National contaminant mapping of soil losses from surficial erosion: an analysis of livestock grazing pressures on soil losses across NZ

For the first time, a national-scale soil loss model of New Zealand has captured both inherent landuse properties, alongside high-resolution calculations of livestock grazing densities and landuse pressures.

The model captures spatial and temporal variability in soil loss rates (t ha-1 yr-1) at high resolution (15m2) and seasonal time steps. The analyses demonstrate the variability in soil degradation and soil loss rates from different land uses, farm classes, and catchments across New Zealand. These analyses generally indicate that broad and sweeping statements about specific landuse classes are generally unfounded unless specific context and conditions are met. Exceptions to this are forage crop paddocks, landslide scars, and to a lesser degree, exotic forestry land, which had relatively high surficial erosion rates.

We conclude that an individual forage crop is not a significant contributor to overall sediment yields when considered at catchment scales due to its relatively small area, but the conditions of these paddocks are such that even if 1% of lands were forage crops they could account for 10-20% of surficial erosion across the catchment.

Thus, when considered as a whole, winter-forage crop paddocks represent a ‘low hanging fruit’ when considering options for reducing sediment loads in catchments.