Groundwater Recharge Rates Research Articles

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.

Land‐use comparison of depression‐focussed groundwater recharge in the Canadian prairies

AbstractIn the cold semiarid Canadian prairies, groundwater recharge is focussed under numerous topographic depressions, in which snowmelt runoff converges. Agricultural land uses on the uplands surrounding the depressions affect snow accumulation, snowmelt infiltration, evapotranspiration (ET) and soil moisture dynamics, thereby influencing snowmelt runoff and depression‐focussed recharge. The objective of this study is to compare the differences in hydrological processes under two common land uses in the Canadian prairies, namely grazed grass and annual crop, and examine how they affect groundwater recharge. A short‐term (3 years) paired catchment study was used for detailed observation of hydrological processes in two depressions, supplemented by a longer‐term (17 years) data set covering a larger scale to quantify the differences in snowmelt runoff between the two land uses. Compared to the grazed grassland, the cropland had a shorter and more intense period of ET, and root water uptake restricted to the shallower (top 0–80 cm) soil zone. The amount of snowmelt runoff was greater in the grazed grassland primarily due to a higher amount of snow accumulation, which was dictated by differences in topography. This finding was contrary to previous studies in the Canadian prairies that indicated substantially smaller snowmelt runoff in ungrazed grassland, but was consistent with the larger‐scale remote sensing results, which showed only a marginal difference between grazed grasslands and croplands. Groundwater recharge rates were estimated using the chloride mass balance method for the present condition using “modern” pore water containing tritium. The rates were similar between the grazed grassland and croplands, implying similarity in snowmelt runoff characteristics. These results suggest that groundwater recharge will continue to be focussed under depressions in the future, though the amount and seasonality of recharge may be influenced by warmer winters.

Potential of Shallow Groundwater for Household Level Irrigation Practices in Tahtay Koraro Woreda, Tigray, Northern Ethiopia

This paper deals with the results of a pilot study conducted to estimate the shallow groundwater resource potential and irrigation capacity at the household level in Tahtay Koraro Woreda, northwestern zone of Tigray, Ethiopia. The potential evapotranspiration and actual evapotranspiration of the study area are estimated to be 1484 mm/year and 814 mm/year respectively. The runoff is approximately calculated to be 280 mm/year and the annual groundwater recharge is estimated to be 29 mm/year. The total annual groundwater abstraction for human, livestock, and irrigation is estimated to be 25 mm/year. It should be noted that the groundwater recharge rate is expected to remain constant while the total annual groundwater discharge is expected to increase from year to year. This relation when projected over a long period may result in a negative groundwater budget which can result in depletion of groundwater (lowering of groundwater levels), reduced baseflow to streams, and deterioration of water quality. The computed values for hydraulic conductivity of the aquifers range from 1.63 m/day to 7.27 m/day with an average value of 4.9 m/day and transmissivity from 48.9 m2/day to 218.1 m2/day with an average value of 147.14 m2/day. The aquifers in the highly weathered basalt and highly weathered siltstone – sandstone intercalation have transmissivity values ranging from 99 m2/day to 218.1 m2/day with an average value of 157 m2/day and are grouped into the moderate potentiality aquifers category. The aquifers in the slightly weathered and fractured metavolcanics grouped under low potentiality based on the lower transmissivity values (<50 m2/day). The study area has low to moderate groundwater potentiality, hence, large-scale groundwater pumping is not possible. Therefore, the current activity of using hand dug wells for household-level irrigation is the best way of using groundwater for irrigation and other uses as well. Increasing the depth of the existing hand dug wells that are constructed in highly weathered basalt and highly weathered siltstone – sandstone intercalation can also enhance the yield of the hand dug wells. It is recommended to use water-saving irrigation technologies rather than increasing the number of wells. This will also help in increasing the irrigation area. Groundwater recharge enhancement structures such as trenches, percolation ponds, and check dams be constructed in scientifically selected localities to further enhance the groundwater potential.

