Sparrow Modeling to Understand Water-Quality Conditions in Major Regions of the United States: A Featured Collection Introduction1

Management of the quality of the Nation’s water requires large quantities of information describing current conditions and related environmental factors. Such information is expensive to collect and difficult to interpret over large scales. To support the efficient use and interpretation of available water-resource information, the U.S. Geological Survey (USGS) developed a spatial water-quality modeling framework known as SPAtially Referenced Regressions On Watershed attributes or SPARROW (Smith et al., 1997). SPARROW is a hybrid empirical ⁄process-based mass-balance model that can be used to estimate the major sources and environmental factors that affect the long-term supply, transport, and fate of contaminants in streams. The spatially explicit model structure is defined by a river reach network coupled with contributing catchments. The model is calibrated by statistically relating watershed sources and transport-related properties to monitoring-based waterquality load estimates. The model results can inform scientific understanding and management by providing a tool for identifying sources of constituents that affect water quality over a wide range of spatial scales, providing estimates of mass contributions from sources to streams and downstream receiving waters. Further details on SPARROW are available at: http://water.usgs.gov/nawqa/sparrow/. SPARROW models have been previously developed in the United States (U.S.) over spatial extents ranging from the conterminous U.S. (Smith et al., 1997; Alexander et al., 2000, 2008) to large regions such as the Chesapeake Bay watershed (Preston and Brakebill, 1999) and smaller watersheds such as those draining to the North Carolina coast (McMahon et al., 2003). SPARROW models have been applied in many ways to improve the understanding of waterquality conditions and controlling factors, including: (1) identifying major sources of nutrients in streams of the conterminous U.S. (Smith et al., 1997; Alexander et al., 2008) and in individual watersheds in support of Total Maximum Daily Load (TMDL) assessments (McMahon et al., 2003; Moore et al., 2004), (2) understanding the role of stream processing in the delivery of nutrients to coastal waters, such as the Gulf of Mexico (Alexander et al., 2000, 2008), (3) identifying the sources of salinity affecting water supply in the southwest (Anning et al., 2007), and (4) understanding the environmental factors affecting sediment loading to the Chesapeake Bay