Empirical Methods for Predicting Eutrophication in Impoundments. Report 3. Phase II. Model Refinements.

Empirical eutrophication models are useful tools for some aspects of reservoir water quality assessment and management. This report modifies existing model structures and parameter estimates to improve their generality and permit application under a wider spectrum of reservoir conditions. A network of models is assembled for predicting reservoir-average concentrations of total phosphorus, total nitrogen, chlorophyll-a, transparency, organic nitrogen, particulate phosphorus, and hypolimnetic oxygen depletion rate (near-dam) as functions of reservoir mean depth, hydraulic residence time, and inflow concentrations of total phosphorus, ortho-phosphorus, total nitrogen, and inorganic nitrogen. Models are tested against several independent lake and reservoir data sets compiled from the literature. An error analysis indicates that the prediction of chlorophyll-a, the most direct measure of eutrophication response, is limited more by variabilities in the biological responses to a given set of nutrient concentrations and other environmental conditions than by uncertainties in predicting pool nutrient levels from external loadings. Inflow available phosphorus concentration and mena depth are shown to explain most of the variance in reservoir tropic state indicators and hypolimnetic oxygen status. Keywords include: Eutrophication; Reservoirs; Reservoir operation; Water quality--measurement; Water quality management.