Key Hydrological Processes Research Articles

An evaluation of approaches for modelling hydrological processes in high‐elevation, glacierized Andean watersheds

AbstractWe use two hydrological models of varying complexity to study the Juncal River Basin in the Central Andes of Chile with the aim to understand the degree of conceptualization and the spatial structure that are needed to model present and future streamflows. We use a conceptual semi‐distributed model based on elevation bands [Water Evaluation and Planning (WEAP)], frequently used for water management, and a physically oriented, fully distributed model [Topographic Kinematic Wave Approximation and Integration ETH Zurich (TOPKAPI‐ETH)] developed for research purposes mainly. We evaluate the ability of the two models to reproduce the key hydrological processes in the basin with emphasis on snow accumulation and melt, streamflow and the relationships between internal processes. Both models are capable of reproducing observed runoff and the evolution of Moderate‐resolution Imaging Spectroradiometer snow cover adequately. In spite of WEAP's simple and conceptual approach for modelling snowmelt and its lack of glacier representation and snow gravitational redistribution as well as a proper routing algorithm, this model can reproduce historical data with a similar goodness of fit as the more complex TOPKAPI‐ETH. We show that the performance of both models can be improved by using measured precipitation gradients of higher temporal resolution. In contrast to the good performance of the conceptual model for the present climate, however, we demonstrate that the simplifications in WEAP lead to error compensation, which results in different predictions in simulated melt and runoff for a potentially warmer future climate. TOPKAPI‐ETH, using a more physical representation of processes, depends less on calibration and thus is less subject to a compensation of errors through different model components. Our results show that data obtained locally in ad hoc short‐term field campaigns are needed to complement data extrapolated from long‐term records for simulating changes in the water cycle of high‐elevation catchments but that these data can only be efficiently used by a model applying a spatially distributed physical representation of hydrological processes. Copyright © 2013 John Wiley & Sons, Ltd.

Simulation of water balance in a clayey, subsurface drained agricultural field with three-dimensional FLUSH model

Summary Water flow is a key component in the evaluation of soil erosion and nutrient loads from agricultural fields. Field cultivation is the main non-point pollution source threatening water quality of surface waters in Nordic and many other countries. Few models exist that can describe key hydrological processes in clayey soils, i.e. overland flow, preferential flow in macropores and soil shrinkage and swelling. A new three-dimensional (3-D) distributed numerical model called FLUSH is introduced in this study to simulate these processes. FLUSH describes overland flow with the diffuse wave simplification of the Saint Venant equations and subsurface flow with a dual-permeability approach using the Richards equation in both macropore and matrix pore systems. A method based on the pentadiagonal matrix algorithm solves flow in both macropore and matrix systems directly in a column of cells in the computational grid. Flow between the columns is solved with iteration accelerated with OpenMP parallelisation. The model validity is tested with data from a 3-D analytical model and a clayey subsurface drained agricultural field in southern Finland. According to the simulation results, over 99% of the drainflow originated from the macropore system and drainflow started in some cases within the same hour when precipitation started indicating preferential flow in the profile. The moisture content of the clay soil had a profound effect on runoff distribution between surface runoff and drainflow. In summer, when the soil was dry and cracked, drainflow dominated the total runoff, while in autumn, when the shrinkage crack network had swollen shut, surface runoff fraction clearly increased. Observed differences in surface runoff fraction before and after tillage indicated that the operation decreased hydraulic conductivity of the profile.

Regional flood frequency analysis in arid regions: A case study for Australia

Summary A large part of the world faces the problem of aridity, e.g. in Australia, the semi-arid and arid regions occupy about 50–75% of the country. There is a serious lack of data on many key hydrological processes in the arid regions, which makes it difficult to understand the hydrological processes in these regions. Although arid regions are characterised by a very low mean annual rainfall, it can experience severe flooding causing widespread damage. In hydrological literature, there is a limited presence of the application of regional flood frequency analysis (RFFA) studies for the world arid regions due to limited data availability. This paper presents a RFFA study for the semi-arid and arid regions of Australia using the most up-to-date data set and compares the results with that of the world data. It has been found that arid catchments exhibit a higher loss and lower runoff coefficient values as expected. Based on four model selection criteria and L -moments ratio diagram, it has been found that a Generalised Pareto distribution is preferable to the Exponential distribution to fit the partial duration series flood data in the arid regions of Australia. It has also been found that the arid region exhibits much steeper flood frequency growth curve than that of the humid region and the growth curve gets steeper with the decreasing mean annual rainfall. The developed growth curve for the semi-arid and arid region of Australia is found to be similar to other typical semi-arid and arid regions of the world. The developed prediction equation for the mean annual flood in the semi-arid and arid region of Australia uses two readily obtainable predictor variables (catchment area and design rainfall intensity), which can be used along with the developed growth curve to estimate design floods for the ungauged catchments in these regions with reasonable accuracy. The growth curve developed in this study is primarily applicable to the semi-arid and arid regions of Australia; however, this may be adapted to other similar regions of the world along with the locally developed prediction equation for the scaling factor (i.e. mean annual flood in this case) to derive preliminary design flood estimates.

