Geosciences Journal Vol. 20, No. 5, p. 731  744, October 2016 DOI 10.1007/s12303-016-0001-5 ⓒ The Association of Korean Geoscience Societies and Springer 2016

REVIEW

Estimating groundwater recharge in the humid and semi-arid African regions: review Korea Institute of Civil Engineering and Building Technology, Goyang, Gyeonggi-do 10223, Republic of Korea Il-Moon Chung* Marios A. Sophocleous Kansas Geological Survey (Previous), 1930 Constant Avenue, Lawrence, Kansas 66047, USA Dereje Birhanu Mitiku Department of Construction Environment Engineering, University of Science & Technology, Daejeon 34113,

Nam Won Kim

Republic of Korea Korea Institute of Civil Engineering and Building Technology, Goyang, Gyeonggi-do 10223, Republic of Korea

ABSTRACT: Reliable estimation of groundwater recharge rate is crucial for the assessment of groundwater resource potential in Africa. In this study, we reviewed existing studies on groundwater recharge, especially in the semi-arid and humid regions of Africa. After the assessment of the main advantages and disadvantages of each method, we strongly agree that among the distinct existing methods, Water-Table Fluctuations (WTF), Recession-Curve Displacement, and Chloride Methods can be used with a better certainty of improved estimation of groundwater recharge in these regions. In addition, the features of existing studies on groundwater recharge are outlined. The major challenge of these regions on recharge study is the lack of basic data. Therefore, this paper suggests methods for dealing with this limitation and also the future outlook using recently developed technologies such as Remote Sensing (RS) and Geographical Information System (GIS). Watershed hydrologic modeling, which is a robust method for recharge estimation that is widely applied around the world, should also be applied for future perspective by solving the problems of its use and data requirements to find a better result. Strictly speaking, the key to the successful estimation of groundwater recharge lies in the utilization of a variety of independent methods. Therefore, by bringing together the advantages, limitations, and cost of each method, the study of the recharge estimation in different climatic environments of African regions can enter a new era. Key words: humid and semi-arid African regions, groundwater recharge, sustainable implementation, groundwater resource planning

1. INTRODUCTION Groundwater is the main source of water supply in most rural Africa and assists the community with poverty reduction through irrigation (Macdonald et al., 2011). Rapid change of climate as well as population growth affects all water resources; however, these factors will affect groundwater slower than surface water. This could be a potential buffer, especially during droughts (Macdonald et al., 2009; Calow et al., 2010). In Sub-Saharan Africa, groundwater supplies 75% of the total of drinking water from safe sources (Foster et al., 2006). Hence, groundwater is critical for human survival and economic development. As groundwater use is very import*Corresponding author: [email protected]

ant to meet the increasing demand of water supply, reliable estimation of groundwater recharge rate is crucial for the assessment of the potential of groundwater resource to develop long-term management schemes (Sophocleous, 1989). Especially, groundwater recharge is very important in arid and semiarid regions, where water resources are often crucial for the economic development (De Vries and Simmers, 2002). Generally recharge is defined as the downward flow of water reaching the water table and is classified as direct recharge, indirect recharge, and localized recharge, which is an intermediate type of groundwater recharge resulting from the horizontal near surface (Robins, 1998). As it is difficult to measure groundwater recharge directly like precipitation, it needs to be determined by a variety of indirect methods (Lerner, 1990; Scanlon et al., 2002). Because groundwater recharge can be affected by various factors such as rainfall intensity and frequency, soil type etc. it is known to be highly variable in both space and time (De Vries and Simmers, 2002). There are several methods to estimate groundwater recharge worldwide. According to United States Geological Survey (USGS, 2014) these methods can be divided into five different categories: (i) groundwater methods (Groundwater Modeling and Water Table Fluctuation method (WTF)), (ii) streamflow methods (Seepage Meters, Stream-flow Gain/ Loss Measurements), Recession-Curve Displacement Method and Watershed Models), (iii) Tracer methods (Chloride, Chlorofluorocarbons, Temperature and Tritium), (iv) Unsaturated zone methods (Darcian Method, Zero-Tension Lysimeters, and Zero-Flux Plane), and (v) water budget methods (Deep Percolation Model and Hydrologic Evaluation of Landfill Performance (HELP3) Model). The purpose of this study is to review the existing studies on groundwater recharge in the humid and semi-arid African regions and to suggest useful methodologies to quantify groundwater recharge in the African context taking into account the recently developed technologies.

