Variability Of Groundwater Recharge Research Articles

Improving the water table fluctuation method to estimate groundwater recharge below thick vadose zones

ABSTRACT The water table fluctuation (WTF) method is popular for groundwater recharge (GR) estimation, but its accuracy is challenged when applied in areas with thick vadose zones because of the signal lag and attenuation with depth and uncertainties from barometric pressure effect and lateral flow. Improvement of the WTF method used the linear regression method and Darcy’s law, and has been assessed to give satisfactory results. In particular, the improved method presented lower GR (20–34%) relative to the conventional method. GR decreased from the centre to the edge of the tableland. The regional average GR was 63–81 mm year−1, equivalent to 11–14% of annual average rainfall. Lag times between recharge and rainfall ranged from 1 to 9 months. Rainfall and vegetation dominated the spatiotemporal variability of GR. Our study provides reference and technical support for GR estimation with the WTF method in regions with a thick vadose zone.

Characteristics and Indicative Significance of Groundwater Stable Isotopes in the Loess Plateau at the Regional Scale

Regional groundwater recharge is a critical scientific issue for sustainable groundwater resource development and management. However, spatial variations in groundwater recharge in the Loess Plateau （LP） remain poorly understood. To fill this knowledge gap, a systematic sampling campaign and stable isotope analysis were carried out for groundwater （shallow aquifer） in 13 major catchments during July 2019. The main objectives of this study were： ① to understandthe spatial distribution and influencing factors of stable isotopes in groundwater and ② to reveal the groundwater recharge sources and pathways and their spatial variations, combined with the precipitation stable isotope datasets. Stable isotopes in groundwater had poor spatial variations at the regional scale； however, they became isotopically depleted with the increase in annual average precipitation on the catchment scale （r = -0.87）. Compared with the stable isotope of precipitation, stable isotopes of groundwater were generally depleted and were similar to the precipitation of the rainy season （July-September）. These together indicated that there was pronounced seasonality of groundwater recharge, and the main recharge period was the rainy season. In particular, the recharge seasonality index （δP/G） was closely related to the catchment's average annual precipitation （r = -0.77） and leaf area index （r = -0.63）. In addition, groundwater lc-excess was generally negative, with the catchment-mean value ranging from -4.3‰ to -0.7‰. Hydrologically, this indicated that groundwater recharge pathways （ratio of matrix flow vs. preferential flow） were different among these catchments, which should be quantitatively determined by combining the saturated zone （groundwater） and the unsaturated zone （soil） in future work. Our findings can improve the understanding of groundwater recharge in LP and provide a scientific basis for sustainable management of groundwater resources at the regional scale.

Indirect effects of revegetation dominate groundwater recharge change at the basin scale

Attribution of groundwater recharge (GR) change is important for understanding subsurface hydrological processes and managing regional groundwater resources. Despite vegetation change directly influencing GR, its indirect effects are mostly incorporated into other effects because of multicollinearity in environmental factors. Taking the 59 hydrological stations from 13 basins in the Loess Plateau of China as an example, GR was estimated with daily streamflow data and recession-curve-displacement method for 1961–2019. Geographical detector was employed to qualitatively attribute the spatial variability of GR, while structural equation modeling and elasticity coefficient method were utilized for qualitative and quantitative analysis of the direct and indirect impacts of environmental factors on the temporal variation of GR. The annual mean GR in 13 basins ranged from 11 to 53 mm/year, and the spatial heterogeneity of GR can be well explained by loess thickness. GR significantly decreased in 68% of hydrological stations by 38% ± 15%. Averaged over the 13 basins, the direct and indirect effects of NDVI change respectively accounted for 10% and 48% of GR reduction, which greatly exceeded other factors. The indirect effects of vegetation change dominated GR change, but they were getting smaller with increasing latitude. This study highlights the importance in considering indirect effects of vegetation change in hydrological variability, and the results can improve the understanding of hydrological variability.

