Transient Recharge Estimability Through Field‐Scale Groundwater Model Calibration

The estimation of recharge through groundwater model calibration is hampered by the nonuniqueness of recharge and aquifer parameter values. It has been shown recently that the estimability of spatially distributed recharge through calibration of steady‐state models for practical situations (i.e., real‐world, field‐scale aquifer settings) is limited by the need for excessive amounts of hydraulic‐parameter and groundwater‐level data. However, the extent to which temporal recharge variability can be informed through transient model calibration, which involves larger water‐level datasets, but requires the additional consideration of storage parameters, is presently unknown for practical situations. In this study, time‐varying recharge estimates, inferred through calibration of a field‐scale highly parameterized groundwater model, are systematically investigated subject to changes in (1) the degree to which hydraulic parameters including hydraulic conductivity (K) and specific yield (S_y) are constrained, (2) the number of water‐level calibration targets, and (3) the temporal resolution (up to monthly time steps) at which recharge is estimated. The analysis involves the use of a synthetic reality (a reference model) based on a groundwater model of Uley South Basin, South Australia. Identifiability statistics are used to evaluate the ability of recharge and hydraulic parameters to be estimated uniquely. Results show that reasonable estimates of monthly recharge (<30% recharge root‐mean‐squared error) require a considerable amount of transient water‐level data, and that the spatial distribution of K is known. Joint estimation of recharge, S_y and K, however, precludes reasonable inference of recharge and hydraulic parameter values. We conclude that the estimation of temporal recharge variability through calibration may be impractical for real‐world settings.     

Response of deep groundwater to land use change in desert basins of the Trans‐Pecos region,Texas, USA: Effects on infiltration,recharge, and nitrogen fluxes

Quantifying the effects of anthropogenic processes on groundwater in arid regions can be complicated by thick unsaturated zones with long transit times. Human activities can alter water and nutrient fluxes, but their impact on groundwater is not always clear. This study of basins in the Trans‐Pecos region of Texas links anthropogenic land use and vegetation change with alterations to unsaturated zone fluxes and regional increases in basin groundwater NO₃^? concentrations. Median increases in groundwater NO₃^? (by 0.7–0.9 mg‐N/l over periods ranging from 10 to 50+ years) occurred despite low precipitation (220–360 mm/year), high potential evapotranspiration (~1570 mm/year), and thick unsaturated zones (10–150+ m). Recent model simulations indicate net infiltration and groundwater recharge can occur beneath Trans‐Pecos basin floors, and may have increased due to irrigation and vegetation change. These processes were investigated further with chemical and isotopic data from groundwater and unsaturated zone cores. Some unsaturated zone solute profiles indicate flushing of natural salt accumulations has occurred. Results are consistent with human‐influenced flushing of naturally accumulated unsaturated zone nitrogen as an important source of NO₃^? to the groundwater. Regional mass balance calculations indicate the mass of natural unsaturated zone NO₃^? (122–910 kg‐N/ha) was sufficient to cause the observed groundwater NO₃^? increases, especially if augmented locally with the addition of fertilizer N. Groundwater NO₃^? trends can be explained by small volumes of high NO₃^? modern recharge mixed with larger volumes of older groundwater in wells. This study illustrates the importance of combining long‐term monitoring and targeted process studies to improve understanding of human impacts on recharge and nutrient cycling in arid regions, which are vulnerable to the effects of climate change and increasing human reliance on dryland ecosystems.     

Application of the water table fluctuation method to characterize groundwater recharge in the Pampa plain,Argentina

Abstract

The water table fluctuation (WTF) method is based on accepting that rises of a water table are due to recharge water reaching the groundwater. To apply the method, an estimate of the specific yield of the zone of fluctuation of the groundwater level is required. In this paper, a method for estimation of the specific yield (S_y) is proposed; it consists of a graphical procedure which relates rises in groundwater level to the precipitation from which they originated. The method presents more reliable S_y values as the number of events measured increases. Eighteen years of daily measurements were analysed to obtain a S_y value of 0.09, which was used to apply the WTF method. The obtained recharge values show consistency with values calculated by other authors for the same region.