High-resolution Data Based Groundwater Recharge Estimations of Aynalem Well Field

Due to the ever-increasing demand for water in Aynalem catchment and its surrounding, there has been an increased pressure on the Aynalem well field putting the sustainability of water supply from the aquifer under continuous threat. Thus, it is vital to understand the water balance of the catchment to ensure sustainable utilization of the groundwater resource. This in turn requires proper quantification of the components of water balance among which recharge estimation is the most important. This paper estimates the groundwater recharge of the Aynalem catchment using high-resolution hydro-meteorological data. Daily precipitation and temperature measurement data for years 2001-2018; groundwater level fluctuation records collected at every 30 minutes; and soil and land use maps were used to make recharge estimations. In the groundwater level fluctuation, three boreholes were monitored, but only two were utilized for the analysis because the third was under operation and does not represent the natural hydrologic condition. Thornthwaite soil moisture balance and groundwater level fluctuation methods were applied to determine the groundwater recharge of the Aynalem catchment. Accordingly, the annual rate of groundwater recharge estimated based on the soil-water balance ranges between 7mm/year and 138.5 mm/year with the weighted average value of 89.04 mm/year. The weighted average value is considered to represent the catchment value because the diverse soil and land use/cover types respond differently to allow the precipitation to recharge the groundwater. On the other hand, the groundwater recharge estimated using the groundwater level fluctuation method showed yearly groundwater recharge of 91 to 93 mm/year. The similarity in the groundwater recharge result obtained from both methods strengthens the acceptability of the estimate. It also points out that the previously reported estimate is much lower (36 to 66 mm/year).

Using vadose-zone water stable isotope profiles for assessing groundwater recharge under different climatic conditions

ABSTRACT Considering three sites under different climate conditions (arid, semi-arid, and subhumid), this study aims to use the vadose-zone water stable isotope profiles to estimate the groundwater recharge rate. High-resolution vertical subsurface soil sampling along the vadose zone of the investigated sites was conducted. The collected samples were analysed to determine their stable isotope ratios (δ2H and δ18O) that were used along with the piston displacement method to estimate recharge. Annual recharge rates of 0.2% (± 0.1%), 2.5%, and 18% of the total annual precipitation were obtained for the arid, semi-arid, and subhumid sites, respectively. Recharge rates at the semi-arid and subhumid sites are comparable to those previously estimated using water balance-based methods. The recharge rate at the arid site is lower than that previously estimated for that site using the water budget-based method, revealing difficulties in applying the piston displacement method under an arid regime.

An alternative method for groundwater recharge estimation in karst

Aquifers with time variant catchment boundaries, regional flows, and inflows from surface streams and accumulations are common in karst areas. Due to the lack of reliable methods for determination of water balance in the phreatic zone of such aquifers, groundwater recharge estimation is a challenge task. This paper introduces a new method for estimation of groundwater recharge in karst – the so-called partial correlation method (PCM). The basic hypothesis is that the groundwater recharge rates can be determined from the time series of precipitation, air temperature, relative humidity, and spring discharge, by using a partial cross-correlation function as objective function. Following this hypothesis, a simple conceptual model of water balance in the soil cover, epikarst and vadose zone is developed, and an objective function for estimation of the optimal value of parameter of this model is proposed. PCM is applied on the catchment of Žrnovnica Spring located in the Dinaric karst area of Croatia. The results of application are compared with the results of previous studies, as well as with the results obtained by five empirical models that have been used for estimation of groundwater recharge in the Mediterranean karst. The effective infiltration coefficients obtained by PCM for the Žrnovnica Spring are generally in accordance with the previously published ones.

Hydrogeochemical evolution and groundwater recharge processes in arsenic enriched area in central Gangetic plain, India

