North Atlantic Oscillation (NAO) after 1970. Similarly, in the 3–6 year band, the streamflows are depicted mostly by the NAO, the sea surface temperature anomalies over the Nino-3 region (ENSO3) and the PNA during the period 1950–1999, and the NAM prior to 1950. Furthermore, strong local correlations between teleconnection patterns and western, central, and eastern streamflows are also revealed in both the 2–3 and 3–6 year bands with striking changes around 1950 and 1970. The correlation analysis in the 2–3 year and 3–6 year bands revealed the presence of two change points in the west and east streamflows occurring around 1950 and 1970.

Development of effective and efficient rainfall-runoff models using integration of deterministic, real-coded genetic algorithms and artificial neural network techniques

Abstract: [1] Many researchers have reported about the problems in modeling low-magnitude flows while developing artificial neural network (ANN) rainfall-runoff models trained using popular back propagation (BP) method and have suggested the use of alternative training methods. This paper presents the results of a new approach employing real-coded genetic algorithms (GAs) to train ANN rainfall-runoff models, which are able to overcome such problems. The paper also presents a new class of models termed gray box models that integrate deterministic and ANN techniques for hydrologic modeling. Daily rainfall and streamflow data from the Kentucky River watershed were employed to test the new approach. Many standard statistical measures were employed to assess and compare various models investigated. The results obtained in this study demonstrate that ANN rainfall-runoff models trained using real-coded GA are able to predict daily flow more accurately than the ANN rainfall-runoff models trained using BP method. The proposed approach of training ANN models using real-coded GA can significantly improve the estimation accuracy of the low-magnitude flows. It was found that the gray box models that are capable of exploiting the advantages of both deterministic and ANN techniques perform better than the purely black box type ANN rainfall-runoff models. A partitioning analysis of results is needed to evaluate the performance of various models in terms of their efficiency in modeling and effectiveness in accurately predicting varying magnitude flows (low, medium, and high flows).

A comparative study of Markov chain Monte Carlo methods for conceptual rainfall‐runoff modeling

Abstract: [1] One challenge that faces hydrologists in water resources planning is to predict the catchment's response to a given rainfall. Estimation of parameter uncertainty (and model uncertainty) allows assessment of the risk in likely applications of hydrological models. Bayesian statistical inference provides an ideal means of assessing parameter uncertainty, whereby prior knowledge about the parameter is combined with information from the available data to produce a probability distribution (the posterior distribution) that describes uncertainty about the parameter and serves as a basis for selecting appropriate values for use in modeling applications. Widespread use of Bayesian techniques in hydrology has been hindered by difficulties in summarizing and exploring the posterior distribution. These difficulties have been largely overcome by recent advances in Markov chain Monte Carlo (MCMC) methods that involve Monte Carlo sampling of the posterior distribution. This study compares four MCMC sampling algorithms in the context of rainfall-runoff modeling. The algorithms compared include a conventional Metropolis-Hastings algorithm used previously in hydrological applications which uses a combination of block and single-site updating and an adaptive Metropolis algorithm that has characteristics that are well suited to model parameters with a high degree of correlation and interdependence, as is often evident in hydrological models. In addition to these, two other algorithms are evaluated to clarify the relative importance of updating all model parameters as a block versus updating each parameter one at a time. The MCMC techniques are compared for simplicity, ease of use, statistical efficiency in exploration of the parameter space, and speed of implementation, using 11 years of daily rainfall-runoff data from the Bass river catchment in Australia. The results show that the adaptive Metropolis algorithm is superior in many respects and can offer a relatively simple basis for assessing parameter uncertainty in hydrological modeling studies and that the efficiency of the adaptive algorithm is not solely attributed to the block-updating element of the algorithm.

