Recharge Model Research Articles

Using a Distributed Recharge Model to Quantify Recharge Processes in a Semi-Arid Karst Catchment: An Example from Wadi Natuf, West Bank

The Wadi Natuf catchment is situated to the west of the Palestinian capital city of Ramallah which is in the West Bank. The catchment has been instrumented since 2003 to identify and examine recharge processes in semi-arid upland karst terrain, in which both direct and indirect recharge are important. The key recharge processes are direct rainfall recharge, and indirect recharge via wadis including the lateral routing of potential recharge in the unsaturated zone to springs which supply the wadis. A conceptual model describing these processes was developed. A distributed recharge model was then employed to test this conceptual model and to calculate recharge. A semi-arid wetting threshold method, based on local field experiments was used for recharge estimation. The model was calibrated by comparing simulated wadi flows to those recorded during a relatively short historical event. The study demonstrates that short-term monitoring can enable a sensible validation of a conceptual model leading to the estimation of recharge. Confidence in the model simulation requires further field work to strengthen the understanding of processes taking place in semi-arid climates and karstic flow environments.

Groundwater recharge, flow and stable isotope attenuation in sedimentary and crystalline fractured rocks: Spatiotemporal monitoring from multi-level wells

The use of stable isotopes from precipitation as tracers in unconsolidated porous media can be limited due to attenuation in the unsaturated zone, yet this process has been poorly constrained in fractured rock. The complexities of fractured rock systems necessitate the use of multi-level monitoring wells for the measurement of isotopic tracers with depth. These methods, however, are rarely employed in groundwater flow and recharge studies, particularly involving temporal isotope measurements in multiple rock types. To understand the attenuation of isotopic snowmelt tracers in fractured rock and explore surface connections and recharge, an investigation was undertaken at three geologically-distinct field sites in south-eastern Ontario, Canada. Results of constant head injection tests at 2-m intervals were used to identify and isolate high transmissivity zones (2–3 in each well) for the construction of eight multi-level wells (22 intervals) used for groundwater sampling. Deuterium and oxygen-18 were measured in rain (n = 64), snow (n = 67), soil water (n = 118) and groundwater (n = 594) on a bi-weekly to monthly basis over two seasonal snowmelt cycles (>1.5 years). In combination with stable isotopes, measurements of hydraulic head, groundwater temperature, specific conductance and two rounds of tritium sampling contributed to the development of a conceptual model for snowmelt recharge and flow in sandstone, limestone and crystalline rock. Seasonal variation of δ2H and δ18O was observed in all rock types indicating recent recharge and strong surface connections. The dampening and residence time of the snowmelt tracer increased with depth in crystalline rock. Tracer attenuation was highest in higher-porosity sedimentary rock and strong confining conditions were observed to inhibit recent recharge. Substantial isotopic attenuation measured in the overburden indicated that the strong snowmelt signal measured in groundwater stemmed from water infiltrating through areas with thin or no overburden. Identification of the periodic trends and distinct isotopic horizons in this study would not have been possible without the use of multi-level wells.

Application of Modified <i>MAR</i> Model in Barnai River Basin, NW Bangladesh: A Performance Study

Present study aims to evaluate performance analysis of modified recharge well (RW) technique of the Managed aquifer Recharge (MAR) models for rain water from rooftop of office building and that of runoff water from Kharoil <i>Beel</i> (Swamp) harvesting approach in Barnai river basin, north-western part of Bangladesh. In the area, the lower amount of rainfall than country-wide average as well as less infiltration capacity of thick top clay layer lies above the potential aquifer at higher depth put constrain for groundwater resource development against its ever increasing demand day by day along with limited scope to conserve surface water in rivers, <i>Kharies</i> (canals), <i>Beels</i> etc. The performance of modified MAR model for recharge rate calculation with a goal to restore declining trend of groundwater level (GWL) are found good without any quality hazard. Moreover, for continuous operation and better performance of MAR model, regular monitoring and maintenance are necessary along with in-depth research activities to address the burning water scarcity for drinking and irrigation management issue as viable option in sustainable way.

