Groundwater budget for the upper and middle parts of the River Gash Basin, eastern Sudan

Abstract

The River Gash Basin is filled by the Quaternary alluvial deposits, unconformably overlying the basement rocks. The alluvial deposits are composed mainly of unconsolidated layers of gravel, sand, silt, and clays. The aquifer is unconfined and is laterally bounded by the impermeable Neogene clays. The methods used in this study include the carry out of pumping tests and the analysis of well inventory data in addition to the river discharge rates and other meteorological data. The average annual discharge of the River Gash is estimated to be 1,056 × 106 m3 at El Gera gage station (upstream) and 587 × 106 m3 at Salam-Alikum gage station (downstream). The annual loss mounts up to 40% of the total discharge. The water loss is attributed to infiltration and evapotranspiration. The present study proofs that the hydraulic conductivity ranges from 36 to 105 m/day, whereas the transmissivity ranges from 328 to 1,677 m2/day. The monitoring of groundwater level measurements indicates that the water table rises during the rainy season by 9 m in the upstream and 6 m in the midstream areas. The storage capacity of the upper and middle parts of the River Gash Basin is calculated as 502 × 106 m3. The groundwater input reach 386.11 × 106 m3/year, while the groundwater output is calculated as 365.98 × 106 m3/year. The estimated difference between the input and output water quantities in the upper and middle parts of the River Gash Basin demonstrates a positive groundwater budget by about 20 × 106 m3/year