Prediction of water quality in lakes and reservoirs: Part II - Model calibration, sensitivity analysis and application

A range was assigned to each of the parameters used in the ecological component of the DYRESM Water Quality model based on values found in the literature. The sensitivity of the model to changes in these parameters was determined by individually adjusting parameters to the maximum or minimum of their assigned ranges whilst keeping all other parameters at their assigned means. The effects of these changes were quantified through the mean, the vertical distribution and the temporal variation in chlorophyll a and dissolved oxygen concentrations over a 200 day period, using data for Prospect Reservoir. Parameters that influenced kinetics of phosphorus by phytoplankton, the minimum internal concentration, the half saturation constant and the maximum uptake rate, were amongst the most important determinants of all three measures of chlorophyll a. These parameters were also important determinants of the mean concentration and the vertical distribution of dissolved oxygen, through effects on photosynthetic oxygen production. Sediment oxygen demand had a significant effect on the mean concentration and vertical distribution of dissolved oxygen, phytoplankton density altered the vertical distribution of chlorophyll a, and rates of phytoplankton growth, respiration and mortality influenced the mean concentration of chlorophyll a. The model parameters were calibrated for the 200 day period and the model was validated over an additional 306 days. The mean errors between simulated and measured temperatures, and dissolved oxygen and chlorophyll a concentrations, as percentages of the measured values, were 5.2, 9.9 and 24.1% respectively. Simulated nutrient concentrations (PO4P, NO3N, NH4N, total phosphorus and total nitrogen) all reflected the general temporal and spatial trends observed in the measured data from Prospect Reservoir.