Editor D. Koutsoyiannis

Citation Varni, M., Comas, R., Weinzettel, P., and Dietrich, S., 2013. Application of water table fluctuation method to characterize the groundwater recharge in the Pampa plain, Argentina. Hydrological Sciences Journal, 58 (7), 1445–1455.     

Performance of the White method for estimating groundwater evapotranspiration under conditions of deep and fluctuating groundwater

The White method is a simple but the most frequently applied approach to estimate groundwater evapotranspiration (ET_g) from groundwater level diurnal signals. Because of a lack of direct measurements of ET_g, it is difficult to evaluate the performance of the White method, particularly in field environments with variable groundwater fluctuations. A 2‐year field observation in a hyper‐arid riparian tamarisk (Tamarix spp.) stand with deep groundwater depth in the lower Tarim River basin of China was conducted to measure the surface evapotranspiration (ET_s) and groundwater table. The performance of the White method and the influences of the variable groundwater fluctuations on the determinations of the specific yield (S_y) and recharge rate of groundwater (r) in the White method were investigated. The results showed that the readily available S_y determined by Loheide's method was feasible but must be finely determined based on the soil textures in the layers in which the groundwater level fluctuated. A constant S_y value for a defined porous medium could be assumed regardless of the discharge or recharge processes of groundwater. The time span of 0000 h to 0600 h for r estimation for the White method worked best than other time spans. A 2‐day moving average of r values further enhance ET_g estimation. Slight effects of environmental or anthropogenic disturbances on the diurnal fluctuations of groundwater level did not influence the ET_g estimations by the White method. Our results provide valuable references to the application of the White method for estimating daily ET_g in desert riparian forests with deep groundwater depth. Copyright © 2015 John Wiley & Sons, Ltd.     

Quasi-Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling

This study presents an extension of the concept of “quasi-saturation” to a quasi-saturated layer, defined as the uppermost dynamic portion of the saturated zone subject to water table fluctuations. Entrapped air here may cause substantial reductions in the hydraulic conductivity (K) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result of groundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specific yield (S_y), such as the water table fluctuation method (WTF), and reductions in K values. These effects impact estimation of fluid flow velocities and contaminant migration rates in groundwater. In order to quantify actual groundwater recharge rates and the effects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using a quasi-saturated layer for a pilot area in Rio Claro, Brazil. The calculated recharge rate represented 16% of the average precipitation over an 8-year period, approximately half of estimates using the WTF method. Air entrapment amounted to a fillable porosity of 0.07, significant lower that the value of 0.17 obtained experimentally for S_y. Numerical results showed that the entrapped air volume in the quasi-saturated layer can be very significant (0.58 of the air fraction) and hence can significantly affect estimates of groundwater recharge and groundwater flow rates near the water table.     

Nitrate transport in the unsaturated zone: a case study of the riverbank filtration system Karany,Czech Republic

Nitrate transport in the unsaturated zone of a riverbank filtration (RBF) system in Karany, Czech Republic, was studied. Previous study of the system estimated RBF recharge as 60% riverbank filtrate and 40% local groundwater contaminated by nitrates. Nitrate concentrations observed in RBF recently cannot be explained by simple groundwater contamination and a new conception of groundwater recharge is suggested. A two‐component model based on water ¹⁸O data modelled recharge of local groundwater. One component of groundwater recharge is rainfall and irrigation water moving through the unsaturated zone of the Quaternary sediments in piston flow. The second component is groundwater from the Cretaceous deposits with a free water table. Both the components of groundwater recharge have different nitrate concentrations, and resulting contamination of groundwater depends on the participation of water from Quaternary and Cretaceous deposits. Nitrates' origins and their mixing in the subsurface were traced by ¹⁵N data. Nitrate transport from the unsaturated zone is important and time variable source of groundwater contamination. Copyright © 2011 John Wiley & Sons, Ltd.     