The present study dealt with understanding hydrogeochemical evolution, ascertaining distribution, fate and spatio-temporal variation of arsenic along with comprehending recharge processes and quantification of recharge rate in the central Gangetic plain, India. The arsenic enriched area was observed mostly in the fluvial deposits with younger alluvium. The depth to water levels maps for 1996 and 2016 showed marked spatio-temporal variation and the groundwater recharge rate was estimated to be varied between 0.05 m/year and 0.07 m/year in the study area. The elevated arsenic concentration was noticed in the region, having declined groundwater recharge. A total of 147 water samples were collected from hand pumps (n = 141) and rivers (n = 6) during the pre-monsoon period (May 2016). In addition, about 81 groundwater samples were collected from 27 locations during the pre-monsoon, monsoon and winter 2019 for studying seasonal variability in the hydrogeochemical parameters and isotopic composition of water. Arsenic concentration was found more in the area where deposits of coarser sediment of the Quaternary period was present along the rivers Ganga and Ghaghra. The arsenic concentration was observed higher in the pre-monsoon (maxm. As 641 μg/L), followed by the post-monsoon (425 μg/L) and monsoon season (375 μg/L). The depleted isotopic value and higher D-excess values in groundwater suggested active recharge conditions with precipitation as the major source of recharge in the study area. It is hypothesized that rainwater induced oxygenated water into the aquifer by the process of recharge, which may prompted various biogeochemical reactions due to change in redox conditions and endorsed arsenic sorption in the monsoon season. Thereafter, anoxic conditions prevailed in the post-monsoon season, and finally, in the pre-monsoon season, reducing conditions continued and arsenic released at a rapid rate, which was justified with the seasonal variation of arsenic concentration.

Dynamic groundwater recharge simulations based on cosmic‐ray neutron sensing in a tropical wet experimental basin

AbstractAlthough cosmic‐ray neutron sensing (CRNS) is probably the most promising noninvasive proximal soil moisture measurement technique at the field scale, its application for hydrological simulations remains underexplored in the literature so far. This study assessed the use of CRNS to inversely calibrate soil hydraulic parameters at the intermediate field scale to simulate the groundwater recharge rates at a daily timescale. The study was conducted for two contrasting hydrological years at the Guaraíra experimental basin, Brazil, a 5.84‐km², a tropical wet and rather flat landscape covered by secondary Atlantic forest. As a consequence of the low altitude and proximity to the equator low neutron count rates could be expected, reducing the precision of CRNS while constituting unexplored and challenging conditions for CRNS applications. Inverse calibration for groundwater recharge rates was used based on CRNS or point‐scale soil moisture data. The CRNS‐derived retention curve and saturated hydraulic conductivity were consistent with the literature and locally performed slug tests. Simulated groundwater recharge rates ranged from 60 to 470 mm yr–1, corresponding to 5 and 29% of rainfall, and correlated well with estimates based on water table fluctuations. In contrast, the estimated results based on inversive point‐scale datasets were not in alignment with measured water table fluctuations. The better performance of CRNS‐based estimations of field‐scale hydrological variables, especially groundwater recharge, demonstrated its clear advantages over traditional invasive point‐scale techniques. Finally, the study proved the ability of CRNS as practicable in low altitude, tropical wet areas, thus encouraging its adoption for water resources monitoring and management.

Groundwater recharge in hillslopes on the Chinese Loess Plateau

Study regionYuanzegou Watershed in Qingjian, Wangdonggou watershed in Changwu, the Chinese Loess Plateau. Study focusHillslopes are the predominant land form unit on Earth, and despite its importance in maintaining groundwater supplies in many parts of the world, groundwater recharge on hillslopes is poorly understood. We hypothesize that groundwater recharge in the deep unsaturated zones of hillslopes is facilitated mainly by soil matrix flow, and the recharge rates are much smaller in hillslopes than on flat landscapes. To test the hypothesis, seven 15–20 m-long cores were collected from areas of different land uses to determine the groundwater recharge rates and its controls in deep unsaturated zones of hillslopes at a sub-humid and a semi-arid watershed located on the Chinese Loess Plateau. New hydrologic insights for the regionThe tritium distribution in each soil profile exhibits a well-defined bell-shape, signifying that soil matrix displacement is the main recharge mechanism. The tritium peak was located between depths of 9.33 m–11.01 m at the semiarid watershed and 6.29 m–7.22 m at the sub-humid watershed, and the recharge rates varied from 24.5 to 33.8 mm yr−1. These recharge rates account for 4 %–7 % of the long-term precipitation, which is approximately 56 % of the recharge typically seen on flat landscapes. Groundwater recharges were mainly controlled by soil texture, and land use changes, but exhibited little impact from the climatic difference between the two watersheds.

