Estimation of safe yield from potential groundwater balance. Case study: Yumbe district.
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Yumbe district as one of the largest refugee hosting districts in the country is experiencing a deficit in the water required to meet the demand of the current population. Due to unsustainability of the surface water, groundwater has become the major source of water to rely on for both domestic and agricultural use in the district. However, the district experiences seasonal availability of water especially groundwater challenges with water quality resulting from contamination and depletion resulting from uncontrolled abstraction. Several groundwater studies have been done in the area but the concept of safe yield has not been given much attention which has led to the uncontrolled abstraction. Therefore, this research aimed at estimating the safe yield from the long- term average annual and seasonal groundwater recharge using GIS based WetSpass model which will help to evaluate water withdrawals and their sustainability for human needs in the district. The long-term (5years) hydro-meteorological (Rainfall, temperature, windspeed and potential evapotranspiration) and biophysical (soil, land use, topography, slope and groundwater depth) data of the study area were collected, their grid maps were generated, resampled and converted to ASCII format and then input in to the model to simulate the long-term average annual and seasonal groundwater recharge of the study area. The simulated groundwater recharge was then used to estimate the safe yield which is 25% of the recharge. The results showed that, the average annual groundwater recharge ranges from14.3 to 257.74 mm/year, meanwhile the seasonal recharge ranges from 0 to 13.69 mm/year and 24 to244.43 mm/year for dry and wet seasons respectively. The annual safe yield ranges from 3.5 to 64.44 m3/d/ha with the mean value of 32.644 m3/d/ha/year meanwhile the seasonal safe yield ranges from 0 to 3.4 m3/d/ha and 6 to 61.1 m3/d/ha for dry and wet season respectively. The sub-counties of Kei, Drajini, Lodonga, and Ariwa were found to be the most promising areas for sustainable groundwater abstraction. Low recharge and safe yield rates are found in the central and eastern parts (sub-counties of Apo, Yumbe town council, Romogi, Kululu, and Kochi) of the district. Therefore, Yumbe district decision-makers, humanitarian organizations working with refugees in the district, and the Ministry of Water and Environment can use the above information when developing water resource programs such as hand-dug wells, springs, and boreholes, as areas with sustainable and insufficient groundwater sources are clearly identified from the study results.