Estimating groundwater recharge in a cold desert environment in northern China using chloride

Understanding sources and rates of recharge to the Badain Jaran Desert in northern China is important for assessing sustainability of the area’s oasis lake ecosystem and its water resources in general. For this purpose, direct recharge was investigated with the chloride mass balance method for 18 unsaturated zone profiles (6–16 m depth). Spatial variability is low across the area (range in mean Cl in profiles: 62–164 mg/L Cl), largely attributable to the uniformity of sandy unsaturated zone conditions. No strong correlations between environmental factors of profile locations and recharge rates were found, though a weak relationship between recharge and vegetation density was suggested. The study area’s complex dune morphology appears to have no measurable impact on recharge variability. Mean estimated diffuse recharge is 1.4 mm/year (1.0–3.6 mm/year for 95% confidence level), approximately 1.7% of mean annual precipitation. Temporal fluctuations in recharge due to climate variability are apparent and there is good correspondence in temporal trends over a time span of 200–300 years. Water balance considerations indicate that direct recharge is insufficient to support the numerous perennial lakes in the study area, suggesting that diffuse recharge presently plays a minor role in the overall water balance of the desert’s shallow Quaternary aquifer.     

Choosing appropriate techniques for quantifying groundwater recharge

Various techniques are available to quantify recharge; however, choosing appropriate techniques is often difficult. Important considerations in choosing a technique include space/time scales, range, and reliability of recharge estimates based on different techniques; other factors may limit the application of particular techniques. The goal of the recharge study is important because it may dictate the required space/time scales of the recharge estimates. Typical study goals include water-resource evaluation, which requires information on recharge over large spatial scales and on decadal time scales; and evaluation of aquifer vulnerability to contamination, which requires detailed information on spatial variability and preferential flow. The range of recharge rates that can be estimated using different approaches should be matched to expected recharge rates at a site. The reliability of recharge estimates using different techniques is variable. Techniques based on surface-water and unsaturated-zone data provide estimates of potential recharge, whereas those based on groundwater data generally provide estimates of actual recharge. Uncertainties in each approach to estimating recharge underscore the need for application of multiple techniques to increase reliability of recharge estimates. Electronic Publication     

Movement of water through the thick unsaturated zone underlying Oro Grande and Sheep Creek Washes in the western Mojave Desert,USA

Previous studies indicate that a small quantity of recharge occurs from infiltration of streamflow in intermittent streams in the upper Mojave River basin, in the western Mojave Desert, near Victorville, California. Chloride, tritium, and stable isotope data collected in the unsaturated zone between 1994 and 1998 from boreholes drilled in Oro Grande and Sheep Creek Washes indicate that infiltration of streamflow occurs to depths below the root zone, and presumably to the water table, along much of Oro Grande Wash and near the mountain front along Sheep Creek Wash. Differences in infiltration at sites along each wash are the result of hydrologic variables such as proximity to the mountain front, quantity of streamflow, and texture of the subsurface deposits. Differences in infiltration between the washes are the result of large-scale geomorphic processes. For example, Oro Grande wash is incised into the Victorville fan and infiltration has occurred at approximately the same location over recent geologic time. In contrast, Sheep Creek Wash overlies an active alluvial fan and the stream channel can move across the fan surface through time. Infiltration does not occur to depths below the root zone at control sites outside of the washes. Electronic Publication     

Multiple-method estimation of recharge rates at diverse locations in the North Carolina Coastal Plain, USA

Recharge rates determined at diverse study sites in a shallow, unconfined aquifer differed from one another depending on the analytical method used and on each method’s applicability and limitations. Total recharge was quantified with saturated-zone methods using water-table fluctuations at seven sites in North Carolina, USA and using groundwater-age dating at three of the seven sites; at two of the sites, potential recharge was quantified with an unsaturated-zone method using Darcy’s law; and at five of the sites, net recharge was quantified with a stream hydrograph-separation method using streamflow-recession curves. Historical mean net recharge was 25 to 69% of the historical total recharge rates. The large disparity is attributed to groundwater losses between recharge and discharge areas, primarily by evapotranspiration and seepage to underlying aquifers. The spatial distribution of historical mean annual total recharge did not vary between landscape units, as suggested in a previous study. Similarly, total recharge did not correlate significantly with mean annual rainfall, mean annual water table depth, or the surficial soil properties of percent clay and bulk density. Total recharge did correlate significantly with the surficial soil properties of percent sand and percent silt.

