Spatial Distribution Of Recharge Research Articles

A Stepwise Modelling Approach to Identifying Structural Features That Control Groundwater Flow in a Folded Carbonate Aquifer System

This paper concerns a stepwise modelling procedure for groundwater flow simulation in a folded and faulted, multilayer carbonate aquifer, which constitutes a source of good quality water for human consumption in the Apennine Range in Central Italy. A perennial river acts as the main natural drain for groundwater while sustaining valuable water-related ecosystems. The spatial distribution of recharge was estimated using the Thornthwaite–Mather method on 60 years of climate data. The system was conceptualized as three main aquifers separated by two locally discontinuous aquitards. Three numerical models were implemented by gradually adding complexity to the model grid: single layer (2D), three layers (quasi-3D) and five layers (fully 3D), using an equivalent porous medium approach, in order to find the best solution with a parsimonious model setting. To overcome dry-cell problems in the fully 3D model, the Newton–Raphson formulation for MODFLOW-2005 was invoked. The calibration results show that a fully 3D model was required to match the observed distribution of aquifer outflow to the river baseflow. The numerical model demonstrated the major impact of folded and faulted geological structures on controlling the flow dynamics in terms of flow direction, water heads and the spatial distribution of the outflows to the river and springs.

Hummock-scale controls on groundwater recharge rates and the potential for developing local groundwater flow systems in water-limited environments

We use two-dimensional numerical models to identify controls on groundwater recharge, water table dynamics, and the sink-source function of fine-textured hummocks in water-limited environments. A silty clay loam hummock with aspen forest cover in the sub-humid Boreal Plains region of Canada is used to develop a conceptual model and calibrate a variably-saturated flow model to better investigate the dynamics of recharge, water table position, and lateral flow between the forested hummock and adjacent peatland. Scenario testing (achieved by raising and lowering the hydraulic conductivity by an order of magnitude and altering the hummock height and length) provides further understanding of the controls that hydraulic conductivity and hummock morphometry exert on recharge and forestland-peatland interactions. We find that the hydraulic conductivity of the glacial till has the largest control over recharge, followed by the hummock height and length and, lastly, annual atmospheric fluxes. Hummocks with higher hydraulic conductivity or long morphometric profiles are most often sinks of water as they promote increased root-water uptake, while taller hummocks with lower hydraulic conductivity tend to act as hydrologic sources for adjacent wetlands. The magnitude of annual recharge and water table elevation are poorly correlated with annual atmospheric fluxes due to the strong effects of multi-year storage deficits or surpluses; however, they are appreciably affected by decadal wet-dry cycles typical of the Boreal Plains climate. Additionally, we show that both distance along a hummock and elevation are key factors in controlling the spatial distribution of recharge at a single hummock. It is common to conceptualize the water table as a shallow and subdued replica of surface topography, where water flows from topographic highs (i.e., forestlands) to topographic lows (i.e., peatlands) in humid environments; however, as we show here, recharge rates, water table elevation, and the sink-source function of a forested hummock in water-limited environments is highly variable in time and space, and strongly dependent on both the hydrogeological and morphometric properties of the hummock.

Groundwater recharge to a structurally complex aquifer system on the island of Tobago (Republic of Trinidad and Tobago)

Trinidad and Tobago is a developing two-island nation in the Lesser Antilles of the Caribbean. Tobago is the smaller island and has small highly heterogeneous aquifers composed of igneous and metamorphic crystalline rock with strong structural controls on the spatial distribution of permeability. Hydrogeologic analyses of the water budget and groundwater production suggest that portions of the island are underlain by prolific fractured-rock aquifer systems. This study quantifies the amount and spatial distribution of recharge, as well as the fraction of recharge captured by groundwater pumping, using historical data, new field data, remote sensing data, multiple storage quantification methods and stable isotope analysis. Despite extensive freshwater withdrawals, groundwater production reaches only ~10% of annual groundwater recharge. Groundwater capture zones are created using a first-order hydrologic balance approach and with backward particle tracking in a steady-state groundwater model. Both approaches to generating capture zones suggest that many wells capture water from outside their topographic watershed. The location of sustainable, high yield, fresh potable groundwater wells less than 1 km from the coast, that have fractured bedrock intakes well below sea level, supports the concept of a rigorous and active groundwater flow system. Understanding the hydrogeology of small bedrock island aquifers is critical to evaluating groundwater resources, especially in the Caribbean where there is strong seasonality in precipitation, finite surface-water storage and increases in potable water demand.