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2. FEATURES OF GROUNDWATER RECHARGE STUDIES AND MANAGEMENT ISSUES IN THE HUMID AND SEMI-ARID AFRICAN REGIONS 2.1. Purpose of Groundwater Recharge Studies In Africa, the lack of a basic understanding of the hydrological system of the basin causes a critical problem for water resource management (Wenner, 1973; Tenalem, 2002). Groundwater recharge studies have been used for assessing the vulnerability of groundwater resources. Quantitative groundwater map is an example of such assessments, which enables the understanding of the resilience of groundwater to climate changes. A global map of long term average diffuse groundwater recharge for the period 1961–1990 was presented by Döll and Fiedler (2008), which is the ensemble mean of two WaterGAP Global Hydrology Model (WGHM) running with either Global Precipitation Climatology Centre (GPCC) or Climate Research Unit (CRU) precipitation data as input. The estimated groundwater recharge and thus the renewable groundwater resources in Africa are estimated to be 2,072 km3/yr. Another example of the assessment methods is the index methods. Döll (2009) used WGHM selecting four climate scenarios to assess the impact of climate change on groundwater recharge and estimated the number of affected people. Döll (2009) also used a water scarcity index, an index of the dependence of water supply on groundwater, and a human development index to quantify the vulnerability of the population to decrease groundwater resources. Wang et al. (2010) developed a novel practical index method, which made allowances for groundwater recharge, groundwater recharge acceptance, as well as groundwater storage and it is also assumed to be controlled by basic hydrogeologic properties. The method has been applied in east Africa to produce groundwater availability map during drought season for various precipitation and aquifers, which can be used to manage groundwater resource efficiently. 2.2. Groundwater Management Status and Environmental Concerns in the Humid and Semi-Arid African Regions For the sustainable management of groundwater resources, the most important factor required is the amount of recharge received by an aquifer. However, this is usually the least well-known quantity in hydrogeology, especially in arid and semi-arid environments. To date, the African continent has used only a small proportion (5%) of its available water resources (Molden, 2007). The African water crisis is therefore far more complex than the continental water availability and has been classified as economic scarcity rather than just physical water scarcity (Faurès et al., 2007). A very good indicator of water security is the investment in water storage capacity for the regulation of variable hydrological inputs. Compared to international countries, African countries are still performing very

poorly in this regard (Grey and Sadoff, 2006). Currently, most of the humid and semi-arid regions in Africa are experiencing “economic water scarcity” due to lack of investment in infrastructure development (Tuinhof et al., 2011). Thus, the current priority must be more effective planning and sustainable implementation of groundwater development. In these regions, more emphasis should be given to identify and evaluate the relationship between groundwater systems and aquatic ecosystems (Tuinhof et al., 2011). Soil compaction and soil erosion, which lead to infiltration losses in groundwater and baseflow, should be reduced. Methods are required to be found for enhancing groundwater recharge through agricultural land management and small scale engineering measures. In addition, accurate estimation of groundwater recharge is required for sustainable groundwater development in the planning stage (Fig. 1). Furthermore, a number of aquatic ecosystems in these regions depend upon groundwater discharge from aquifers (e.g., the Nech Sar of Ethiopia). The occurrence and value of groundwater-dependent ecosystems need to be better characterized, and the impact of groundwater use for water supply needs to be monitored to arrive at balanced approaches to their conservation (Tuinhof et al., 2011). 2.3. Historical Development of Estimating Recharge According to a recent report by the British Geological Survey (Wang et al., 2010), numerous studies on African recharge have been carried out during the last few decades. The important distinct features in Africa are two different ways of estimating recharge according to the climate conditions. In typical humid regions, precipitation generally exceeds evapotranspiration during most seasons; hence, the recharge occurs continuously. However, in arid regions potential evapotranspiration generally exceeds precipitation. Therefore, groundwater recharge in desert areas is generally negligible (Gee and Hillel, 1988). If the average annual potential evaporation is greater than precipitation, there may be no groundwater recharge. However, the time scale of the calculations is important. The use of long time scales, such as yearly or monthly scales, can lead to an underestimation of the recharge. Water-budget estimates should be conducted using data and time steps that are not greater than a day because precipitation at such smaller scales can greatly exceed evapotranspiration and result in effective recharge (Sophocleous and Merriam, 2012). In Africa, annual recharge rates vary on averaged from less than 1 mm per annum to more than 60 mm per annum (Braune and Xu, 2010). The distribution of aquifers is now reasonably mapped for most parts of Africa (Richts et al., 2011). Quantitative information on the aquifer characteristics, groundwater recharge rates, flow regimes, quality controls is still rather patchy, however, it is improving in some countries as a result of more recent efforts (Tuinhof et al., 2011).

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Fig. 1. Schematic illustration of natural and managed processes of aquifer recharge (Modified after Tuinhof et al., 2011).

Recently, the first quantitative continent wide maps of aquifer storage and potential borehole yields in Africa based on an extensive review of available maps, publications and data were presented by MacDonald et al. (2012). The maps are designed to give a continent wide view of groundwater and encourage the development of national and sub national quantitative maps and assessments to maintain the development of groundwater based adaptation strategies for current and future climate variability (MacDonald et al., 2012). 3. COMMONLY USED RECHARGE ESTIMATION METHODS IN THE HUMID AND SEMI-ARID AFRICAN REGION

computed by multiplying the water-level rise in a well with the specific yield of the aquifer. This method assumes that a water-level rise is caused by the recharge arriving at the water table and that the specific yield is constant (Risser et al., 2005). If the existing observation wells have recorded water level, it is inexpensive and simple to apply the method. These methods for estimating the recharge using groundwater level data have been adopted in Africa and according to Healy and Cook (2002) the WTF method is among the most widely used methods for this purpose. The WTF method uses the following equation to estimate recharge for unconfined aquifers: dh R = Sy ------ , dt

(1)

