Assessing the importance of hypsometry for catchment soil erosion: A case study of Yanze watershed, Rwanda

Abstract

Implementing a watershed erosion control programme requires resource-intensive and time-consuming preliminary studies to prioritize interventions and target them to sub-catchments where they are most likely to yield the most effective results.

In this study, we explore and document the effectiveness of using hypsometric analysis as a method to prioritize erosion control measures and we apply this method to Yanze watershed located in central Rwanda.

Hypsometric integral values were estimated based on a 30m-resolution DEM of the watershed using ArcGIS and R software. Soil loss estimates were calculated by means of RUSLE modelling using data obtained from different sources, namely 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 young stage of its erosional cycle.

The result of the RUSLE model showed that the average potential soil loss in Yanze watershed is 55.63 tonnes.ha^-1.year^-1 which is comparable to the national average estimated at 62 tonnes.ha^-1.year^-1.

A correlation analysis that was conducted between hypsometric integral values of Yanze sub-catchments and their respective mean soil loss values revealed no correlation between the two variables. From the results of this study, we conclude that in watersheds where lithology affects soil erosion significantly, morphology can indeed indicate erosion potential. However, future studies that try to characterize erosion potential using morphometry should use more morphometric parameters in the regression model.