Nutrient flux modelling in the Upper Waikato Catchment, New Zealand

Wairakei Pastoral Limited operate a significant pastoral and forestry holding within the Upper Waikato River Catchment. Conversion from forestry to dairy enterprises results in a significant change to land use and subsequent nutrient mobilisation, threatening the freshwater river systems and shallow groundwater quality.

In addition to modelling to estimate the impacts on freshwater receptors from the proposed expansion of the dairy industry, we are undertaking farm-scale nutrient flux modelling using the Agricultural Production Systems Simulator (APSIM) to provide daily estimates of nitrogen and phosphorous loss to shallow groundwater. This is then coupled to deeper groundwater flow and subsequent discharge to surface water creeks and rivers.

We are also using groundwater geochemistry to characterise and understand the chemical changes that take place and the attenuation of nutrients through the groundwater systems.

The results will be used to both justify future expansion licensing and provide mitigation strategies if excess nutrient leakage is expected.

This work is being carried out in partnership with Jacobs Ltd and WWA P/L.