Impact of Nonpoint Sources on Water Quality Spokane River Case Study: Long Term Modeling Zubayed Rakib1 and Michael Barber2 1. Graduate Student, Civil and Environmental Engineering, University of Utah, Salt Lake City 2. Professor and Department Chair, Civil and Environmental Engineering, University of Utah, Salt Lake City Introduction Spokane River Streamflow Trend Analysis Model Calibration Recent and projected urban, suburban, industrial, and commercial growth coupled with large agricultural demands have raised concerns about water availability and water quality in watersheds throughout the world. In the U.S., this is leading to more instream flow requirements to protect quantity and total maximum daily load (TMDL) rules to maintain quality. Solutions to these issues have not adequately addressed the implications of climate change and population growth which may significantly alter choices for long-term solutions. This study use CE-QUAL-W2 model applied to the Spokane River as a case study to simulate water quality changes in response to various climate and population growth scenarios, with particular attention to phosphorus, nitrogen, dissolved oxygen and river temperature. The Spokane River and its tributaries represent an ideal model watershed since previous studies have detected trends of decreasing monthly mean streamflow and annual 7-day low streamflows. TMDLs have been established for the Spokane River-Long Lake system for phosphorus to control excessive algae blooms and low dissolved oxygen considering low flow event. However, when nonpoint sources from stormwater runoff and nutrient cycling represent significant loading conditions, decisions based on low flow analysis may provide inadequate or incomplete information and thus invalidate mitigation decisions. Kendall’s tau trend analyses (=0.05) detected trends of decreasing monthly mean streamflow at Spokane River near Post Falls (August and September), Spokane River at Spokane (September), and Little Spokane River at Dartford (September and October); and decreasing annual 7-day low streamflows at Spokane River near Post Falls, and Spokane River at Spokane gages (Hortness and Covert, 2005). Model Results Long Term simulation WISDM The WISDM approach integrates a system of existing and widely-applied models into an interdisciplinary framework that allows to address the key issues surrounding the sustainability of water resources. The project aims to understand how climate and land use changes have affected water quantity and quality, and how will climate variability impact water quantity and quality in the next few decades. Spokane River near Post Falls, Idaho Spokane River at Spokane, Washington Reduction of Cells In order to reduce simulation time, we successfully reduced the vertical cells by nearly 50% while maintaining nearly identical results. Results below are for year 2001 at segment 188 (Long Lake). Applying point and nonpoint Load Reduction CE-QUAL-W2 Model, Study Area – Spokane River - Long Lake Conclusions CE-QUAL-W2 is a two dimensional, longitudinal/vertical, hydrodynamic and water quality model that is appropriate for water bodies where lateral variations in flow velocity, temperature, and water quality are insignificant. The study revealed that hydrologic conditions outside low flow period may be cause for future concern, and perhaps the impact of nonpoint source loading on nutrient cycling still needs better understanding. Contribution ratios of nutrient loads from nonpoint loading increase as streamflow increase, a fact that does not became apparent from low flow analysis. The wasteload and load allocations under existing TMDLs may not be inadequate in supporting water quality standards over long term. Model results suggest the need for significant reduction of nonpoint source loading in order to prevent low dissolved oxygen in the river-lake systems. Spokane River stretches 111 miles from Coeur d’Alene, Idaho to Franklin D. Roosevelt Lake Study area extends 39.2 miles from Stateline Bridge (RM 96.0) to Long Lake (RM 33.9). The river has both gaining reach and loosing reaches Tributaries: Hangman Creek, Little Spokane River, Coulee/Deep Creek Four NPDES Dischargers within study area Acknowledgements We would like to thank USDA National Institute of Food and Agriculture for providing us with the funding through its NSF/USDA Water Sustainability and Climate program. We would also like to thank John Covert and Michael Hepp (ECY), Tim Vore and Steve Esch (Avista Utilities), Stephen Burns and Lloyd Brewer (City of Spokane) for providing us with necessary data. Reference Portland State University, Spokane River-Long Lake CE-QUAL-W2 Model. Available from: http://www.ce.pdx.edu/w2/ Watershed Integrated Systems Dynamics Modeling (WISDM). Available from: http://www.cereo.wsu.edu/wisdm/ Hortness, J.E., and Covert, J.J., 2005, Streamflow trends in the Spokane River and tributaries, Spokane Valley/Rathdrum Prairie, Idaho and Washington: U.S. Geological Survey Scientific Investigations Report 2005-5005, 17 p.