Testing the reproducibility of active-distributed temperature sensing for measuring groundwater specific discharge beneath a braided river

Braided rivers are a major contributor of groundwater recharge, yet little is known about how recharge rates vary in time. Existing methods for estimating groundwater recharge from rivers (i.e., river loss) are inadequate for studying highly heterogeneous braided river systems at a sufficient spatiotemporal resolution. To do so, active-distributed temperature sensing (A-DTS) is employed, which combines fibre optic temperature measurements with an active heat source, enabling high-resolution quantification of water fluxes. In this study, twelve successive A-DTS surveys were conducted during a 24-hour experiment on a 100 m horizontal subsurface hybrid fibre optic cable installed at 5 m depth beneath a braided river. The experiment was carried out under conditions where the river stage and flow were relatively stable to demonstrate the reproducibility and effectiveness of the A-DTS method for measuring groundwater specific discharge. This foundational work will provide a high level of confidence in the method for future studies aimed at evaluating temporal variations in groundwater recharge. The median groundwater specific discharge values calculated over the 24-hour period had a very narrow range from 3.5 to 4.0 m d−1 across the wetted footprint of the river, which is within the measurement error of the installation (6 %), indicating relatively stable groundwater recharge during the experiment. This provides confidence in the repeatability of the A-DTS method as an effective technology for quantifying river loss over longer time periods, to understand seasonal variability of groundwater recharge in braided river systems.

Proxy modeling approach to evaluate groundwater recharge potentiality zones in the data scarce area of upper Blue Nile Basin, Ethiopia.

Prioritization of groundwater recharge potentiality evaluation is critical for sustainable water resources management. Since recharge is a main source for enhancing groundwater availability. Water scarcity is extremely severe in the upper Blue Nile Basin (i.e., Gunabay watershed). Therefore, this study emphasizes groundwater recharge delineating and mapping 3920.25 km2 in the data-limited area of the upper Blue Basin using proxy modeling (i.e., WetSpass-M model and geodetector model) and tools. The driving/influencing factors are rainfall, temperature, wind speed, evapotranspiration, elevation, slope, land cover, soil, groundwater depth, drainage density, geomorphology, and geology that control the movement of groundwater recharge. However, the first nine factors were used as inputs in the WetSpass-M model to evaluate groundwater recharge. To validate the groundwater recharge availability, water table fluctuation was established based on recorded groundwater levels. Furthermore, the major influencing factors and their interaction have been quantified using geodetector model. Spatiotemporal recharge distribution (in mm) is classified as very low (0-6), low (6-30), moderate (30-51), high (51-83), and very high (83-508) comprising 21%, 20%, 20%, 20%, and 19% of the total area, respectively. Very high groundwater recharge zone has been found in the northwest part of the area. The geodetector results showed that soil (0.841) and temperature (0.287) had larger individual contributions, but the interaction between soil and temperature (0.962) was more significant. It indicates that the interaction between climate and soil has the largest influence on groundwater recharge variability. Generally, the overall approach of this study can be applied to water sectors, policymakers, and decision-makers to overcome future water scarcity.

Understanding process controls on groundwater recharge variability across Africa through recharge landscapes

• Classifications can help consolidate our understanding of recharge controls. • Wetter Recharge Landscapes generate more recharge than dry Recharge Landscapes. • Mean annual recharge rates increase with increasing recharge ratios. • Global datasets limit our ability to explain the spatial variability of recharge. Groundwater is critical in supporting current and future reliable water supply throughout Africa. Although continental maps of groundwater storage and recharge have been developed, we currently lack a clear understanding on how the controls on groundwater recharge vary across the entire continent. Reviewing the existing literature, we synthesize information on reported groundwater recharge controls in Africa. We find that 15 out of 22 of these controls can be characterised using global datasets. We develop 11 descriptors of climatic, topographic, vegetation, soil and geologic properties using global datasets, to characterise groundwater recharge controls in Africa. These descriptors cluster Africa into 15 Recharge Landscape Units for which we expect recharge controls to be similar. Over 80% of the continents land area is organized by just nine of these units. We also find that aggregating the Units by similarity into four broader Recharge Landscapes (Desert, Dryland, Wet tropical and Wet tropical forest) provides a suitable level of landscape organisation to explain differences in ground-based long-term mean annual recharge and recharge ratio (annual recharge / annual precipitation) estimates. Furthermore, wetter Recharge Landscapes are more efficient in converting rainfall to recharge than drier Recharge Landscapes as well as having higher annual recharge rates. In Dryland Recharge Landscapes, we found that annual recharge rates largely varied according to mean annual precipitation, whereas recharge ratio estimates increase with increasing monthly variability in P-PET. However, we were unable to explain why ground-based estimates of recharge signatures vary across other Recharge Landscapes, in which there are fewer ground-based recharge estimates, using global datasets alone. Even in dryland regions, there is still considerable unexplained variability in the estimates of annual recharge and recharge ratio, stressing the limitations of global datasets for investigating ground-based information.