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Résumé Les taux de recharge déterminés sur divers sites d’études dans un aquifère phréatique libre, varient les uns par rapport aux autres selon la méthode analytique utilisée et les limites et l’applicabilité de chaque méthode. La recharge totale a été quantifiée avec des méthodes en zone saturée utilisant les fluctuations des nappes sur sept sites de la Caroline du Nord, USA, et en utilisant les datations des eaux souterraines sur trois des sept sites ; sur deux des sites, la recharge potentielle a été quantifiée avec une méthode en zone non-saturée utilisant la loi de Darcy, et sur deux sites, la recharge nette a été quantifiée suivant une méthode de séparation des hydrographes de cours d’eau utilisant les courbes de récession du débit. La moyenne historique de la recharge nette est comprise entre 25 et 69% des taux historiques de recharge totale. L’importante disparité est attribuée aux pertes en eaux souterraines entre les zones de recharge et les zones d’exutoire, d’abord par évapotranspiration et ensuite par infiltration vers les aquifères sous-jacents. La distribution spatiale de la recharge historique moyenne annuelle et totale ne varie pas selon les unités paysagères, comme cela était suggéré dans une étude antérieure. De même, la recharge totale n’est pas corrélée significativement avec la moyenne annuelle des précipitations, la moyenne annuelle de la profondeur de la nappe, ou les propriétés de surface des sols que sont le pourcentage d’argile ou la densité apparente. La recharge totale est corrélée significativement avec les propriétés de surface du sol et les teneurs en sable et en silt.

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Resumen Las tasas de recarga determinadas en diversos sitios de estudio en un acuífero somero no confinado difieren entre sí dependiendo del método analítico usado y de las limitaciones y aplicabilidad de cada método. Se cuantificó la recarga total con métodos de zona saturada usando fluctuaciones del nivel freático en siete sitios en Carolina del Norte, Estados Unidos, y usando datación de edades de agua subterránea en tres de los siete sitios; en dos de los sitios se cuantificó la recarga potencial con un método de zona no saturada usando la Ley de Darcy y en cinco de los sitios se cuantificó la recarga neta con el método de separación hidrográfica usando curvas de recesión de flujo en arroyos. La recarga neta media histórica varió entre 25 y 69% de las tasas de recarga total histórica. La diferencia tan grande se atribuye a pérdidas de agua subterránea entre áreas de descarga y recarga, principalmente por evapotranspiración y escurrimiento hacia acuíferos subyacentes. La distribución espacial de la recarga total anual media histórica no varió entre las unidades de paisaje, como ya se había sugerido en un estudio previo. De modo similar, la recarga total no guarda correlación significativa con la lluvia media anual, la profundidad media anual del nivel freático, o las propiedades superficiales del suelo, particularmente porcentaje de arcilla y densidad volumétrica. La recarga total si tuvo una correlación significativa con las propiedades superficiales del suelo tal como porcentaje de arena y porcentaje de limo.

     

