Characterization of a catchment erosion potential through hypsometric analysis: case of Yanze watershed, Rwanda

Abstract

Implementing a watershed erosion control program necessitates extensive and time-consuming preliminary investigations to strategically prioritize interventions, so that sub-catchments that are likely to yield the most effective results are targeted. This study aims at exploring and documenting the efficacy of utilizing hypsometric analysis to prioritize erosion control measures, using Yanze watershed in central Rwanda as a case study. Utilizing a 30-meter resolution Digital Elevation Model (DEM) of the watershed and employing ArcGIS and R software, we calculated hypsometric integral values of each sub-catchment of Yanze watershed. We then calculated annual soil loss estimates using the Revised Universal Soil Loss Equation (RUSLE) model. Data from diverse sources were used during the soil loss modeling, including the Rwanda Meteorological Agency (rainfall data), ISRIC (soil data), and Sentinel-2 images (land cover maps). The hypsometric integral values of Yanze sub-catchments were found to be high, ranging from 0.5 to 0.936. This, combined with the overall convex upward hypsometric curves, indicates that Yanze watershed is still at a youthful stage in its erosional cycle. The results of the soil loss model showed that the average potential soil loss in the Yanze watershed is 55.63 tonnes.ha-1.year-1, which is comparable to the national average that has been estimated at 62 tonnes.ha-1.year-1 Correlation analysis between the hypsometric integral values and mean soil loss values of Yanze sub-catchments revealed no significant correlation. From the results of this study, we conclude that in watersheds where lithology significantly affects soil erosion, morphology can indeed indicate the potential for erosion. However, we further concluded that future studies to characterize erosion potential using morphometry should employ additional morphometric parameters in the regression model.