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Watershed and Lake Response Modeling <br />In water quality studies, two model types are typically used; a watershed model to estimate <br />annuals loads of the pollutant of interest, and a lake response model to estimate the in -lake <br />concentration of the pollutant. <br />Watershed model: In order to estimate runoff generated phosphorus loads entering Langton <br />Lake, a watershed loading model was constructed (Appendix A, Table 4). The Pond Network <br />( PONDNET) model (Walker, 1987) utilizes information on the annual volume, and mean total <br />phosphorus concentrations of runoff for each type of watershed landuse. The resulting loads are <br />then routed through the system to generate estimates of incoming loads to the downstream <br />receiving body, or cell as in the case of Langton Lake. PONDNET is especially suitable to <br />evaluate the effectiveness of upstream detention basins in removing phosphorus based on area, <br />wet volume, and configuration of the pond and characteristics of the incoming runoff. Table 3 in <br />Appendix A shows the values that were used in the PONDNET model to represent the quality <br />and quantity of runoff from different landuses, both in the Langton and Bennet Lake watershed <br />models. <br />The annual export rates for this study lie well within the range of accepted literature values for <br />each landuse. An average annual precipitation of 28 inches /year, based on long -term <br />precipitation records from the National Weather Service was also used as an input to the model. <br />Based on the model, the lake receives an estimated average annual phosphorus load of 186 <br />pounds. The model output also provides the basis to estimate the benefit of the treatment pond <br />serving the industrial area south of the lake. Without the treatment pond, the estimated average <br />annual phosphorus load to Langton Lake would be about 250 lbs /yr. Thus watershed phosphorus <br />load to Langton Lake would be 64 lbs or almost 35% higher than it is now. Clearly, the pond <br />provides a very significant benefit to the lake. <br />Langton Lake enjoys relatively high water quality for a very shallow urban lake, reflected by the <br />prevalence of rooted vegetation and relatively good water clarity. This can be attributed to the <br />pre- treatment provided by the north cell and the industrial pond, and the relatively small <br />contributing watershed area. <br />City of Roseville 46 <br />Parks Natural Resource Management <br />