3.1. Method of Estimating Recharge from Groundwater Data where Sy is the specific yield, h is the water-table height, and dh/dt is the derivative of water table height with respect to These methods include groundwater modeling and WTF time. Equation (1) is applied only over periods of water-level methods. Groundwater models such as Modular Three-Dimen- rise (i.e., h is positive). This method works best over short sional Finite-Difference Groundwater Flow Model (MOD- periods for shallow water tables that display instances of sharp FLOW) (McDonald and Harbaugh, 1988) can be used to rise and decline in the water level. The method inherently estimate groundwater recharge by the calibration of the model assumes that the recharge occurs only as a result of transient to appropriate values of aquifer characteristics such as hydraulic events; recharge occurring under steady flow conditions canconductivity, storativity, hydraulic head. Using this method not be estimated (Nimmo et al., 2005). groundwater recharge can be estimated for different spatial Leduc et al. (1997) found that the annual recharge is about and temporal scale. However, hydraulic conductivity and 10% of the annual rainfall for a Sahel site in South Niger. specific storage data taken from are not often exact, therefore, Sibanda et al. (2009) used several methods, including the water the user should be aware of the potential uncertainties (McDonald level fluctuation method (Nyagwambo, 2006), in the semiand Harbaugh, 1988). Moreover, to use the model adequately arid Umguza district in Matebeleland North Province, Zimmodeling software as well as well trained professionals is babwe. Gavigan et al. (2009) estimated groundwater recharge required. However, the WTF method provides a point recharge using the observed groundwater level fluctuations and gave

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the best estimates for groundwater recharge decreasing from 140 mm/yr in the Teso region to less than 30 mm/yr in the semi-arid Karamoja plain, Uganda. 3.2. Streamflow Methods Seepage meters, streamflow gain/loss measurements (seepage run), recession-curve displacement methods, and watershed models are categorized under these methods. However, in the Sub-Saharan Africa to our knowledge baseflow separation method has been applied in Zimbabwe, Uganda and Ethiopia for groundwater recharge estimation. In the Zimbabwe, estimation of recharge has been made by three methods: river baseflow analysis; hydrochemical analysis of groundwater and simulation modeling. All three methods produce similar results, suggesting a recharge amounts from 2–5% of the annual rainfall, which varies from 2–82 mm per annual (Houston, 1988). Previous recharge studies in Uganda have employed a number of methods including soil moisture balance, WTF, isotope methods, chloride balance, flow modeling and baseflow separation (Taylor and Howard, 1999). The estimated annual groundwater recharge rates in Uganda are highly variable and it varies from 10% of the annual rainfall in the deep weathering zone of central Uganda (Taylor and Howard, 1996) to 1% of the annual rainfall in the zone of stripping in western Uganda (Taylor and Howard, 1999). Similarly, in Ethiopia, the natural recharge of groundwater in 14 major river basins has been estimated using the baseflow separation method from 17 selected river gauging stations and the estimated recharge varies from 10 mm/yr to 120 mm/yr (Pavelic et al., 2012). Tenalem (2008) also estimated the annual recharge of Meki basin using base flow index (BFI). The estimated recharge is approximately 62.7 mm and the indirect recharge or channel loss obtained from the Meki river discharge measurement is 14 mm. Base flow is not exactly equivalent to recharge, however, it is sometimes used as an approximation of long-term recharge assuming minimum underflow, evapotranspiration from riparian vegetation, and other ground water losses from the watershed (Risser et al., 2005). When used as a proxy for recharge, base flow has sometimes been referred to as “effective recharge (Daniel, 1996)”, “base recharge (Szilagyi et al, 2003)” or “observable recharge (Holtschlag, 1997)” to acknowledge the fact that it represents an amount that is less than that which recharged the aquifer. Recently, several types of hydrograph separation techniques have been developed. But users should properly consider the drawbacks arise due to simplification, external water chemistry factors, and arbitrary or non-physical assumption of turning point (Winston and Criss, 2002; Nathan and McMahon, 1990). 3.3. Estimation by Watershed Hydrologic Model The Soil Water Assessment Tool (SWAT) (Arnold et al.,

