| Summary: | Runoff farming is reported to improve land productivity and crop yields in hot and dry climates. This study was conducted to assess the available rainwater that can be harvested in a conserved catchment in a drought prone agro-ecological zone. The study was carried out in the Cyili sub-catchment, southern province of Rwanda, which has an irregular rainfall pattern and unexploited runoff water. Soil Conservation Service-Curve Number method (SCS-CN), CROPWAT model version 8 and Hazen model with an average daily rainfall data recorded from 1971 to 2016 were applied to estimate the runoff and water requirements in the study area. Findings of the study revealed that more than half rainfall water received in the catchment was lost through runoff (229.8 mm) and effective rainfall was lower (246.9 mm) than the actual crop water requirement for maize (330 mm). The expected seasonal surface runoff volume to be harvested by the farmers was 3008 m3 ha−1 per season and 1.29 × 106 m3 per season for the entire whole sub-catchment (430 ha). Based on Hazen model, the return period of low rainfall (dry spell) event would be expected every 2 years with a 98% probability of occurrence. Cyili sub-catchment has higher potential runoff volume to stabilize the deficit of water demand in the period of short rainy season. This suggests that rainwater harvesting through supplementary irrigation is an option to improve the crop yield in the dry period as well as in the annual dry season. Keywords: Available rainwater, Cyili sub-Catchment, Curve number, Runoff, Rwanda
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