Application of grey correlation method to evaluate potential groundwater recharge sites

Artificial recharge is a practical tool available for increasing the groundwater storage capacity. The efficiency of artificial recharge is related to various hydrogeological factors of the target area. In this study, a variable saturated groundwater flow model, FEMWATER, was used to evaluate the arrival times of recharged water that infiltrates from an artificial recharge pond to the groundwater table under various hydrogeological conditions. Forty-five arrival times were generated by FEMWATER. The relationships between the arrival times and hydrogeological factors used in the simulation of FEMWATER were analyzed by the grey correlation method. The results show the order of importance of the factors as they influence the arrival time. In order from high to low importance, they are α, D _g, θ _e, D _p, K _S and β. D _g and D _p are interpreted as the potential for movement of the recharge water; θ_e is the water storage capacity of soil, and K _S represents the ability of soil to transport water. α and β describe the characteristic curve of the unsaturated soil. The method was applied to evaluate a suitable site for artificial recharge in the Yun-Lin area. Grey correlation analysis was performed to obtain the grey correlation grade using the minimum arrival time as a reference sequence. An index is proposed herein to determine the recharge efficiency of 20 sampling sites. A contour mapping of index values at the 20 sampling sites identified three areas for artificial aquifer recharge in Yun-Lin. Area A in the upper plain is considered more appropriate for groundwater recharge than areas B and C in the coast.     

Groundwater recharge and discharge in a hyperarid alluvial plain (Akesu,Taklimakan Desert,China)

Groundwater recharge and discharge in the Akesu alluvial plain were estimated using a water balance method. The Akesu alluvial plain (4842 km²) is an oasis located in the hyperarid Tarim River basin of central Asia. The land along the Akesu River has a long history of agricultural development and the irrigation area is highly dependent on water withdrawals from the river. We present a water balance methodology to describe (a) surface water and groundwater interaction and (b) groundwater interaction between irrigated and non‐irrigated areas. Groundwater is recharged from the irrigation system and discharged in the non‐irrigated area. Uncultivated vegetation and wetlands are supplied from groundwater in the hyperarid environment. Results show that about 90% of groundwater recharge came from canal loss and field infiltration. The groundwater flow from irrigated to non‐irrigated areas was about 70% of non‐irrigated area recharge and acted as subsurface drainage for the irrigation area. This desalinated the irrigation area and supplied water to the non‐irrigated area. Salt moved to the non‐irrigation area following subsurface drainage. We conclude that the flooding of the Akesu River is a supplemental groundwater replenishment mechanism: the river desalinates the alluvial plain by recharging fresh water in summer and draining saline regeneration water in winter. Copyright © 2007 John Wiley & Sons, Ltd.     

Applicability of spectral analysis to determine hydraulic diffusivity

This study is to evaluate the applicability of estimating the one-dimensional horizontal hydraulic diffusivity of an unconfined aquifer with time-dependent fluctuation of lateral head and vertical recharge boundaries using observed water level spectra. Different models of boundary condition are imposed to evaluate the statistical significance between the calculated hydraulic diffusivity (ξ^′) with the given hydraulic diffusivity (ξ). The auto-spectra of the water level in observation wells tapping the same aquifer are closely related to those at the disturbed boundaries. For an aquifer with a constant hydraulic diffusivity, the water level fluctuation in the monitoring wells is linearly related to the water level spectra observed at the boundaries. The spectral density function of aquifer hydraulic head varies inversely with specific yield (S _y) and directly with recharge. Given small variation in water level spectra at the disturbed boundaries, the water level fluctuation in the aquifer is affected by the recharge condition and the aquifer spectral density function is sensitive to S _y. Using an iterative technique to estimate ξ from 1400 sets of given parameters, 99% of the ξ^′/ξ values deviated within only one order of magnitude with the model length (L) being equal to 1 km and 10 km. For L equal to 100 m, approximately 82% of the ξ^′/ξ population falls within two orders of magnitude. Therefore, spectral analysis of aquifer hydraulic head response can be used to estimate the hydraulic diffusivity of an unconfined aquifer which is affected by periodic variations in recharge and head at boundaries.     