1998) and Precipitation Runoff Modeling System (PRMS) (Leavesley et al., 1983) have been applied by Arnold et al. (2000) and Dripps and Bradbury (2007) in different climatic regions to quantify groundwater recharge. The basic data required to successfully apply the models include meteorological, topography, soils, land use and streamflow records for calibration and validation. Recent advances in Geographical Information System (GIS) and Remote Sensing (RS) technology and the availability of spatially distributed data on climate, geomorphology and geology enable to use distributed watershed models that can be calibrated with the readily available stream flow data. Examples of such models are Hydrological Simulation Program-Fortran (HSPF) (Bicknell et al., 1997), Chemicals Runoff and Erosion from Agricultural Management Systems (CREAMS) (Knisel, 1980), and Precipitation Runoff Modeling System (PRMS) (Leavesley et al., 1983). Nevertheless, due to the lack of data for hydrologic modeling in most regions of Africa, the use of spatial information using GIS and RS is encouraging. Mileham et al. (2009) applied validated precipitation and temperature data from the regional climate model (RCM) Providing Regional Climates for Impact Studies (PRECIS) to a semi distributed soil moisture balance model (SMBM) to quantify the impacts of climate change on groundwater recharge and runoff for a medium-sized catchment (2,098 km2) in the humid tropics of southwestern Uganda. To obtain useful spatial information on groundwater recharge, it may be necessary to use distributed hydrologic modeling analysis. Alemaw and Chaoka (2003) suggested a distributed GIS-based hydrological model developed using GIS and computational hydrology techniques. Their model was based on the water balance consideration of the surface and subsurface processes considering soil moisture accounted on a monthly basis. The model estimated runoff of 151 mm/yr from a matrix of specific geo-referenced grids representing Southern Africa. Bonsor et al. (2009) estimated groundwater recharge in the Nile River basin using zoom an object oriented distributed recharge model (ZOODRM) (Mansour and Hughes, 2004). The model estimated annual groundwater recharge varies from 0 to 5 mm/yr. Nyenje and Batelaan (2009) investigated the effects of climate change on groundwater recharge and baseflow in the upper Ssezibwa catchment in Uganda by using a physically based distributed rainfall-runoff model, WetSpa (Liu et al., 2004). They estimated the current average annual groundwater recharge in the catchment is 245 mm/yr, accounting 17% of the mean annual precipitation. McDonald et al. (2009) pointed out that negligible recharge occurs in areas with annual rainfall of less than 200 mm, recharge of approximately 50 mm can occur in areas with annual rainfall in the range of 200–500 mm; whereas recharge will be greater in areas where rainfall exceeds 500 mm. Obuobie and Barry (2010) suggested the recharge for the Ghana varies from 1.5% to 19% of the annual rainfall, which is estimated using several methods, including water balance, chloride mass

Estimating groundwater recharge in the humid and semi-arid African regions

balance, WTF, and hydrological model. 3.4. Tracer Method These categories include chloride, chlorofluorocarbons, temperature, and tritium methods. However, in this review, more emphasis is given to those applied in humid and semiarid regions of Africa. The chloride and tritium tracer are among the widely applied method which provides an estimate of recharge by using a mass-balance equation. Chloride is chemically neutral due to this behavior, it has been applied as an ideal tracer frequently (Scanlon, 1991) and worldwide in most semiarid regions, the method has been used to assess moisture fluxes (Sharma and Hughes, 1985; Johnston, 1987). The main principle of this method is based on chloride transport and readers can find more detail explanation in Bresler (1973) and Peck et al. (1981). The tritium method uses a tritium as a tracer, which is a naturally occurring radioactive isotope of hydrogen with a half-life of 12.43 years (Stonestrom et al., 2007). Similar to chloride, tritium is also an excellent tracer particularly for an unsaturated region, but compared to chloride tracer it needs professional sampling and analysis (Gaye and Edmunds, 1996), which limits its application in the African regions due to lack of well trained professional and sampling costs. As chloride sampling can be done by nonspecialists, it has the advantage to be applied for estimating recharge in most semi-arid African regions using the mass balance approach, but the main drawback is the requirement of long-term records of rainfall chemistry (Gaye and Edmunds, 1996). Traditionally, three major methodologies have been applied in semi-arid and tropical regions in Africa. The most popular one is a chemical tracer method using chloride, stable isotope, and tritium profiles (Gaye and Edmunds, 1996; Cook et al., 1992). Geochemical tracers that have been used for recharge estimation include tritium (3H), oxygen-18 (18O), and deuterium (2H), which are constituents of the water molecule (H2O); naturally occurring anions such as chloride (Cl–) and bromide (Br–); agriculturally introduced chemicals including nitrate (NO3–); applied organic dyes such as fluorescein (C20H12O5); and dissolved gases including chlorofluorocarbons (CFCs), sulfur hexafluoride (SF6), and noble gases such as helium (He) and argon (Ar). Concentrations of these constituents in pore water can be related to the recharge by applying chemical mass-balance equations, taking advantage of the distinctive temporal patterns in the composition of infiltrating water, or by determining the “age” of the water (Nimmo et al., 2005). Sami (1992) used isotope (18O and 2H) and geochemical signatures to identify the recharge and groundwater salinization processes in a semi-arid rangeland catchment, Eastern Cape, South Africa. De Vries et al. (2000) estimated a recharge using environmental tracer and groundwater flow modeling, which indicated that recharge is about 5 mm/yr at the eastern

