Modelling of restoration scenarios for Lake Ngaroto

Abstract

Waipa District Council is investigating restoration options for Lake Ngaroto, a shallow lake that has become hypertrophic through high external (catchment) nutrient loading from its catchment, which is associated with land use that is predominantly dairy farming. Shallow lakes like Lake Ngaroto, which have high external loading, typically have high internal nutrient loading, which may also need to be addressed to gain immediate improvements in water quality. By contrast, reduction of external nutrients is more likely to be associated with gradual and long-lasting effects on water quality.

This report is a culmination of data collection and analysis, and model development and simulation of restoration scenarios for Lake Ngaroto. Data were gathered on flow, nutrient concentrations, temperature, dissolved oxygen, suspended sediments, meteorological variables and the lake morphology for the lake and its catchment. These data were used to produce a water balance and input files for DYRESM-CAEDYM, a one dimensional, coupled hydrodynamic-water quality model. Once calibrated, using the measured lake data (ammonium (NH4-N), nitrate (NO3-N), phosphate (PO4-P), total phosphorus (TP), total nitrogen (TN), chlorophyll a (Chl a), suspended solids (SS), temperature, dissolved oxygen (DO)) as a means of comparison for modelled data, a number of scenarios were carried out to simulate outcomes of lake restoration measures.