Hydromechanical triggering of landslides: From progressive local failures to mass release

Water infiltrating during intense rainfall on steep slopes gradually weakens the wet soil mass, inducing localized failures that may initiate a cascade of load redistributions and successive failures propagating across a hillslope. The challenge of linking the progressive nature of local events culminating in an abrupt landslide is addressed by a new hydromechanical triggering model that links key hydrologic processes with threshold‐based mechanical interactions. A hillslope is represented as assembly of soil columns interconnected by frictional and tensile mechanical bonds represented as virtual bundles of fibers. Increasing water load exerted on mechanical bonds causes gradual failure of fibers until restraining forces are exceeded. Following failure at the soil‐bedrock interface, the load on a column is redistributed to its neighbors via intact mechanical (primarily tensile) bonds which, in turn, may also fail and transmit the load downslope as compressive stresses. When soil internal compressive strength is exceeded, a load‐bearing column may liquefy and initiate a landslide release that could propagate downslope or retrogressively upslope. The model reproduces observed power law frequency magnitude relationships of landslides with exponents ranging between −1.0 and −2.2 in agreement with landslide inventory data. We applied a criticality measure defined by Ramos (2011) to evaluate the specific influences of slope angle, soil texture, and root reinforcement on attainment of hillslope criticality in which a small local failure may trigger release of a large soil mass. The model provides new insights on the conditions giving rise to an abrupt transition from a seemingly stable hillslope to a catastrophic landslide.

Local calibration of remotely sensed rainfall from the TRMM satellite for different periods and spatial scales in the Indus Basin

The availability of accurate rainfall data at proper temporal and spatial scales is vital for knowledge of renewable water resources and safe withdrawals for irrigation. Rain gauge networks in mountainous basins such as the Indus are sparse and insufficient to plan withdrawals and water management applications. Satellite rainfall estimates can be used as an alternative source of information but need area-specific calibration and validation due to the indirect nature of the radiation measurements. In this study, a calibration protocol is worked out for rainfall data from the Tropical Rainfall Measuring Mission (TRMM) satellite because uncalibrated TRMM rainfall data are inaccurate for use in rainfall–runoff studies and in soil water balance studies. Two alternative techniques, regression analysis (RA) and geographical differential analysis (GDA), were used to calibrate TRMM rainfall data for different periods and spatial distributions. The validity of these techniques was tested using Nash–Sutcliffe efficiency and the standard error of estimate. The GDA technique proved to be better, with higher efficiency and smaller error in complex mountainous terrains. The deviation between TRMM data and rain gauge data was decreased considerably from 10.9% (pre-calibration at 625 km2) to 6.1% (post-calibration at 3125 km2) for annual time periods. For monthly periods, the deviation of 34.9% (pre-calibration at 625 km2) was decreased to 15.4% (post-calibration at 3125 km2). Calibration can be improved further if more rain gauges are available. The GDA technique can be applied to calibrate TRMM rainfall data in regions with limited rain gauge data and can provide a sufficiently accurate estimate of the key hydrological process that can be used in water management applications.

Quantifying mountain block recharge by means of catchment‐scale storage‐discharge relationships

