A Post-Audit Anaylsis of the NWRI Nine-Box Water Quality Model for Lake Erie

Abstract

Computed results from the Lake Erie nine-box water quality model developed at the National Water Research Institute (NWRI) are compared to observed data for three different periods: calibration (1978), verification (1967–1977), and post audit (1979–1982). It is shown that the median mean relative errors for these three stages of model development are essentially the same, demonstrating the model validity. In particular, the post-audit analysis supports the hypothesis that the sediment oxygen demand rate is related to the phosphorus loading. However, this relationship can only be made evident following the accurate simulation of the dynamic changes in the thermal layers and diffusion processes. Since such processes are directly affected by weather episodes, the anoxic occurrences in the central basin hypolimnion are subjected more to short-term meteorological uncertainties than to the long-term effects of the phosphorus removal program. Post-audit results on previously proposed phosphorus control curves, through the use of a completely independent data set, also confirms this phenomenon. It is concluded that a family of control curves or a set of probability indicators incorporating both loading and weather influences are more meaningful than a single control curve that is commonly proposed.