Soil erosion estimation using ¹³⁷Cs and ²¹⁰Pb radionuclides in the Upper Ikopa catchment, Madagascar

In Madagascar, the Ikopa River flows through the capital, Antananarivo, and serves as the city's main water source. Soil degradation in the watershed reduces agricultural productivity and pollutes downstream waters by transporting eroded material. During heavy rains, suspended solids accumulate in the river, leading to sediment buildup and flooding across the Antananarivo plain, especially in low-lying urban areas. This study estimates average soil erosion in the 4559 km² upper Ikopa watershed using remote sensing, GIS, a physical erosion model, and atmospheric-fallout radionuclides (FRNs), marking the first large-scale application of FRNs in Madagascar. Soil susceptibility to erosion was assessed using an isosector approach integrating remote sensing and GIS data, classifying pixels into four sensitivity levels (low to very high). Two sites per class were selected for soil sample collection, each with three transects and 21-32 samples analyzed for ¹³⁷Cs and ²¹⁰Pb. Results interpreted with the diffusion-migration model (adequate for non-cultivated land conditions) yielded erosion rates (t.ha^-1.yr^-1) for the four sensitivity classes: low = 0.28-0.48, medium = 0.95-1.18, high = 3.72-4.47, and very high = 4.60-5.72. Rates derived from ¹³⁷Cs were slightly higher than those from ²¹⁰Pb, likely due to forest degradation and differing integration periods. Applying class-based rates to the watershed produced an average soil loss of 3.2-4.0 t ha^-1 yr^-1 (1.4-1.8 Mt. yr^-1 total). These rates are lower than previous model-based estimates for cultivated areas, providing complementary insights for soil management across the Ikopa watershed.