U.S. Geological Survey

Data Series 152

National Water-Quality Assessment Program

Water-Quality, Streamflow, and Ancillary Data for Nutrients in Streams and Rivers Across the Nation, 1992–2001

Ancillary Data

The summary data set also contains selected ancillary data for the watershed upstream from each site. Land-use data include a classification into one of six categories as well as the percentages of major land-cover classes. These percentages were determined from National Land Cover Data (NLCD) digital maps, as revised by Nakagaki and Wolock (2005). Land-use categories are based primarily on land cover but are modified in some cases on the basis of local conditions, such as the presence of significant point sources. In general, the land cover upstream from sites in the “Ag” category is more than 50 percent agricultural (cropland plus pasture) and less than 5 percent urban. Land cover upstream from sites in the “Urban” category usually is more than 25 percent urban and less than 25 percent agricultural. “Undeveloped” sites are downstream from land cover less than 25 percent agricultural and less than 5 percent urban. For sites in the “Mixed” category, there are substantial percentages of both agricultural (greater than 25%) and urban (greater than 5%) land cover upstream. Sites with moderate percentages of either agricultural (25 to 50%) or urban (5 to 25%) land cover are classified as partially developed (“Partial”).

In large basins, the effects of specific land uses can be moderated by complex mixture of land cover, multiple point sources, and temporally variable sources of streamflow. Thus, 105 sites were classified simply as “Large” on the basis of a combination of upstream area and long-term mean annual streamflow. The criteria for this classification were:

• Mean annual streamflow greater than 3,000 cubic feet per second (81 sites with a minimum area of about 1,500 square kilometers), or
• Basin area greater than 5,000 square kilometers and mean annual streamflow greater than 2,000 cubic feet per second (17 additional sites), or
• Basin area greater than 25,000 square kilometers and mean annual streamflow greater than 500 cubic feet per second (7 additional sites).

The selected sites represent basins that are large, in terms of area and streamflow, relative to other basins in the National Water-Quality Assessment Program (NAWQA). However, even the largest of these NAWQA sites have a much smaller area and streamflow than many sites that are sampled for other programs on major rivers of the Nation.

Land-cover data were incomplete or unavailable for a few sites. At six sites, more than 10 percent of the upstream area is in Canada or Mexico, which are not included in the NLCD coverage. Also, nine sites in Alaska and Hawaii have no NLCD coverage. Land-use categories for these sites were determined on the basis of local information provided by the study unit teams. In addition, the classifications for 45 other sites were modified on the basis of local information, such as the presence of point sources in the basin.

Nutrient inputs are based on county-level estimates from the report: County-level estimates and use rates of nutrient sources from fertilizer, manure, and atmospheric deposition in the conterminous United States, 1982–2001 (B.C. Ruddy, U.S. Geological Survey, written commun, 2005). These estimates were assigned to specified NLCD land uses within each county and then summed for the area within each basin boundary. Inputs include nitrogen and phosphorus in commercial fertilizers used on farms and in urban settings, nitrogen and phosphorus in livestock manure, and nitrogen in atmospheric deposition. The time periods used for determining annual nutrient inputs are: