Multistage hierarchical optimization for land use allocation to control nonpoint source water pollution

Nonpoint source (NPS) water pollution, the leading cause of water quality impairment in the U.S and Lake Erie, has been difficult to address because of technical and administrative problems. The nature and characteristics of NPS require site-specific land-use planning within multiple political jurisdictions, and the coordination of conservation management efforts among stakeholders within watershed boundaries. However, this watershed-based approach has been inefficient because there is no systematic way to guide decision makers in selecting appropriate management practices and their spatial locations. This task requires a comprehensive understanding of the physical and geographical characteristics of the watershed and an assessment of the cumulative impacts of the proposed management plans.; To effectively and systematically analyze complex spatial watershed systems, a hierarchical optimization approach is proposed. At the top level, a stormwater runoff simulation model is used to generate peak discharge pseudo data, that are input to a regression analysis, where the functional relationship between peak discharge and land-use variables is approximated as a quadratic function (R2=0.94%). This function is then used in an optimization model to allocate future land uses (urban, conservation, and agriculture) at the subwatershed level, in a way that minimizes the resulting peak discharge at the watershed outlet. The same process is repeated at the drainage area level, for each subwatershed. Finally, an integrated hydrological-land-use optimization (IHLUO) model is developed at the level of 30-meter cells for each drainage area. Each optimization level takes as exogenous inputs the land allocations generated by the previous optimization. This modeling methodology (1) investigates the NPS pollution generation/transport mechanisms and the spatial variability/interdependencies of land uses and watershed hydrology, (2) systematically evaluates different land-use patterns and their response to rainfall, and (3) searches for the optimal land-use pattern at the cell level. The results are very promising with a 46% reduction of the peak discharge rate, as compared to the rate corresponding to the current land-use pattern. The most downstream and upstream areas are to be protected with more conservation, and most urban activities are allocated to the minimal impact subwatershed. Sensitivity analyses are performed and suggest maintaining at least 30% of the land in a conservation state, developing no more than 12% for urban purposes, and no more than 70% for agricultural purposes. The results from the IHLUO model produce site-specific detailed land conservation guidelines. The proposed methodology is applied to the Old Woman Creek watershed, located in the southwestern basin of Lake Erie (Ohio).; The proposed research provides a new way to evaluate the locational impacts of alternative land-use patterns on NPS runoff, delineating site-specific land-use plans and identifying critical areas for preservation according to their suitability for conservation or susceptibility to pollution. This research is the first to demonstrate how to integrate a runoff simulation model into an optimization scheme, illustrating how NPS pollution levels can be reduced by changing the spatial configuration of land uses in a watershed.