Application of SedNetNZ in the Waikato region to support NPS-FM 2020 implementation

Report: TR 2024/05

Authors: Simon Vale, Hugh Smith (Manaaki Whenua – Landcare Research)

Waikato Regional Council  (WRC) contracted Manaaki Whenua – Landcare Research to model erosion and suspended sediment loads across the region using SedNetNZ for a range of erosion mitigation and climate change scenarios.

The project was undertaken to support implementation of the 2020 National Policy Statement for Freshwater Management (NPS-FM 2020).

The present report updates and replaces previous SedNetNZ modelling completed in the Waikato region on a catchment or water management zone basis. The project applied an updated version of the SedNetNZ model that contains improvements to the model’s components, including representation of riverbank and surficial erosion, floodplain sedimentation, and lake sediment trapping.

Objectives

The project has three objectives:

• to model region-wide mean annual suspended sediment loads to provide:
  • a contemporary baseline (2022) using recent land cover and erosion mitigations completed to date
  • past land cover and riparian fencing, based on the WRC regional riparian survey, matched to the nearest Land Cover Database (LCDB) mapping year
  • future erosion mitigation scenarios based on a targeted approach that ranks watersheds draining to individual stream segments by sediment load, and applies mitigation works sequentially to pastoral land based on its Land Use Capability (LUC) class; future riparian fencing was applied to watersheds according to the requirements of the stock exclusion regulations (2020).
• to model the effect of future climate change projections on region-wide erosion and suspended sediment loads at mid- (2040) and late (2090) century for the baseline land cover and mitigations to date, and each of the future erosion mitigation scenarios.
• to assess the load reductions required to meet NPS-FM 2020 attribute bands and the national bottom line (NBL) for suspended fine sediment (visual clarity) for the baseline and future mitigation scenarios, with and without the effects of climate change.