Combined use of caesium-137 isotope and the universal soil loss equation to establish a soil loss budget along hill slopes in El Maasra basin

Abstract

Soil degradation is a global problem. Over the last several decades, the effects of land degradation in Tunisia are starting to have increasingly serious socio-economic and environmental repercussions. The agricultural sector is most seriously affected. The Tunisian Sahel, despite its modest topographical conditions, has not escaped this threat, especially upstream of the El Maasra Wadi. The fertile soils and homogeneous topography of this area, which dates to Roman times, are highly favorable to a wide range of agricultural practices. The Central Sahel has a long history of olive-growing, followed by a shift to irrigated farming in the 1970s. Today, it is given a multifunctional and multidimensional importance due to the complexity of conflicts of interest between different users. Rural abandonment, mining activity and pastoral pressure have all been behind the soil crisis, which is still largely unknown in detail in terms of the processes involved, the mechanisms behind them and the pace at which they are occurring. Today, there is an urgent need for tools and reliable methods to diagnose the state of soils, to elucidate the various facets of erosive processes and create the structural conditions necessary for their preservation. In the same vein, this research project was initiated in the Wadi El Maasra watershed to quantify soil losses using two methods: empirical model (USLE equation) and experimental based on the isotopic tracer of the Caesium-137. The two methods showed an almost identical geographical distribution, but slightly different loss rates. The specific soil loss obtained from the USLE model was of the order of 15 t ha-1 year-1, which appears to be largely underestimated compared with the loss rate obtained by the Caesium-137 method, which was of the order of 32 t ha-1 year-1.