Groundwater recharge: an overview of processes and challenges

Since the mid-1980s, a relative explosion of groundwater-recharge studies has been reported in the literature. It is therefore relevant to assess what is now known and to offer further guidance to practitioners involved in water-resource development. The paper summarizes current understanding of recharge processes, identifies recurring recharge-evaluation problems, and reports on some recent advances in estimation techniques. Emphasis is accorded to (semi-)arid regions because the need for information is greatest in those areas – groundwater is often the only water source, is vulnerable to contamination, and is prone to depletion. Few studies deal explicitly with groundwater recharge in temperate and humid zones, because recharge is normally included in regional groundwater investigations as one component of the water balance. The resolution of regional water-balance studies in (semi-)arid areas is, in contrast, often too low to quantify the limited recharge component with sufficient precision. Despite the numerous studies, determination of recharge fluxes in (semi-)arid regions remains fraught with uncertainty. Multiple tracer approaches probably offer the best potential for reliable results in local studies that require 'at-point' information. However, many investigations indicate that these approaches are not straightforward, because in some cases preferential flow contributes as much as 90% of the estimated total recharge. Tracer results (e.g. Cl^–, ³H) must therefore be interpreted with care in areas with multi-modal flow in the vadose zone. Moreover, accurate estimation of total chloride deposition is essential, and tritium may be influenced by vapour transport at low flux rates. In addition, paleoclimatic and paleohydrological conditions may cause discrepancies between measured actual processes and calculated long-term averages. The frequently studied issues of localized recharge and spatial variability need not be a problem if concern is with regional estimates. The key for practitioners is the project objective, which dictates whether 'at-point' or area-/groundwater-based estimation methods are appropriate. Many indirect (wadi) recharge studies reported in the literature are site specific; the relationship between 'at point' hydraulic properties and channel-reach losses demands further investigation. Electronic Publication     

Multiple-methods investigation of recharge at a humid-region fractured rock site,Pennsylvania, USA

Lysimeter-percolate and well-hydrograph analyses were combined to evaluate recharge for the Masser Recharge Site (central Pennsylvania, USA). In humid regions, aquifer recharge through an unconfined low-porosity fractured-rock aquifer can cause large magnitude water-table fluctuations over short time scales. The unsaturated hydraulic characteristics of the subsurface porous media control the magnitude and timing of these fluctuations. Data from multiple sets of lysimeters at the site show a highly seasonal pattern of percolate and exhibit variability due to both installation factors and hydraulic property heterogeneity. Individual event analysis of well hydrograph data reveals the primary influences on water-table response, namely rainfall depth, rainfall intensity, and initial water-table depth. Spatial and seasonal variability in well response is also evident. A new approach for calculating recharge from continuous water-table elevation records using a master recession curve (MRC) is demonstrated. The recharge estimated by the MRC approach when assuming a constant specific yield is seasonal to a lesser degree than the recharge estimate resulting from the lysimeter analysis. Partial reconciliation of the two recharge estimates is achieved by considering a conceptual model of flow processes in the highly-heterogeneous underlying fractured porous medium.     

Artificial recharge of groundwater: hydrogeology and engineering

Artificial recharge of groundwater is achieved by putting surface water in basins, furrows, ditches, or other facilities where it infiltrates into the soil and moves downward to recharge aquifers. Artificial recharge is increasingly used for short- or long-term underground storage, where it has several advantages over surface storage, and in water reuse. Artificial recharge requires permeable surface soils. Where these are not available, trenches or shafts in the unsaturated zone can be used, or water can be directly injected into aquifers through wells. To design a system for artificial recharge of groundwater, infiltration rates of the soil must be determined and the unsaturated zone between land surface and the aquifer must be checked for adequate permeability and absence of polluted areas. The aquifer should be sufficiently transmissive to avoid excessive buildup of groundwater mounds. Knowledge of these conditions requires field investigations and, if no fatal flaws are detected, test basins to predict system performance. Water-quality issues must be evaluated, especially with respect to formation of clogging layers on basin bottoms or other infiltration surfaces, and to geochemical reactions in the aquifer. Clogging layers are managed by desilting or other pretreatment of the water, and by remedial techniques in the infiltration system, such as drying, scraping, disking, ripping, or other tillage. Recharge wells should be pumped periodically to backwash clogging layers. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s10040-001-0182-4. Electronic Publication     

Evidence for structurally controlled recharge in the Blue Ridge Province,Virginia, USA