Shallow aquifer groundwater dynamics due to land use/cover change in highly urbanized basin: The case of Kathmandu Valley

Study regionKathmandu Valley, Nepal. Study focusIn the context of rapid urbanization in Kathmandu Valley (KV), this study quantifies spatial distribution of recharge and changes in drawdown under pumping rate and land use/cover change scenarios. Both scenarios are incorporated in a groundwater flow model in MODFLOW. New hydrological insights for this regionCurrent groundwater recharge rate is spatially distributed across the basin in a range of 95.3–85.3 % of precipitation in dry season and 48−38% of precipitation in wet season as per recharge area distributions. This results in groundwater recharge of 1.052 MCM and 1.949 MCM in dry and wet seasons respectively. In the case of increase in both pumping rate and encroachment of recharge areas (due to land use/cover change) and only encroachment of recharge areas, drawdown is projected to change respectively by −5.25 m in dry season and −7.39 m in wet season and −1.09 m in dry season and -3.63 m in wet season in 2050. The central urban, northern foothills, and southern parts are projected to observe more draw down. Meanwhile, the boundary forest area has the potential to increase the water table in the future.

Field-based estimation and modelling of distributed groundwater recharge in a Mediterranean karst catchment, Wadi Natuf, West Bank

Abstract. While groundwater recharge is one of the most prominently covered subjects in hydrogeology, the spatial distribution of recharge has been given relatively little attention, especially in semi-arid, karstic aquifers. Under conditions of highly diverse geology, relief, vegetation and land use, the complexity and variability of spatially distributed hydrological processes remains a challenge in many regions around the world. This is particularly true for hitherto ungauged basins, such as Wadi Natuf, a 103 km2 large karstic Eastern Mediterranean watershed in the Palestinian upstream mountain and recharge area of the Western Aquifer Basin (WAB), which is shared with Israel in the coastal plain. In this first in a series of two papers, distributed recharge is estimated and represented, based on 7 years of extensive field observations and measurements and based conceptually on observable physical landscape features such as geology, land use and land cover (LU/LC) and especially soil conditions. For the first time in the WAB, a forward calculated soil moisture and percolation model (SMSP) was set up with parameters directly gained from field observations. The model was parameterised in a strictly parsimonious manner, as a one-dimensional model (a.k.a. “tank”, bucket or box model). This is based on dominant hydrological processes, in particular saturation excess in the soil column, and identifying patterns of linkage between different landscape features. Average soil thickness was encountered at the range of decimetres, rarely above one metre. Both soil thickness and LU/LC features, such as terraced olive groves or forests as well as grassland or barren rock outcrops, were found to be highly formation specific. This linkage allowed us to further simplify the model and its requirements in a realistic manner for eight soil moisture stations, chosen at six different geological formations with typical soil and LU/LC representations. The main result of the model was the determination of formation-specific recharge coefficients, spatially ranging between 0 % and almost 60 % of annual rainfall or up to 300 mm a−1 in Wadi Natuf's climate. The karstified main aquifers showed recharge coefficients (RC) above 40 % and even the less prominent slightly aquitardal local aquifers reached RC values above 30 %. The model was separately tested on two conceptual levels: on the level of basin form (soil moisture) and basin response (signatures of peak recharge and local spring discharge events) under well-controlled conditions in isolated sub-catchments. In principle, our approach is applicable in many of the scarcely gauged karstic groundwater basins around the world with a highly diverse landscape and geology.