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Kalahari where annual rainfall exceeds 400 mm. Scanlon et al. (2006) conducted a global recharge studies, including Africa, and pointed out the importance of preferential flow in controlling the recharge as shown in many studies on areas of Botswana and S. Africa (Selaolo et al., 1996; De Vries et al., 2000). Weaver and Talma (2005) found that recharge is about 17.4% of the mean annual rainfall at the Table Mountain Group Aquifer in Western Cape, Africa. 3.5. Water Budget Methods The Deep Percolation Model and HELP3 Model are categorized under the water budget methods. The deep percolation model was primarily developed to estimate long term average groundwater recharge for large scale areas using different weather, land uses and soils, the model operate on a daily basis using precipitation as the main input and it can be also applied to different spatial and temporal scales (Bauer and Vaccaro, 1987). The HELP is a quasi-two-dimensional hydrologic model capable of estimating the basic water balance components such as runoff, evapotranspiration and liner leakage using weather, soil and design data, though the model was primarily developed to compare design alternatives based on their water balances components, it can also be applied to several sites with spatial scale to estimate the potential recharge at a particular place (Schroeder et al., 1994). The above two methods are based on hydrologic water balance and they can be applied to estimate recharge if appropriate rainfall and land use data are available. However, the main disadvantage is the high cumulative errors from component fluxes due to the fact that some fluxes, such as runoff and evapotranspiration, cannot be measured easily (Lerner et al., 1990; Scanlon et al., 2002). Taylor and Howard (1996) applied the soil moisture balance method, stable isotope data, and groundwater flow modeling for estimating groundwater recharge in the Aroca catchment of the Victoria Nile basin in central Uganda, and they found similar results. The study indicated that the estimated average groundwater recharge is on the order of 200 mm/yr, and is more dependent on the number of heavy rainfall (greater than 10 mm/day) events than the total annual volume of rainfall. Rushton et al. (2006) introduced a novel concept and an algorithm to account for continuing evapotranspiration on days following heavy rainfall. The method is applied to a semi-arid region in northeast Nigeria and recharge occurred during the period of main crop growth because the soil moisture deficit became zero. Mileham et al. (2008) estimated groundwater recharge in the humid tropics of equatorial Uganda and reported that there is a mean annual recharge of 104 mm/yr and mean annual surface runoff of 144 mm/yr. 3.6. Intercomparison of Each Method Although the recharge can be reasonably estimated, the

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methods have inherent errors and uncertainties. Many authors have addressed the uncertainties of the different recharge estimation methods. Holtschlag (1997) reported glaring discrepancies between different recharge estimation methods. Rushton and Ward (1979) concluded that 15% uncertainties should be expected for the soil water balance approach while estimating recharge rate. Winter (1981) also discussed various errors that are inherent while measuring and computing various components of the water balance, indicating that long-term average has fewer errors than short-term values and suggested that errors in annual estimates of precipitation, stream flow, and evaporation vary from 2–15%, whereas monthly rates vary from 2–30%. In general, the nonlinear recharge response with time, the highly variable areal distribution of groundwater recharge, the scarcity of hydrogeological data, and the complexities of the hydrologic balance are the major sources of difficulty in estimating recharge (Sophocleous, 2004). In humid regions, groundwater recharge can be estimated from the baseflow component of the measured river discharge. However, computed baseflow values are largely dependent on the method applied for baseflow separation (Tallaksen, 1995). Likewise, lumped hydrologic models are not well suited for estimating groundwater recharge, because they give a single average recharge value for the whole catchment. As a result, more detailed information is essential for proper planning and managing of groundwater supplies in a catchment (Beven, 1989). However, having limited observational data in Africa, gives rise to considerable uncertainties in broad and continental statistics relating to freshwater availability and withdrawals (Taylor et al., 2009). Uncertainty in the renewable groundwater resources ranges up to 51% (Döll and Fiedler, 2008). Furthermore, lack of water-level monitoring networks leads into a problem for basic data collection, which is crucial for model calibration and validation. Estimating aquifer recharge is not simple either, especially for deeper and semiconfined aquifer systems (Van Camp et al., 2013). In reality, comparisons are important for indicating methods that seem to be more realistic in displaying importance towards the recharge process in different physiographic regions and temporal variations. However, choosing a method largely depends on the objectives of recharge estimation, spatial and temporal scales, availability of data, and the available resources in terms of time and expense. It is desirable to apply and compare multiple independent methods for increasing the reliability of recharge estimates (De Vries and Simmers, 2002; Scanlon et al., 2002). The reviewed recharge characteristics of the humid and semi-arid African regions are summarized in Table 1, and the corresponding locations are overlaid with their potential annual recharge (Fig. 2) taken from the Water Gap Global hydrological model (Döll and Fiedler, 2008). Even though there exists a big difference in spatial and temporal scale between the methods applied in Water Gap Global

hydrological model and the reviewed recharge in this study, the result analysis and comparison indicates that a very good agreement with the global scale model. More detail comparison of recharge estimation methods reviewed in this study, including their advantage and the major drawbacks are summarized in Table 2. 4. RECOMMENDED RECHARGE ESTIMATION METHODS IN THE HUMID AND SEMI-ARID AFRICAN REGIONS Technically enhanced approaches for groundwater recharge estimation are still in development process. In this section, we discuss methods for recharge estimation in the humid and semi-arid African regions by carefully assessing the advantages and disadvantages of each method. The following methods can be applied with a better certainty: WTF, Recession Curve Displacement, and Chloride Method. The other most robust method is the watershed hydrologic modeling, which is a widely used method and is discussed in detail below. The method represents an increasing complexity in its use as well as data requirements from a future perspective. 4.1. Enhanced Water Table Fluctuation Method In addition to African regions, the WTF method has been applied in various studies (Meinzer and Stearns, 1929; Rasmussen and Andreasen, 1959; Gerhart, 1986; Hall and Risser, 1993) and is described in detail by Healy and Cook (2002). The enhanced version of the WTF method is useful in African areas where sufficient groundwater level data are available. Sophocleous (1991) introduced the novel concept of combining the soil-water balance and WTF methods for estimating the natural groundwater recharge. This combination method is termed the “hybrid WTF method.” Using a simple average of several such estimates, results in a site-calibrated effective storativity value that can be used to translate each major water table rise tied to a specific storm period for a corresponding amount of groundwater recharge. The estimated errors in the hybrid water-fluctuation method are reduced by running a storm period-based soil-water balance throughout the year in combination with the associated water-level rise and by employing the Complementary Relationship Areal Evapotranspiration (CRAE) methodology (Morton et al., 1985) for areal evapotranspiration estimates, thus avoiding the soil plant system complexities. The concept of the hybrid-WTF method introduced by Sophocleous (1991) was revisited focusing on the definite concept of fillable porosity and an improved WTF model (Park, 2012). For this purpose, a physically based equation of the WTF method was proposed, and the concept of transient fillable porosity was systematically addressed. Kim et al. (2013) proposed a transient water table fluctuation model (TWTFM) along with a watershed hydrologic model for estimating the