V2Karst V1.1: a parsimonious large-scale integrated vegetation–recharge model to simulate the impact of climate and land cover change in karst regions

Abstract. Karst aquifers are an important source of drinking water in many regions of the world. Karst areas are highly permeable and produce large amounts of groundwater recharge, while surface runoff is often negligible. As a result, recharge in these systems may have a different sensitivity to climate and land cover changes than in other less permeable systems. However, little is known about the combined impact of climate and land cover changes in karst areas at large scales. In particular, the representation of land cover, and its controls on evapotranspiration, has been very limited in previous karst hydrological models. In this study, we address this gap (1) by introducing the first large-scale hydrological model including an explicit representation of both karst and land cover properties, and (2) by providing an in-depth analysis of the model's recharge production behaviour. To achieve these aims, we replace the empirical approach to evapotranspiration estimation of a previous large-scale karst recharge model (VarKarst) with an explicit, mechanistic and parsimonious approach in the new model (V2Karst V1.1). We demonstrate the plausibility of V2Karst simulations at four carbonate rock FLUXNET sites by assessing the model's ability to reproduce observed evapotranspiration and soil moisture patterns and by showing that the controlling modelled processes are in line with expectations. Additional virtual experiments with synthetic input data systematically explore the sensitivities of recharge to precipitation characteristics (overall amount and temporal distribution) and land cover properties. This approach confirms that these sensitivities agree with expectations and provides first insights into the potential impacts of future change. V2Karst is the first model that enables the study of the joint impacts of large-scale land cover and climate changes on groundwater recharge in karst regions.

Infiltration capacity of cracked pavements

Understanding the hydrological behaviour of urban surfaces is imperative in the design of surface water drainage systems and flood mitigation strategies, as well as for the modelling of groundwater recharge and pollution. This study has examined the hydrological behaviour of cracked impervious surfaces through field infiltration testing and image analysis of the cracks themselves. Infiltration tests were undertaken on a section of concrete slab pavers paving. The results showed that cracks in impervious surfaces allow significant volumes of water to infiltrate through them, with infiltration rates comparable to those found in sands and gravels. Using a regression model, infiltration rates were related directly to crack characteristics obtained from image processing software, thereby enabling the first published quantitative link between percentage cracked area and infiltration capacity. The implications of accounting for this infiltration for surface water management systems are estimated to be in the order of £20 million annually for the construction industry in England.

No Direct Linkage Between Event‐Based Runoff Generation and Groundwater Recharge on the Maimai Hillslope

AbstractHillslope hydrological investigations in humid regions to date have focused mostly on runoff generation during events. The few papers that have also examined groundwater recharge processes associated with subsurface stormflow production have found strong linkages between episodes of runoff and recharge to the aquifer. But the range of climate, vegetation, and geological conditions examined thus far has been limited. Here we explore how geologic characteristics, timing of subsurface stormflow, and hydroclimatic conditions relate to the timing of bedrock groundwater recharge at the well‐studied Maimai watershed. We hypothesized that recharge would be determined by subsurface stormflow frequency in this system with high rainfall and little seasonality of the hydrologic response. Unexpectedly, isotopic analysis and noble gas measurements indicated that recharge occurred almost exclusively during winter months despite previous work at Maimai showing subsurface stormflow occurs in all seasons and rainfall‐runoff ratios are high year‐round. A sprinkler and dye experiment conducted directly on open bedrock identified groundwater recharge mechanisms and rates, and a simple empirical recharge model suggests almost 90% of recharge occurred from only 55% of annual precipitation. We found no correlation between the timing and magnitude of groundwater recharge and total precipitation, direct runoff or subsurface stormflow. The catchment effectively converted rainfall to runoff during all seasons, but the unfractured low permeability bedrock (2.3 × 10−8 m/s) required long durations of extended catchment wetness for appreciable recharge to occur—a condition satisfied only during winter months with lower evapotranspiration. These findings suggest the need to better understand the geologic controls of recharge in headwaters.

Mechanism of groundwater recharge in the middle-latitude desert of eastern Hunshandake, China: diffuse or focused recharge?