The effects of subterranean estuary dynamics on nutrient resource ratio availability to microphytobenthos in a coastal lagoon

Causal links between subterranean estuary (STE) dynamics, their climatological drivers, and the ecology of coastal ecosystems have remained elusive. Yet, establishing these connections is essential for fully integrated management of coastal ecosystems. We test, in a semi-arid climate, whether the composition of submarine groundwater discharge (SGD) to a lagoon can be regulated by the annual oscillation of the local STE, itself driven by groundwater recharge variability. We study STE outflow samples gathered monthly for a year in the Ria Formosa lagoon, examining the temporal dynamics of salinity, EH, pH (Total scale), dissolved oxygen and nutrient (PO43−, NO2−, NO3−, NH4+, and Si(OH)4) concentrations under the local hydrological regime. The objectives were threefold: (1) to determine the annual variability of nutrient content and N:P:Si stoichiometry in SGD into the lagoon; (2) to identify the main drivers of variability in SGD composition and stoichiometry and their interactive effects; (3) to discuss links to, and implications for, ecosystem function that could help define expectations of cause-effect relationships and be useful for environmental management of the lagoon and similar systems elsewhere.We find that the terrestrial groundwater recharge cycle drives the expansion and contraction of the subterranean estuary on annual timescales, causing the pH of SGD to fluctuate in opposition to continental groundwater level. The annual dynamics of the STE and the resulting pH oscillation determine the annual variability of nutrient composition ratio in SGD and shape benthic primary production dynamics. When saltwater intrusion occurs, the pH within SGD increases, enhancing nitrification and desorption of exchangeable phosphorus, while silicate fluxes increase with seawater retreat. The result is that nutrient resource ratio availability for coastal primary production depends on the fresh groundwater level. This implies that ecosystem function in such systems is more tightly related to the dynamics of linked groundwater reservoirs than previously thought.

Quantifying sensitivity of groundwater recharge to land use and land cover changes by improving model performance on the wetland dominated Tikur Wuha Watershed, Ethiopia

Quantifying the spatial variability of groundwater recharge is crucial for water resource management under changing land use conditions. Hydrological model performance is highly influenced particularly for peak discharge simulation for the wetland-dominated watersheds. This study aimed at improving the performance of a numerical model at a wetland-dominated watershed by explicitly accounting for wetland processes. The Soil and Water Assessment Tool (SWAT) was used to simulate the spatial variability of groundwater recharge for the two Land Use and Land Cover (LULC) maps classified in 1996 and 2017. An acceptable model performance during the calibration and validation periods is found by incorporating wetland parameters. The trend of LULC change showed a significant reduction for Wetland and Forest going along with an increase of agricultural land at the rate of 9.56 ​km2/yr. Accounting for the LULC changes in the model, the simulated median groundwater recharge to the aquifer has reduced from 506.25 ​mm to 483.34 ​mm per year. During these periods, the extent of Wetland extremely converted towards Grassland, resulting in a reduction of median groundwater recharge from 508.6 ​mm to 495.3 ​mm per year in wetland-dominated sub-watersheds. In addition, this study demonstrates spatial variability in groundwater recharge with differences in catchment properties such as elevation and slope. Therefore, this study provides spatially quantified values for groundwater recharge under the LULC conditions, which are useful in the management and development of groundwater recharge regions.

Coupled modelling approach to assess effects of climate change on a coastal groundwater system