Interfacial area measurements for unsaturated flow through a porous medium

Abstract: [1] Multiphase flow and contaminant transport in porous media are strongly influenced by the presence of fluid-fluid interfaces. Recent theoretical work based on conservation laws and the second law of thermodynamics has demonstrated the need for quantitative interfacial area information to be incorporated into multiphase flow models. We have used synchrotron based X-ray microtomography to investigate unsaturated flow through a glass bead column. Fully three-dimensional images were collected at points on the primary drainage curve and on the secondary imbibition and drainage loops. Analysis of the high-resolution images (17 micron voxels) allows for computation of interfacial areas and saturation. Corresponding pressure measurements are made during the course of the experiments. Results show the fluid-fluid interfacial area increasing as saturation decreases, reaching a maximum at saturations ranging from 20 to 35% and then decreasing as the saturation continues to zero. The findings support results of numerical studies reported in the literature.

Analytical solutions for leakage rates through abandoned wells

Abstract: [1] Disposal of waste fluids via injection into deep saline aquifers is practiced in a variety of industries. Injection takes place in sedimentary basins that often have a history of oil and gas exploration and production, which means that wells other than those used for waste disposal may exist in the vicinity of the injection site. These existing wells provide possible pathways for leakage of waste fluids toward the shallow subsurface and the land surface. For single-phase flows of liquids with essentially constant properties, the equations governing the system are linear, and solutions may be written using the superposition principle. Because leakage through existing wells produces a time-varying flux rate, the solution of the governing equations involves convolution integrals. Previous solutions have addressed the problem of one injection well, one existing (passive) well, and a simple geometry of two aquifers separated by an aquitard by use of Laplace transforms. Even for this simple case, inversion of the transform is difficult. Solutions involving more than one passive well have not been developed. Nor have solutions been developed for more than two aquifers and one aquitard. Realistic injection cases often involve layered systems with multiple aquifers and aquitards, as well as multiple passive wells, sometimes numbering in the hundreds. Solutions for the general case of multiple aquifers and wells may be developed through introduction of approximations to the well function and appropriate simplification of the convolution integral. Such a solution is computationally simple. Comparison to solutions using the full (Laplace transform) solution indicates that the new solution procedure produces excellent results. Application of the new solution to a case of multiple passive wells shows that the cumulative leakage flux in the passive wells is not a simple sum of the single-well case, owing to leakage-induced drawdown around the passive wells. In addition, application to the case of multiple aquifers and aquitards demonstrates the importance of leakage into intervening aquifers as a mechanism to mitigate leakage into shallow zones, a process referred to as the “elevator model.” The new analytical solution provides a tool to analyze practical injection problems and forms a foundation on which more complex solutions, such as those involving injection of a nonaqueous fluid into a deep brine formation, may be based.

Landscape controls on nitrate removal in stream riparian zones

Abstract: [1] We examined how landscape hydrogeologic characteristics influence groundwater nitrate removal by eight stream riparian sites on glacial till and outwash landscapes in southern Ontario, Canada. During high water table periods in 2000–2002, mean NO3−-N input concentrations from adjacent cropland to the riparian sites ranged from 0.15 to 44.7 mg L−1. Seven of the eight sites had a mean nitrate removal efficiency of >90%. This removal occurred within the first 15 m of the riparian zone at three sites with loamy sand and sandy loam soils overlying a shallow confining layer at 1–2 m. However, at four of five sites with more conductive sand and cobble sediments the width required for 90% nitrate removal varied from >25 m to a maximum of 176 m at a site with a confining layer at 6 m. Sites linked to an extensive thick (>6 m) upland aquifer with a slope gradient of >15% at the riparian perimeter had high nitrate inputs throughout the year and were large nitrate sinks. Sites with gentle topography (<4–5%) and <2 m of permeable sediments were minor nitrate sinks because of small nitrate inputs that were limited to the late autumn-spring period. A conceptual model linking landscape hydrogeologic characteristics to riparian zone nitrate removal capacity is developed to understand and predict the effectiveness of riparian buffers at the landscape scale.

