This report details a third piece of work in a series prepared for Bay of Plenty Regional Council. It outlines recent refinements made to the ROTAN model and describes several scenarios of land use change and mitigation. Findings are intended to help managers develop policy by estimating the extent of export reduction required to meet the lake target of 435 tN/yr and how quickly the load to the lake is likely to respond to such reductions. The results will be used by the University of Waikato to predict likely changes in lake water quality.
Previous reports provided the following:
This report is focused on the required reductions in diffuse sources of nitrogen in the catchment, which are largely on agricultural land. This report does not discuss possible reductions in point source loads – these are included in each of the ROTAN models but do not vary between scenarios. This report provides the following:
The report does not set out to identify parcels of land where it would be best to effect change in land use in or to reach the target in the least time and/or at least cost – although ROTAN has the potential to be used for that purpose in the future.
Input data included:
In this report, new input data has been introduced to the original model and changes were made to the model to facilitate long model runs and the model was recalibrated. A new version of the model called ROTAN-1 was the result. Some of the original model problems remained. These included matching groundwater nitrogen loads owing to uncertainties in the extent of aquifer boundaries, groundwater age, land use patterns and nitrogen export rates. However, the model fit was deemed sufficiently good for scenario modeling. Several alternative versions of the ROTAN model were developed to test the sensitivity of predictions to uncertainties in key model coefficients and input data. These versions were named ROTAN-2 to ROTAN-9. While they were not calibrated as carefully as ROTAN-1 they provided valuable insights into model behaviour and reliability.
This report presents four scenarios of possible land use change (R-0, R-250, R-300 and R-350), aimed at achieving the target nitrogen load to the lake. The focus is on predicting the magnitude (load reduction) and timing (response time) of changes in lake load.
The four scenarios are:
ROTAN-1 provides a satisfactory match to key features of observed flows and concentrations
Prediction of Nitrogen loads to Lake Rotorua using the ROTAN model
Rutherford, K; Palliser, C; Wadwha, S.
Prepared by NIWA for Bay of Plenty Regional Council, 2011