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An Environmental Trading Ratio for Water Quality Trading

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Transactions of the ASABE. 54(5): 1599-1614. (doi: 10.13031/2013.39838) @2011
Authors:   M. Lee, K. R. Douglas-Mankin
Keywords:   Hydrologic modeling, SWAT, Water quality, Watershed

Uncertainty is considered the major barrier that limits water quality trading (WQT) programs. One aspect of WQT uncertainty is caused by differences between point source (PS) and nonpoint source (NPS) timing and processes. A method that could separate NPS pollutant and environmental process uncertainties from other WQT program structure and market uncertainties would improve the design of a WQT program and provide a consistent procedure for other applications, such as achieving total maximum daily load (TMDL) targets, that require site-specific quantification of source loads and reductions. The objectives of this study were to develop and demonstrate a method to explicitly define an environmental trading ratio (eTR) that quantifies the environmental equivalence of PS to NPS effluent trades with statistical analysis of watershed modeling results. A pilot study modeled five selected scenarios with the SWAT watershed model and 36 years historical climate data in the Lower Kansas watershed in northeastern Kansas. Minimum tillage, surface fertilizer application, and no edge-of-field vegetative filter strips (VFS) produced greater total N loads, whereas no-till with surface fertilizer application and no edge-of-field VFS produced greater total P loads. The potential pollutant load reductions for a given shift in scenarios varied by subbasin. The eTR values of potential pollutant load reductions showed similar trends, indicating that the best alternative scenario might change from one subbasin to another. The eTR values in this study reflected significant total N and total P load reductions at a known confidence level and were substantially less than the programmatic trading ratios typically fixed at 2 to 1. These analyses show solid evidence that using a site-specific eTR system can address the uncertainty of environmental equivalence of source loads in a WQT program.

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