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Lake Modelling

Models of lakes are used to provide insights into water quality at some future point in time, so that management actions may be targeted and cost-effective. In the past, small-scale physical models were used to simulate lake environments, but nowadays computer models are used to test potential management options. Computer models use a series of mathematical equations to describe the complex interactions amongst physical, chemical and biological processes that affect the water quality of a lake. The equations are stitched together consecutively in a computer program, allowing millions of calculations to take place in a single simulation. Use of models has become a standard practice to support community decision making for managing lake water quality. These models will increasingly be part of the National Policy Statement for Freshwater Management which will necessitate that freshwater management units are ’maintained or improved’.

Graphical Display of Reference and Current Trophic Level Index of New Zealand Lakes

This interactive graphical display represents modelled Trophic Lake Index values for baseline (pre-human disturbance) and present-day lake trophic conditions. This work was produced by Abell et al. (2018) as part of the Ministry of Business, Innovation and Employment funded Lakes Resilience Programme.

Mean annual Trophic Level Index (TLI) values predicted for lakes prior to human disturbance ('reference' conditions) were estimated and compared to predicted current TLI for lakes that could be modelled. The results, expressed as deviation from reference, are displayed below as an interactive graphical representation for 1,030 mapped lakes >1 ha.

National Water Quality Monitoring Network lake data were used to develop statistical models to predict average values of the four constituent TLI variables: total nitrogen (TN), total phosphorus (TP), chlorophyll a and Secchi depth. Separate models were developed to predict water quality corresponding to current and reference states. Multiple predictor variables were used. For estimating current water quality, these included estimated TN and TP concentrations for lake inflows that were calculated using output from the Catchment Land Use for Environmental Sustainability (CLUES) model. McDowell et al. (2013) was used to predict reference nutrient concentrations for lake inflows under reference conditions.

Model predictions of TLI variables were then used to estimate TLI for reference and current states (see interactive map below), with the difference between these values interpreted as a measure of change in trophic status from reference conditions. Current TLI was greater than reference TLI in 89% of lakes. On average, lake TLI was 0.67 units greater under current conditions than reference conditions (standard deviation = 0.67 units). Based on predictions, 32% of lakes were oligotrophic under current conditions, compared with 68% of lakes under a reference state. Under current conditions, 38% of lakes were mesotrophic, compared with 24% of lakes under a reference state. The predominant lake trophic status under a reference state was oligotrophic and the predominant trophic state under current conditions was mesotrophic. Under current conditions, 28% of lakes have a trophic status of eutrophic or greater (TLI > 4) compared with 5% of lakes under reference conditions.

Research lead: Troy Baisden

Click on the image below to view interactive map