Delineation of potential groundwater recharge zones using analytic hierarchy process-guided GIS in the semi-arid Motloutse watershed, eastern Botswana

Study regionThe Motloutse Watershed is located in eastern Botswana. Study FocusThis study investigates the spatial distribution of recharge in the semi-arid watershed using Weighted Overlay Analysis.The overuse of groundwater in the Motloutse catchment to support domestic and industrial expansion has generated the need for the determination of areas where renewable storage to groundwater may occur. A select of parameters known to have influence on recharge were considered for input into the analytic process and included lithology, lineament density, ground slope, soil type, soil thickness, and drainage density. A thematic map for each parameter was prepared, reclassified into high, moderate and low potential for recharge. Analytic Hierarchy Process was applied to rank the importance of each parameter relative to one another concerning groundwater recharge. Potential groundwater recharge sites were delineated using Weighted Overlay tool of ArcGIS software. New Hydrological Insights for the RegionHigh recharge sites cover 8% of the catchment and correspond to places underlain by sandstones and fractured basaltic outcrops as well as areas covered by thick sandy soils. Moderate recharge occurs in 78 % and low recharge in 14 % of the catchment area. The results from this study are useful for selecting areas for more focused studies on recharge and also form a significant decision support tool for sustainable and equitable utilization of groundwater resources within the Motloutse catchment.

Transient resistivity response to infiltrating water front through vadose zone

In a groundwater regime, the unsaturated zone acts as a complex system, for the simulation of subsurface water movement. Numerous attempts have been made in the field of hydrology, during the recent past in modeling the subsurface flow dynamics. Recharge through vadose zone takes place through three distinct parts viz. upper (recharge area), middle and lower (discharge parts). In order to understand the characteristic behavior of vadose zone over the infiltrating water front, an integrated approach of infiltration test and resistivity survey was attempted at 6 different sites with distinct geomorphology, in a semi-arid granite terrain. In this paper the authors have documented the compilation of double ring infiltration test results and its impact on shallow subsurface transient resistivity signature. Similar attempt can be extended in different geological terrain, wherein it would be helpful to ascertain the spatial distribution of recharge with limited spot recharge measurements.

Modelling spring discharge and solute transport in conduits by coupling CFPv2 to an epikarst reservoir for a karst aquifer

Karst aquifers differ from unconsolidated granular aquifers with respect to their groundwater flow behavior because of the dual-permeability characteristics (discrete conduit network coupled to a fissured matrix), the duality of recharge (point vs. diffuse recharge) as well as the duality of flow (laminar vs. turbulent state). In many karst aquifers, the epikarst frequently plays an important role in redistributing precipitation as diffuse recharge into the fissured matrix and as point recharge into the conduit network as well as for the temporal distribution of recharge input. In this paper, a coupled groundwater model CE (CFPv2 + ERCH) is developed, consisting of a combination of a simple reservoir model generating the recharge source function assuming the concept of an epikarst horizon (ERCH) with CFPv2, a discrete conduit-continuum groundwater flow model of the phreatic karst aquifer. The CE model is applied to simulate spring discharge and a tracer breakthrough in a karst aquifer near Guilin City in southwest China. In order to examine the effect of an added epikarst reservoir model, simulations are carried out with CFPv2 but without including ERCH. Simulation results of both, spring hydrograph and tracer breakthrough, indicate that CFPv2 with ERCH performs much better. A sensitivity analysis of the CE reveals the relative importance of parameters characterizing the epikarst reservoir. Point recharge is a main factor determining the large variation in spring discharge and its proportion to total recharge can reach 58%. These simulation results highlight the importance of the temporal and spatial distribution of recharge to karst spring discharge response. The coupled model provides an effective approach to include the effect of an epikarst zone in CFPv2 and it can easily be applied for the simulation of karst aquifers with similar characteristics as the study site.

Protecting groundwater levels and ecosystems with simple management approaches

Groundwater quantity is often managed using simple tools. The most common are (1) basin or sub-basin scale volumetric allocations, usually based on either historic use or estimates of recharge, (2) trigger-level management which regulates use according to observations of groundwater level, and (3) buffer zones, which control the location of wells, particularly around groundwater-dependent ecosystems (GDEs). The volumetric approach limits the long-term impact of abstraction and provides a stable, secure supply for groundwater users. However, this approach does not consider the spatial distribution of recharge and discharge, and so is poor at protecting GDEs. Buffer zones provide an effective means of limiting the short-term impact of abstraction on GDEs, and can also be used to shift impact from high to low priority GDEs. However, buffer zones mostly delay the impacts of abstraction on groundwater level and flow, and are less effective for managing long-term impacts. Groundwater response triggers aim to directly control groundwater levels, although the success of this approach is highly dependent on the location of the observation well, and the trigger value. This makes its successful implementation extremely difficult. Used alone, none of these approaches will successfully protect the environment. In combination, they can provide reasonable protection for ecosystems and reliability of groundwater supply for users.