Impact of land-use land cover dynamics on runoff in Panchnoi River basin, North East India

Abstract Alteration of land-use land cover pattern causes severe consequences on the hydrological system by modifying the rainfall-runoff pattern in a region. The study aimed to investigate the impact of land-use land-cover dynamics on runoff generation in different geomorphic divisions of Panchnoi River basin. The study used the Soil Conservation Service-Curve Number method to estimate runoff generation in the Panchnoi River basin in a GIS platform. This study observed that the conversion of the land-use pattern in the geomorphic zones significantly enhances runoff. The Piedmont experience highest land-use change, where 64.17 km2 forest cover lost to cropland and built-up lands, leads to a notable increase in runoff generation, i.e. from 1 076 mm (52.82% of rainfall) in 1990 to 1 467 mm (70.46% of rainfall) in 2015. The Flood plain and New alluvial plain generates high runoff in the basin as it mostly occupied by human-induced land-uses, i.e. 1 444 mm (72.72% of rainfall) and 1 360 mm (71.70% of rainfall) respectively in 1990, which increase to 1588 mm (79.20%) and 1507 mm (78.69%) runoff respectively in 2015, due to alteration of cropland to built-up lands. In the Old alluvial plain, a marginal land-use change observed resulted in moderate growth in runoff from 1 272 mm (62.35%) to 1 404 mm (66.79%). The study indicates land-use land-cover change invokes to increase runoff generation can give rise severe environmental and economic problems in the river basin, through the occurrence of flashflood and soil erosion. Highlights for public administration, management and planning: • Evaluation of the impact of land-use land cover dynamics on runoff is essential for containing flash flood and water resource management on a basin scale. • Alteration of natural land covers has severe implications in the form of flood, soil erosion, and loss of biodiversity. • Enhanced runoff due to land-use dynamics reduces groundwater recharge rate that may cause drinking water scarcity in the dry season shortly.

Stormwater harvesting in ephemeral streams: how to bypass clogging and unsaturated layers

To cope with water scarcity in drylands, stormwater is often collected in surface basins and subsequently stored in shallow aquifers via infiltration. These stormwater harvesting systems are often accompanied by high evaporation rates and hygiene problems. This is commonly a consequence of low infiltration rates, which are caused by clogging layers that form on top of the soil profile and the presence of a thick vadose zone. The present study aims to develop a conceptual solution to increase groundwater recharge rates in stormwater harvesting systems. The efficiency of vadose-zone wells and infiltration trenches is tested using analytical equations, numerical models, and sensitivity analyses. Dams built in the channel of ephemeral streams (wadis) are selected as a study case to construct the numerical simulations. The modelling demonstrated that vadose-zone wells and infiltration trenches contribute to effective bypassing of the clogging layer. By implementing these solutions, recharge begins 2250–8100% faster than via infiltration from the bed surface of the wadi reservoir. The sensitivity analysis showed that the recharge rates are especially responsive to well length and trench depth. In terms of recharge quantity, the well had the best performance; it can infiltrate up to 1642% more water than the reservoir, and between 336 and 825% more than the trench. Moreover, the well can yield the highest cumulative recharge per dollar and high recharge rates when there are limitations to the available area. The methods investigated here significantly increased recharge rates, providing practical solutions to enhance aquifer water storage in drylands.

Point- and reach-scale measurements are important for determining accurate seepage rates in controlled flow channels

A critical component of water-resources management in the irrigated agriculture landscape, particularly those landscapes dependent on groundwater availability, is determining groundwater recharge rates from streams and other channels. In California, flows in many such channels are “controlled” by upstream reservoir releases to meet downstream urban, irrigation and environmental water requirements. Seepage volumes from these channels and how they might vary during controlled release periods is a key component of meeting downstream riparian and groundwater-pumping needs. Understanding annual seepage from streamflow channels is also important in developing water budgets as part of the management of groundwater resources under the Sustainable Groundwater Management Act (SGMA) in California. However, direct measurements of channel seepage rates are infrequent or unavailable, and these rates, or associated volumes, are most often only estimated. Here we describe direct point- and reach-scale field measurements of channel seepage rates in Lower Putah Creek (Solano County) and in distribution lateral channels of the Oakdale Irrigation District on the east side of the San Joaquin Valley (San Joaquin and Stanislaus counties). We measured overall average seepage rates of about 2 feet (610 mm) per day at both locations and determined how these rates varied spatially and temporally during the summer when channel flows are controlled for downstream requirements.

