Loading estimate methods to support integrated watershed-lake modelling: Duffins Creek, Lake Ontario

Integrated watershed-lake modelling requires high quality data for calibration and validation. The two-phase loading estimate approach presented here provides a more confident estimate of nutrient loads for these models. Phase 1 establishes the initial range of the loading estimates using averaging algorithms, ratio estimators, event mean concentration (EMC) and regression-based methods. For Duffins Creek outlet, the 2007, 2008 and 2009 ranges are 6.2–30, 22.3–78 and 19.5–242 tonnes of total phosphorus (TP), respectively. After combining the Beale ratio estimator and the regression-based methods in Phase 2, the 2007, 2008 and 2009 ranges are reduced to 13–17, 57–73 and 69–92 tonnes TP, respectively. The reduction represents the 0 and 28.07% upper bound bias of the regression-based method. Applying this information to the regression-based methods, daily and monthly ranges with a lower bound with no adjustment and with upper bound as 1.2807 times the regression-based TP load estimates are established. These loads are then used in integrated watershed-lake model calibration and validation to improve the model predictions.