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TRADABLE CREDITS FOR STORM WATER VOLUME: AN ALTERNATIVE APPROACH FOR SUSTAINABLE URBAN WATERSHED MANAGEMENT
Citation:
Szlag*, D C., H Thurston**, P F. Harten*, B Lemberg**, AND H C. Goddard. TRADABLE CREDITS FOR STORM WATER VOLUME: AN ALTERNATIVE APPROACH FOR SUSTAINABLE URBAN WATERSHED MANAGEMENT. Presented at Watershed Management Workshop, Columbus, OH, 5/1/2001.
Description:
The increased storm water runoff rate and volume caused by urbanization, and their detrimental effects on stream habitat and morphology, is well documented. In most cases, current storm water management policies are focused on attenuating peak flow rates. While these policies may allegedly prevent erosion or downstream flooding, they do not address the total volume of storm water runoff. consequently, the frequency and duration of flows capable of moving substantial amounts of sediment increases and the quality of in-stream habitat often degrades. Reduction of storm wate runoff volume (SWRV) can be achieved through establishment of a number of distributed watershed-wide best management practices (BMPs). These perform the task of retaining water during a storm event for gradual discharge later or infiltration. To minimize the costs of establishing such a network of distributed BMPs we propose the use of a system of tradable storm water volume credits (TSVCs). Assuming that the efficiency of BMPs as well as their costs of establishment and maintenance vary across the watershed, and that marginal abatement costs are increasing, we show that TSVCs provide incentive to build and maintain BMPs within the watershed where they are most efficient, thereby lowering the cost of SWRV control. Determination of the cost of the TSVC system required estimation of the demand and supply of credits to establish their predicted equilibrium price. The pertinent cost is the marginal (or incremental) cost of abating a given volume of runoff. The watershed-wide supply of credits is based on factors affecting the stream channel stability. We will discuss several approaches, based on stream power and watershed morphology, for setting the supply of credits so that the in-stream habitat is protected. Using individual property level data for the Shepherd Creek subwatershed of the Mill Creek (specific attributes such as land-use, soil type and BMP cost information), we demontrate the TSVC methodology with an ArcView decision support tool (based on NRCS TR-55) that permits calculation of permissilble storm water volume on a property-by-property basis under stream flow and ecological constraints. Mathematical representation of the property owners' cost minimizing objective is implemented in Excel. Although the aim of TSVC is protection of in-stream habitat and base flow, and not explicitly improvement of water quality, distributed SWRV control offers potential benefits and opportunities for improving water quality. Synergies between the TSVC program and traditional watershed pollutant trading will also be briefly discussed.