Despite the importance of mountainous catchments for providing freshwater resources, especially in semi‐arid regions, little is known about key hydrological processes such as mountain block recharge (MBR). Here we implement a data‐based method informed by isotopic data to quantify MBR rates using recession flow analysis. We applied our hybrid method in a semi‐arid sky island catchment in southern Arizona, United States. Sabino Creek is a 91 km2 catchment with its sources near the summit of the Santa Catalina Mountains northeast of Tucson. Southern Arizona's climate has two distinct wet seasons separated by prolonged dry periods. Winter frontal storms (November–March) provide about 50% of annual precipitation, and summers are dominated by monsoon convective storms from July to September. Isotope analyses of springs and surface water in the Sabino Creek catchment indicate that streamflow during dry periods is derived from groundwater storage in fractured bedrock. Storage‐discharge relationships are derived from recession flow analysis to estimate changes in storage during wet periods. To provide reliable estimates, several corrections and improvements to classic base flow recession analysis are considered. These corrections and improvements include adaptive time stepping, data binning, and the choice of storage‐discharge functions. Our analysis shows that (1) incorporating adaptive time steps to correct for streamflow measurement errors improves the coefficient of determination, (2) the quantile method is best for streamflow data binning, (3) the choice of the regression model is critical when the stage‐discharge function is used to predict changes in bedrock storage beyond the maximum observed flow in the catchment, and (4) the use of daily or night‐time hourly streamflow does not affect the form of the storage‐discharge relationship but will impact MBR estimates because of differences in the observed range of streamflow in each series.

Streamflow response to headwater reforestation in the Ganaraska River basin, southern Ontario, Canada

AbstractIncreased attention is being paid to the role of reforestation in strategies for sustainable forest management, landscape restoration and carbon sequestration. Reforestation of drainage basins is generally assumed to reduce annual streamflow as well as peak and low flows. However, most studies of reforestation effects on streamflow have been conducted on small experimental basins, and the applicability of their results to larger basins is unclear. This study revisits an earlier examination of streamflow response to headwater reforestation in a 267 km2 basin (Ganaraska River) in southern Ontario, Canada for 1945–2007. Forest cover in the basin headwaters increased from 13 km2 in 1945 to 31 km2 by 1990, with most of this change between 1950 and 1965. Streamflow metrics from the reforested basin and two headwater sub‐basins were compared to those from a nearby basin and sub‐basin of similar size and physiography to the Ganaraska basins but without extensive headwater reforestation. No temporal trends were found for inter‐basin differences in annual runoff or runoff ratios for the entire Ganaraska basin or its largest sub‐basin; however, reforestation appears to have reduced several metrics of peak streamflow at the basin and sub‐basin scale. Relationships between high flows classified according to generating event type and the associated precipitation depth suggest that expansion of forest cover in the Ganaraska River basin (GRB) and associated changes in microclimatic conditions have reduced the potential for frozen soil to generate surface runoff and high flows in response to rain‐on‐snow during spring snowmelt. This was accompanied by increased low flows from the GRB and its sub‐basins from 1960 to 2007 relative to the control basin, suggesting that the expanded forest cover has enhanced groundwater recharge by prolonging spring snowmelt and promoting infiltration of rainfall and snowmelt inputs relative to non‐forested areas. Some discrepancies between these results and those of other studies of streamflow response to reforestation may be attributed to the relatively large size of the GRB and the limited extent of reforestation. Nevertheless, the results highlight the importance of considering how reforestation may affect key hydrologic processes in a given landscape when predicting the resulting streamflow response. Copyright © 2011 John Wiley & Sons, Ltd.

Distributed modelling of future changes in hydrological processes of Spencer Creek watershed

AbstractThe hydrologic impact of climate change has been largely assessed using mostly conceptual hydrologic models. This study investigates the use of distributed hydrologic model for the assessment of the climate change impact for the Spencer Creek watershed in Southern Ontario (Canada). A coupled MIKE SHE/MIKE 11 hydrologic model is developed to represent the complex hydrologic conditions in the Spencer Creek watershed, and later to simulate climate change impact using Canadian global climate model (CGCM 3·1) simulations. Owing to the coarse resolution of GCM data (daily GCM outputs), statistical downscaling techniques are used to generate higher resolution data (daily precipitation and temperature series). The modelling results show that the coupled model captured the snow storage well and also provided good simulation of evapotranspiration (ET) and groundwater recharge. The simulated streamflows are consistent with the observed flows at different sites within the catchment. Using a conservative climate change scenario, the downscaled GCM scenarios predicted an approximately 14–17% increase in the annual mean precipitation and 2–3 °C increase in annual mean maximum and minimum temperatures for the 2050s (i.e., 2046–2065). When the downscaled GCM scenarios were used in the coupled model, the model predicted a 1–5% annual decrease in snow storage for 2050s, approximately 1–10% increase in annual ET, and a 0·5–6% decrease in the annual groundwater recharge. These results are consistent with the downscaled temperature results. For future streamflows, the coupled model indicated an approximately 10–25% increase in annual streamflows for all sites, which is consistent with the predicted changes in precipitation. Overall, it is shown that distributed hydrologic modelling can provide useful information not only about future changes in streamflow but also changes in other key hydrologic processes such as snow storage, ET, and groundwater recharge, which can be particularly important depending on the climatic region of concern. The study results indicate that the coupled MIKE SHE/MIKE 11 hydrologic model could be a particularly useful tool for understanding the integrated effect of climate change in complex catchment scale hydrology. Copyright © 2010 John Wiley & Sons, Ltd.