Geophysical, geological, and hydrogeological characterization of a tributary buried bedrock valley in southern Ontario

Buried bedrock valleys infilled with Quaternary-aged sediment have the potential to become productive aquifers owing to prevalent sand and gravel deposits often associated with these topographic lows. In areas where groundwater is drawn from the underlying bedrock aquifer, buried bedrock channels may significantly affect the spatial distribution of recharge and localized contaminant pathways. Therefore, understanding the form, distribution, and the nature of Quaternary infill sediments within these buried bedrock river valleys, and their relationship to hydraulically transmissive bedrock features is an important aspect of groundwater resource management. Here, we evaluate the effectiveness of electrical resistivity and seismic refraction collected over a partially urbanized 150 ha area with variable vegetation, roads, and structures, to map the spatial distribution of sediments and delineation of a channel segment associated with a regional bedrock valley. Electrical resistivity and seismic refraction was performed along 13 (covering ∼11.6 km) and seven transects (covering ∼0.9 km), respectively, to map and characterize the bedrock surface morphology beneath a variable thickness of unconsolidated deposits. Three continuously cored holes and downhole geophysical logs, supplemented with four nearby water well records captured the in-channel as well as adjacent Quaternary stratigraphy (∼15–40 m). Cores recorded multiple glacial till deposits and ice-marginal processes associated with ice advances and retreats. Hydraulic transmissivity of the bedrock around the valley feature was evaluated using a FLUTe hydraulic transmissivity profiling technique. This study demonstrates the potential of combining several surface geophysical methods with sedimentological analysis of continuous cores and hydraulic data for characterizing tributary bedrock channel morphology and Quaternary infill at a scale relevant to localized studies of municipal production well recharge zones and contaminant transport and fate.

Inter-Basin Groundwater Transfer and Multiple Approach Recharge Estimation of the Upper Awash Aquifer System

Multiple approaches have been used to estimate groundwater recharge in the Upper Awash river basin. The amount of recharge reaching the Upper Awash aquifer system from the Blue Nile sub-basins is also estimated. Water Balance, Chloride Mass Balance and HYDRUS 1D infiltration model are used to estimate recharge. A total of 29 sites were selected for the HYDRUS 1D multiple “at point” recharge simulations. Base Flow Separation (BFS) methods, using both River Analysis Package software Version 3.0.3 and Excel-based Time Plot program are also used as a proxy for recharge. Besides, overlay analysis in Processing MODFLOW, ArcGIS, and SURFER environments has been done to thoroughly consider spatial heterogeneity between any two point estimates and appreciate the effect of lineament density, topography, slope and major urbanized land on pattern of spatial distribution of recharge. Because of differences inherent in the assumptions and datasets used, the various methods employed give wide range of differences in recharge estimates. Recharge estimated for the Upper Awash basin ranges from 51.5 mm/year to 157 mm/year and for the two southern left-bank sub-basins of the Middle Blue Nile basin (Mugher and Jema) ranges from 86 mm/year to 239 mm/year. Consequently, annual average volumetric recharge in the Upper Awash and annual groundwater flux from portion of the Blue Nile sub basins to the Upper Awash aquifer system are estimated to be 983 Mm3 and 365 Mm3 respectively. The significant flux joining the Upper Awash groundwater system from part of the Middle Blue Nile basin, which is almost 37% of the total annual recharge to the Upper Awash basin makes this part of the Middle Blue Nile basin an important recharge zone for the Upper Awash groundwater. Estimating recharge using integrated approaches was found to be useful to identify range of plausible recharge rates in the two basins. Besides, the new methodological approach of superimposing recharge governing factors on interpolation of point recharge estimates helps to produce physical based spatial distribution of recharge.

