ASSESSMENT OF LAND SUITABILITY, SURFACE WATER POTENTIAL AND QUALITY FOR SURFACE IRRIGATION: THE CASE STUDY OF ROBI JIDA WATERSHED, ETHIOPIA

Abstract

Irrigation and land resources play an important role in the development of countries around the world. The Robi Jida watershed, located in the North Shoa Oromia region, covers an area of 76,719 ha. Hence, this study focuses on assessing the potential land suitability, estimating irrigation water demand for site-specific crops over the study area, total surface water availability, and river quality for surface irrigation. Potential land suitability is assessed using standard predefined land evaluation parameters such as slope, LULC, soil texture, soil drainage, soil depth, soil type, soil stoniness, AWSC of soil, pH of soil, OC of soil, EC of soil, CEC of soil, river proximity, road proximity, and market proximity. To obtain the overall suitability factors, the Analytical Hierarchy Process (AHP) was applied to the Weight Overlay Analysis (WOA) tool. Suitability was determined by considering Food and Agricultural Organization (FAO) criteria for surface irrigation development. Land suitability was assessed using GIS tools. Irrigation water demand was estimated using the CROPWAT 8.0 model. Total water availability is estimated by combining the water availability of the catchment, as estimated by the SCS-CN method, and the water availability of the stream flow. River water quality was assessed based on irrigation water quality parameters such as EC, TDS, SAR, Na%, SSP, KR, MH, PI, RSC, RSBC, and TH, as well as the Irrigation Water Quality Index (IWQI). From total land area of 76719ha, highly suitable area is (41.7%), moderately suitable (58%), and marginally suitable (0.3%). In the dry season, crops need 18.4, 15.3, 3.1, 10.7, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 6.1, and 14.6 m³/s in Jan, Feb, Mar, April, May, June, July, Aug, Sep, Oct, Nov, and Dec, respectively. From May to Oct, crops do not need water because it is the rainy season, but from Nov to April, crops need more water because it is the dry season in these watersheds. The total water availability in watersheds is 3.10, 3.94, 10.44, 17.29, 14.07, 28.61, 142.28, 199.69, 61.84, 7.23, 3.82, and 2.65 m³/s in Jan, Feb, Mar, April, May, June, July, Aug, Sept, Oct, Nov, and Dec, respectively. Maximum total water availability occurs from June to Sept, while it is minimal during the remaining months. Out of the 11 irrigation water quality parameters (EC, TDS, SAR, Na%, SSP, RSC, KR, PI, MH, RSBC, and TH), all are deemed suitable for surface irrigation except MH. MH >50 meq/L in the middle and downstream areas in both seasons, which is unsuitable for surface irrigation. The IWQI during the wet season is 43.84, which is suitable for surface irrigation. However, during the dry season, the IWQI is 68.20, which is moderately suitable for surface irrigation. This evaluation would enable the discovery of more farming land for sustainable irrigation development.