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 NZ

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 and soil erodibility into the RUSLE framework. Surface erosion rates for winter-forage paddocks were substantially higher than pastoral grasslands, woody grasslands, forests and natural soil production rates. Validation with empirical measurements from sediment traps, sediment cores, and chemical fingerprinting demonstrated strong linear regressions. Terrain impacted soil erosion directly through slope steepness and flow convergence and indirectly through strong orographic effects on rainfall erosivity. Annual surface erosion across Aotearoa New Zealand could reach 16.5-29.2 Mt/y, representing $20M annually and up to 24-31% of sediment yield for two catchments.