A new topographic index to quantify downslope controls on local drainage

Abstract: [1] Topography is an important control on hydrological processes. One approach to quantify this control is the topographic ln(a/tanβ) index. This index has become widely used in hydrology, but it utilizes a relatively small portion of the information contained in a digital elevation model (DEM). One potentially important feature not considered in the implementation of the ln(a/tanβ) index is the enhancement or impedance of local drainage by downslope topography. This effect could be important in some terrain for controlling hydraulic gradients. We propose a new way of estimating the hydraulic gradient by calculating how far downhill (Ld, [m]) a parcel of water must move in order to lose a certain amount of potential energy (d, [m]). Expressed as a gradient, tanαd = d/Ld, values tend to be lower on concave slope profiles and higher on convex slope profiles compared with the local gradient, tanβ. We argue that the parameter d controls the deviation of hydraulic gradient from surface slope. While we determine this subjectively, landscape relief, DEM resolution, and soil transmissivity should be considered at the selection of d. We found the downslope index values to be less affected by changes in DEM resolution than local slope. Three applications are presented where the new index is shown to be useful for hydrological, geomorphological, and biogeochemical applications.

Hydrological flow paths during snowmelt: Congruence between hydrometric measurements and oxygen 18 in meltwater, soil water, and runoff

Abstract: soil during the spring. The approximately sixtyfold increase in runoff, from 0.13 mm d � 1 to 8 mm d � 1 , was generated by a 30–40 cm rise of the groundwater level. The total runoff during the snowmelt period from late April to late May was 134 mm, of which 75% was preevent water. Mass balance calculations based on hydrometric and isotopic data independently, both using upscaling of a hillslope transect to the entire 13-ha catchment, provided similar results of both water storage changes and the amount of event water that was left in the catchment after the snowmelt. In general, groundwater levels and runoff were strongly correlated, but different functional relationships were observed for frozen and unfrozen soil conditions. Although runoff generation in the catchment generally could be explained by the transmissivity feedback concept, the results suggest that there is a temporal variability in the flow pathways during the spring controlled by soil frost during early snowmelt. INDEX TERMS: 1860 Hydrology: Runoff and streamflow; 1836 Hydrology: Hydrologic budget (1655); 1821 Hydrology: Floods; 1823 Hydrology: Frozen ground; KEYWORDS: hydrograph separation, oxygen 18, snowmelt, spring flood, boreal, northern Sweden, transmissivity feedback

Control‐volume method for numerical simulation of two‐phase immiscible flow in two‐ and three‐dimensional discrete‐fractured media

Abstract: [1] We provide a numerical procedure for the simulation of two-phase immiscible and incompressible flow in two- and three-dimensional discrete-fractured media. The concept of cross-flow equilibrium is used to reduce the fracture dimension from n to (n-1) in the calculation of flow in the fractures. This concept, which is often referred to as the discrete-fracture model, has a significant effect on the reduction of computational time. The spatial discretization is performed with the control-volume method. This method is locally conservative and allows the use of unstructured grids to represent complex geometries, such as discrete-fracture configurations. The relative permeability is upwinded with a criterion based on the evaluation of the flux direction at the boundaries of the control volumes, which is consistent with the physics of fluid flow. The system of partial differential equations is decoupled and solved using the implicit-pressure, explicit-saturation (IMPES) approach. The algorithm has been successfully tested in two- and three-dimensional numerical simulations of wetting phase fluid injection (such as water) in discrete-fractured media saturated by a nonwetting phase (such as nonaqueous phase liquid or oil) with mild to high nonlinearity in relative permeability and capillary pressure. To the best of our knowledge, results for simulations of two-phase immiscible and incompressible flow in three-dimensional discrete-fractured media, including capillary and gravity effects, are the first to appear in the literature.

A geological framework for interpreting the low‐flow regimes of Cascade streams, Willamette River Basin, Oregon

Abstract: [1] In ungauged basins, predicting streamflows is a major challenge for hydrologists and water managers, with approaches needed to systematically generalize hydrometric properties from limited stream gauge data. Here we illustrate how a geologic/geomorphic framework can provide a basis for describing summer base flow and recession behavior at multiple scales for tributaries of the Willamette River in Oregon. We classified the basin into High Cascade and Western Cascade provinces based on the age of the underlying volcanic bedrock. Using long-term U.S. Geological Survey stream gauge records, we show that summer streamflow volumes, recession characteristics, and timing of response to winter recharge are all linearly related to the percent of High Cascade geology in the contributing area. This analysis illustrates how geology exerts a dominant control on flow regimes in this region and suggests that a geological framework provides a useful basis for interpreting and extrapolating hydrologic behavior.