Monitoring soil degradation and siltation using spectral indices in a semi-arid watershed: (Case of the Mohamed V dam in the Moulouya basin, (Morocco)

Water erosion is a natural phenomenon that alters the characteristics of the Earth's surface and specifically impacts dams, water resources, and soil resources. The fight against water erosion has always been a major concern for human societies worldwide. This work aims to characterize the state of soil degradation and its impact on the Mohamed V Dam in the Moulouya basin. The methodology used relies on remote sensing(RS) and geographic information systems (GIS) to delineate soil degradation due to water erosion and highlight its effect on the Mohamed V Dam over three distinct periods: 1985, 2020, and 2025. The approach is based on the extraction of indices related to the Normalized Difference Vegetation Index (NDVI), the Difference Water Index (NDWI), and light intensity (BI) from the analysis of Landsat satellite images from the aforementioned years. The results show an increase in the area of severely degraded soils during the study period, while areas of slightly and moderately degraded soils gradually expanded. This soil degradation led to increased siltation of the Mohammed V Dam, resulting in a significant loss of its water storage capacity and necessitating its raising. This simple and rapid approach is a valuable decision-making tool, and its results are crucial for establishing priority zones for erosion control measures.