Estimating groundwater recharge in the humid and semi-arid African regions

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Table 1. The results of groundwater recharge studies in Sub-Saharan Africa Location

Country Climatic type codea

Eastern Cape, SAF S. Africa

Land use and soil features

Fractured lower Beaufort sandstones and mudstones Peninsula formation Western Cape, Mediterra- Bredasdorp formation SAF S. Africa nean TMG Aquifer Nardouw formation Botswana, Semi-arid BWA Sand Kalahari (Savanna) Botswana

Annual precip- Estimated annual itation (mm) recharge (mm)

Semi-arid

BWA Semi-arid

Method

Reference

460

N/A

Tracer

Sami (1992)

600–700

5% 9.7~13.5% 17.4% 24~46%

Chloride Mass Balance (CMB), Isotope

Weaver and Talma (2005)

N/A

1~5

400

0.5~3.8

N/A

HAPEXPrecambrian basement NER Semi-arid 500–600 Sahel in Niger gneisses, schists and granites South Kalahari Sand, Karoo Basalt, ZWE Semi-arid 555 Zimbabwe Forest Sandstone Central Agricultural, UGA Monsoon 1400 Uganda regolith and gneiss Precambrian gneisses, schists, Uganda UGA Monsoon phyllites and granites of the 1190 Buganda–Toro formation Agriculture (about 63%) and Central Tropical UGA evergreen broad-leaf forest. 1600–2000 Uganda humid Sandy clay-loam. Quaternary Chad Nigeria NGA Semi-arid 450 Formation sediments

50–60 15~20 200

DeVries et al. (2000) Selaolo et al. CMB, Isotope (1996) Leduc et al. WTF (1997) CMB, WTF, Sibanda et al. Groundwater modeling (2009) Soil moisture balance, Taylor and HowGroundwater modeling ard (1996) Modeling and tracer

Mileham et al. (2008)

104

Soil moisture balance

245

Modeling by WetSpa Nyenje and (Liu, 2004) Batelaan (2009)

65

Soil moisture balance

Rushton et al. (2006)

Various methods, including water balObuobie and ance, CMB, WTF and Barry (2010) hydrological modelling Ethiopia, Pavelic et al. (2012); Tropical Lithosols, nitosols, <100 and Baseflow separation Meki & Akaki ETH 10–120 Tenalem (2008); monsoon cambisols and regosols 2000> Method,CMB Catchment Demlie et al. (2007) a The corresponding country code locations with their potential annual recharge is shown in Figure 2. Ghana

GHA

Tropical Acrisols, Lixisols, Plintho800–2200 humid sols, Luvisols, and Leptosols

daily groundwater recharge. The developed model was tested by applying to the water-table fluctuation data from Hancheon, Korea. In the applications, it was observed that the recharge and fillable porosity estimates were most sensitive to non-linearity in the unsaturated water content profile and permeability. The WTF method is best applied over short time periods in regions having shallow water tables that display sharp rises and declines in water levels. The method has been applied over a wide variety of climatic conditions. Recharge rates are also estimated by this technique, and they range from 5 mm/yr in the Tabalah Basin of Saudi Arabia (Abdulrazzak et al., 1989) to 247 mm/yr for a small basin of a humid region in the eastern US (Rasmussen and Andreasen, 1959). Owing to the simplicity of the method and wide availability of the waterlevel hydrographs from observation wells, the WTF method has been used worldwide for several years (Meinzer, 1923; Rasmussen and Andreasen, 1959), and it is strongly recommended for the humid and semi-arid African regions.

