Assessment of Rainwater Harvesting Potential for Irrigation at Kabiruini in Nyeri, Kenya

Abstract

Constructed reservoirs sometimes fail to provide the required amount of water through unforeseen loses. This is the case with Kabiruini reservoir which failed to provide adequate harvested rainwater for irrigation on the second year of desiltation necessitating this research. Assessment of rainwater harvesting potential and determination of irrigation area were carried out using rainfall and evapotranspiration data obtained from SWAT GLOBAL. The rainfall data ranging from 1979 to 1989 obtained from Water Resources Authority (WRA) was used as calibrating runoff for the runoff simulation using Australian Water Balance Model (AWBM). The simulated runoff and the potential irrigation area were processed in Microsoft excel where monthly values were tabulated. The reservoir was found to runoff harvest of 78309 m3 per year on average out of which 30955 m3 (40%) was lost through seepage and evapotranspiration. The remaining 48114 m3 was split into three cropping seasons whose total irrigation area was found to be 6 Ha per year. This proved that that the catchment can provide adequate runoff for the intended irrigation area. However, a good apportionment plan was found to be necessary to ensure that the reservoir can meet the irrigation demand which will avoid loses in excess land preparation. Increasing the capacity of the dam from the current 23302 m3 to 35075m3 would increase it storage capacity by 50%. This study recommends that seepage and evapotranspiration loses should be mitigated using appropriate means to reduce water depletion in the reservoir.