Predicting Background and Risk-based Sedimentation for Forest Watershed TMDLs

Abstract

Abstract. The Water Erosion Prediction Project (WEPP) model is a physically-based soil erosion tool INTRODUCTIONIn many forested watersheds, managers carry out fuel reduction activities, such as thinning and prescribed fire, to reduce the likelihood or severity of a wildfire. These management activities may cause increased hillslope erosion, and increased sediment yields from watersheds the year following the treatment. Watershed managers need to estimate erosion as part of their planning activities. In some cases, this estimate may need to be a Total Maximum Daily Load (TMDL) of sediment associated with a treatment.Water quality regulations frequently base the quality of our surface water on the TMDL of a pollutant. This practice has evolved from early regulation of industrial polluters who had the ability to increase or decrease the flows of polluted effluents into our nation's surface water. This approach to regulation was well suited to industrial enterprises, and allowed them to set goals to balance the cost of treatment versus the quantity of effluent that they could discharge.Although not ideal, the TMDL approach was subsequently applied to agriculture. Pollution from sediments from agriculture are not generated from a single point, but are dispersed across the landscape and generally described as "nonpoint source pollution." The TMDL approach assumed that a manager would be able to develop farming practices that would limit daily sediment generation. Typically, the Universal Soil Loss Equation (USLE) aided in developing management plans, even though it is an average annual technology. The TMDL approach stretched the validity of the USLE technology. This extrapolation was acceptable because of agency and public familiarity with the USLE. Another assumption dealt with management practices and assumed the same level of disturbances every season, or in some cases, in every rotation sequence.A more recent erosion prediction technology is the Water Erosion Prediction Project (WEPP) model (Laflen et al., 1997). WEPP was developed to replace the USLE. WEPP estimates average annual erosion after simulating a number of years with each year containing a number of storms. The daily weather file that drives WEPP is generated stochastically, and has a similar statistical distribution to that observed on nearby weather stations. In recent years, there have been numerous suggestions to apply the WEPP technology to predicting sediment TMDLs (Conroy et al., 2003;Elliot, 2002).The "average annual erosion" approach does not lend itself to management of wildlands, such as forests and rangelands. In these ecosystems, erosion is relatively low to non-existent when the site is not disturbed. Wildlands are disturbance-driven ecosystems, and occasionally, a wildfire will occur during a dry year, or landslides during a wet season. In nature, these disturbances dominate the erosion processes. Years with large runoff events dominate sediment delivery at a watershed scale. Human disturbances can also increase runoff and erosion, incl...