Physical and geophysical data collected at a fractured-rock research site in the Blue Ridge Province of Virginia, USA indicate that recharge rates to a fractured-rock aquifer are controlled by a small-scale thrust fault associated with regional thrust faulting. Recharge rates appear to be correlated to spatial variation in the hydraulic conductivity of the regolith, which has been influenced by weathering rates and the metamorphic and structural history of the underlying parent material. The angle of dip of the thrust-fault shear zone and the fracturing within the crystalline rock adjacent to the fault plane appear to serve as geologic controls that preferentially direct infiltrated meteoric water to a deeper confined fractured-rock aquifer. In-situ analysis of electrical resistivity, matric potential and moisture content shows two distinctly different recharge processes which are spatially correlated with the structure of the shallow subsurface (regolith overlying the vertically oriented shear zone and regolith overlying the thrust-fault hanging wall), and which have strong temporal correlations with the dynamics of the underlying saturated conditions. Recharge flux within the regolith overlying the vertically oriented shear zone is strongly controlled by the orientation and areal extent of the thrust-fault shear zone, highlighting the importance of accurate delineation of recharge areas in crystalline rock aquifer systems.     

Identifying and quantifying urban recharge: a review

The sources of and pathways for groundwater recharge in urban areas are more numerous and complex than in rural environments. Buildings, roads, and other surface infrastructure combine with man-made drainage networks to change the pathways for precipitation. Some direct recharge is lost, but additional recharge can occur from storm drainage systems. Large amounts of water are imported into most cities for supply, distributed through underground pipes, and collected again in sewers or septic tanks. The leaks from these pipe networks often provide substantial recharge. Sources of recharge in urban areas are identified through piezometry, chemical signatures, and water balances. All three approaches have problems. Recharge is quantified either by individual components (direct recharge, water-mains leakage, septic tanks, etc.) or holistically. Working with individual components requires large amounts of data, much of which is uncertain and is likely to lead to large uncertainties in the final result. Recommended holistic approaches include the use of groundwater modelling and solute balances, where various types of data are integrated. Urban recharge remains an under-researched topic, with few high-quality case studies reported in the literature. Electronic Publication     

Environmental isotopic study on the recharge and residence time of groundwater in the Heihe River Basin, northwestern China

The recharge and origin of groundwater and its residence time were studied using environmental isotopic measurements in samples from the Heihe River Basin, China. δ¹⁸O and δD values of both river water and groundwater were within the same ranges as those found in the alluvial fan zone, and lay slightly above the local meteoric water line (δD=6.87δ¹⁸O+3.54). This finding indicated that mountain rivers substantially and rapidly contribute to the water resources in the southern and northern sub-basins. δ¹⁸O and δD values of groundwater in the unconfined aquifers of these sub-basins were close to each other. There was evidence of enrichment of heavy isotopes in groundwater due to evaporation. The most pronounced increase in the δ¹⁸O value occurred in agricultural areas, reflecting the admixture of irrigation return flow. Tritium results in groundwater samples from the unconfined aquifers gave evidence for ongoing recharge, with mean residence times of: less than 36 years in the alluvial fan zone; about 12–16 years in agricultural areas; and about 26 years in the Ejina oasis. In contrast, groundwater in the confined aquifers had ¹⁴C ages between 0 and 10 ka BP.     

Sensitivity analysis of hydrological parameters in modeling flow and transport in the unsaturated zone of Yucca Mountain, Nevada, USA

The unsaturated fractured volcanic deposits at Yucca Mountain in Nevada, USA, have been intensively investigated as a possible repository site for storing high-level radioactive waste. Field studies at the site have revealed that there exist large variabilities in hydrological parameters over the spatial domain of the mountain. Systematic analyses of hydrological parameters using a site-scale three-dimensional unsaturated zone (UZ) flow model have been undertaken. The main objective of the sensitivity analyses was to evaluate the effects of uncertainties in hydrologic parameters on modeled UZ flow and contaminant transport results. Sensitivity analyses were carried out relative to fracture and matrix permeability and capillary strength (van Genuchten α) through variation of these parameter values by one standard deviation from the base-case values. The parameter variation resulted in eight parameter sets. Modeling results for the eight UZ flow sensitivity cases have been compared with field observed data and simulation results from the base-case model. The effects of parameter uncertainties on the flow fields were evaluated through comparison of results for flow and transport. In general, this study shows that uncertainties in matrix parameters cause larger uncertainty in simulated moisture flux than corresponding uncertainties in fracture properties for unsaturated flow through heterogeneous fractured rock.     

