Evolution of Hydric Erosion in the Medjerda Catchment in Northern Tunisia and Its Impact on Soil Degradation and Plant Cover

In northern Tunisia, the phenomenon of hydric erosion leads to both soil loss and a depletion of water reserves. Over the course of several decades, the fertile soils of the Oued Medjerda watershed have continued to deteriorate due to hydric erosion on its slopes, posing a real threat to the agricultural sector. In this research, we present a new methodological assessment that was applied to several dates linked to periods of flooding, highlighting the evolution of this erosive occurrence over a span of 30 years. Using digital mapping, several resulting maps were developed, including (1) maps of the erosive power index “SPI” and the sediment transport index “STI” through a DEM (SRTM, 90 m) and (2) maps of surface change using multi-date digital terrain models (DEM) from Google Earth between 1985 and 2015. The obtained results enabled us to (1) locate the land most threatened by hydric erosion and (2) analyze and quantify the spatiotemporal evolution of this destructive phenomenon, where 387984 ha of land were eroded by hydric erosion between 2000 and 2005 and 228938 ha between 1985 and 2015. This reveals the interaction between tectonics, erosion, and sedimentation, where active faults enabled the infiltration of water from torrential rains (2003, 2012, and 2015 floods), leading to the supersaturation of sediments (soft layers) in the vicinity of these faults. The selection of SPI and STI indexes (with a correlation of 0.9583) was confirmed by the accumulation of sediments on flat land. The Medjerda watershed is evidence of the development of an erosive situation that represents a serious threat to agricultural crops, specifically, and sustainable development in general.