The effect of future climate change on groundwater resources was assessed using a modelling approach for a coastal area in Vietnam. In this study, the distributed hydrological model (PANTA RHEI) was coupled with the finite element subsurface flow system model (FEFLOW) to predict the future variation of groundwater resources. The assessment of the seasonal variability of climate conditions on the variation of groundwater recharge and groundwater levels (GWLs) in different aquifers was analysed for the historical (1986–2005), present (2013–2014), and future (2046–2065) period. The domain groundwater model covers approximately 50 km2. The groundwater model was calibrated and validated using hourly measured groundwater levels at 11 monitoring wells within the study area. The necessary information on geological formations, hydrogeological parameters, and groundwater abstraction rate were implemented in the groundwater model set up. One representative concentration pathway (RCP8.5) is considered for projecting future conditions of groundwater resources.The results of this study showed that future rainfall was projected to decrease both in wet and dry seasons. Groundwater recharge was projected to decrease significantly in the dry season (10.9%) compared to the wet season (2.6%). As a result, groundwater levels were also projected to decline in future. Stronger declining trends were detected for deeper groundwater, especially in the dry season with declines of 6.7–20.2 m. The model results also showed that the GWLs are largely influenced by the natural recharge through precipitation in the study area. These findings may help decision–makers and stakeholders for devising sustainable groundwater management strategies in coastal area.

Climate and land-use change impacts on spatiotemporal variations in groundwater recharge: A case study of the Bangkok Area, Thailand

Groundwater contributes to the socioeconomic development of the Thai capital Bangkok and its vicinity. However, groundwater resources are under immense pressure due to population growth, rapid urbanisation, overexploitation, and climate change. Therefore, evaluating the combined impact of climate change and land-use change on groundwater recharge can be useful for developing sound groundwater management systems. In this research, the future climate is projected using three Regional Climate Models (RCMs), namely ACCESS-CSIRO-CCAM, CNRM-CM5-CSIRO-CCAM, and MPI-ESM-LR-CSIRO-CCAM for three future periods: near future (2010–2039), mid future (2040–2069), and far future (2070–2099) under two Representative Concentration Pathway (RCP) scenarios 4.5 and 8.5 as suggested in the IPCC's Fifth Assessment Report. All RCMs project the temperature to rise incessantly, although future precipitation is predicted to fluctuate (increase and decrease) among the various RCMs and RCP scenarios. A Dyna-CLUE model is employed to analyse the future land-use change scenarios (low, medium, and high urbanisation), with the aim of expanding the built-up area and creating land-use maps covering the period to 2099. A hydrological model, WetSpass, is used to estimate groundwater recharge under future climate and land-use change. The findings reveal that groundwater recharge is expected to decrease in high and medium urbanisation areas, ranging from 5.84 to 20.91 mm/yr for the RCP 4.5 scenario and 4.07 to 18.72 mm/yr for RCP 8.5. In contrast, for the low urbanisation scenario, the model projects an increase in groundwater recharge ranging from 7.9 to 16.66 mm/yr for the RCP 4.5 scenario and 5.54 to 20.04 mm/yr for RCP 8.5.

Appraisal of groundwater recharge in Neelum watershed (Upper Indus Basin) using geospatial water balance technique

Water availability is important for survival of millions of people living in the Himalayan region of Upper Indus Basin and adequate monitoring system is for better water resources management. In the present study, groundwater recharge appraisals in the Neelum watershed (Upper Indus Basin) were investigated by using water balance and geospatial modeling techniques on monthly time-scale climate data from 1989 to 2015. Results demonstrated that on an average out of total annual rainfall (i.e., 2028 mm), about 46% of the rainfall convert to surface runoff and 35% loss to atmosphere via evapo-transpiration (ET), while the remaining 18% contribute to infiltrate the groundwater recharge. Groundwater recharge enhanced during snow-melt from December to March and the rainfall infiltration increased during July and August months. Similarly, the infiltration ranges 106–177 mm from January to March and 45–51 mm from December to July. The groundwater discharge in the form of oozing from the spring occurred during the remaining six months, which ultimately contributed to the baseflow of the stream. Findings from the study revealed variations in groundwater recharge during the years and hence recommended more hydrological studies to predict future changes in climate and land use for sustainable development of freshwater resources in the Upper Indus Basin.