Optimizing urban stormwater control strategies and assessing aquifer recharge through drywells in an urban watershed

A coupled simulation-optimization model (SOM) is developed in this work that links the US Environmental Protection Agency’s Storm Water Management Model (SWMM) with a genetic algorithm. The SOM simulates rainfall-runoff processes in urban watersheds and optimizes the implementation of drywells (DWs), bio-retention cells (BCs), and permeable pavement (PP) for stormwater control and aquifer recharge in District 6 of Tehran Municipality, Iran. Feasible DWs are selected through site inspection and considering stormwater quality criteria to prevent aquifer contamination. This study compares the current rates of urban runoff and groundwater recharge (baseline scenario) with new stormwater management strategies, which were designed based on several levels of funding. Results show the highest rate of runoff reduction and infiltration, as well as the most cost-effective options, would be achieved when DWs are added to the combination of BCs and PP for stormwater management. The runoff reduction rate in the presence of DWs would rise by 11.7, 7.0, and 6.1% in comparison to their absence for 12-, 17-, and 22-million-dollar budget levels, respectively. Implementation of BCs and PP would cause infiltration of about 235, 274, and 279 thousand m3 for the three cited budget levels, while combining DWs with BCs and PP would increase infiltration by 19, 15.6, and 14% for the three levels of investment, respectively. These results demonstrate the benefits of using local nonfunctional wells and qanats to reduce peak flows, replenish urban aquifers, and improve the economic efficiency of urban stormwater management projects.

Conversion of degraded farmlands to orchards decreases groundwater recharge rates and nitrate gains in the thick loess deposits

Conversion of farmlands to orchards has been reported to greatly influence groundwater recharge and quality; however, it has not been fully discussed in regions with arid climate and thick unsaturated zones. This study attempted to connect land use with groundwater recharge and quality in an arid region covered by loess up to 350 m deep. We collected 1300 soil samples for 26 profiles of 10 m deep, which is deep enough for the water and solutes in soil to be under steady state. We selected sampling sites under farmlands and different deep-rooted fruit trees (i.e., apple and apricot) of varying ages. Based on mass balance methods, we determined the chloride concentrations in soil pore water to estimate the recharge rates, and estimated the groundwater pollutant gains by employing nitrate as pollution index. The mean diffuse recharge was estimated as 3.2 ± 1.5–82.3 ± 44.5 mm yr−1 accounting for 1.7 ± 0.8–13.7 ± 7.4% of mean annual precipitation, and the values under farmlands can be reduced by up to 60% after conversion to deep-rooted fruit trees. The mean groundwater nitrate gains ranged 0.7 ± 0.4–16.5 ± 10.4 kg hm−2 yr−1, and the conversion decreased groundwater nitrate gains by up to 90%. The reduced nitrate pollution to groundwater is different from those regions with shallow vadose zones or humid climate where groundwater pollution is getting more severe with increasing fertilizer application. The reduced recharge was found to be the primary control, but it would extend the lifetime of the risk associated with groundwater pollution from nitrate gains. Our results provide useful information for management of agriculture and groundwater resources in regions with thick vadose zones.

Spatiotemporal Variation of Groundwater Recharge in the Lower Reaches of the Poyang Lake Basin, China: Insights From Stable Hydrogen and Oxygen Isotopes

AbstractUnderstanding of groundwater recharge and associated hydrological processes is important for sustainable management and utilization of water resources, especially in the monsoon regions. However, few systematic studies have been conducted on the spatiotemporal variation of groundwater recharge. In this study, stable isotopes of precipitation and groundwater were used to explore the spatiotemporal variation of groundwater recharge in the lower reaches of the Poyang Lake Basin of China, a region affected by the interplay of different monsoons. The results indicated that isotopic values (δ2H and δ18O) of precipitation exhibited obvious seasonal variations, higher in winter and lower in summer, controlled by different monsoons. The stable isotopes of groundwater also presented distinct seasonal variations, revealing the dominant influence of seasonal precipitation, but they had different characteristics in space. In the northern study area and Rao and Xin Basins, the isotopic signatures of groundwater showed the shift to lower values in summer and higher values in winter, consistent with those of precipitation, while in the Ganfu Plain, which is characterized by poorly permeable strata, isotopic signatures of groundwater presented opposite temporal variations than those of precipitation (lagged behind precipitation). The characteristics of groundwater isotopes might be linked with the spatial heterogeneity of groundwater recharge rate, which was affected by recharge mechanisms, lithology of aquifer materials, and interaction between groundwater and lake water. This study provides insights into the groundwater recharge of the Poyang Lake Basin and other similar areas.

