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Title:	Empirical methods for predicting eutrophication in impoundments. Report 3, phase II, Model refinements
Authors:	Environmental and Water Quality Operational Studies (U.S.) Walker, William W.
Keywords:	Eutrophication Reservoirs Reservoir operation Water quality Water quality measurement Environmental management Computer programs Mathematical models Numerical models
Publisher:	Environmental Laboratory (U.S.) Engineer Research and Development Center (U.S.)
Description:	Technical Report Abstract: 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 mean depth are shown to explain most of the variance in reservoir trophic state-indicators and hypolimnetic oxygen status. For a given chlorophyll-a concentration and morphometry, hypolimnetic oxygen depletion rates are found to average about 40 percent higher in reservoirs, as compared with northern lakes. The difference may be. attributed to effects of oxygen demands exerted by allochthonous organic materials, spatial gradients in chlorophyll, and/or reservoir outlet configuration. Areal depletion rates are shown to be an increasing function of mixed-layer chlorophyll-a concentration, but independent of hypolimnetic temperature and morphometry, for lakes and reservoirs with mean hypolimnetic depths between 2 and 30 m. Because of the general magnitude of the areal depletion rates; all of the thermally stratified reservoirs tested with mean depths less than 10 m reached anoxic conditions (< 2 mg/1 dissolved oxygen) at some point during the stratified season. Predicting the hypolimnetic oxygen status in these relatively shallow systems is limited more by ability to predict thermal stratification than by ability to predict depletion rates from trophic status and morphometry. Metalimnetic oxygen depletion is shown to be generally more important than hypolimnetic depletion in deeper reservoirs. A principal components analysis leads to a two-dimensional classification system for eutrophication-related water quality. The first two components explain 95 percent of the variance in pool nutrient, chlorophyll-a, organic nitrogen, and transparency levels. The first component is interpreted as a quantitative factor which reflects the total nutrient supply. The second is interpreted as a qualitative factor which reflects light-limited primary productivity and the partitioning of nutrients between organic and inorganic forms. Information on both dimensions provides a more complete description of reservoir water quality and its controlling factors than any single variable or index. Average concentrations inadequately characterize many reservoirs with pronounced spatial gradients in water quality. A computer simulation model is developed and tested for predicting longitudinal gradients in phosphorus and related trophic state indicators. The model accounts for the advection, dispersion, and sedimentation of phosphorus along a given tributary arm. Second-order sedimentation kinetics are shown to be more realistic than first-order kinetics for predicting within-reservoir spatial variations, as well as among-reservoir spatially averaged variations. Previous reports in this series include: "Report 1, Phase I: Data Base Development" and "Report 2, Phase II: Model Testing." NOTE: This file is large. Allow your browser several minutes to download the file.
Rights:	Approved for public release; distribution is unlimited.
URI:	http://hdl.handle.net/11681/4514
Appears in Collections:	Technical Report