Integrating Diffuse and Concentrated Recharge in Karst Models

Distributed parameter models of mature karst aquifers need to take into consideration both the dual character of flow in the saturated zone and the heterogeneous infiltration conditions. Usually, in the modeling of unconfined aquifers, recharge due to infiltration is considered as uniformly distributed over the entire domain. The present study evaluates the contribution of distinct infiltration zones and examines different hypotheses on the spatial distribution of recharge zones. The applied methodology comprises two parts: i) the development of a 3D transient model and ii) the development and examination of different scenarios on the spatial distribution of recharge. The model is developed using the finite element code FEFLOW which allows for the integration of discrete features such as channels in a porous matrix. The scenarios for the spatial distribution of the recharge zones describe three hypotheses. In the first scenario, recharge is exclusively introduced through faulted zones. In the second scenario, the entire model domain receives the recharge. In the third scenario, recharge is distributed in sub-areas selected based on geomorphological and hydrogeological criteria. For each scenario, simulation model results for the water table are compared with field measurements and discussed. The derived results indicate that greater model accuracy is achieved for Scenarios 2 and 3. Finally, a forth scenario, designed by combining Scenarios 2 and 3, exhibits the best fitting between simulation results and observation data. The methodology is applied to the karst aquifer of Palaikastro-Chochlakies in Eastern Crete, Greece, for the period 2008–2012.

From precipitation to groundwater baseflow in a native prairie ecosystem: a regional study of the Konza LTER in the Flint Hills of Kansas, USA

Abstract. Methods are developed to study hydrologic interactions across the surficial/groundwater interface in a native prairie ecosystem. Surficial ecohydrologic processes are simulated with the USDA's EPIC model using daily climate data from the Kansas Weather Data Library, vegetation and soil data from the USDA, and current land-use management practices. Results show that mean annual precipitation (from 1985–2005) is partitioned into 13% runoff regionally and 14% locally over the Konza LTER, lateral flow through soil is 1% regionally and 2% locally, groundwater recharge is 11% regionally and 9% locally, and evapotranspiration accounts for the remaining 75%. The spatial distribution of recharge was used in a regional Modflow groundwater model that was calibrated to existing groundwater observations and field measurements gathered for this study, giving a hydraulic conductivity in the Flint Hills region of 1–2 m day−1 with a local zone (identified here) of 0.05–0.1 m day−1. The resistance was set to fixed representative values during model calibration of hydraulic conductivity, and simple log-log relations correlate the enhanced recharge beneath ephemeral upland streams and baseflow in perennial lowland streams to the unknown resistance of the streambeds. Enhanced recharge due to stream transmission loss (the difference between terrestrial runoff and streamflow) represents a small fraction of streamflow in the ephemeral upland and the resistance of this streambed is 100 000 day. Long-term baseflow in the local Kings Creek watershed (2% of the groundwater recharge over the watershed) is met when the resistance of the lowland streambed is 1000 day. The coupled framework developed here to study surficial ecohydrological processes using EPIC and groundwater hydrogeological processes using Modflow provides a baseline hydrologic assessment and a computational platform for future investigations to examine the impacts of climate change, vegetative cover, soils, and management practices on hydrologic forcings.

Groundwater Recharge Assessment in the Chateauguay River Watershed

The objective of this study was to evaluate groundwater recharge in the Chateauguay River watershed. The Hydrologic Evaluation of Landfill Performance (HELP) model was used to assess daily values of recharge, evapotranspiration and runoff. The study area was divided into a regular grid, 250 m × 250 m, for a total of 47,616 grid elements. The input parameters included soil physical properties, land use, vegetation and climate data. Calibration of HELP was carried out against runoff and baseflow estimates obtained from separation of five river hydrographs. Over a 39 year period, the mean annual recharge rate was estimated at 86 mm, or 9% of the total precipitation. Areas characterized by high water level elevations and unconfined flow conditions were identified as the main recharge areas. Daily estimates show that recharge takes place mainly in spring and fall. Over the observed period, the annual variations of evapotranspiration and runoff were directly related to changes in precipitation, whereas the annual recharge response was subdued, with much lower variations. HELP was also used to assess potential climate change scenarios using data for the driest and most humid years. The mean annual recharge was 51 mm for the driest year and 99 mm for the most humid year. Differences in the spatial distribution of recharge for the predictive scenarios indicate that the areas most sensitive to climate change correspond to the preferential recharge areas.