Scaling and physiographic controls on streamflow behaviour on the Precambrian Shield, south-central Ontario

Summary Relationships explaining streamflow behaviour in terms of drainage basin physiography greatly assist efforts to extrapolate streamflow metrics from gauged to ungauged basins in the same landscape. The Dorset Environmental Science Centre (DESC) has monitored streamflow from 22 small basins (3.4–190.5 ha) on the Precambrian Shield in south-central Ontario, in some cases since 1976. The basins exhibit regional coherence in their interannual response to precipitation; however, there is often a poor correlation between streamflow metrics from basins separated by as little as 1 km. This study assesses whether inter-basin variations in such metrics can be explained in terms of basin scale and physiography. Several characteristics (annual maximum, minimum and average flow) exhibited simple scaling with basin area, while magnitude, range and timing of annual maximum daily runoff showed scaling behaviour consistent with the Representative Elementary Area (REA) concept. This REA behaviour is partly attributed to convergence of fractional coverage of the two dominant and hydrologically-contrasting land cover types in the DESC region with increasing basin size. Three Principal Components (PCs) explained 82.4% of the variation among basin physiographic properties, and several runoff metrics (magnitude and timing of annual minimum daily runoff, mean number of days per year with 0 streamflow) exhibited significant relationships with one or more PC. Significant relationships were obtained between basin quickflow ( QF ) production and the PCs on a seasonal and annual basis, almost all of which were superior to simple area-based relationships. Basin physiography influenced QF generation via its control on slope runoff, water storage and hydrologic connectivity; however, this role was minimized during Spring when QF production in response to large rain-on-snow events was relatively uniform across the DESC basins. The PC-based relationships and inter-seasonal changes in their form were consistent with previous research conducted at point, slope and basin scales in the DESC region, and perceptions of key hydrological processes in these small basins may not have been as readily obtained from scaling studies using streamflow from larger basins. This process understanding provides insights into scaling behaviour beyond those derived from simple scaling and REA analyses. The physiography of the study area is representative of large portions of the Precambrian Shield, such that basin streamflow behaviour could potentially be extended across much of south-central Ontario. This would assist predictions of streamflow conditions at ungauged locations, development and testing of hydrological models for this landscape, and interpretation of inter-basin and intra-annual differences in hydrochemical behaviour on the southern Precambrian Shield.

Water Table and Vegetation Response to Ditch Blocking: Restoration of a Raised Bog in Southwestern British Columbia

Large-scale peat harvesting operations alter hydrology of raised bogs so that natural regeneration may not occur without altering the water table. This paper describes efforts to restore key hydrologic and ecologic processes in a southwest BC raised bog (Burns Bog) highly disturbed by decades of peat extraction, drainage, filling, and conversion to agriculture, urban, and industrial uses. The restoration goals are to return a high water table throughout the bog, but particularly in the pine forests at the edge of the bog, to re-establish Sphagnum cover, and to re-start the peat-forming process in degraded peat-harvested sectors. Peripheral and interior ditches are being blocked to increase the retention of winter precipitation into the drier summer months. Piezometer/well measurements have detected water table increases in the past two years, including a relatively immediate response in one site. New Sphagnum colonies have become established in the dry perimeter forest, a first indication that the water table may be rising and that peat-forming vegetation may be responding positively. These preliminary results suggest that it is possible to detect a rapid hydrological response with a comprehensive monitoring network in raised bogs; this observation is key to early prediction of future peatland restoration initiatives.

Thermodynamics and optimality of the water budget on land: A review

The water balance on land plays a critical role in connecting key hydrological processes with climate and ecology. Over the last few years, several advances have been made in applying thermodynamic and optimality approaches to better describe Earth system processes in general, and the water balance on land in particular. Both concepts relate to the proposed principle of Maximum Entropy Production (MEP), which states that complex systems far from thermodynamic equilibrium organize in a way such that the rate of entropy production‐a measure of irreversibility‐is maximized in steady state. MEP provides a foundation to understand optimality in hydrology at a fundamental, thermodynamic level that is applicable across a wide range of Earth systems beyond hydrology. This review describes the foundation of the water balance far from thermodynamic equilibrium and potential applications of MEP. Some of the objections to optimality and thermodynamics are discussed as well as its potential implications.