Although water is scarce in most deserts of the world, the middle-latitude desert of Hunshandake, China, has abundant water resources, mainly groundwater. In this study, isotopic and hydrochemical compositions were investigated to understand the recharge of groundwater in this desert. The groundwaters are fresh and depleted in δ2H and δ18O, compared with modern precipitation, but have high values of tritium (5–25 TU), indicating that these groundwaters are likely less than 70 years old but not of meteoric origin. Clear differences were observed between the north and south parts of the desert. Groundwater in the northern part is characterized by lower landform elevation, lower ion concentrations, higher tritium contents, higher deuterium excess, and more depleted values of δ2H and δ18O than that in the southern part. This indicates a discrepancy between the topographic hydraulic gradient and the isotopic and hydrochemical gradients of groundwater in the desert. It also implies different water sources between the two areas. Combined analysis was further performed on natural waters from the Dali Basin and surrounding mountains. It indicated that groundwater in the north is mainly sourced from the Daxin’Anling Mountains, by leaking of the Xilamulan River water through a thick faulted aquifer. Groundwater in the south has two sources, the Yinshan Mountains and Daxing’Anlin Mountains. Therefore, modern focused recharge is more significant for groundwater recharge in the desert than the mechanisms of diffuse recharge. A conceptual model of groundwater recharge is proposed: the MTVG (mountain water – tectonic fault hydrology – unconfined vadose zone – groundwater) mechanism.

Effect of spatial and temporal scale on simulated groundwater recharge investigations

Abstract Hydrologic model input datasets such as climate, land use, elevation, soil, and geology information are available in a range of scales for use in water resources investigations. Smaller spatial and temporal scale input data allow groundwater recharge models to simulate more physically realistic processes and presumably result in more accurate estimates of groundwater recharge. Projected climate data are, therefore, often downscaled to smaller spatial and temporal scales for use in these models. It is unknown, however, if increasingly smaller-scale climate data produce substantially different simulated recharge results, either in magnitude or trend. Also, even if simulated recharge results are different at a higher space and time resolution, simulation at coarser resolution might be adequate to provide recharge information at decision scales (e.g., meeting Colorado River compact requirements on a ten-year moving average basis). Historical climate datasets at three spatial (∼800 m, ∼4 km, and ∼12 km) and two temporal (daily and monthly) scales were used in a Soil Water Balance (SWB) model of the upper Colorado River basin (UCRB) to simulate groundwater recharge over the water-year 1982–2014 time period. The magnitude of annual and moving ten-year annual average recharge results for daily climate data were within 5% and 7% of ∼4 km results for ∼800 m and ∼12 km climate data, respectively, with deviations from 1982 to 2014 means within 1% and 3% (median), respectively. Comparison of simulated recharge results using the coarsest spatial and temporal climate data with results from the finest scale data indicated similar small differences over ten-year moving annual averages, over water years, and during high recharge months. While differences in simulated groundwater recharge magnitude, which may be important for groundwater-flow simulations, were substantial during some seasonal comparisons, trends in recharge were almost identical across scales, leading to similar conclusions about change from “normal”. Considering the uncertainty inherent in projected climate data, coarser spatial and longer temporal scale input data may be sufficient for water resources managers who need to understand changes in trends in groundwater recharge over water-year or longer time periods.

Modelling of Climate-Induced Groundwater Recharge for Assessing Carbon Emission from Groundwater Irrigation

In this study impact of climate change on groundwater recharge is investigated and the carbon emission from groundwater irrigation is assessed under projected climate change scenarios for Karnal district of Haryana state in India. HYDRUS-1D and MODFLOW models were used to simulate the climate change impacts on groundwater recharge for different projected climate change scenarios. Simulation results showed that groundwater recharge would increase marginally by 2030 over the baseline year of 2008 under the scenario based on ARIMA predictions, which considered the effect of all climate parameters. However, under the scenarios, which considered only rise in temperature, groundwater recharge would decrease by 0.07–0.22 m. Rise in temperature by 3.5°C and 4.3°C along with 9% and 16% increase in rainfall over the base year would increase the recharge by 0.09 m and 0.14 m respectively. The study also revealed that the effect of climate change on cumulative recharge would be more in sugarcane fields than in rice fields. Carbon emission of groundwater irrigation under the scenarios based on rise in temperature only would increase by a minimum of 12 kg CO 2 /ha in pearl millet crop by the year 2030 to a maximum of 3250 kg CO 2 /ha for sugarcane crop by the end of this century. Estimated total carbon emission in 2030 would be 345,857 metric tonne from groundwater irrigation in Karnal district which is 87,474 metric tonne more than the baseline emission.