Using Ground Water Responses to Improve Understanding of Climate Variation Impacts and Salinity Risk

Dryland salinity is a serious threat to the sustainable management of land and water in many parts of Victoria. However, the onset of dry climate conditions has reduced its priority as a natural resource management issue in recent years. Apart from the elevated need for water taking the spotlight, the dry conditions reduce the risk of salinity, temporarily at least. Questions are naturally posed about what impact that these conditions have on salinity incidence. Does salinity retreat to some or any extent? How significant is the recession or improvement, if any? Obviously, a number of other factors besides climate influence whether, and to what degree, a salt affected area alters. This paper concentrates on investigating the climate variation impacts on several ground water flow systems to gain some further insight into salinity risk in time and space, as well as management of this risk. Close observation of the landscape and analysis of the ground water level record improves knowledge about the ground water flow systems and their responsiveness to changes in climate and landscape management. A review of observation bore data has revealed considerable declines in ground water levels across northern Victoria since 1996. The dry climate conditions during this time are hypothesised to have been largely responsible for these declines despite a number of landscape factors. The declines have been accompanied by marked changes in the occurrence and spatial distribution of recharge. In upland salt-affected areas of north central Victoria, cleared crest and upper slope landscape positions, which have typically had a high potential recharge status, have recorded very little recharge since 1996. During the same time, many lower slope and discharge positions in these areas have continued to record some seasonal recharge in most years. This recharge is argued to be due more to local infiltration than aquifer throughflow and has significant implications for salinity management priorities.

Modeled impacts of predicted climate change on recharge and groundwater levels

A methodology is developed for linking climate models and groundwater models to investigate future impacts of climate change on groundwater resources. An unconfined aquifer, situated near Grand Forks in south central British Columbia, Canada, is used to test the methodology. Climate change scenarios from the Canadian Global Coupled Model 1 (CGCM1) model runs are downscaled to local conditions using Statistical Downscaling Model (SDSM), and the change factors are extracted and applied in LARS‐WG stochastic weather generator and then input to the recharge model. The recharge model simulated the direct recharge to the aquifer from infiltration of precipitation and consisted of spatially distributed recharge zones, represented in the Hydrologic Evaluation of Landfill Performance (HELP) hydrologic model linked to a geographic information system (GIS). A three‐dimensional transient groundwater flow model, implemented in MODFLOW, is then used to simulate four climate scenarios in 1‐year runs (1961–1999 present, 2010–2039, 2040–2069, and 2070–2099) and compare groundwater levels to present. The effect of spatial distribution of recharge on groundwater levels, compared to that of a single uniform recharge zone, is much larger than that of temporal variation in recharge, compared to a mean annual recharge representation. The predicted future climate for the Grand Forks area from the downscaled CGCM1 model will result in more recharge to the unconfined aquifer from spring to the summer season. However, the overall effect of recharge on the water balance is small because of dominant river‐aquifer interactions and river water recharge.

Scale Effects of Hydrostratigraphy and Recharge Zonation on Base Flow

Uncertainty regarding spatial variations of model parameters often results in the simplifying assumption that parameters are spatially uniform. However, spatial variability may be important in resource assessment and model calibration. In this paper, a methodology is presented for estimating a critical basin size, above which base flows appear to be relatively less sensitive to the spatial distribution of recharge and hydraulic conductivity, and below which base flows are relatively more sensitive to this spatial variability. Application of the method is illustrated for a watershed that exhibits distinct infiltration patterns and hydrostratigraphic layering. A ground water flow model (MODFLOW) and a parameter estimation code (UCODE) were used to evaluate the influence of recharge zonation and hydrostratigraphic layering on base flow distribution. Optimization after removing spatial recharge variability from the calibrated model altered base flow simulations up to 53% in watersheds smaller than 40 km(2). Merging six hydrostratigraphic units into one unit with average properties increased base flow residuals up to 83% in basins smaller than 50 km(2). Base flow residuals changed <5% in watersheds larger than 40 and 50 km(2) when recharge and hydrostratigraphy were simplified, respectively; thus, the critical basin size for the example area is approximately 40 to 50 km(2). Once identified for an area, a critical basin size could be used to guide the scale of future investigations. By ensuring that parameter discretization needed to capture base flow distribution is commensurate with the scope of the investigation, uncertainty caused by overextending uniform parameterization or by estimating extra parameter values is reduced.