A catchment water balance model for estimating groundwater recharge in arid and semiarid regions of south-east Iran

This paper presents a new model of the rainfall-runoff-groundwater flow processes applicable to semiarid and arid catchments in south-east Iran. The main purpose of the model is to assess the groundwater recharge to aquifers in these catchments. The model takes into account main recharge mechanisms in the region, including subsurface flow in the valley alluvium in mountainous areas and recharge from the bed of ephemeral rivers. It deals with the effects of spatial variation in the hydrological processes by dividing the catchment into regions of broad hydrologic similarity named as highland, intermediate and aquifer areas. The model is based on the concept of routing precipitation within and through the catchment. The model has been applied to the Zahedan catchment and the results indicate that the groundwater level estimated by the recharge model generally is in agreement with the behaviour of groundwater levels in observation wells. The sensitivity analysis indicates that when the rainfall in the aquifer area is used to replace the values recorded in the intermediate area and the highland area, the recharge estimates are reduced by 42-87%. This result supports the division of the catchment into different zones of hydrological similarity to account for spatial variability of hydrological processes. Electronic Publication     

Using CRD method for quantification of groundwater recharge in the Gaza Strip, Palestine

Rainfall is the main source of groundwater recharge in the Gaza Strip area in Palestine. The area is located in the semi-arid zone and there is no source of recharge other than rainfall. Estimation of groundwater recharge from rainfall is not an easy task since it depends on many uncertain parameters. The cumulative rainfall departure (CRD) method, which depends on the water balance principle, was used in this study to estimate the net groundwater recharge from rainfall. This method does not require much data as is the case with other classical recharge estimation methods. The CRD method was carried out using optimisation approach to minimise the root mean square error (RMSE) between the measured and the simulated groundwater head. The results of this method were compared with the results of other recharge estimation methods from literature. It was found that the results of the CRD method are very close to the results of the other methods, but with less data requirements and greater ease of application. Based on the CRD method, the annual amount of groundwater recharge from rainfall in the Gaza Strip is about 43 million m³. An erratum to this article can be found at     

Thermal history of the unsaturated zone at Yucca Mountain,Nevada, USA

Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (T_h) up to ∼80 °C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite- and fluorite-hosted fluid inclusions and calcite δ¹⁸O values, and depositional timing is constrained by the ²⁰⁷Pb/²³⁵U ages of chalcedony or opal in the deposits. Fluid inclusion T_h from 50 samples of calcite and four samples of fluorite range from ∼35 to ∼90 °C. Calcite δ¹⁸O values range from ∼0 to ∼22‰ (SMOW) but most fall between 12 and 20‰. The highest T_h and the lowest δ¹⁸O values are found in the older calcite. Calcite T_h and δ¹⁸O values indicate that most calcite precipitated from water with δ¹⁸O values between −13 and −7‰, similar to modern meteoric waters.     

Soil water movement and deep drainage through thick vadose zones on the northern slope of the Tianshan Mountain: Croplands vs. natural lands

Regional aridity is increasing under global climate change, and therefore the sustainable use of water resources has drawn attention from scientists and the public. Land-use changes can have a significant impact on groundwater recharge in arid regions, and quantitative assessment of the impact is key to sustainable groundwater resources management. In this study, the changes of groundwater recharge after the conversion of natural lands to croplands were investigated and compared in inland and arid region, i.e., the northern slope of the Tianshan Mountain. Stable isotopes suggest that soil water in topsoil (< 2 m) has experienced stronger evaporation under natural lands than croplands, and then moves downward as a piston flow. Recharge was estimated by the tracer-based mass balance method, i.e., chloride and sulfate. Recharge rates under natural conditions estimated by the chloride mass balance (CMB) method were estimated to be 0.07 mm/a in deserts and 0.4 mm/a in oases. In contrast, the estimated groundwater recharge ranged from 61.2 mm/a to 44.8 mm/a in croplands, indicating that groundwater recharge would increase significantly after land changes from natural lands to irrigated croplands in arid regions. Recharge estimated by the sulfate mass balance method is consistent with that from the CMB method, indicating that sulfate is also a good tracer capable of estimating groundwater recharge.     