Soil profile evolution following land‐use change: implications for groundwater quantity and quality

Soil and vadose zone profiles are used as an archive of changes in groundwater recharge and water quality following changes in land use in an area of the Loess Plateau of China. A typical rain‐fed loess‐terrace agriculture region in Hequan, Guyuan, is taken as an example, and multiple tracers (chloride mass balance, stable isotopes, tritium and water chemistry) are used to examine groundwater recharge mechanisms and to evaluate soil water chloride as an archive for recharge rate and water quality. Results show that groundwater recharge beneath natural uncultivated grassland, used as a baseline, is about 94–100 mm year^?1 and that the time it takes for annual precipitation to reach water table through the thick unsaturated zone is from decades to hundreds of years (tritium free). This recharge rate is 2–3 orders of magnitude more than in the other semiarid areas with similar annual rainfall but with deep‐rooted vegetation and relatively high temperature. Most of the water that eventually becomes recharge originally infiltrated in the summer months. The conversion from native grassland to winter wheat has reduced groundwater recharge by 42–50% (50–55 mm year^?1 for recharge), and the conversion from winter wheat to alfalfa resulted in a significant chloride accumulation in the upper soil zone, which terminated deep drainage. The paper also evaluates the time lag between potential recharge and actual recharge to aquifer and between increase in solute concentration in soil moisture and that in the aquifer following land‐use change due to the deep unsaturated zone. Copyright © 2012 John Wiley & Sons, Ltd.     

Integrating lysimeter drainage and eddy covariance flux measurements in a groundwater recharge model

Abstract

Field-scale water balance is difficult to characterize because controls exerted by soils and vegetation are mostly inferred from local-scale measurements with relatively small support volumes. Eddy covariance flux and lysimeters have been used to infer and evaluate field-scale water balances because they have larger footprint areas than local soil moisture measurements. This study quantifies heterogeneity of soil deep drainage (D) in four 12.5-m² repacked lysimeters, compares evapotranspiration from eddy covariance (ET_EC) and mass balance residuals of lysimeters (ETwb_Lys), and models D to estimate groundwater recharge. Variation in measured D was attributed to redirection of snowmelt infiltration and differences in lysimeter hydraulic properties caused by surface soil treatment. During the growing seasons of 2010, 2011 and 2012, ETwb_Lys (278, 289 and 269 mm, respectively) was in good agreement with ET_EC (298, 301 and 335 mm). Annual recharge estimated from modelled D was 486, 624 and 613 mm for three calendar years 2010, 2011 and 2012, respectively. In summary, lysimeter D and ET_EC can be integrated to estimate and model groundwater recharge.

Editor D. Koutsoyiannis     

Net Recharge vs. Depth to Groundwater Relationship in the Platte River Valley of Nebraska,United States

One‐km resolution MODIS‐based mean annual evapotranspiration (ET) estimates in combination with PRISM precipitation rates were correlated with depth to groundwater (d) values in the wide alluvial valley of the Platte River in Nebraska for obtaining a net recharge (Rn) vs. d relationship. MODIS cells with irrigation were excluded, yielding a mixture of predominantly range, pasture, grass, and riparian forest covers on sandy soils with a shallow groundwater table. The transition depth (d_t) between negative and positive values of the net groundwater recharge was found to be at about 2 (±1) m. Within 1 (±1) m of the surface and at a depth larger than about 7 to 8 (±1) m, the mean annual net recharge became independent of d at a level of about ?4 (±12)% and 13 (±10)%, respectively, of the mean annual precipitation rate. The obtained Rn(d) relationship is based on a calibration‐free ET estimation method and may help in obtaining the net recharge in shallow groundwater areas of negligible surface runoff where sufficient groundwater‐depth data exist.     