Estimating Ground Water Recharge from Topography, Hydrogeology, and Land Cover

Proper management of ground water resources requires knowledge of the rates and spatial distribution of recharge to aquifers. This information is needed at scales ranging from that of individual communities to regional. This paper presents a methodology to calculate recharge from readily available ground surface information without long-term monitoring. The method is viewed as providing a reasonable, but conservative, first approximation of recharge, which can then be fine-tuned with other methods as time permits. Stream baseflow was measured as a surrogate for recharge in small watersheds in southeastern Wisconsin. It is equated to recharge (R) and then normalized to observed annual precipitation (P). Regression analysis was constrained by requiring that the independent and dependent variables be dimensionally consistent. It shows that R/P is controlled by three dimensionless ratios: (1) infiltrating to overland water flux, (2) vertical to lateral distance water must travel, and (3) percentage of land cover in the natural state. The individual watershed properties that comprise these ratios are now commonly available in GIS data bases. The empirical relationship for predicting R/P developed for the study watersheds is shown to be statistically viable and is then tested outside the study area and against other methods of calculating recharge. The method produces values that agree with baseflow separation from streamflow hydrographs (to within 15% to 20%), ground water budget analysis (4%), well hydrograph analysis (12%), and a distributed-parameter watershed model calibrated to total streamflow (18%). It has also reproduced the temporal variation over 5 yr observed at a well site with an average error < 12%.

GIS-based decision support system for groundwater assessment in large irrigation project areas

In large canal irrigation project areas, integrated management of surface and groundwater resources can improve water use efficiencies and agricultural productivity and also control water logging. Such integrated management requires an estimation of spatial distribution of recharge and ground water flow in the underlying aquifer. Recharge occurs both as percolation losses from fields and seepage losses from the water distribution network. Percolation losses are influenced by weather, soil properties, land use, and canal water and groundwater use. Seepage losses depend on the conditions of flow in the water distribution system. In large irrigation project areas all the factors influencing the recharge of groundwater vary spatially. In this study, a geographical information systems (GIS) is used to map the spatial distribution of recharge which then serves as input to a regional groundwater flow model for simulating the behavior of the underlying aquifer. The basis is that the project area can be divided into a set of basic simulation units (BSUs) that are homogenous with respect to the conditions that influence the recharge processes. A daily field soil water balance model and a simple canal flow model are used to estimate the percolation and seepage losses, respectively. The combination of models and GIS can be used as an integrated decision support system to assess the groundwater resources and derive strategies for integrated management of canal and groundwater resources in the project area.

Estimating recharge at Yucca Mountain, Nevada, USA: comparison of methods

Obtaining values of net infiltration, groundwater travel time, and recharge is necessary at the Yucca Mountain site, Nevada, USA, in order to evaluate the expected performance of a potential repository as a containment system for high-level radioactive waste. However, the geologic complexities of this site, its low precipitation and net infiltration, with numerous mechanisms operating simultaneously to move water through the system, provide many challenges for the estimation of the spatial distribution of recharge. A variety of methods appropriate for arid environments has been applied, including water-balance techniques, calculations using Darcy's law in the unsaturated zone, a soil-physics method applied to neutron-hole water-content data, inverse modeling of thermal profiles in boreholes extending through the thick unsaturated zone, chloride mass balance, atmospheric radionuclides, and empirical approaches. These methods indicate that near-surface infiltration rates at Yucca Mountain are highly variable in time and space, with local (point) values ranging from zero to several hundred millimeters per year. Spatially distributed net-infiltration values average 5 mm/year, with the highest values approaching 20 mm/year near Yucca Crest. Site-scale recharge estimates range from less than 1 to about 12 mm/year. These results have been incorporated into a site-scale model that has been calibrated using these data sets that reflect infiltration processes acting on highly variable temporal and spatial scales. The modeling study predicts highly non-uniform recharge at the water table, distributed significantly differently from the non-uniform infiltration pattern at the surface.