Phenomenological models of hydrologic processes in south Florida

Summary The objectives of this study were to use existing hydrologic data to identify phenomenological models appropriate for describing key hydrologic processes in south Florida, and collect field data to develop improved process equations. Analyses of 3300 storm events within a 250-km 2 study area in south Florida indicate that rainfall amounts and durations are typically correlated over distances on the order of 2 km. For 1-h duration storms, significant correlation exists for length scales on the order of 1 km, and for 24-h duration storms, significant correlation exists for length scales on the order of 10 km. Double-ring infiltrometer tests indicate that asymptotic infiltration capacities can be reasonably estimated from soil texture. Infiltration capacities are characterized by relatively rapid decay rates and typically approach a constant value within 10 minutes of ponding. Rainfall-recharge data indicate a linear relation between rainfall and recharge characterized by a specific yield of 0.26 and a threshold rainfall of 1 cm. Evaporation from the saturated zone during persistent dry periods can be described by a linear model in which the saturated-zone evaporation rate is equal to the potential evaporation rate for a depth of 1.4 m and decreases to zero at a depth of 2.5 m.

Calibration of the Interaction Soil Biosphere Atmosphere land‐surface scheme on a small tropical high‐mountain basin (Cordillera Real, Bolivia)

In a continuing effort to test the behavior of Meteo France's land‐surface scheme, Interaction Soil Biosphere Atmosphere (ISBA), under a wide range of hydrometeorological conditions, the scheme was applied to a small tropical high‐mountain river basin of the Cordillera Real in Bolivia. The surface scheme simulates the key hydrological processes, such as evaporation, infiltration, surface runoff, and snow and soil ice freeze‐thaw processes, which are essential for accurately simulating the hydrological behavior of the river basin in this type of location. The 12.5 km2 nonglacierized subbasin has been divided into homogeneous surface units with irregular shapes, on the basis of their elevation and soil surface characteristics. The ISBA scheme was applied to each unit and the water fluxes were transferred from one unit to another using a system dynamics modeling approach. The method was applied step by step, improving its efficiency based on several hypotheses. This special application of the ISBA scheme, combining the high‐mountain conditions and the tropical seasonality is discussed.

Comparison of 1D models of water flow in unsaturated soils

Understanding the interaction between soil, vegetation and atmosphere processes and groundwater dynamics is of paramount importance in water resources planning and management in many practical applications. Hydrological models of complex water resource systems need to include a number of components and should therefore seek a balance between capturing all relevant processes and maintaining data requirement and computing time at an affordable level. Water transfer through the unsaturated zone is a key hydrological process connecting atmosphere, surface water and groundwater. The paper focuses on the analysis of the modelling approaches that are generally used to describe vertical water transfer through unsaturated soil in hydrological models of water resource systems: a physically based approach, using numerical solutions of Richards' equation, and two conceptual models, based on reservoir cascade schemes, are compared. The analysis focuses on the soil water content in the top soil (first meter) and on the outflow from the profile (i.e. recharge to the aquifer). Results show that the water contents simulated by the mechanistic and conceptual models are in good agreement, unless the capillary fringe reaches the top soil (i.e. groundwater table very close to the soil surface). The ability of conceptual models to capture the daily recharge dynamics is generally rather poor, especially when fine textured soils and thick profiles are considered; a better agreement is found when recharge is accumulated over longer time periods (e.g. months). Improvements can be achieved by allowing the number of reservoirs in cascade to vary with changing profile depth, although scientifically sound rules for fixing the number of reservoirs need to be established.

Application of isotope tracers in continental scale hydrological modeling

Isotope tracers are widely used to study hydrological processes in small catchments, but their use in continental-scale hydrological modeling has been limited. This paper describes the development of an isotope-enabled global water balance and transport model (iWBM/WTM) capable of simulating key hydrological processes and associated isotopic responses at the large scale. Simulations and comparisons of isotopic signals in precipitation and river discharge from available datasets, particularly the IAEA GNIP global precipitation climatology and the USGS river isotope dataset spanning the contiguous United States, as well as selected predictions of isotopic response in yet unmonitored areas illustrate the potential for isotopes to be applied as a diagnostic tool in water cycle model development. Various realistic and synthetic forcings of the global hydrologic and isotopic signals are discussed. The test runs demonstrate that the primary control on isotope composition of river discharge is the isotope composition of precipitation, with land surface characteristics and precipitation-amount having less impact. Despite limited availability of river isotope data at present, the application of realistic climatic and isotopic inputs in the model also provides a better understanding of the global distribution of isotopic variations in evapotranspiration and runoff, and reveals a plausible approach for constraining the partitioning of surface and subsurface runoff and the size and variability of the effective groundwater pool at the macro-scale.