Influences of the unsaturated,saturated, and riparian zones on the transport of nitrate near the Merced River,California, USA

Transport and transformation of nitrate was evaluated along a 1-km groundwater transect from an almond orchard to the Merced River, California, USA, within an irrigated agricultural setting. As indicated by measurements of pore-water nitrate and modeling using the root zone water quality model, about 63% of the applied nitrogen was transported through a 6.5-m unsaturated zone. Transport times from recharge locations to the edge of a riparian zone ranged from approximately 6 months to greater than 100 years. This allowed for partial denitrification in horizons having mildly reducing conditions, and essentially no denitrification in horizons with oxidizing conditions. Transport times across a 50–100-m-wide riparian zone of less than a year to over 6 years and more strongly reducing conditions resulted in greater rates of denitrification. Isotopic measurements and concentrations of excess N₂ in water were indicative of denitrification with the highest rates below the Merced River. Discharge of water and nitrate into the river was dependent on gradients driven by irrigation or river stage. The results suggest that the assimilative capacity for nitrate of the groundwater system, and particularly the riverbed, is limiting the nitrate load to the Merced River in the study area.     

Sources of groundwater recharge at the confluence of the Niles, Sudan

 Due to the rapid expansion of the Sudanese capital city far away from its three Niles, it has become a necessity to estimate the sustainability of the groundwater resources in this arid area. The purpose of this study – based on the stable-isotopic composition of groundwater – is to identify the sources of recharge and their relative contribution. The results show that groundwater infiltrated from the Niles under the present prevailing arid climate, with greater contribution from the White Nile compared to the Blue Nile, occupies an extension of 12 km from the Niles within the two main aquifers. Paleogroundwater of meteoric origin, infiltrated in the cooler pluvial earlier Holocene, occupies the outer zones beyond the Niles' effect. A rational utilization policy is recommended to preserve the finite groundwater resources. Received: 27 August 1998 · Accepted: 2 March 1999     

Unsaturated zones as archives of past climates: toward a new proxy for continental regions

Natural chemical and isotopic tracers contained in unsaturated-zone moisture profiles are being developed as potential new archives for reconstructing recharge history, as well as palaeoclimatic or palaeobotanical conditions over time scales ranging from 20–120,000 years. Results worldwide to date are reviewed, and examples from northern Africa and the western USA are discussed in detail. Encouraging results are obtained from relatively homogeneous deposits such as Quaternary dune sands, where Cl profiles are compared both with the instrumental record, such as rainfall and river-gauging records, and ³H profiles. Model studies have helped to define the persistence time of unsaturated-zone signals, where evidence of a 20-year event such as the Sahel drought may persist for 1,000 years. Significant questions remain regarding the assumptions used in interpreting profiles, particularly the extent to which preferential flow may occur, transient flow phenomena, and stability of tracer-input function. Unsaturated zones that exhibit strong preferential flow are probably unsuitable as archives. Questions remain also on the assumption that flow remains downward, especially in deep unsaturated zones where non-isothermal vapour transport may occur. Estimation of depositional flux for Cl and other parameters is probably the greatest source of uncertainty, both at the spatial scale and also in the long term. Advances are being made in all areas, however, and the use of multiple tracers (chemical, especially Cl and NO₃) and isotopic signals (δ¹⁸O, δ²H, ³⁶Cl), together with soil hydraulic properties, is promising for palaeohydrological reconstruction. Electronic Publication     

Estimating aquifer recharge in Mission River watershed, Texas: model development and calibration using genetic algorithms

Soil moisture balance studies provide a convenient approach to estimate aquifer recharge when only limited site-specific data are available. A monthly mass-balance approach has been utilized in this study to estimate recharge in a small watershed in the coastal bend of South Texas. The developed lumped parameter model employs four adjustable parameters to calibrate model predicted stream runoff to observations at a gaging station. A new procedure was developed to correctly capture the intermittent nature of rainfall. The total monthly rainfall was assigned to a single-equivalent storm whose duration was obtained via calibration. A total of four calibrations were carried out using an evolutionary computing technique called genetic algorithms as well as the conventional gradient descent (GD) technique. Ordinary least squares and the heteroscedastic maximum likelihood error (HMLE) based objective functions were evaluated as part of this study as well. While the genetic algorithm based calibrations were relatively better in capturing the peak runoff events, the GD based calibration did slightly better in capturing the low flow events. Treating the Box-Cox exponent in the HMLE function as a calibration parameter did not yield better estimates and the study corroborates the suggestion made in the literature of fixing this exponent at 0.3. The model outputs were compared against available information and results indicate that the developed modeling approach provides a conservative estimate of recharge.     

