Simulation of Lake Water Quality Using a One-Dimensional Model with Watershed Input: Model Description and Application to Lake Riley and Lake Elmo

Abstract

This report Covers development of a model called MINLAKE98, which combines Riley's (1988) lake water quality model with a year-round model of temperature and dissolved oxygen (Fang and Stefan, 1994). Once validated this combined model will be used to simulate· the effect of runoff from two rural watersheds on the water quality of several ponds in the watershed. For many lakes in the US, point source inputs (such as municipal and industrial effluents) have been monitored, modified, diverted, and modeled. Today, non-point sources are the concern. Current problems include changes in land use, runoff quantity and quality, and population density. The results are seen e.g. in Lake Sammamish, Washington; In 1968 wastewater effluent was diverted out of the lake decreasing the annual mean total phosphorus concentration by the late 1970's. However, since the early 1980's the total phosphorus concentrati0l1 has begun to increase due to land use changes (Perkins, et al. 1997). An added dimension, is the effect of climate change has on runoff quality and the subsequent impact on lake water quality. Lake water, quality models which simulate year-round temperature and concentrations of phytoplankton, dissolved oxygen, and nutrients (phosphorus, nitrogen, and silica) can be used to study changes in trophic status of a lake. Examples include predicting changes resulting from global, warming or different ,landuse management, practices. For many lakes, noticable changes in the trophic state due to changes in management practices or changes due to global warming take more than a single openwater season to be observed. A model which includes simulation tlu'ough the winter icecover period can provide a prediction oftl~e following spring as opposed to an open-water model which requires re-initialization in the spring. Year-round simulation is also necessary for the prediction of long-term changes to a lake in response to changes in the watershed or due to climate change. Additionally, year-round models can predict anoxic periods during the winter which may be potential winter fish-kill events.