GIS-based gravity-fed surface irrigation potential assessment for river catchments in Kobo District, Ethiopia

Planning for irrigation development requires information about the land suitability and adequacy of the available water resources. This study was conducted to assess gravity-fed surface irrigation potential of river catchments in Kobo District, Ethiopia. To identify the suitability of the land for gravity-fed surface irrigation, parameters which include slope, soil characteristics (type, depth and texture), land use land cover (LULC) and distance from diversion sites were considered. The effects of the first three parameters on land suitability were evaluated using the FAO method. Proximity to diversion sites was evaluated from consideration of economic distances. The integrated effect of the parameters on land suitability was determined based on weighted overlay analysis in a GIS environment. Four dominantly grown crops in the area, namely maize, tomato, cabbage and watermelon were chosen and their gross irrigation demands were estimated using CROPWAT 8.0 model. The water sources in the area are Aradom and Kelekeli Rivers. As the potential assessment is for gravity-fed surface irrigation, suitable diversion sites were selected at the two rivers based on the site selection criteria. The available flow at the diversion points was estimated by transferring discharges from gauged sites using the runoff coefficient method. Independent analysis of the parameters for land suitability indicated that 58.5% of the slope, 100% of the soil and 70% of the LULC in the study area are in the range of highly suitable to marginal suitable for surface irrigation. From the combined analysis of the parameters including proximity to the diversion sites, the suitable land area for gravity-fed surface irrigation was found to be 9819 ha which is only 5.53% of the study area. The water availability estimate showed that the 90% dependable flows at the diversion sites are adequate to satisfy the irrigation demands of the crops. Thus, the suitable land area was taken as the gravity-fed surface irrigation potential of the study area. Of this potential, 5375 ha was identified to be supplied by the Aradom River and the remaining 4444 ha by the Kalkeli River. Finally, the study has demonstrated a realistic approach to assessing the effective irrigation potential of an area for gravity-fed surface irrigation. It is, thus, believed that the methodology and findings of the study will help policymakers make better decisions for developing gravity-fed surface irrigation in the area.