TECHNICAL INVESTIGATION AND ENGINEERING DESIGN OF RAINWATER HARVESTING PONDS IN BIYO WATERSHED, SOMALI REGION, ETHIOPIA

Abstract

There is high water scarcity problem in Somali region especially in low land areas where pastoralists and agro-pastoralists’ have been existed. In these areas, constructions of rainwater harvesting structures like embankment ponds are indispensable for the survival of both humans and livestock’s. To construct these ponds, identifying the suitable sites for constructions of ponds are necessary and they are mandatory for any rain water harvesting structures. Therefore, this study was initiated with the objective of technically investigate the suitable sites and design of rainwater harvesting ponds in Biyo watershed. The watershed delineated at the outlet of the watershed and estimated as an area of 38,790.40ha. The watershed comprises of four sub watersheds, namely: Eljogen sub watershed 3,503.08ha, Kansakaror sub watershed 9,470.84 ha, Melowle sub watershed 9,013.38ha, and Daraseye sub watershed 16,803.10 ha. The mean annual rainfall data of the study area was analyzed from four metrological stations (Jigjiga, Tuli-guled, Hadew and Chinakson) by using Thiessen polygon method and the value was 612.56 mm. The amount of runoff generated from Biyo watershed was estimated using Curve number method and was obtained 200,276,445.84m3/year. Determination of suitable sites selection for rainwater harvesting ponds in Biyo watershed were done based on various factors like rainfall, runoff, slope, soil property, availability of local construction materials and land use land cover. Based on these selection criterion, Daraseye sub watershed was selected and further analysis was conducted. Designed rainwater harvesting ponds were done based on the total water demands of Human and livestock including 5% of irrigation water requirement. The total demands of water required for human, livestock and irrigation were estimated as 32,000m3/year. The detail design of main pond and its appurtenant structures were done accordingly with the total water demand estimated. The estimated total cost of water supply system for construction of rainwater harvesting ponds in the selected watershed was 9.7 Million ETB. Hence, the result of this study could help stakeholders to plan and implement appropriate construction of rainwater harvesting ponds in Biyo watershed.