Abstract:
In order to propose various water resource projects for hydropower, agriculture, industry, water
supply, and other uses in the area, it is essential to assess the water resource potential available in
a watershed. The Upper Bilate watershed in southern Ethiopia faces high and temporal variability
in water resources due to multi-weather systems, rainfall, and the Ethiopian rift. A systematic
assessment of groundwater and surface water potential was undertaken by using soil and water
assessment tool (SWAT) and MCDA, a GIS-based technique. The water resource potential of the
watershed was assessed by using the Arc SWAT hydrologic model and historical hydro
meteorological data from 1989 to 2020. The model was calibrated and validated using the observed
stream flow data of 23 years from 1991 to 2013 from the gauged station near Alaba Kulito. For
model calibration 16 years, and for model validation, 7 years of data were used and indicated an
acceptable model performance range. In order to evaluate the groundwater potential, the GIS-based
MCDA technique was used to analyze the recharge, geomorphology, lithology, soil, land use/land
cover, slope, and topographic parameters generated from DEM-derived. The surface water
potential of the study area was estimated by Arc SWAT hydrologic model in the forms of annual
surface runoff 468.93mm, Lateral flow in the watershed is 9.92mm and Base flow output is
206.42mm.The groundwater resource was estimated an average annual groundwater recharge of
239.47 mm. The shallow aquifer contributes an estimated average annual recharge of 205.89 mm
and the deep aquifer contributes an estimated average annual recharge of 11.95 mm. The
spatiotemporal variation of groundwater recharge was modelled using the estimated monthly
average recharge, which ranges from 2.66 to 47.07 mm. The result is divided into low, moderate,
and high zones of upper Bilate watershed and validated using data on the nearby wells and springs.
While covering an area of 1517.84 km2, More than 76% of the territory has moderate groundwater
potential, more than 20% of the area has low groundwater potential, whereas cover an area of
402.58 km2 and more than 3% of the area has high groundwater potential. The validation analysis
revealed that 63.1 % agreement between the groundwater inventory data and the developed
groundwater potential zone. Hence, the results are reliable and enable water users and decision
makers to sustainably utilize the available groundwater in the study area. Furthermore, this study
is one of the rare groundwater investigations in the hydrogeological setting of the study area.