Usefulness of Soil Moisture and Actual Evapotranspiration Data for Constraining Potential Groundwater Recharge in Semiarid Regions

AbstractUnsaturated flow modeling is widely used to estimate potential groundwater recharge. In semiarid regions, modeled recharge is usually highly uncertain due to the uncertainty in model parameters and also because recharge is often a small fraction of overall rainfall and actual evapotranspiration (AET). This study attempted to assess the usefulness of soil moisture and AET data on reducing the recharge uncertainty using an experimental site in semiarid southeastern Australia as an example. A biophysically based model capable of simulating unsaturated flow, energy balance, and plant growth was employed to conduct recharge modeling. Soil moisture measured at five different depths of the vadose zone and AET derived from an eddy covariance tower were used to constrain the recharge modeling. The best‐calibrated model shows reasonable agreement between modeled and simulated soil moisture and AET time series, with three‐year average recharge of 50.9 mm. The model uncertainty analysis using Monte Carlo simulations indicates that recharge estimated from the unsaturated flow modeling could be highly uncertain (0–213 mm/year). The uncertainty analysis shows that the soil moisture was more useful than AET in constraining groundwater recharge, particularly the measurements at the top of both soil horizons. The combined use of both the soil moisture and AET led to the most constrained recharge uncertainty (14.8–87.3 mm/year). This study suggests that for practical applications, soil moisture data from at least the top of each soil horizon and AET data (either remotely sensed or directly measured) should be used together to constrain recharge modeling.

Evaluations of the downward velocity of soil water movement in the unsaturated zone in a groundwater recharge area using δ18 O tracer: the Kumamoto region, southern Japan

Water and substances from the surface infiltrate the unsaturated zone before reaching groundwater. Yet, little study has been done on the unsaturated zone due to the difficulty of sampling. A lot of studies have been carried out on the top soil down to a depth of one metre and on shallow aquifers because they are easily accessible for sampling. The unsaturated zone of the Kumamotoregion recharge areas is important due to concerns about groundwater pollution from agriculture. The aim of this study was to estimate the downward velocity of soil water movement through the unsaturated zone and the recharge rate using δ18O as a tracer. Five sampling sites were selected and a core was taken from each site. The cores were cut into 0.1 m pieces and soil water was extracted from each to analyze for δD and the δ18O content. Average δD and δ18O compositions of soil water were similar to the isotopic compositions of summer precipitation. Annual average recharge rate and the downward velocity of soil water in each site were estimated by fitting a vertical δ18O profile pattern to a precipitation δ18O time series as a theoretical water displacement flow model for recharge. An estimated annual average recharge rate in the recharge area ranged from 745 to 1058 mm/yr with the annual average downward velocity of 1.37 to 2.34 m/yr. Based on the estimated downward velocity, the infiltration time for soilwater to reach the aquifer was determined as ranging from 9 to 24 years, which corresponds with previous groundwater age estimations presented in an earlier published study on the same area. It was assumed that contaminants will reach aquifers in 9 to 25 years if the effects of diffusion and microbiological reaction are not taken into account.

Groundwater recharge predictions in contrasted climate: The effect of model complexity and calibration period on recharge rates

We systematically evaluated the effect of model complexity and calibration strategy on estimated recharge using four varyingly complex models and a unique long-term recharge data set. A differential split sample test was carried out by using six calibration periods with climatically contrasting conditions in a constrained Monte Carlo approach. All models performed better during calibration than during validation due to differences in model structures and climatic conditions. The two more complex, physically-based models predicted the observed recharge with relatively small uncertainties, even when calibration and prediction periods had different climatic conditions. In contrast, the more simplistic soil-water balance model significantly underestimated the recharge rates. The fourth, semi-mechanistic model captured the observed recharge rates, but with a larger uncertainty range than the physically-based models. Our results may have relevant implications for a broad range of applications when recharge models are used as decision-making tools.

Does hillslope trenching enhance groundwater recharge and baseflow in the Peruvian Andes?

