Climate change impacts on erosion and suspended sediment loads in New Zealand

• Modelling climate change impacts on national erosion and sediment loads is challenging.

• Framework for estimating future load contributions from predominant erosion sources presented

• Regional sediment loads increase 1–233 % by end-century across New Zealand.

• Policymakers need to consider spatial variability in climate impact on erosion and sediment loads.

Soil is a critical resource that provides many ecosystem services and is highly valued by indigenous cultures as key for supporting essential human needs. Land degradation processes such as erosion are depleting soil resources while increased sediment loads impact downstream receiving environments, with compounding effects due to land use and climate change. Models are required to estimate the magnitude and extent of climate impacts on soil erosion and sediment loads at national scale for policymakers and catchment managers to assess the future effectiveness and feasibility of policies and mitigation plans, and to prioritise mitigation efforts. Commonly used soil erosion and sediment load models are often unable to represent the diversity of erosion processes and the future trajectory of their hydroclimatic drivers at such scales. We present a modelling framework that estimates suspended sediment load contributions from three predominant erosion processes: mass movement, surficial erosion, and streambank erosion within their respective spatial erosion domains by differencing models of surficial erosion and total load within the domains. We estimate how catchment suspended sediment loads may change under future climate using change factors derived from the main hydroclimatic driver of each erosion process.

Applying this framework at national scale for Aotearoa New Zealand, we demonstrate a disproportionate increase in mass movement erosion expected in soft-rock hill country, with <1–28 % of North Island watersheds and <1–8 % of South Island watersheds estimated to experience a 100% increase in sediment yield by end-century, primarily driven by the impact of increasing storm magnitude-frequency on mass movement erosion. This results in regional increases in sediment load delivered to the coast ranging from 1 to 233%.

Our results highlight the need for policymakers and catchment managers to recognise spatial variations in the response of erosion processes and catchment loads to climate change when developing policy and prioritising mitigation efforts, as combating future erosion may require different methods to those used in contemporary management to achieve catchment objectives.