Dual-porosity modeling of groundwater recharge: testing a quick calibration using in situ moisture measurements,Areuse River Delta,Switzerland

A simple method for calibrating the dual-porosity MACRO model via in situ TDR measurements during a brief infiltration run (2.8 h) is proposed with the aim of estimating local groundwater recharge (GR). The recharge was modeled firstly by considering the entire 3 m of unsaturated soil, and secondly by considering only the topsoil to the zero-flux plane (0–0.70 m). The modeled recharge was compared against the GR obtained from field measurements. Measured GR was 313 mm during a 1-year period (15 October 1990–15 October 1991). The best simulation results were obtained when considering the entire unsaturated soil under equilibrium conditions excluding the macropore flow effect (330 mm), whereas under non-equilibrium conditions GR was overestimated (378 mm).Sensitivity analyses showed that the investigation of the topsoil is sufficient in estimating local GR in this case, since the water stored below this depth appears to be below the typical rooting depth of the vegetation and is not available for evapotranspiration. The modeled recharge under equilibrium conditions for the 0.7-m-topsoil layer was found to be 364 mm, which is in acceptable agreement with measurements.

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Resumen Se propone un método sencillo para calibrar el modelo de doble porosidad "MACRO" mediante medidas in-situ obtenidas por TDR durante un breve ensayo de infiltración (2,8 horas), con el objetivo de estimar la recarga local al acuífero. Ésta ha sido modelada de dos formas: considerando los 3 m de suelo no saturado y empleando sólo desde la capa superior hasta el plano de flujo nulo (de 0 a 0,70 m). Se compara la recarga modelada con la recarga local medida en campo, la cual fue de 313 mm durante un ciclo anual (del 15 de octubre de 1990 al 15 de octubre de 1991). Las mejores simulaciones corresponden a la hipótesis de columna entera no saturada en condiciones de equilibrio, excluyendo el efecto de macroporos (valor de 330 mm), mientras que el resultado obtenido para condiciones de no equilibrio en la recarga local está sobreestimado (378 mm).Los análisis de sensibilidad muestran que la investigación del horizonte superior del suelo es suficiente para estimar la recarga local en este caso, ya que el agua almacenada por debajo de esta profundidad parece estar fuera del alcance típico de las raíces de la vegetación y no puede ser evapotranspirada. La recarga modelada en condiciones de equilibrio para la capa superior de 0,70 m de espesor es de 364 mm, valor aceptable respecto a las medidas.

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Résumé Une méthode simple pour la calibration du modèle à double porosité MACRO par des mesures TDR in situ durant un bref essai d'infiltration (2.8 h) a été proposée pour l'estimation locale de la recharge de la nappe (RN). La RN a été d'abord simulée en tenant compte de toute la zone non saturée (3 m) et ensuite, en considérant uniquement la couverture du sol entre zéro et le plan du flux nul (0.70 m). La RN simulée a été comparée à la RN observée. La RN mesurée durant une année (15 octobre 1990–15 octobre 1991) était de 313 mm. Les meilleures simulations ont été obtenues en tenant compte de toute la zone non saturée sous les conditions d'équilibre excluant le flux préférentiel (330 mm). Sous les conditions de non équilibre, la RN a été surestimée (378 mm).Les analyses de sensitivité ont montré que l'investigation de la couverture du sol est suffisante pour l'estimation locale de la RN du fait que l'eau traversant le plan du flux nul se trouverait sous la zone des racines et échapperait à l'évapotranspiration. La RN simulée sur les 0.70 m du sol sous les conditions d'équilibre était de 364 mm, ce qui est comparable aux mesures.