Baseline elevation and siltation of Ribb reservoir in the Upper Abbay Basin, Ethiopia: a good threshold for change detection and sustainability

Water reservoir construction for irrigation agriculture is becoming an important strategic solution for food security. Consequently, large numbers of reservoirs have been constructed at a global scale. However, sedimentation is a prime problem. The Ribb reservoir located at the Upper Abbay Basin in the Ethiopian Northwest Highlands was designed to store 234 Mm³ of water for irrigation agriculture and is at high risk of sedimentation because of land degradation in the upstream watersheds. Hence, this study estimated runoff and sediment influx into the reservoir, established baseline elevation of the reservoir surface as a reference for temporal change analysis, and identified erosion-prone sub-watersheds for priority intervention using the SWAT model and GIS analysis. As a result, the watershed was delimited into 47 sub-watersheds, and model calibration and validation were performed using gauge station data at the watershed outlet. The watershed contribution of surface runoff, flow, and sediment yield (mean annual) to the reservoir was 267.1 mm, 13.16 m³/s, and 49 t ha⁻¹ yr⁻¹, respectively. The result also revealed that sub-watersheds 1, 2, 3, 7, 12, 16, 26, 29, 36, 37, and 41 were found the sources of 61% of sediment export (109.1 t ha⁻¹ yr⁻¹). The annual sediment production rate of the entire watershed (41.6 t ha⁻¹ yr⁻¹) is high. Therefore, specific and sub-watershed scale erosion and sediment management methods must be designed and implemented to minimize the runoff speed and the associated soil erosion and sediment removal to extend the reservoir life and irrigation agriculture.