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

Abstract

The study aims at evaluating physical suitability (land and water resource) for surfaceirrigation development using GIS techniques. The Segen sub-basin was selected as acase study for this thesis work which is located at the Southern part ofthe 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 basedon the method as described in FAO guideline for land evaluation. The essence of landevaluation 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 uniqueinformation of land qualities for which the suitability is based on. The selected themelayers for this study include soil types, with their corresponding physical and chemicalcharacteristics that affect irrigation agriculture, land cover, and slope layer, which isderived from the Digital Elevation Model of the study area. As its being the mostlimiting factor for the realization of, especially for surface irrigation method, the slopelayer was used as the base map for the overlaying analysis. The suitable land identified categorized according to the different climatic zones alongthe 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 waterin the respective climatic zones was estimated and the area that can be irrigated withthis 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 thestudy area are in the range of highly suitable to marginal suitable for surface irrigation systemwhereas the remaining 29.1% (soil) and 47.5% (slope) in the area are not suitable. In terms ofland cover/use, land covered by settlement, degraded shrub land, forest, wetland, water bodyand grazing land covering 17.2% of the study area were restricted from irrigationdevelopment. When these factors were weighted using weighted overlay in Arc GIS, thepotential irrigable land for surface irrigation was reduced to 2.12%.