Investigating the Effects of Agricultural Water Management in a Mediterranean Coastal Aquifer under Current and Projected Climate Conditions

Coastal delta plains are areas with high agricultural potential for the Mediterranean region because of their high soil fertility, but they also constitute fragile systems in terms of water resources management because of the interaction of underlying aquifers with the sea. Such a case is the Pinios River delta plain located in central Greece, which also constitutes a significant ecosystem. Soil and Water Assessment Tool (SWAT) and SEAWAT models were combined in order to simulate the impact of current water resources management practices in main groundwater budget components and groundwater salinization of the shallow aquifer developed in the area. Moreover, potential climate change impact was investigated using climate data from Regional Climate Model for two projected periods (2021–2050 and 2071–2100) and two sea level rise scenarios (increase by 0.5 and 1 m). Modeling results are providing significant insight: although the contribution of the river to groundwater inflows is significant, direct groundwater recharge from precipitation was found to be higher, while capillary rise constitutes a major part of groundwater outflows from the aquifer. Moreover, during the simulation period, groundwater flow from the aquifer to the sea were found to be higher than the inflows of seawater to the aquifer. Regarding climate change impact assessment, the results indicate that the variability in groundwater recharge posed by the high variability of precipitation during the projected periods is increasing the aquifer’s deterioration potential of both its quantity and quality status, the latter expressed by the increased groundwater Cl− concentration. This evidence becomes more significant because of the limited groundwater storage capacity of the aquifer. Concerning sea level rise, it was found to be less significant in terms of groundwater salinization impact compared to the decrease in groundwater recharge and increase in crop water needs.

Seasonal and Spatial Variation of Groundwater Quality Vulnerable Zones of Yellareddygudem Watershed, Nalgonda District, Telangana State, India.

Evaluation of seasonal and spatial variations in vulnerable zones for poor groundwater quality is essential for the protection of human health and to maintain the crop yields. With this objective, groundwater samples were collected seasonally (i.e., pre- and post-monsoon) from the Yellareddygudem watershed of Telangana, India. These samples were analysed for major chemical parameters (pH, TDS, Ca2+, Mg2+, Na+, K+, [Formula: see text], Cl-, [Formula: see text], [Formula: see text], and F-). Geographical information system (GIS) tools were used to delineate the seasonal and spatial variations for vulnerable zones related to the drinking groundwater quality index (DGQI) and irrigation groundwater quality index (IGQI). Geochemical facies and relations, Piper diagrams, and principal component analysis indicated that the weathering, dissolution, leaching, ion exchange, and evaporation were the primary processes controlling the groundwater quality seasonally. Human influences were the secondary factors. The TDS, Na+, K+, Cl-, [Formula: see text], and F- parameters were observed to be within the drinking water quality standard limits in most post-monsoon groundwater samples. However, the DGQI showed an increase in the number of samples with unsuitable quality for drinking in the post-monsoon period compared with the pre-monsoon period. The IGQI demonstrated that the number of samples with unsuitable quality for irrigation increased in the post-monsoon period compared to the pre-monsoon period. The differences in the vulnerable zones between the pre- and post-monsoon periods were due to variations in groundwater recharge, following the topography. Thus, the present study will help decision makers to plan groundwater treatment measures within vulnerable zones.

Clustering of Groundwater Wells and Spatial Variation of Groundwater Recharge in Sina Basin, India

A spatial and temporal analysis of groundwater levels, topography, and precipitation is required to properly manage the groundwater resource. The present paper explains it in two parts: (1) spatial analysis of groundwater levels and selection of suitable clustering approach for selection of representative wells and (2) spatial and temporal variations of groundwater recharge calculated by three numerical models: Chaturvedi model, Amritsar model and ERAS model. Four clustering techniques including K-Means clustering algorithm, Hierarchical clustering technique, canopy and expectation maximisation (EM) were used for the clustering of groundwater levels. Among these, the canopy technique presents more reliable results compared to the other techniques for the spatial analysis of groundwater levels and the formation of representative wells in the Sina basin. For the groundwater recharge estimation, Chaturvedi model and ERAS model values were found very close. The recharge values show consistency with the precipitation data and found that 15% of precipitation contributes to annual groundwater recharge. Spatio-temporal variation of groundwater recharge correlated with precipitation is also carried out for the selected basin. The results show a drastic decline in the groundwater recharge from the year 1990 to 2008. An empirical expression is also developed for groundwater recharge estimation in terms of groundwater level. This provides regional scale information about the basin and helps to understand the groundwater exploitation scenario for instance.