Recharge,upflux and water table response for shallow water table conditions in southwest Florida

A disproportionate increase or decrease in water table in response to minor water input or drainage is observed in shallow water table conditions inside drainage lysimeters. This increase happens because the capillary fringe of the shallow water table reaches up to or near the surface (Wieringermeer effect). The correlations between water table level changes and rainfall, seepage irrigation, drip irrigation, and drainage were analysed. Correlations with rainfall, seepage irrigation, and drainage were high (R² ranged from 0·46 to 0·97). Drip irrigation had low correlations due to the low rates of application (R² ranged from 0·26 to 0·44). Conventional methods of calculating recharge, such as multiplying the specific yield with the water table fluctuations, cannot be used for Wieringermeer effect situations. A method using water balance data and soil moisture at different depths in the lysimeters was developed to estimate recharge and upflux. The recharge results were used to develop the apparent specific yield S_ya, which could be used to calculate consequent recharge events from water table fluctuation data. Combining the water table fluctuation relationships developed with the S_ya value will allow the prediction of recharge from rainfall and irrigation events without the need for soil moisture equipment. Copyright © 2005 John Wiley & Sons, Ltd.     

A Geochemical Approach to Determine Sources and Movement of Saline Groundwater in a Coastal Aquifer

Geochemical evaluation of the sources and movement of saline groundwater in coastal aquifers can aid in the initial mapping of the subsurface when geological information is unavailable. Chloride concentrations of groundwater in a coastal aquifer near San Diego, California, range from about 57 to 39,400 mg/L. On the basis of relative proportions of major‐ions, the chemical composition is classified as Na‐Ca‐Cl‐SO₄, Na‐Cl, or Na‐Ca‐Cl type water. δ²H and δ¹⁸O values range from ?47.7‰ to ?12.8‰ and from ?7.0‰ to ?1.2‰, respectively. The isotopically depleted groundwater occurs in the deeper part of the coastal aquifer, and the isotopically enriched groundwater occurs in zones of sea water intrusion. ⁸⁷Sr/⁸⁶Sr ratios range from about 0.7050 to 0.7090, and differ between shallower and deeper flow paths in the coastal aquifer. ³H and ¹⁴C analyses indicate that most of the groundwater was recharged many thousands of years ago. The analysis of multiple chemical and isotopic tracers indicates that the sources and movement of saline groundwater in the San Diego coastal aquifer are dominated by: (1) recharge of local precipitation in relatively shallow parts of the flow system; (2) regional flow of recharge of higher‐elevation precipitation along deep flow paths that freshen a previously saline aquifer; and (3) intrusion of sea water that entered the aquifer primarily during premodern times. Two northwest‐to‐southeast trending sections show the spatial distribution of the different geochemical groups and suggest the subsurface in the coastal aquifer can be separated into two predominant hydrostratigraphic layers.     

Groundwater Recharge at Five Representative Sites in the Hebei Plain,China

Accurate estimates of groundwater recharge are essential for effective management of groundwater, especially when supplies are limited such as in many arid and semiarid areas. In the Hebei Plain, China, water shortage is increasingly restricting socioeconomic development, especially for agriculture, which heavily relies on groundwater. Human activities have greatly changed groundwater recharge there during the past several decades. To obtain better estimates of recharge in the plain, five representative sites were selected to investigate the effects of irrigation and water table depth on groundwater recharge. At each site, a one‐dimensional unsaturated flow model (Hydrus‐1D) was calibrated using field data of climate, soil moisture, and groundwater levels. A sensitivity analysis of evapotranspirative fluxes and various soil hydraulic parameters confirmed that fine‐textured surface soils generally generate less recharge. Model calculations showed that recharge on average is about 175 mm/year in the piedmont plain to the west, and 133 mm/year in both the central alluvial and lacustrine plains and the coastal plain to the east. Temporal and spatial variations in the recharge processes were significant in response to rainfall and irrigation. Peak time‐lags between infiltration (rainfall plus irrigation) and recharge were 18 to 35 days in the piedmont plain and 3 to 5 days in the central alluvial and lacustrine plains, but only 1 or 2 days in the coastal plain. This implies that different time‐lags corresponding to different water table depths must be considered when estimating or modeling groundwater recharge.     

