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 km 2 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 137 Cs and 210 Pb. 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 137 Cs were slightly higher than those from 210 Pb, 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. • Soil erosion sensitivity was mapped using remote sensing in Madagascar's Ikopa upper watershed • Radionuclide fallout tracers ( 137 Cs and 210 Pb ex ) independently validated erosion sensitivity classes • Inventories reveal multi-decadal scale soil degradation with a mean erosion of 4 t ha −1 ·yr −1 on uncultivated land • Topography and land cover are the primary drivers of erosion in this tropical highland
Tsaravola et al. (Wed,) studied this question.