Data Compilation

Data were retrieved from the USGS NWIS database for water years 1973–2005 and the USEPA STORET database for water years 1975–2004. A water year is defined as the 12-month period from October through September and is designated by the year in which the period ends. Ancillary information on regional landscape variables and agricultural variables was compiled for each of the NWIS sites. The data used for analysis are available in Staub and others (2009).

NWIS Dataset

The 44 NWIS sites (fig. 1) that were analyzed for trends are listed in table 1. The water-quality data contained in NWIS include data from multiple projects, analyzed at several USGS laboratories. Important sources of long-term water-quality data include the NASQAN (Hooper and others, 1997) and NAWQA (Hamilton and others, 2004) Programs.

The data retrieved include flow; physical properties including dissolved oxygen, pH, alkalinity, and specific conductance; nutrient concentrations including total nitrogen, nitrite plus nitrate, ammonia, total ammonia plus organic nitrogen, and total phosphorus; dissolved organic carbon; suspended-sediment concentrations; and major-ion concentrations including chloride, calcium, sulfate, sodium, and silica.

STORET Dataset

Nitrogen and phosphorus concentration data for the 1975–2004 and 1993–2004 periods were retrieved from STORET and subsequently analyzed for trends for 290 water-quality monitoring sites (fig. 1). A list of the STORET stations is presented in Staub and others (2009). The STORET data retrieval included total nitrogen, total organic plus ammonia nitrogen, total and dissolved ammonia, total organic nitrogen, and total and dissolved nitrite plus nitrate. Phosphorus data included total phosphorus, dissolved phosphorus, and suspended phosphorus.

Ancillary Variables Dataset

Ancillary information, including regional landscape and agricultural variables, were compiled for each drainage basin among the NWIS sites. The ancillary data include variables for atmospheric deposition of nitrogen, nitrogen and phosphorus from fertilizer and from manure, land cover, soil characteristics, surficial geology, hydrologic characteristics, and ecoregions. The data sources and methods used to compile basin ancillary data are reported in Ruddy and others (2006) and in Staub and others (2009). These data were compiled to investigate the possible association of annual variation in water quality with annual variation in basin ancillary characteristics.