Groundwater Recharge Estimation in Ethiopia
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Date
2020-12-17
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Addis Ababa University
Abstract
Understanding the rate of groundwater recharge is crucial to studies of water availability, wellhead protection, contaminant transport, ground-water and surface-water interactions, effects of urbanization, and aquifer vulnerability to contamination. The aim of this study was to determine the rate of groundwater recharge in the study area at long term monthly temporal and 250-meter spatial resolutions by applying the WetSpass-M model, recursive digital filter base flow separation, and chloride mass balance methods, and by developing a groundwater conceptual model before implementing the recharge estimation methods.
The multi criteria decision analysis (MCDA) combined the rainfall, soil texture, land use/land cover (LULC), lithology, lineament density, drainage density, and slope factors in the GIS environment to develop a conceptual model of groundwater recharge to the study area and evaluate the appropriateness of the recharge estimation methods selected for the estimation. The WetSpass-M model requires the land use/land cover (LULC), slope, soil texture, depth to groundwater, and climatological variables (rainfall, temperature, potential evapotranspiration, and wind speed) for the estimation of physically distributed groundwater recharge in the study area. The base flow separation, on the other hand, used river flow data on 18 river gauge stations located at the outlets of the major rivers of the study area. The chloride mass balance method requires precipitation amount, chloride concentration in rainfall, chloride concentration in groundwater as input datasets.
The MCDA result showed that both the spatial and temporal characteristics of the groundwater recharge potential in the study area was highly controlled by the rainfall characteristics in the study area. Similarly, the highest estimations of WetSpass-M and base flow separation methods were observed in months and areas that receive the highest rainfall.
Accordingly, the long term average estimation to the rainy months in the study area (June to September) by WetSpass-M model was found to be 10.5 mm, 18 mm, 15.7 mm and 10 mm, but by the base flow separation method, it was 3 mm, 5.5 mm, 10.9 mm, and 14.2 mm. Some particular areas such as Goro in the western part of the study area, due to their higher and extended rainfall characteristics, receive higher amount of groundwater recharge almost throughout the year. In Goro, groundwater recharge reached up to 400 mm/yr which was around 20% of the average rainfall in that particular location. The long term annual average groundwater recharge in the study area from WetSpass-M, Base flow separation, and chloride
mass balance were found to be 73 mm (81 BCM), 63 mm (40 BCM), 65 mm (72 BCM) respectively.
The outputs of this study, due to its finer spatial and temporal resolution, can be very useful to better understand the characteristics of rate of both the spatial and temporal groundwater recharge in the country as well as to studies related to groundwater management, contamination susceptibility, landslide, and subsidence.
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Keywords
Groundwater, Recharge Estimation, Ethiopia