AbstractAs Andean glaciers rapidly retreat due to climate change, the balance of groundwater and glacial meltwater contributions to stream discharge in tropical, proglacial watersheds will change, potentially increasing vulnerability of water resources. The Shullcas River Watershed, near Huancayo, Peru, is fed only partly by the rapidly receding Huaytapallana glaciers (&lt;20% of dry season flow). To potentially increase recharge and therefore increase groundwater derived baseflow, the government and not‐for‐profit organizations have installed trenches along large swaths of hillslope in the Shullcas Watershed. Our study focuses on a nonglacierized subcatchment of the Shullcas River Watershed and has 2 objectives: (a) create a model of the Shullcas groundwater system and assess the controls on stream discharge and (b) investigate the impact of the infiltration trenches on recharge and baseflow. We first collected hydrologic data from the field including a year‐long hydrograph (2015–2016), meteorological data (2011–2016), and infiltration measurements. We use a recharge model to evaluate the impact of trenched hillslopes on infiltration and runoff processes. Finally, we use a 3‐dimensional groundwater model, calibrated to the measured dry season baseflow, to determine the impact of trenching on the catchment. Simulations show that trenched hillslopes receive approximately 3.5% more recharge, relative to precipitation, compared with unaltered hillslopes. The groundwater model indicates that because the groundwater flow system is fast and shallow, incorporating trenched hillslopes (~2% of study subcatchment area) only slightly increases baseflow in the dry season. Furthermore, the location of trenching is an important consideration: Trenching higher in the catchment (further from the river) and in flatter terrain provides more baseflow during the dry season. The results of this study may have important implications for Andean landscape management and water resources.

Incorporation of Satellite Data and Uncertainty in a Nationwide Groundwater Recharge Model in New Zealand

A nationwide model of groundwater recharge for New Zealand (NGRM), as described in this paper, demonstrated the benefits of satellite data and global models to improve the spatial definition of recharge and the estimation of recharge uncertainty. NGRM was inspired by the global-scale WaterGAP model but with the key development of rainfall recharge calculation on scales relevant to national- and catchment-scale studies (i.e., a 1 km × 1 km cell size and a monthly timestep in the period 2000–2014) provided by satellite data (i.e., MODIS-derived evapotranspiration, AET and vegetation) in combination with national datasets of rainfall, elevation, soil and geology. The resulting nationwide model calculates groundwater recharge estimates, including their uncertainty, consistent across the country, which makes the model unique compared to all other New Zealand estimates targeted towards groundwater recharge. At the national scale, NGRM estimated an average recharge of 2500 m 3 /s, or 298 mm/year, with a model uncertainty of 17%. Those results were similar to the WaterGAP model, but the improved input data resulted in better spatial characteristics of recharge estimates. Multiple uncertainty analyses led to these main conclusions: the NGRM model could give valuable initial estimates in data-sparse areas, since it compared well to most ground-observed lysimeter data and local recharge models; and the nationwide input data of rainfall and geology caused the largest uncertainty in the model equation, which revealed that the satellite data could improve spatial characteristics without significantly increasing the uncertainty. Clearly the increasing volume and availability of large-scale satellite data is creating more opportunities for the application of national-scale models at the catchment, and smaller, scales. This should result in improved utility of these models including provision of initial estimates in data-sparse areas. Topics for future collaborative research associated with the NGRM model include: improvement of rainfall-runoff models, establishment of snowmelt and river recharge modules, further improvement of estimates of rainfall and AET, and satellite-derived AET in irrigated areas. Importantly, the quantification of uncertainty, which should be associated with all future models, should give further impetus to field measurements of rainfall recharge for the purpose of model calibration.

Managing the environmental problems of irrigated agriculture through the appraisal of groundwater recharge

This study employed an easy groundwater recharge model which requires input values that are effectively on hand or attainable and exactly measured. The estimated long-term seasonal groundwater recharges have been used for the assessment of environmental issues of agricultural lands in an irrigated semiarid area of northwest India, where groundwater level has risen. The groundwater recharge model performed very nicely in calculating the watertable depths in the course of the entire study period (1996–2016) which is also substantiated statistically through the lower values of root mean squared error (0.2133 m) and mean error (−0.0325 m) and high model efficiency (0.88) and regression coefficient (0.979). Various scenarios have been studied to look at the impact of model's input variation on its output. The analysis of various recharge components shows an average annual watertable rise of 0.08 m in the command due to a higher positive average net recharge in the monsoon season which cannot be fully counterbalanced by a lower negative mean net recharge during the winter season. Among the scenarios analyzed, the better agricultural practices with modified cropping pattern have delivered the best effects for containing the environmental issues of salinization and waterlogging.

