Identification of potential irrigation development sites in the upper Aswa catchment, northern Uganda.

Abstract

Development of irrigation systems requires information on available, water supply, terrain, soils, land use, climate and socio-economic factors. This research. aimed at identification of potential irrigation infrastructure development sites within the upper Aswa catchment through determination of suitable water abstraction points, quantification of river flows available for irrigation and selection of optimum irrigation method for each of the sites. Suitability criteria for water abstraction points was the. presence of surface water accumulation points at the outlet of sub-watersheds of the catchment, soil of moderate infiltration, presence of arable land as well as relatively gentle slopes. The DEM land use, slope and the soil map of the catchment were obtained and processed SWAT in QGIS 2.6 environment was used to burn the streams onto the already processed DEM and to divide the area into sub-basins. Twenty-three sub-basins were delineated in which eleven sites evaluated were found to be highly suitable for irrigation infrastructure development on the basis of land, soil and slope factors. These sites are located along the three main river systems; Agago, Moroto/Aswa I and Aswa Il that drain the catchment. Hydrological Assessment was done by constructing the flow duration curves for historical river discharges, for the three main river systems within the study area. A high variability in the daily discharges was observed, depicting unstable flows. The 80% reliable flow for Agago was less than Im3 /s, Aswa I (Moroto) was 0.1m 3 /s and that of Aswa II was slightly more than 5m 3 /s. Comparison of the long-term average monthly discharges with the monthly irrigation requirements of rice crop for small (<100ha), medium(100-500ha) and large scale (>500ha) irrigation development showed that there is need for off-farm storage: infrastructure to cater for the December-March dry season corresponding to the lowest discharges and highest irrigation requirements of 9.5mm/day. For this period the storage requirement for the river systems in. ate 1436572, 2205610 and 1730241 for Agago, Moroto and Aswa Il respectively as determined by subtracting the seasonal irrigation water need to the available discharge throughout the season allowing 25% environmental flow. Optimum irrigation methods were determined using analytical-technical and. technical economic steps in which the acceptability indices drip, sprinkler and Surface irrigation methods against crop, field and human factors were compared at each of the sites to generate the VIMs of the different irrigation technologies. Across all. the sites, surface irrigation method was the most adaptable with VIM ranging from 4 to 5.5. This was followed by sprinkler with VIM ranging from 3 to 4 and the least adaptable method was drip with VIM of 2 to 31 in conclusion, whereas upper Aswa has expansive land area suitable for irrigation, the acreage that can actually be, irrigated is limited to less than 1% due to law river flows during the dry season. Therefore, to increase the acreage, under irrigation, it is necessary to consider conjunctive use of' groundwater, construction of surface reservoirs alone can increase the irrigation command area by a very small margin.