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

Project and client

• 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.

Methods

• The updated SedNetNZ model was applied to the Waikato Region to estimate mean annual suspended sediment loads across the River Environment Classification v2 (REC2) digital stream network.
• The contemporary baseline simulation uses recent land cover (LCDBv5, 2018), erosion mitigations completed to date, and the estimated regional riparian fencing extent based on the 2017 WRC regional riparian survey. The baseline includes an assessment of model sensitivity to variations in the spatial arrangement of winter-forage cropping based on regional mapping of forage crops in 2021 and 2022.
• The backward-looking scenarios use the WRC regional riparian survey data for 2002, 2007, 2012, and 2017 (referred to as B2002–B2017) and the nearest past land-cover maps from LCDB, as well as available river management and soil conservation data.
• Future erosion mitigation scenarios were applied to watersheds separately, ranked by sediment loads from pastoral areas on (a) LUC class 7e and 8e land, and (b) LUC class 6e land. These scenarios apply targeted mitigations to 20, 30, 40, 50, and 100% (referred to as M20-M100) of (a) LUC class 7e and 8e (afforested with natives or exotics) and (b) LUC class 6e (space-planted trees) pastoral land by sequentially selecting from load-ranked watersheds. Riparian fencing was applied across all mitigation scenarios.
• The effect of future climate change on erosion and suspended sediment loads for the contemporary baseline and future mitigation scenarios was modelled following a similar approach to that described by Basher et al. (2020) and Neverman et al. (2023). This involved the use of rainfall and temperature grids from six regionally downscaled climate models (RCMs) and four representative concentration pathway (RCP) climate trajectories at mid- and late century to modify projected future erosion process rates under climate change. The M100 mitigation scenario was only modelled at late century in recognition of the longer timeframe required to plausibly achieve this level of erosion mitigation across the region.
• Proportional and absolute load reductions required to meet NPS-FM 2020 attribute bands and the NBL for state of the environment (SOE) monitoring sites were assessed for contemporary and future mitigation scenarios.

Conclusions and recommendations

• Region-wide total erosion is estimated at 2.32 Mt yr^–1, with the highest sediment yields primarily found in areas prone to earthflows, shallow landslides, and gullies in the West Coast, Waipā, and Upper Waikato CMZs.
• Estimated changes in riparian fencing since 2002 produced a minor, region-wide decrease in modelled mean annual sediment load (4.1%) between 2002 and 2017, although changes in land cover from woody cover to pasture increased erosion during this time, partly offsetting reductions in load related to fencing.
• Potential modelled future erosion mitigations produced significant reductions in total erosion, with decreases of up to 41% under the most ambitious scenario (M100). These reductions are predominantly expected in the West Coast, Upper Waikato, and Waipā CMZs, areas that currently make a substantial contribution (1.43 Mt yr^–1 or 62%) to the overall sediment load.
• Climate change is anticipated to exacerbate erosion and increase suspended sediment loads by late century under contemporary land cover and erosion mitigations. However, the implementation of additional erosion mitigation (M20, M50, M100) can substantially offset these increases, demonstrating the potential effectiveness of targeted erosion control measures in negating the worst impacts of climate change on sediment loads.
• To meet NPS-FM 2020 visual clarity limits, 56% of the 105 SOE sites across the Waikato region require sediment load reductions to achieve the NBL. This decreases to 27% of sites under the M100 erosion mitigation scenario for contemporary climate.
• Under climate change, the number of SOE sites requiring reductions to achieve the NBL increases, and most SOE sites require reductions to at least maintain baseline visual clarity with the implementation of the most ambitious erosion mitigations (M100) by late century.
• Continued investment in erosion mitigations is necessary to limit the potential impacts of climate change on suspended sediment loads by late century.
• Improvements in model predictions could be made with further acquisition of comprehensive data on the effectiveness of erosion control measures, particularly region-specific data. Additionally, use of region-wide LiDAR-derived terrain data would enable improved representation of the stream network and erosion processes at higher spatial resolutions.