Assessing impacts of future potential climate change scenarios on aquifer recharge in continental Spain

Climate change will modify the availability of groundwater resources in the future. Thus the evaluation of average aquifer recharge from precipitation, and its uncertainty, becomes a key subject in determining suitable countrywide water policies. The confident prediction of renewable groundwater resources requires an accurate evaluation of aquifer recharge over time and space, especially in large territories with varied conditions for aquifer recharge such as continental Spain. This study asses impacts of future potential climatic change scenarios on distributed net aquifer recharge (NAR) from precipitation over continental Spain. For this, the used method (1) generates future time series of climatic variables (precipitation, temperature) spatially distributed over the territory for potential aquifer recharge (PAR), and (2) simulates them within previously calibrated spatial PAR or NAR recharge models from the available historical information to provide distributed PAR or NAR time series. The information employed comes from the Spain02 project for the historical climatic data, from Alcalá and Custodio (2014, 2015) for the historical spatial NAR, and from the CORDEX EU project (2013) regional climate models (RCMs) simulations for the future climate scenarios. A distributed empirical precipitation-recharge model is defined by using a regular 10km×10km grid, and assuming that precipitation (P) and temperature (T) are the most important climatic variables determining PAR, while their spatiotemporal variabilities determine the impacts of future potential climatic scenarios on renewable groundwater resources. Potential plausible pictures of future climate scenarios are defined by combining information coming from different RCMs and General Circulation models (GCMs), downscaling techniques, and ensemble hypothesis. These scenarios were simulated within the used precipitation-recharge model to estimate impacts on NAR. The results show that global mean NAR decreases by 12% on average over continental Spain. Over 99.8% of the territory, a variable degree of recharge reduction is obtained; the reduction is quite heterogeneously distributed in line with the variety of conditions for aquifer recharge over continental Spain. The standard deviation of annual mean NAR will increase by 8% on average in the future. The dependence of these changes regarding potential explanatory variables, such as elevation and latitude was also analysed.

Mechanisms of recharge in a fractured porous rock aquifer in a semi-arid region

Eleven porewater profiles in rock core from an upland exposed sandstone vadose zone in southern California, with thickness varying between 10 and 62 m, were analyzed for chloride (Cl) concentration to examine recharge mechanisms, estimate travel times in the vadose zone, assess spatial and temporal variability of recharge, and determine effects of land use changes on recharge. As a function of their location and the local terrain, the profiles were classified into four groups reflecting the range of site characteristics. Century- to millennium-average recharge varied from 4 to 23 mm y−1, corresponding to <1–5% of the average annual precipitation (451 mm over the 1878–2016 period). Based on the different average Cl concentrations in the vadose zone and in groundwater, the contribution of diffuse flow (estimated at 80%) and preferential flow (20%) to the total recharge was quantified. This model of dual porosity recharge was tested by simulating transient Cl transport along a physically based narrow column using a discrete fracture-matrix numerical model. Using a new approach based on partitioning both water and Cl between matrix and fracture flow, porewater was dated and vertical displacement rates estimated to range in the sandstone matrix from 3 to 19 cm y−1. Moreover, the temporal variability of recharge was estimated and, along each profile, past recharge rates calculated based on the sequence of Cl concentrations in the vadose zone. Recharge rates increased at specific times coincident with historical changes in land use. The consistency between the timing of land use modifications and changes in Cl concentration and the match between observed and simulated Cl concentration values in the vadose zone provide confidence in porewater age estimates, travel times, recharge estimates, and reconstruction of recharge histories. This study represents an advancement of the application of the chloride mass balance method to simultaneously determine recharge mechanisms and reconstruct location-specific recharge histories in fractured porous rock aquifers. The proposed approach can be applied worldwide at sites with similar climatic and geologic characteristics.

