A Digital Hydrologic Network Supporting NAWQA MRB SPARROW Modeling--MRB_E2RF1

A digital hydrologic network was developed to support SPAtially Referenced Regression on Watershed attributes (SPARROW)
models within selected regions of the United States. These regions correspond with the U.S. Geological Survey's National Water
Quality Assessment (NAWQA) Program Major River Basin (MRB) study units 2, 3, 4, 5, and 7 (Preston and others, 2009).
MRB2, covers the South Atlantic-Gulf and Tennessee River basins. MRB3, covers the Great Lakes, Ohio, Upper Mississippi,
and Souris-Red-Rainy River basins. MRB4, covers the Missouri River basins. MRB5, covers the Lower Mississippi,
Arkansas-White-Red, and Texas-Gulf River basins. MRB7, covers the Pacific Northwest River basins.

The digital hydrologic network described here represents surface-water pathways (MRB_E2RF1) and associated catchments
(MRB_E2RF1WS). It serves as the fundamental framework to spatially reference and summarize explanatory information
supporting nutrient SPARROW models (Brakebill and others, 2011; Wieczorek and LaMotte, 2011). The principal geospatial
dataset used to support this regional effort was based on an enhanced version of a 1:500,000 scale digital stream-reach
network (ERF1_2) (Nolan et al., 2002). Enhancements included associating over 3,500 water-quality monitoring sites to the
reach network, improving physical locations of stream reaches at or near monitoring locations, and generating drainage
catchments based on 100m elevation data.

A unique number (MRB_ID) identifies each reach as a single unit. This unique number is also shared by the catchment area
drained by the reach, thus spatially linking the hydrologically connected streams and the respective drainage area characteristics.
In addition, other relevant physical, environmental, and monitoring information can be associated to the common network and
accessed using the unique identification number.