Environment Southland contracted NIWA to simulate the effects of land use change and farm mitigation practices on water quality (loads and concentrations of total nitrogen (TN) and total phosphorous (TP) for the Mataura River catchment (5350 km2) using the CLUES 3.0 model.

Comparisons between the base-case and the future scenarios were carried out to determine the possible impacts of changes in land use and farm practices

Land use change is mostly from sheep and beef farming to dairying and cropping and is restricted to the Waimea and Mid Mataura, Upper Mataura and Waikaia sub-catchments.  The mitigation strategies include stock exclusion, nitrification inhibitors, herd shelters, improved farm dairy effluent management and constructed wetlands.  Whether or not a particular type of mitigation is applied to a location depends on the land use, the LUC and the soil drainage class.

Methodology

  • Monitored water quality was used for comparison with model predictions and was provided by Environment Southland monitoring and by NIWA as part of the National River Water Quality Network.   CLUES simulates long term water quality including annual average nutrient concentrations.  These results were compared to five year median concentrations from the monitoring sites.
  • Shape files of dominant land use classes were provided to NIWA by Aqualinc Ltd. for four land use scenarios with a spatial resolution comparable to CLUES.  The land use scenarios are: Consented (current) land use; 2030 conservative demand growth;  2030 accelerated demand growth and 50% 2030 conservative demand growth.
  • Soil drainage classes are required for two of the mitigation practices.  Soil data available for the Matura catchment was supplied by ES from their Tope-climate database.  However, this data did not cover the entire catchment.  For the areas where the Topo-climate soil data was not available, soil data was taken from the Land Resource Inventory: fundamental soil layer to create a catchment wide combined soil layer.
  • Land Use Capability (LUC), which is used for the farm mitigation scenarios, was taken from the LRI.
  • CLUES includes nutrient yields from point sources in its geo-database.  Point source information was provided by Environment Southland and updated for inclusion in this application.
  • The effect of implementing mitigations in a particular sub-catchment or selection of sub-catchments was simulated using CLUES 3.0 by specifying the percentage reduction in nutrient loss that would be expected given the land use, LUC and soil drainage class.  Two scenarios were developed based on the scenarios created by Monaghan et al. (2010) for the Oreti Catchment
  1. Current mitigation practice (stock exclusion from streams);
  2. Future mitigation comprising an amalgamation of practices including  stock exclusion, nitrification inhibitors, herd shelters, improved farm dairy effluent (FDE) management and constructed wetlands.
  • The nutrient reductions which represent the farm mitigation practices are the same as those developed by AgResearch for the Oreti River catchment (Monaghan et al., 2010), however, unlike the Oreti study, the farm practice scenarios assume that the mitigations are applied in combination.  

 
Outputs and findings

CLUES was run for the four land use and two farm practice scenarios giving eight sets of results for changes in TN and TP.  The results for these scenarios show the possible impact of the land use change on water quality and the extent to with mitigation can reduce those impacts.

Clues MAt


Recommendations

The overriding result for the future land use change scenarios is that while there can be substantial changes in predicted nutrient yields associated with land use change, which in turn affect localised water quality, the net impact on water quality is fairly minimal in the lower reaches.  The results predicted with assumed mitigation suggests that mitigation can be used to offset increases in nutrient loads associated with land use change.  Without mitigation, the catchment TN load could increase by around 1% and the TP load by 2 to 4% due to land use change.  With mitigation and no land use change reductions of 32% for TN and 7% for TP are predicted.  With both land use change and mitigation, the loads are decreased by around 31% for TN and 4-6% for TP.

Associated Models

Links

http://www.es.govt.nz/media/13839/mataura-water-study-appendices.pdf

(Note – this is the full report – scroll down through the appendices in the pdf for the full report)