NASQAN National Stream Quality Accounting Network
The major impetus for establishing the NASQAN program in 1974 was to develop a baseline water chemistry data set that was long-term and systematically collected throughout the nation. The original network consisted of over 500 stations sampled at monthly intervals. Measured constituents included nutrients, major ions, and suspended sediment. Over time, the program was constrained by budget cuts and corresponding reductions were made in sampling, both in terms of station numbers and sampling frequency. By 1994, the program was limited to quarterly sampling at roughly 275 stations.
Data was collected at uniform time intervals, without concern for the hydrologic patterns of high or low flow, and therefore provide a fairly representative description of conditions on any given day. These data are appropriate for trend detection and can be used for load estimation ONLY if a sufficient number of years are considered together so as to cover a broad range of discharges.
Additional water-quality monitoring was conducted by the Hydrologic Benchmark Network (HBN), which was established in 1963. This program focused on relatively small and minimally disturbed watersheds. It provides data that is used to evaluate trends in water-quality over time and serves as a control for distinguishing natural variability in small streams from effects induced by human activity.