Assessing Temporal Changes in Groundwater Recharge Using Spatial Variations in Groundwater Ages

AbstractDetermining variations in groundwater replenishment over a variety of time scales remains a challenge in the management and protection of groundwater resources. Specifically, capacity to use hydraulic data collected in small windows of time to infer long‐term changes can be limited by system responses. Groundwater ages offer an alternative approach as they represent the time since recharge occurred. Here we use spatial variations in groundwater ages and environmental tracer concentrations to infer temporal variations in groundwater recharge and apply the method to a mine site in northwest Australia, where a stream has been modified from ephemeral to perennial, resulting in enhanced recharge to groundwater. Measurements of 14C and CFC‐12 at five transects along an ephemeral stream were interpreted with the new model to identify recharge rates and the proportion of recharge attributable to enhanced versus natural recharge from flood events. Enhanced recharge varied between 0.03 and 0.66 m/year compared to flood‐generated recharge values ranging between 0.07 and 1.3 m/year. Our results show that spatial variations of groundwater ages and environmental tracer concentrations preserve information about past flow regimes. While our study has demonstrated decadal variations in recharge, application of the method on larger scales could infer much greater extent of temporal variability in recharge, with the potential for significant insight into climate effects on groundwater.

Review: The influence of global change on Europe’s water cycle and groundwater recharge

Roughly a third of Europe’s water demand is satisfied by groundwater abstraction. Understanding how future changes in climate, weather, vegetation and land use will affect the transport of atmospheric water to the subsurface is critical for successful implementation of Europe’s Water Framework Directive and to maintain groundwater as a high-quality water resource. This paper summarizes the known drivers of trends and variations in groundwater recharge (precipitation, evapotranspiration and vegetation, land use) in Central Europe and how they have changed in recent decades. From past observations and future climate projections, the foreseeable consequences for groundwater recharge under a changing climate are discussed. The paper focuses on the complex role of soils and vegetation at the interface between atmosphere and groundwater, and addresses open questions and possible new directions for research. Summarizing the evidence, land use and land-use change have a large control on recharge, but the influence of climate change is increasingly recognized. Central Europe’s current transition from a temperate and relatively moist climate towards a more variable and Mediterranean-like climate may shift recharge patterns and increase the ratio of focused-to-diffusive recharge as precipitation patterns change and the frequency and intensity of climatic extremes (e.g., heavy rainfall, heatwaves, droughts, floods and wild fires) increase. However, uncertainty remains with regard to the dynamic response of Europe’s vegetation to climate change as well as to human modifications of the water cycle (e.g., through irrigation, forest management, artificial recharge or urbanization), which currently challenges model-based predictions of future recharge.

Impacts of Climate Change on Groundwater Recharge in Wyoming Big Sagebrush Ecosystems are Contingent on Elevation

Water is the most limiting and important natural resource in drylands, where low precipitation, high evaporative demand, and drought events are common. Groundwater is the critical resource for human livelihoods to persist through the intra-annual dry periods in dryland ecosystems. Overexploitation of groundwater resources and the externalities associated with depleted aquifers make understanding the ecohydrology of drylands an essential issue. We focus on the water balance and climatic drivers of big sagebrush ecosystems, an important dryland ecosystem type in Wyoming that covers a large spatial extent. The goal of this project was to understand how groundwater recharge (GWR) may change in magnitude and seasonality across multiple sites in Wyoming in the future. We used a combination of fieldwork and simulation modeling to explore key climatic and ecohydrological drivers of GWR. We simulated soil water balance using SOILWAT2, a process-based ecosystem-scale soil water model, and future climate data to estimate change in GWR through 2100. We found that mean annual temperature and precipitation explained 65% of the variation in future change in GWR. High-elevation (>2200 m) wet sites had larger increases in GWR in the future compared to low-elevation dry sites. The among-site variability in GWR was also higher for sites >2200 m, which indicates that mean annual precipitation and perhaps snowpack are important explanatory variables for GWR. Our research suggests that GWR for high-elevation big sagebrush sites may increase in magnitude from current values and may occur earlier in the year, with important implications for the timing and availability of water resources.