1.5~19%

4.2. Recession-curve Displacement Method In watersheds with gaining streams, groundwater recharge can be estimated through stream hydrograph separation (Meyboom, 1961; Rorabough, 1964; Mau and Winter, 1997; Rutledge, 1997; Halford and Mayer, 2000). Various approaches are used for hydrograph separation, including digital filtering (Nathan and McMahon, 1990; Arnold et al., 1995) and recession-curve displacement methods (Rorabough, 1964). The accuracy of the reported recharge rates depends on the validity of the various assumptions. The recharge estimates in the U.S. based on hydrograph separation range from 152 mm/yr to 1,270 mm/yr in 89 basins (Rutledge and Mesko, 1996) and from 127 mm/yr to 635 mm/yr in 15 basins (Rutledge and Daniel, 1994). The recession-curve displacement method can be used to estimate groundwater recharge from streamflow records. The method is based on the premise that the streamflow-recession curve is displaced upward during periods of groundwater recharge. Groundwater recharge during an event

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Fig. 2. Reviewed locations onto the potential recharge map in Sub-Saharan Africa (Modified after Döll and Fiedler, 2008). Table 2. Comparison between selected methods of recharge estimation in this study (Adopted from Scanlon et al., 2002; USGS, 2014a) Category

Method Spatial Scale Groundwater Modeling Local to Regional Groundwater Water-Table Fluctuations Local Recession-Curve Displacement Watershed Seepage Meters Point Stream Base-Flow Watershed Streamflow Streamflow Gain/ Local Loss Measurements Watershed Models Watershed to Regional Chloride Point Tracer Tritium Point Deep Percolation Model Regional Water budget HELP3 Model Point to Regional

Temporal Scale Estimated Quantity Month to Years Recharge Day to Years Recharge Event to Years Net Recharge Event to Months Potential Recharge Years Net Recharge Instantaneous Days to Years Years Month to Years Day to Years Day to Years

Potential Recharge

Data Needs High Low Low Low Low

Relative Cost High Low Low Low Low

Low

Low

Recharge High High Recharge Moderate Moderate Recharge Moderate High Potential Recharge Moderate Moderate Potential Recharge Low to Moderate Moderate

a

This table is adapted from USGS groundwater resources program (http://water.usgs.gov/ogw/gwrp/methods/compare/) regarding comparison of groundwater recharge estimation methods in humid region.

can be estimated on the basis of the work by Rorabaugh (1964) and Glover (1964). 4.3. Chloride Method The use of both natural and artificial tracers usually leads to a considerable gain in information about water movement

(WGSHS, 2003). For estimating the mean average direct groundwater recharge, chloride method is a fair approximation to the long term average value. It has been applied by Allison and Hughes (1978) and Edmunds and Gaye (1994). Chloride is a conservative tracer that indicates evaporation and during evaporation, the concentration increases and the increase provides a measure of the evaporation. Chloride

Estimating groundwater recharge in the humid and semi-arid African regions

does not participate in water-rock ion exchange interactions, and once it moves to the groundwater it is difficult to remove it, so together with rainfall data and under the assumption of negligible runoff, the recharge can be computed (Winter, 2006). The method is often applied in the unsaturated soil zones between the groundwater table and the zero upward flux planes (Winter, 2006). It can also be applied between surface and shallow groundwater for rough estimation under certain limitations (Brunner et al., 2004). Significant errors are inherent while estimating chloride concentration precipitations (including dry deposition) and while assuming that other sources of chloride are insignificant. However, the method is easy to apply and inexpensive. 4.4. Watershed Hydrologic Modeling Watershed modeling is used to estimate recharge rates over large areas throughout the world. Singh (1995) reviewed many watershed models that generally provide recharge estimates as a residual term in the water-budget equation (Arnold et al., 1998; Leavesley and Stannard, 1995; Hatton, 1998). Some lumped models are only capable of estimating a single recharge for the whole catchment (Kite, 1995). However, other methods are spatially disaggregated into hydrologic response units (HRUs) or hydrogeomorphological units (HGUs) (Salama et al., 1993; Leavesley and Stannard, 1995). Soil-moisture balance models (SMBMs) have a demonstrated efficacy in simulating the terrestrial water balance in the humid tropics of Africa (Taylor and Howard, 1999; Rushton et al., 2006; Mileham et al., 2008). SMBMs use widely available hydrometeorological observations (e.g., precipitation, evaporation, soil and vegetation types) and enable quantitative assessment of the impacts of climate change and variability on basin stores (e.g., soil water, groundwater recharge) and fluxes in data-sparse regions. A physically-based distributed rainfall-runoff model like WetSpa (Liu et al., 2004) is capable of simulating river flow hydrographs and spatially-distributed hydrological characteristics, such as soil moisture, infiltration rates and groundwater recharge. The model can be coupled with ArcView to enable pre-processing of GIS data, such as delineation of the spatial inputs of land use, soil and elevation, and derivation of the model’s spatial parameters, such as Manning’s and runoff coefficients, time of travel, flow velocity and surface routing parameters (Nyenje and Batelaan, 2009). To simulate the water resources availability in Africa, the semi-physically based, semi-distributed, and basin-scale model SWAT (Arnold et al., 1998) would be a possible alternative for larger watersheds (Schuol et al., 2008). SWAT is a continuous time model and that operates on a daily time step. In SWAT modeling, the sub basin can be characterized by dominant land-use, soil, and slope classes. For each of the subunits, water balance can be simulated for four storage volumes: snow, soil profile, shallow aquifer, and deep aquifer.