Tidal creeks as hot-spots for hydrological exchange in a coastal landscape

Coastal ecosystem health and sustainability is tightly coupled to submarine groundwater discharge (SGD) and associated nutrient, carbon and pollutant fluxes. However, there are few studies that systematically analyse the interaction between the terrestrial aquifer system, catchment morphology and coastal SGD. The objective of this study was to evaluate the role of catchment morphology and how this influences the spatial distribution, timing and volume of the SGD flux to a branched tidal creek system, on the barrier island Spiekeroog, Germany. The subsurface salinity was mapped using electrical resistivity tomography (ERT) and a hydrogeochemical survey of the shallow groundwater. Temporal and spatial analysis of geochemical tracers (radon-222, chloride) in the tidal creek water was integrated into a transient 222Rn mass balance model for quantification of SGD rates during two field campaigns that encompassed spring and neap tides. The ERT mapping indicated that fresh groundwater dominated under the dune ridges down to about 15 m depth, but became progressively more brackish seawards. This is likely due to frequent tidal and storm flooding of low-lying areas. The highest groundwater fluxes into the creek were indicated by high 222Rn activities (average 468 Bq m−3) towards the dune ridge. Chloride concentrations (up to 17.3 g L−1) increased seawards showing the progressive salinization of water in the creek. The freshwater component of SGD was highly variable in time but was highest at low tide, while the total SGD flux (saline + fresh) was highest when tides changed from inflow to outflow as the rapid pressure release on the local aquifer caused a large hydraulic gradient towards the creek. Comparing the freshwater component of mean daily SGD to the creek with estimated groundwater recharge rates in the catchment (665 m3 d−1) shows that the fresh groundwater discharge exceeds fresh recharge during spring tides (~120%) but is was lower than recharge during neap tides (~27%). In this study we show that tidal creeks and their relation to catchment morphology are relevant for understanding the spatial and temporal exchange of fresh and saline water between the catchment and coastal zone.

Regional groundwater flow and karst evolution\u2013theoretical approach and example from Switzerland

In regional scale aquifers in the Rhine Valley and Tabular Jura east of Basel (Switzerland), the groundwater circulation was investigated using regional-scale geological and hydraulic 3D models. The main aquifers in the area comprise the Quaternary aquifer of unconsolidated gravel deposits along the River Rhine and its tributaries, as well as the regional scale karst aquifer within the Upper Muschelkalk. Land subsidence, a process likely associated with salt solution mining, indicates further subordinate groundwater bearing segments and complex groundwater interactions along fault zones. In the aquifer systems we investigated, regional-scale groundwater circulation was simulated and visualized in relation to the geological settings. Lithostratigraphic units and fault structures were parameterized and analyzed, including the sensitivity of hydraulic properties and boundaries. Scenario calculations were used to investigate the sensitivity that the aquifer systems had to hydraulic parameter changes during Quaternary aggradation and degradation in the main valley. Those calculations were also done for base-level changes in the Rivers Rhine and Birs. For this purpose, this study considered probable historic base-levels before river regulation occurred, and before river dams and power plants were constructed. We also focused on scenarios considering increased groundwater recharge rates, e.g. due to exceptional long-lasting precipitation, or heavy rainfall events in the catchment area. Our results indicate that increased groundwater recharge rates in the catchment areas during such events (or periods) are associated with orders of magnitude increases of regional inflow into the Upper Muschelkalk karst aquifer. Furthermore, the groundwater fluctuations and groundwater saturated regions within the karst aquifer shift to places where high densities of sinkholes are documented. When the surface water base-levels adapt to probable historic levels, it leads to increased hydraulic gradients (i.e. local lowering of the groundwater level by up to 7 m). Those increased gradients are associated with increased groundwater flow within some aquifer regions that are particularly prone to karst development.