Spatial and temporal variability of groundwater recharge in a sandstone aquifer in a semiarid region

Abstract. With the aim to understand the spatial and temporal variability of groundwater recharge, a high-resolution, spatially distributed numerical model (MIKE SHE) representing surface water and groundwater was used to simulate responses to precipitation in a 2.16 km2 upland catchment on fractured sandstone near Los Angeles, California. Exceptionally high temporal and spatial resolution was used for this catchment modeling: hourly climate data, a 20 m×20 m grid in the horizontal plane, and 240 numerical layers distributed vertically within the thick vadose zone and in the upper part of the groundwater zone. The finest practical spatial and temporal resolutions were selected to accommodate the large degree of surface and subsurface variability of catchment features. Physical property values for the different lithologies were assigned based on previous on-site investigations, whereas the parameters controlling streamflow and evapotranspiration were derived from calibration to continuous streamflow at the outfall and to average hydraulic heads from 17 wells. Confidence in the calibrated model was enhanced by validation through (i) comparison of simulated average recharge to estimates based on the applications of the chloride mass-balance method to data from the groundwater and vadose zones within and beyond the catchment, (ii) comparison of the water isotope signature (18O and 2H) in shallow groundwater to the variability of isotope signatures for precipitation events over an annual cycle, and (iii) comparison of simulated recharge time series and observed fluctuation of water levels. The average simulated recharge across the catchment for the period 1995–2014 is 16 mm yr−1 (4 % of the average annual precipitation), which is consistent with previous estimates obtained by using the chloride mass balance method (4.2 % of the average precipitation). However, one of the most unexpected results was that local recharge was simulated to vary from 0 to >1000 mm yr−1 due to episodic precipitation and overland runoff effects. This recharge occurs episodically with the major flux events at the bottom of the evapotranspiration zone, as simulated by MIKE SHE and confirmed by the isotope signatures, occurring only at the end of the rainy season. This is the first study that combines MIKE SHE simulations with the analysis of water isotopes in groundwater and rainfall to determine the timing of recharge in a sedimentary bedrock aquifer in a semiarid region. The study advances the understanding of recharge and unsaturated flow processes and enhances our ability to predict the effects of surface and subsurface features on recharge rates. This is crucial in highly heterogeneous contaminated sites because different contaminant source areas have widely varying recharge and, hence, groundwater fluxes impacting their mobility.

Attributing variations of temporal and spatial groundwater recharge: A statistical analysis of climatic and non-climatic factors

This paper demonstrated the benefits of statistical methods when investigating the climatic and non-climatic drivers responsible for variations in groundwater recharge with a series of up to 43 years of annual recharge for 426 bores in South-East South Australia. We identified the factors influencing groundwater recharge based on 71 climatic metrics and 17 non-climatic metrics (including groundwater abstraction). The results showed: (1) Rainfall during April to October was the most important variable influencing recharge temporal variation, with its decline identified as the most significant factor related to recharge reduction; (2) In contrast, a negative correlation between rainfall during December to February (DJF) and annual groundwater recharge was found. This suggests that a seasonal shift in rainfall (such as decreasing rainfall during April to October and an increase during DJF) can result in a decline in recharge even when the annual rainfall remains unchanged; (3) The length of wet spells (consecutive rain days) and increasing potential evapotranspiration (PET) were additional significant predictors for recharge temporal variation. It demonstrated that a simple empirical relationship (such as recharge as a fixed percentage of rainfall) is not a reliable estimation of renewable groundwater resources under changing climatic conditions; (4) There is a statistically significant correlation between mean groundwater depth and recharge, which implies that if groundwater level fall due to rainfall declines then a positive feedback loop can lead to further recharge declines; (5) Spatially the most statistically significant factors influencing groundwater recharge were soil types and land attributes. The findings of this study can identify which stressors should be included when investigating the impact of climate change on groundwater recharge.

Groundwater recharge assessment in a rural, arid, mid-mountain basin in North-Central Chile

ABSTRACTA basic component of any hydrogeological study is the magnitude and temporal variation of groundwater recharge. This can be difficult to assess accurately, particularly in arid and semi-arid rainfed mid-mountain zones, as is the situation in the rural, low population density zones of North-Central Chile. In this study, recharge in the Punitaqui Basin, North-Central Chile, was characterized, contrasting the results of two methods: a modified Thornthwaite-Mather (MTM) and discharge recession analysis (DRA). We found a recharge rate of between 1 and 4% of average annual precipitation. Average recharge estimated by the MTM method is consistently higher than that estimated by DRA. Also, DRA tends to smooth the recharge values, resulting in a lower inter-annual variation coefficient. Both methods identified a threshold value of total annual precipitation, above which recharge can be expected to occur, of the order of 180 mm year−1, consistent with values reported in similar areas.