Groundwater recharge modelling in a large scale basin: an example using the SWAT hydrologic model

Groundwater constitutes an essential resource that augments surface water resources in meeting the water supply needs of man and the ecosystem. Most importantly in arid and semi-arid environments where rainfall patterns are erratic, groundwater resources are often the preferred source of water. This causes enormous pressure on the resource leading to diminishing groundwater resources. Land use changes also impact on groundwater resources through alterations in the hydrologic regime. It is imperative therefore to evaluate groundwater recharge dynamics under changing land uses to provide for a better resource planning and allocation. We present in this study, an investigation into groundwater recharge dynamics of the Olifants Basin, a water stressed basin in Southern Africa over the past decade with considerations to land use changes. Three land use change scenarios were developed to simulate the groundwater recharge of the basin within the Soil and Water Assessment Tool (SWAT) environment. The SWAT model was calibrated (1988–2001) and validated (2002–2013) with good model performance statistics; NSE, R2, PBIAS, RSR of 0.88, 0.89, −11.49%, 0.34 and 0.67, 0.78, −20.69%, 0.57 respectively for calibration and validation stages. Findings indicate groundwater recharge declined by 10.37 mm (30.3%) and 2.34 mm (9.82%) during the periods 2000–2007 and 2007–2013 respectively. The decline in groundwater recharge was linked to the changes in urban (9.2%), agriculture (6.1%), rangelands (−16.8%) during the period 2000–2007 and urban (1.3%), agricultural (14%), rangelands (−14.8%) during 2007–2013. The SWAT model reveals it capabilities as a decision support tool (DST) in groundwater recharge assessment for large scale basins.

New non-linear model of groundwater recharge: Inclusion of memory, heterogeneity and visco-elasticity

AbstractFractional differentiation has adequate use for investigating real world scenarios related to geological formations associated with elasticity, heterogeneity, viscoelasticity, and the memory effect. Since groundwater systems exist in these geological formations, modelling groundwater recharge as a real world scenario is a challenging task to do because existing recharge estimation methods are governed by linear equations which make use of constant field parameters. This is inadequate because in reality these parameters are a function of both space and time. This study therefore concentrates on modifying the recharge equation governing the EARTH model, by application of the Eton approach. Accordingly, this paper presents a modified equation which is non-linear, and accounts for parameters in a way that it is a function of both space and time. To be more specific, herein, recharge and drainage resistance which are parameters within the equation, became a function of both space and time. Additionally, the study entailed solving the non-linear equation using an iterative method as well as numerical solutions by means of the Crank-Nicolson scheme. The numerical solutions were used alongside the Riemann-Liouville, Caputo-Fabrizio, and Atangana-Baleanu derivatives, so that account was taken for elasticity, heterogeneity, viscoelasticity, and the memory effect. In essence, this paper presents a more adequate model for recharge estimation.

Inverse modeling and uncertainty analysis of potential groundwater recharge to the confined semi-fossil Ohangwena II Aquifer, Namibia

The identification of potential recharge areas and estimation of recharge rates to the confined semi-fossil Ohangwena II Aquifer (KOH-2) is crucial for its future sustainable use. The KOH-2 is located within the endorheic transboundary Cuvelai-Etosha-Basin (CEB), shared by Angola and Namibia. The main objective was the development of a strategy to tackle the problem of data scarcity, which is a well-known problem in semi-arid regions. In a first step, conceptual geological cross sections were created to illustrate the possible geological setting of the system. Furthermore, groundwater travel times were estimated by simple hydraulic calculations. A two-dimensional numerical groundwater model was set up to analyze flow patterns and potential recharge zones. The model was optimized against local observations of hydraulic heads and groundwater age. The sensitivity of the model against different boundary conditions and internal structures was tested. Parameter uncertainty and recharge rates were estimated. Results indicate that groundwater recharge to the KOH-2 mainly occurs from the Angolan Highlands in the northeastern part of the CEB. The sensitivity of the groundwater model to different internal structures is relatively small in comparison to changing boundary conditions in the form of influent or effluent streams. Uncertainty analysis underlined previous results, indicating groundwater recharge originating from the Angolan Highlands. The estimated recharge rates are less than 1% of mean yearly precipitation, which are reasonable for semi-arid regions.