Estimating groundwater recharge from water isotope (δ2H, δ18O) depth profiles in the Densu River basin,Ghana

Abstract

Accurate estimation of groundwater recharge is essential for the proper management of aquifers. A study of water isotope (δ²H, δ¹⁸O) depth profiles was carried out to estimate groundwater recharge in the Densu River basin in Ghana, at three chosen observation sites that differ in their altitude, geology, climate and vegetation. Water isotopes and water contents were analysed with depth to determine water flow in the unsaturated zone. The measured data showed isotope enrichment in the pore water near the soil surface due to evaporation. Seasonal variations in the isotope signal of the pore water were also observed to a depth of 2.75 m. Below that depth, the seasonal variation of the isotope signal was attenuated due to diffusion/dispersion and low water flow velocities. Groundwater recharge rates were determined by numerical modelling of the unsaturated water flow and water isotope transport. Different groundwater recharge rates were computed at the three observation sites and were found to vary between 94 and 182 mm/year (± max. 7%). Further, the approximate peak-shift method was applied to give information about groundwater recharge rates. Although this simple method neglects variations in flow conditions and only considers advective transport, it yielded mean groundwater recharge rates of 110–250 mm/year (± max. 30%), which were in the same order of magnitude as computed numerical modelling values. Integrating these site-specific groundwater recharge rates to the whole catchment indicates that more water is potentially renewed than consumed nowadays. With increases in population and irrigation, more clean water is required, and knowledge about groundwater recharge rates – essential for improving the groundwater management in the Densu River basin – can be easily obtained by measuring water isotope depth profiles and applying a simple peak-shift approach.

Citation Adomako, D., Maloszewski, P., Stumpp, C., Osae, S. & Akiti, T. T. (2010) Estimating groundwater recharge from water isotope (δ²H, δ¹⁸O) depth profiles in the Densu River basin, Ghana. Hydrol. Sci. J. 55(8), 1405–1416.     

Evaluating Groundwater Interbasin Flow Using Multiple Models and Multiple Types of Data

The groundwater interbasin flow, Q_y, from the north of Yucca Flat into Yucca Flat simulated using the Death Valley Regional Flow System (DVRFS) model greatly exceeds assessments obtained using other approaches. This study aimed to understand the reasons for the overestimation and to examine whether the Q_y estimate can be reduced. The two problems were tackled from the angle of model uncertainty by considering six models revised from the DVRFS model with different recharge components and hydrogeological frameworks. The two problems were also tackled from the angle of parametric uncertainty for each model by first conducting Morris sensitivity analysis to identify important parameters and then conducting Monte Carlo simulations for the important parameters. The uncertainty analysis is general and suitable for tackling similar problems; the Morris sensitivity analysis has been utilized to date in only a limited number of regional groundwater modeling. The simulated Q_y values were evaluated by using three kinds of calibration data (i.e., hydraulic head observations, discharge estimates, and constant‐head boundary flow estimates). The evaluation results indicate that, within the current DVRFS modeling framework, the Q_y estimate can only be reduced to about half of the original estimate without severely deteriorating the goodness‐of‐fit to the calibration data. The evaluation results also indicate that it is necessary to develop a new hydrogeological framework to produce new flow patterns in the DVRFS model. The issues of hydrogeology and boundary flow are being addressed in a new version of the DVRFS model planned for release by the U.S. Geological Survey.     

Hydraulic subsurface measurements and hydrodynamic modelling as indicators for groundwater flow systems in the Rotondo granite,Central Alps (Switzerland)

