Effect of projected changes in winter streamflow on stream turbidity, Esopus Creek watershed in New York, USA Rajith Mukundan, 1 * Donald C. Pierson, 2 Lucien Wang, 3 Adao H. Matonse, 1 Nihar R. Samal, 1 Mark S. Zion 2 and Elliot M. Schneiderman 2 1 Institute for Sustainable Cities, City University of New York, New York, NY, 10065, USA 2 New York City Department of Environmental Protection, Kingston, NY, 12401, USA 3 Hazen and Sawyer, P.C., , New York, NY, 10018, USA Abstract: This study focuses on the impact of changes in winter streamflow on in-stream turbidity in the Esopus Creek watershed, one of the New York City water supply watersheds. Projected changes in daily precipitation and air temperature from a suite of five global climate models and three emission scenarios for future periods 2046–2065 and 2081–2100 were downscaled for the study region. The simulated climate scenarios were used to project future streamflows using the Generalized Watershed Loading Functions – Variable Source Area watershed model. Seasonal turbidity rating curves based on measured historical streamflow and stream turbidity were used in combination with the simulated streamflow for generating future stream turbidity scenarios. Results indicate an increase in future ambient stream turbidity from November to March and a decrease during April. These results are the effects of increased winter rainfall, reduced snowfall, and a shift to early timing of spring snowmelt runoff, causing an increase in streamflow during early winter. It also suggests a reduction in the traditional peak streamflow around April that is expected to occur in this region. As a result, our models simulate a consistent increase in the low to medium percentile range of turbidity values associated with low to medium range of streamflows and no apparent change in high-percentile turbidity values associated with high streamflows. Our results may be applicable in regions where snowmelt runoff is an important process and turbidity caused by the suspension of fine clay particles is a water quality concern. Copyright © 2013 John Wiley & Sons, Ltd. KEY WORDS climate change; snowmelt runoff; turbidity load; time series model; autocorrelation; rating curves Received 21 August 2012; Accepted 8 March 2013 INTRODUCTION High suspended sediment loads and the resulting turbidity can impact the sustained use of rivers for water supply and other designated uses. Changes in stream turbidity can be an indication of changes in material fluxes, aquatic geochemistry, water quality, channel morphology, and aquatic habitats (Walling, 2009). Snowmelt runoff is an important component of the hydrologic cycle in many places, and changes in the timing and extent of snowmelt can alter the quantity and quality of seasonal distribution of the streamflow captured for water supply (Marshall and Randhir, 2008) and also influence seasonal variations in landscape and channel erosion. McDonald and Lamoureux (2009) found non-linear increase in sediment yield with increasing snowpack in the Canadian High Arctic due to channel erosion caused by discharge from melting snowpack. Evrard et al. (2011), based on a study in south-eastern France, found a decrease in sediment yield in the presence of deep snow cover that protected the soil against erosion. Riverson et al. (2013) discuss the impact of projected climate change on the spatial distribution of snowpack development and its impact on water quality including sediment in the Sierra Nevada, USA. Understanding the processes and quantifying stream turbidity under present and future conditions will be valuable for watershed-scale management of stream turbidity and maintaining high water quality. In particular, the regional impacts of future climate variability and its seasonal effects on sediment transport need further attention. Regional variations in the direction and magni- tude of hydroclimatic signal especially snow processes have been observed (Stewart, 2009); however, very few studies have reported on the potential impact of climate change on sediment loads of streams and rivers (Intergovernmental Panel on Climate Change (IPCC), 2007). The New York City (NYC) water supply is currently the largest unfiltered water supply in the USA, operating under a renewable filtration avoidance determination granted by the New York State Department of Health and the US *Correspondence to: Rajith Mukundan, City University of New York, New York, NY 10065, USA. E-mail: Rajith.Mukundan@hunter.cuny.edu HYDROLOGICAL PROCESSES Hydrol. Process. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.9824 Copyright © 2013 John Wiley & Sons, Ltd.