Spatio-temporal erosion modelling for gullies control in Kigali city: Case of Kigali Sector

Abstract

Due to land scarcity, hilly topography, and the presence of a significant urban poor population in Rwanda, urban expansion driven by population growth in Kigali city has encroached upon vulnerable ecological areas, resulting in gully erosion. Given the intersection between erosion processes, urban dynamics, and specific locations in urban areas, monitoring temporal variations in erosion is essential for prioritizing resources for controlling gully in areas experiencing the highest erosion. This study aims to model the spatio-temporal patterns of soil erosion to support gully control efforts in Kigali, specifically focusing on the Kigali sector. Potential gully sites were identified using a Digital Elevation Model (DEM) and field measurements. The Integrated Valuation of Ecosystem Services and Tradeoffs-Sediment Delivery Ratio (InVEST-SDR) model was applied to quantify erosion intensity over the period from 2000 to 2022. Data analysis used Pearson’s correlation to evaluate the relationship between estimated erosion levels and observed gully measurements. Zonal statistics analysis was conducted to assess the annual variation in soil erosion intensity at the cell level, aiming to identify areas prone to high erosion. The results indicate that both temporal and spatial erosion intensities increased during the 2010 period, rising to 43 and 150 tons/ha/year, up from 45 and 145 tons/ha/year in 2000. However, in 2015, these rates decreased to 38 and 103 tons/ha/year, before rising again in 2022 to 49 and 133 tons/ha/year. The results from field validation reveal a positive correlation (0.6) between gullies and soil erosion that is exacerbated by rapid urbanization. The findings and analytical approach of this study can support policy and decision-makers in developing cost-effective interventions to manage gullies and reduce soil erosion in the region most affected by soil degradation.