Regional groundwater flow in high mountainous terrain is governed by a multitude of factors such as geology, topography, recharge conditions, structural elements such as fracturation and regional fault zones as well as man‐made underground structures. By means of a numerical groundwater flow model, we consider the impact of deep underground tunnels and of an idealized major fault zone on the groundwater flow systems within the fractured Rotondo granite. The position of the free groundwater table as response to the above subsurface structures and, in particular, with regard to the influence of spatial distributed groundwater recharge rates is addressed. The model results show significant unsaturated zones below the mountain ridges in the study area with a thickness of up to several hundred metres. The subsurface galleries are shown to have a strong effect on the head distribution in the model domain, causing locally a reversal of natural head gradients. With respect to the position of the catchment areas to the tunnel and the corresponding type of recharge source for the tunnel inflows (i.e. glaciers or recent precipitation), as well as water table elevation, the influence of spatial distributed recharge rates is compared to uniform recharge rates. Water table elevations below the well exposed high‐relief mountain ridges are observed to be more sensitive to changes in groundwater recharge rates and permeability than below ridges with less topographic relief. In the conceptual framework of the numerical simulations, the model fault zone has less influence on the groundwater table position, but more importantly acts as fast flow path for recharge from glaciated areas towards the subsurface galleries. This is in agreement with a previous study, where the imprint of glacial recharge was observed in the environmental isotope composition of groundwater sampled in the subsurface galleries. Copyright © 2012 John Wiley & Sons, Ltd.     

Groundwater recharge sources in semiarid irrigated mountain fronts

High-elevation mountains often constitute for basins important groundwater recharge sources through mountain-front recharge processes. These processes include streamflow losses and subsurface inflow from the mountain block. However, another key recharge process is from irrigation practices, where mountain streamflow is distributed across the irrigated piedmont. In this study, coupled groundwater fluctuation measurements and environmental tracers (¹⁸O, ²H, and major ions) were used to identify and compare the natural mountain-front recharge to the anthropogenically induced irrigation recharge. Within the High Atlas mountain front of the Ourika Basin, Central Morocco, the groundwater fluctuation mapping from the dry to wet season showed that recharge beneath the irrigated area was higher than the recharge along the streambed. Irrigation practices in the region divert more than 65% of the stream water, thereby reducing the potential for in-stream groundwater recharge. In addition, the irrigation areas close to the mountain front had greater water table increases (up to 3.5 m) compared with the downstream irrigation areas (<1 m increase). Upstream crops have priority to irrigation with stream water over downstream areas. The latter are only irrigated via stream water during large flood events and are otherwise supplemented by groundwater resources. These changes in water resources used for irrigation practices between upstream and downstream areas are reflected in the spatiotemporal evolution of the stable isotopes of groundwater. In the upstream irrigation area, the groundwater stable isotope values (δ¹⁸O: −8.4‰ to −7.4‰) reflect recharge by the diverted stream water. In the downstream irrigation area, the groundwater isotope values are lower (δ¹⁸O: −8.1‰ to −8.4‰) due to recharge via the flood water. In the nonirrigation area, the groundwater has the highest stable isotope values (δ¹⁸O: −6.8‰ to −4.8‰). This might be due to recharge via subsurface inflow from the mountain block to the mountain front and/or recharge via local low altitude rainfall. These findings highlight that irrigation practices can result in the dominant mountain-front recharge process for groundwater.     

Forthcoming papers

Abstract

The Hai River Basin (HRB) is a heavily irrigated region encompassing the North China Plain (NCP) in northeast China. In the last decades, continuous lowering of groundwater levels had been reported in the NCP. This study used data from the Gravity Recovery and Climate Experiment (GRACE) and in situ measurements to quantify recent changes in groundwater storage from 2003 to 2012. The signal from GRACE observations highlight a sharp decline in the deep subsurface water stores (deep unsaturated zone and groundwater systems) up to a rate of 17.0 ± 4.3 mm year^-1 between 2003 and 2012 over the HRB, equal to a volumetric loss of 5.5 ± 1.4 km³ year^-1. This result shows good consistency with in situ observations of groundwater hydraulic heads compiled from monitoring bores, and emphasizes GRACE’s ability to monitor large-scale groundwater storage variations. Results from GRACE also provide an independent assessment of the effectiveness of water saving programmes that have been implemented by the government so far. Our study indicates that groundwater overdrawal is still prevalent and the dominant factor for the persistent loss in groundwater storage over the HRB/NCP; the current groundwater consumption pattern is far beyond the natural recharge ability in groundwater system.

Editor D. Koutsoyiannis; Associate editor T. Wagener