Morphometric analysis and sub-watershed prioritization of Welmal watershed, Ganale-Dawa River Basin, Ethiopia: implications for sediment erosion

Watershed analysis based on morphometric parameters plays an important role for proper planning and implementation of management program for soil protection and to prevent erosion of sediments. Morphometric analysis of watershed is the best method to identify the relationship of various aspects in the area. The main objective of the study is to critically evaluate morphometric parameters and prioritization of sub-watersheds based on erosion prone area of Welmal watershed located in Bale zone, Ethiopia using Arc GIS10.4.1. For prioritization, nine sub-watersheds are delineated and parameters, such as stream length, stream order, drainage density, stream frequency, bifurcation ratio, length of overland flow, basin perimeter, form factor, compactness coefficient, and elongation ratio, have been considered. The Geographic Information System based on morphometric analysis of River Basin revealed that it is a fourth-order basin, with drainage pattern of the mainly dendritic type, showing homogeneity in texture and little structural control. Welmal River Basin has a total number of 602 streams, where 526 are first order, 64 are second order, 11 are third order and 1 is fourth order. The length of stream segment is maximum for the first-order stream and decreases as the stream order increases. The drainage density (D-d) of the study area of Welmal River Basin is 1.13 km/km(2), which suggests that the River Basins are not much affected by structural disturbance. The entire basin elongation ratio (0.22) indicates that the basin is elongated shape and little prone to overflowing. Based on the value of compound value (C-p), the sub-watershed with the lowest C-p value was given by the highest priority and then categorized the sub-watersheds into three classes as high, medium and low in terms of priority. Accordingly, high-priority zone comprises 2 sub-watersheds, medium 2 sub-watersheds and low 5 sub-watersheds. High-priority sub-watersheds are those that are much more prone to soil erosion and should receive greater attention because they need major land conservation measures.