Advances in Canadian forest hydrology, 1999–2003

AbstractUnderstanding key hydrological processes and properties is critical to sustaining the ecological, economic, social and cultural roles of Canada's varied forest types. This review examines recent progress in studying the hydrology of Canada's forest landscapes. Work in some areas, such as snow interception, accumulation and melt under forest cover, has led to modelling tools that can be readily applied for operational purposes. Our understanding in other areas, such as the link between runoff‐generating processes in different forest landscapes and hydrochemical fluxes to receiving waters, is much more tentative. The 1999–2003 period saw considerable research examining hydrological and biogeochemical responses to natural and anthropogenic disturbance of forest landscapes, spurred by major funding initiatives at the provincial and federal levels. This work has provided valuable insight; however, application of the findings beyond the experimental site is often restricted by such issues as a limited consideration of the background variability of hydrological systems, incomplete appreciation of hydrological aspects at the experiment planning stage, and experimental design problems that often bedevil studies of basin response to disturbance. Overcoming these constraints will require, among other things, continued support for long‐term hydroecological monitoring programmes, the embedding of process measurement and modelling studies within these programmes, and greater responsiveness to the vagaries of policy directions related to Canada's forest resources. Progress in these and related areas will contribute greatly to the development of hydrological indicators of sustainable forest management in Canada. Copyright © 2005 John Wiley & Sons, Ltd.

Development of the conceptual models for chemical conditions and hydrology used in the 1996 performance assessment for the Waste Isolation Pilot Plant

The Waste Isolation Pilot Plant (WIPP) is a US Department of Energy (DOE) facility for the permanent disposal of defense-related transuranic (TRU) waste. US Environmental Protection Agency (EPA) regulations specify that the DOE must demonstrate on a sound basis that the WIPP disposal system will effectively contain long-lived alpha-emitting radionuclides within its boundaries for 10,000 years following closure. In 1996, the DOE submitted the 40 CFR Part 191 Compliance Certification Application for the Waste Isolation Pilot Plant (CCA) to the EPA. The CCA proposed that the WIPP site complies with EPA's regulatory requirements. Contained within the CCA are descriptions of the scientific research conducted to characterize the properties of the WIPP site and the probabilistic performance assessment (PA) conducted to predict the containment properties of the WIPP disposal system. In May 1998, the EPA certified that the TRU waste disposal at the WIPP complies with its regulations. Waste disposal operations at WIPP commenced on 28 March 1999. The 1996 WIPP PA model of the disposal system included conceptual and mathematical representations of key hydrologic and geochemical processes. These key processes were identified over a 22-year period involving data collection, data interpretation, computer models, and sensitivity studies to evaluate the importance of uncertainty and of processes that were difficult to evaluate by other means. Key developments in the area of geochemistry were the evaluation of gas generation mechanisms in the repository; development of a model of chemical conditions in the repository and actinide concentrations in brine; selecting MgO backfill and demonstrating its effects experimentally; and, determining the chemical retardation capability of the Culebra. Key developments in the area of hydrology were evaluating the potential for groundwater to dissolve the Salado Formation (the repository host formation); development of a regional model for hydrologic conditions; development of a stochastic, probabilistic representation of hydraulic properties in the Culebra Member of the Rustler Formation; characterization of physical transport in the Culebra; and, the evaluation of brine and gas flow in the Salado. Additional confidence in the conceptual models used in the 1996 WIPP PA was gained through independent peer review in many stages of their development.

Rock fragments in top soils: significance and processes

This introductory paper reviews various aspects of rock fragments in top soils. Such information is required for various reasons and in particular for predicting the impact of climatic or landuse changes on the response of these soils. Particular attention is paid to the definition and measurement of rock fragment content in top soils, the density of soils containing rock fragments, the spatial distribution and movement of rock fragments in top soils, the effects of rock fragments on some key hydrological processes, thermal properties of top soils, physical soil degradation, soil erosion and soil productivity.