Lake Waikare Water Quality Modeling: Using a New Model to Investigate Flushing Strategies

Numerical modeling was used to investigate the potential impacts of a proposed river water flushing scheme on Lake Waikare water quality. The proposed scheme would divert large volumes of Waikato River water into Lake Waikare to provide flushing for lake nutrients and phytoplankton. To investigate the effectiveness of such a scheme, a lake water quality model was constructed and calibrated using the best available data and information. To support this work, and to augment the existing water quality data set, lake sediment and tributary water sampling was undertaken. The model calibration exercise helped better define and understand existing lake water quality dynamics and highlighted the large impact of catchment agriculture on lake water quality. The calibrated model was then used to simulate three predictive scenarios representing proposed flushing options at three different flushing rates (5, 25, and 50 m3/s). Results indicate the potential for significant reductions in nutrient and phytoplankton concentrations in the lake, and improvements in overall lake trophic status, as a result of the proposed flushing strategies. The caveat to these results, however, is that the quantified mitigation represents only a displacement of pollutant load (to downstream waterways) rather than true mitigation of the problem source (catchment agricultural runoff).