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
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.
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.
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.
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.
CLUES is a modelling system for assessing the effects of land use change on water quality and socio-economic factors at a minimum scale of sub-catchments (~10 km2 and above).
Mataura Catchment Strategic Water Study Report - See Appendix G. Water Quality and Land Use Scenario Modelling (Note – This is the full study report – scroll down through the appendices in the pdf for the full report)