The conventional method for waste load allocations (WLA) employs spatial-differentiation, considering individual point sources, and temporal-integration, using a constant flow, typically 7Q10 low flow. This paper presents a watershed-based seasonal management approach, in which non-point source as well as point sources are incorporated, seasonal design flows are used for water quality analysis, and WLA are performed in a watershed scale. The strategy for surface water quality modeling in the watershed-based approach is described. The concept of seasonal discharge management is discussed and suggested for the watershed-based approach. A case study using the method for the Des Moines River, Iowa, USA is conducted. Modeling considerations and procedure are presented. The significance of non-point source pollutant load and its impact on water quality of the river is evaluated by analyzing field data. A water quality model is selected and validated against field measurements. The model is applied to projections of future water quality situations under different watershed management and water quality control scenarios with respect to river flow and pollutant loading rate. (C) 1998 IAWQ Published by Elsevier Science Ltd. All rights reserved. [References: 15]
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