Assessment of Sediment Yields for a Mixed-landuse Great Lakes Watershed: Lessons from Field Measurements and Modeling

The Soil Water Assessment Tool (SWAT) was implemented to determine annual sediment yields and critical source areas of erosion for the Buffalo River Watershed. Model calibrations were performed by comparing simulated streamflow discharge and sediment concentrations against measured values. Monte-Carlo simulations were performed to identify the most sensitive parameters and the “best-fit” parameter ranges. This study especially highlighted the importance of snow parameters, which, previously had not been identified as sensitive for model simulations. The cover (C) and practice (P) values for croplands had to be reduced considerably from default model values to constrain simulated sediment yields within the observed data range. The model did not simulate an ice-scour event which generated a substantial amount of sediment. The average annual sediment yield simulated by SWAT for the Buffalo River watershed (108,593 ha) amounted to 0.8 tons/ha/yr. The Cazenovia Creek subwatershed contributed the largest portion (45%) of the total sediment yield from the Buffalo River watershed. We attribute the higher sediment yields from Cazenovia Creek to the greater proportion of steep slopes in this subwatershed. The accuracy and reliability of SWAT sediment predictions at the small watershed (second order or less) and storm-event scales will depend on the accuracy of input information, especially the resolution of the landuse-landcover (LULC) layer, the number of rainfall stations used in simulations, and the number of internal sites against which the model has been calibrated. INDEX WORDS: Erosion, sediment yield, GIS, watershed management, watershed models. *Corresponding author. E-mail: [email protected] Current Address: Bioresources Engineering, University of Delaware, 260 Townsend Hall, Newark, DE 19716. INTRODUCTION In 1998, the New York State Department of Environmental Conservation (NYSDEC) placed Buffalo River on the state’s 303(d) list and designated it as a priority for TMDL development (NYSDEC 2004). The Buffalo River has also been identified as one of the 43 Areas of Concern (AOC) in the Great Lakes region by the International Joint Commission (IJC) (IJC 2003). Although “organics in contaminated stream sediments” have been identified as the primary pollutants on the DEC 303(d) list, sediment pollution in the river and its tributaries is a continuous and increasing concern. Recent surveys suggest that various sources of sediment are continuously being introduced due to suburban expansion and development occurring in the headwaters of the watershed (K. Irvine, Department of Geography, Buffalo State College, Buffalo, NY, unpublished data, 2001). These new sources of sediment could complicate the remediation/mitigation of the existing contaminated stream sediments in the downstream reaches. Furthermore, resource agencies such as the USDA Natural Resource Conservation Service (NRCS) and the Erie County Soil and Water Conservation District (SWCD) are very interested in identifying the sediment source areas in the watershed so that appropriate best management practices (BMPs) can be implemented to check sediment contributions at the sources. The BASINS (Better Assessment Science Integrating Point & Nonpoint Sources) suite of models is one of the tools that is being actively promoted by the Environmental Protection Agency (EPA) for developing TMDL plans for the 303(d) list of priority waters (EPA 2001). The Soil Water Assessment Tool (SWAT) model which is provided as a part of the BASINS tools has especially been popular for 472 Inamdar and Naumov developing watershed-scale budgets of water and chemicals and to identify source areas of pollution (Arnold and Fohrer 2005). SWAT has been extensively tested for watersheds in the southeastern U.S. (Santhi et al. 2001, Saleh et al. 2000) and has also been extended to a few watersheds in the northeast and the Midwest (FitzHugh and MacKay 2001, Kirsch et al. 2002, Peterson and Hamlett 1998). However, we are not aware of any SWAT applications for watersheds in the Great Lakes region, especially for watersheds that are affected by lake-effect precipitation (especially snowfall). Unique conditions in lake-effect regions include: considerable snow accumulation over the winter, sediment generation from ice-scour, and elevated discharge and sediment export with spring snowmelts. Furthermore, and unlike many portions of the south, many of the Great Lakes watersheds are not as intensely cultivated and have been experiencing a net loss of agricultural land in some regions due to loss of population. Our interest in this research was to assess the use of SWAT2000 (Arnold and Fohrer 2005; SWAT2000 hereafter referred to as SWAT) for determining sediment yields and for identifying areas of erosion in one of the Great Lakes watersheds— the Buffalo River watershed. The Buffalo River watershed is typical of many of the Great Lakes watersheds in that: (a) it is influenced by lake-effect precipitation with heavy snowfall in the winter; (b) many of the small tributaries ice-up in the winter with ice-scour induced bank erosion at some locations; (c) the watershed is at its wettest during early spring when spring snowmelt along with storm events produce the highest streamflows; and (d) it has urban areas near the watershed outlet and rural and/or agricultural landscapes in the upper headwater reaches. By evaluating SWAT for the Buffalo River watershed we proposed to develop a methodology which would provide guidance for application of SWAT across other Great Lakes watersheds. Key questions that were addressed were: • What are the most sensitive and key parameters that need to be adjusted? • Can SWAT simulate hydrologic and sediment yield patterns observed in a typical mixed-landuse Great Lakes watershed such as the Buffalo River? • What is the impact of resolution and accuracy of the landuse-landcover (LULC) GIS layer on model predicted sediment concentrations and yields? • What are the source areas of sediment in the Buffalo River watershed and what factors dictate their spatial distribution in the watershed?