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For more accurate estimation of groundwater recharge, it is advised to use an integrated surface water and groundwater modeling technique be used. The integrated watershed modeling by SWATMOD (Sophocleous et al., 1997; Sophocleous et al., 1999) is capable of simulating the flow of surface-water, groundwater, and stream-aquifer interactions on a continuous basis. Perkins and Sophocleous (1999) describe drought impact analyses using this system. This system was modified to become a two-way coupling system and was used by Sophocleous and Perkins (2000) to investigate irrigation effects on streamflow and groundwater levels in the lower Republican River watershed in north central Kansas, USA. SWAT can handle larger watersheds and their heterogeneities by representing the effects of such heterogeneities statistically using the concepts of hydrological response units (HRUs), These HRUS are statistically defined soil-vegetation/ land use spatial complexes (within a specified climatic regime) with a distinct hydrologic response (Sophocleous and Perkins 2000). Kim et al. (2008) presented a fully integrated SWAT-MODFLOW model that is capable of describing distributed recharges and evapotranspiration. The methodology of translating the HRU recharge characteristics to the MODFLOW cell values is well described. Furthermore, Chung et al. (2010) estimated the distributed groundwater recharge of the Mihocheon watershed, South Korea using the integrated SWAT-MODFLOW model. Instead of a single recharge value for a large watershed, the subdivided recharge rate with heterogeneous characteristics owing to land use, soil type etc. can be computed on a daily basis. This methodology is generally applicable to humid regions having relatively shallow water tables zone (Chung et al., 2010). However, in arid or semiarid regions having deep water tables this time delay-based methodology may run into difficulties due to the extremely long time delays. Groundwater recharge estimation by fully coupled watershed hydrologic models like Système Hydrologique Européen (MIKE-SHE) (Refsgaard and Storm, 1995) or HydroGeoSphere (Therrien et al., 2010) may be far more accurate, although the models require more detailed and rigorous data, which may be sometimes hard to obtain. Thus, the fully coupled models are most appropriate for watersheds that are heavily influenced by groundwater and surface water interactions (Garraway et al., 2011). Owing to the limitation of data for hydrologic analysis in Africa, Milewski et al. (2009) showed a good example of applied methodologies for rainfall-runoff and groundwater recharge computations that rely on observations extracted from a wide-range of global remote sensing data sets using the arid Sinai Peninsula (area: 61,000 km2) and the Eastern Desert (area: 220,000 km2) of Egypt as their test sites. Milewski et al. (2009) also used this data to the continuous watershed modeling (SWAT) model and calibrated well against observed runoff. Generally, small-scale applications allow more precise

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Il-Moon Chung, Marios A. Sophocleous, Dereje Birhanu Mitiku, and Nam Won Kim

methods for measuring or estimating individual parameters of the water-budget (Healy et al., 1989) on daily, monthly, or yearly time scales. Daily time steps are appropriate for estimation of recharge because recharge is generally a larger component of the water budget at smaller time scales. In summary, although there is limited data available for using watershed models in Africa, we strongly believe that these methods are robust in estimating groundwater recharge in some regions of the humid and semi-arid African regions from a future perspective. However, the complexity in their use, data requirements, and the cost should be taken into consideration for using the models presently. 5. SUMMARY AND CONCLUSIONS For sustainable groundwater management, substantial information on renewable groundwater resources is necessary. Therefore, a thorough understanding of recharge processes and aquifer response for a changing future is necessary (Calow and MacDonald, 2009). In this study, we reviewed existing studies on groundwater recharge, especially in semi-arid and humid regions of Africa. There have been numerous methodologies for African recharge estimation. Among them the distinct methods such as the geochemical approach, a method using groundwater level data, the streamflow method, and the water balance methods were first outlined. Next, the features of existing studies on groundwater recharge were summarized. The major challenge of an African recharge study is the lack of basic data. Thus, we suggests the method to deal with this limitation and the future perspective using recently developed technologies such as RS, GIS, etc. Supplementary information can be obtained from global data sets available on the Internet (such as elevation data, land use, vegetation cover, meteorological data, and Landsat images) (Van Camp et al., 2012). With the rapid growth of information technology, more and more data, in terms of both volume and variety, are expected to be made available on the internet in the near future (Zheng et al., 2006). RS technology has a great potential to revolutionize the groundwater development and management in the future by providing unique and completely new hydrological and hydrogeological data. However, at present, the RS data should be considered along with the conventional field data (Jha and Chowdary, 2007). In spite of the weaknesses of water balance methods in semi-arid areas, recently developed water balance methods combined with GIS technology could be a powerful tool for estimating groundwater recharge, when spatial temporal variability of components in water balance is taken into account (Lerner et al., 1990; De Vries and Simmers, 2002; Eilers et al., 2007).When enough data sets are available, integrated surface-groundwater modeling is recommended for more accurate estimation of groundwater recharge and discharge. Strictly speaking, the key to successful estimation of ground-

water recharge lies in the utilization of a variety of independent methods. We strongly believe that every method has its own strengths and weaknesses, but when combined they become much stronger. Therefore, by bringing together the advantages, limitations, and economy of each method, the proper solution to the recharge estimation in different climatic environments of the humid and semi-arid African regions can be obtained. ACKNOWLEDGMENTS: This work was supported by a grant (1416RDRP-B076275-01-000000) from Infrastructure and transportation technology promotion research Program funded by the Ministry of Land, Infrastructure and Transport of Korean government.

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