SedNetNZ, SLUI and contaminant generation. Part 2: Nitrogen, phosphorus and E. coli

Horizons Regional Council (HRC) requested Manaaki Whenua – Landcare Research (MWLR) to update SedNetNZ for the HRC region and use the results to assess the impact of the Sustainable Land Use Initiative (SLUI) on phosphorus (P), nitrogen (N) and E. coli loads in the region’s rivers.

Conclusions

• The percentage reduction in total P load to date due to SLUI is proportional to the number and area of farm plans implemented per WMZ, with a reduction of 6% across all WMZs. Most WMZs are predicted to have total phosphorus load reductions of <10%, with the greatest load reductions in the East Coast, Kai Iwi, and Lower Rangitikei WMZs. However, by 2043, under both scenarios (0 and 3), many WMZS are predicted to have reductions in P load of 30 to >50%.
• Fencing under SLUI has made very little difference to N loading to streams (<0.1%) and even with complete stock exclusion on SLUI farms there would only be a reduction of <1%. Afforestation and land retirement have achieved larger reductions
  to N load (–3.7%).
• The average reduction in median E. coli values as a result of SLUI fencing to date in WMSZs with an associated E. coli measurement sites is 2.6%. However, if all streams were fenced then median E. coli values cold be reduced by 34%.