Response of surface runoff to land use and land cover change and its impact on Daihai Lake shrinkage in Inner Mongolia, China

Daihai Lake (DL), the third largest inland lake in Inner Mongolia, has shrunk severely given the conditions of no significant decrease in annual precipitation. On the basis of field surveys and Landsat remote sensing images, this study analyzed land use and land cover (LULC) changes in six periods of the DL watershed area during 1986–2019. Then, with the combination of the LULC results and the Soil Conservation Service–Curve Number method, we discussed the change characteristics and influence factors of the surface runoff in the watershed and the reason for DL shrinkage. From 1986 to 2019, the water area of DL decreased rapidly at a rate of −2.22 km2/year, and the area of farmland and grassland in the watershed changed strongly along the opposite trend. The surface runoff in the DL watershed was reduced by approximately 513.6 million m3 during the last 34 years. In addition to local rainfall, groundwater is also an important water supply to the lake and accounts for a greater proportion. The average recharge rate of groundwater to the lake is approximately 64.55 million m3 per year in the past 34 years. The area change of the DL is related to the runoff reduction, huge evaporation, and lake water use in thermoelectric power generation. The local precipitation cannot enter the groundwater in DLW. Large-scale exploitation of external groundwater for agricultural irrigation and industrial production is another important cause of lake degradation.

Mapping groundwater recharge in Africa from ground observations and implications for water security

Groundwater forms the basis of water supplies across much of Africa and its development is rising as demand for secure water increases. Recharge rates are a key component for assessing groundwater development potential, but have not been mapped across Africa, other than from global models. Here we quantify long-term average (LTA) distributed groundwater recharge rates across Africa for the period 1970–2019 from 134 ground-based estimates and upscaled statistically. Natural diffuse and local focussed recharge, where this mechanism is widespread, are included but discrete leakage from large rivers, lakes or from irrigation are excluded. We find that measurable LTA recharge is found in most environments with average decadal recharge depths in arid and semi-arid areas of 60 mm (30–140 mm) and 200 mm (90–430 mm) respectively. A linear mixed model shows that at the scale of the African continent only LTA rainfall is related to LTA recharge—the inclusion of other climate and terrestrial factors do not improve the model. Kriging methods indicate spatial dependency to 900 km suggesting that factors other than LTA rainfall are important at local scales. We estimate that average decadal recharge in Africa is 15 000 km3 (4900–45 000 km3), approximately 2% of estimated groundwater storage across the continent, but is characterised by stark variability between high-storage/low-recharge sedimentary aquifers in North Africa, and low-storage/high-recharge weathered crystalline-rock aquifers across much of tropical Africa. African water security is greatly enhanced by this distribution, as many countries with low recharge possess substantial groundwater storage, whereas countries with low storage experience high, regular recharge. The dataset provides a first, ground-based approximation of the renewability of groundwater storage in Africa and can be used to refine and validate global and continental hydrological models while also providing a baseline against future change.

An analytical methodology to estimate the changes in fresh groundwater resources with sea-level rise and coastal erosion in strip-island unconfined aquifers: illustration with Savary Island, Canada

Closed-form analytical solutions for assessing the consequences of climate change on fresh groundwater oceanic island lenses have been developed by hydrogeologists during the last decade. Based on existing equations, this study focuses on the case of strip oceanic islands when three combined effects of climate change are observed to affect the freshwater lens volume and its groundwater resource renewal: sea-level rise, erosion, and change in groundwater recharge rates. New equations, integrating these combined effects of climate change on fresh groundwater resources are provided. These equations are solved by a novel methodology based on a Dupuit-Forchheimer groundwater flow model that allows for determination of the hydrogeological parameters included in the equations. The approach is illustrated with the strip island of Savary, which is located along the Pacific Coast of Canada in the province of British Columbia. This example illustrates, on the one hand, the volume depletion of the island freshwater lens and, on the other hand, the decrease of the renewal rate of groundwater. The proposed approach can be applied to any strip islands worldwide to assess the cumulative effects of sea-level rise and shore erosion on groundwater resources, depending on the predicted climate change scenarios. The results can then help decision-makers to anticipate the effects of climate change on the groundwater availability in strip oceanic islands and plan future groundwater use accordingly.