Abstract
A multiobjective model is developed to optimally allocate the pollutant load emitted from nonpoint sources in a watershed considering surface and subsurface flows. ArcView GIS (Geographic Information System) software is used to include the effect of geographical variations in flow length and subsurface geology for each land management unit (LMU), which is a grid of regular identical size, of the watershed. The process of self-purification through surface and subsurface flows is differentiated based on the data of subsurface geology for each LMU. The objective function in the model maximizes total allowable discharged loads from LMUs and promotes equal allocation of them, subject to the constraints of effluent limitation at an outlet of sub-watershed, relation among mean effluents from different types of LMUs, relation between effluents through overland and subsurface flow in each LMU and minimum limit of effluent in each LMU. In order to demonstrate the model's applicability, the formulated model is applied to a sub-watershed of the Yasu river basin in Shiga prefecture, Japan, to determine the optimum allocation of discharged loads of total nitrogen from different kinds of LMUs present in the sub-watershed. This model could be helpful to arrive at policies related to sound management of water resources.
