Investigation and Computation of the Soil Erodibility Factor in Karubanda watershed located in Ngoma, Huye and Mbazi Sectors of Huye District, Southern Province of Rwanda using USLE Nomograph (K-factor) Equation

Soil erodiblity signifies the level of resistance of soil to rainfall and runoff erosivity, Soil physico-chemical properties such as SOM (Soil Organic Matter), soil texture, soil structure and soil permeability are key master control of soil erodibility. The present study was conducted to examine the prospective range of soil erodibility in mentioned watershed. Watershed delineation was done using Arc GIS 10.5 software and shape file of Digital Elevation Model (DEM) of Huye District. Systematic soil profiling was performed in order to assess the number of soil horizons that affect permeability of soil. A total of 25 Soil samples were taken on grid of 81 ha within Karubanda watershed covering an area of 1998 ha. The Soil samples were analyzed for soil texture, soil organic carbon, soil structure, and permeability in the University of Rwanda Laboratory of soil and Plant analysis, Wischmeier model was used to determine soil erodibility of Karubanda watershed. The results show moderate range of soil erodibility, the figure show that the soil erodibility ranges from 0.3 to 0.7 Mg h MJ-1mm-1. The mean is 0.454 Mg h MJ-1mm-1 and standard deviation is 0.16 of soil erodibility in entire watershed, Coefficient of Variation (CV) was 35 %, thus, soil erodilility variation is moderate in Karubanda Watershed. Nyakagezi sub watershed was reached maximum value of soil erodibility 0.7 Mg h MJ-1mm-1, while Karubanda sub-watershed revealed low value of soil erodibility with 0.3 Mg h MJ-1mm-1, land use master plan, agronomic techniques like soil conservation practices ( forestation, hedgerow, contouring, strip cropping and application of organic matter) as well as mechanical techniques (terraces and waterway) at Karubanda watershed are needed to reduce the degree of susceptibility of soil which are being eroded specifically in critical area of Nyakagezi sub watershed.