GIS BASED SURFACE IRRIGATION POTENTIAL ASSESSMENT OF RIVER CATCHMENTS FOR IRRIGATION DEVELOPMENT IN SEGEN BASIN

Abstract

The study aims at evaluating physical suitability (land and water resource) for surface irrigation development using GIS techniques. The Segen sub-basin was selected as a case study for this thesis work which is located at the Southern part of the Rift Valley River Basin covering an area of about 20,517 km 2 . The evaluation of land in terms of the suitability classes for surface irrigation was based on the method as described in FAO guideline for land evaluation. The essence of land evaluation for irrigation is to compare or match the physical resource of soil, water, topography and others with the irrigation requirement. A land unit is obtained by the overlaying of selected theme layers, which has unique information of land qualities for which the suitability is based on. The selected theme layers for this study include soil types, with their corresponding physical and chemical characteristics that affect irrigation agriculture, land cover, and slope layer, which is derived from the Digital Elevation Model of the study area. As its being the most limiting factor for the realization of, especially for surface irrigation method, the slope layer was used as the base map for the overlaying analysis. The suitable land identified categorized according to the different climatic zones along the Weito and Segen River in the sub-basin. They are mostly concentrated in the South central and North Eastern part of the sub-basin. The 95, 90, 85, and 80 percentage time exceedance flow of the available surface water in the respective climatic zones was estimated and the area that can be irrigated with this flow was computed using the selected climatic station data and estimated ETo. Surface irrigation land suitability analysis indicate that 70.9 % of soil and 52.5 % slope in the study area are in the range of highly suitable to marginal suitable for surface irrigation system whereas the remaining 29.1% (soil) and 47.5% (slope) in the area are not suitable. In terms of land cover/use, land covered by settlement, degraded shrub land, forest, wetland, water body and grazing land covering 17.2% of the study area were restricted from irrigation development. When these factors were weighted using weighted overlay in Arc GIS, the potential irrigable land for surface irrigation was reduced to 2.12%.