Modelling soil loss from surface erosion at high-resolution to better understand sources and drivers across land uses and catchments; a national-scale assessment of Aotearoa, New Zealand

Soil erosion is a significant challenge for agricultural regions, with cascading impacts to waterways, land productivity, soil carbon, and ecological health. We provide the first national-scale soil erosion model that incorporates the impacts of grazing on ground cover (Cgr) and soil erodibility (Ktr) into the RUSLE framework. Surface erosion rates for winter-forage paddocks (11 t ha−1 y−1) were substantially higher than pastoral grasslands (0.83 t ha−1 y−1), woody grasslands (0.098 t ha−1 y−1), forests (0.103 t ha−1 y−1) and natural soil production rates (≤1–2 t ha−1 y−1). Validation with empirical measurements from sediment traps, sediment cores, and chemical fingerprinting demonstrated strong linear regressions (r2 = 0.86). Terrain impacted soil erosion directly through slope steepness and flow convergence and indirectly through strong orographic effects on rainfall erosivity (r2 = 0.39–0.83). Annual surface erosion across Aotearoa New Zealand could reach 16.5-29.2 Mt y−1, representing ∼$20M annually and up to 24–31% of sediment yield for two catchments.