Estimation of Annual and Seasonal Groundwater Recharge by Using Wetspass‐M and Chloride Mass‐Balance Methods, Upper Bilate Catchment, Rifty Valley Basin, Ethiopia

Abstract

It is difficult to estimate groundwater recharge (GWR) since it cannot be measured directly. Numerous physical and empirical models may be used to calculate the components of the groundwater (GW) balance of a given basin. This study’s goal is to compute the annual and seasonal GWR rates in the upper Bilate catchment using the chloride mass‐balance (CMB) approach and water and energy transfer between soil, plants, and atmosphere under the quasi‐steady state (WetSpass)‐modular three‐dimensional finite‐difference groundwater flow model (MODFLOW). Grid maps were prepared as the input data for the model, and to reflect the catchment condition, the parameter attribute tables were arranged. The water balance (WB)’s constituents for each season and year were simulated by the WetSpass‐M model. As well, the CMB technique was applied; the samples were obtained from wells and around rain gauges of the study area, tested in the Arba Minch University water quality laboratory, and then chloride concentrations were evaluated. The recharge was calculated using the precipitation‐weighted average chlorine content in the rainfall and GW. The mean annual GWR values calculated by using WetSpass‐M and CMB were 157.6 and 147.69 mm, respectively, representing 12.7% and 11.91% of the mean precipitation received in the catchment, respectively. WetSpass‐M, the seasonal average recharge values in winter, spring, summer, and autumn were 29.39, 58.87, 1.92, and 56.28 mm; from the total annual recharge, 2.38%, 4.9%, 0.17%, and 4.55% occurred, respectively. CMB, during the dry and wet seasons are 24.08 and 52.16 mm/year, respectively. This result showed that the recharge estimated by the WetSpass‐M model was slightly higher as compared to the CMB method, and both were in a comparable range and both models are with good performance.