An assessment of areal and temporal variations in streamflow quality using selected data from the National Stream Quality Accounting Network

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Abstract
Streamflow chemical-quality data and stream-temperature data at 88 stations throughout the United States and Puerto Rico were analyzed to develop and to evaluate methodologies for the general assessment of the variation of the Nation's streamflow-quality conditions in space and over time. The spatial variation is described by the use of sample statistics such as minima, maxima, and means of the variables at each station. The temporal variation is described by using nonparametric statistical testing procedures to assess time trends in the data at each station. Evaluation of the coefficients (mean, amplitude, phase coefficient] derived by harmonic analysis of available daily stream-temperature records indicates adequate depiction of yearly seasonal variability and provides information relative to spatial variation in stream temperatures at most sites. Lowest harmonic amplitudes, which are indicative of little seasonal variability in stream temperature, were observed in Florida, along the eastern Gulf coast, and along the Pacific coast. Greatest annual variations in stream temperature exists in the Souris, Red (of the North), and Missouri River basins. Temporal variations in stream temperature denoted by significant changes (at the 99 percent level of confidence) in annual harmonic coefficients were indicated at 15 of 80 stations used in the evaluation. Trends in stream temperature were found to exist at one or more stations in each of the six following areas: the Atlantic coast above Florida; Florida and the eastern Gulf; the Souris River, Red River of the North, and Missouri River basins; the Texas Gulf and Rio Grande area; the Colorado River basin; and the Pacific coast area. Significant trends (at the 99 percent level of confidence) were found in the long-term streamflow chemical quality record at 15 of 88 stations analyzed. Of these stations, 10 showed an increase in levels of specific conductances while 5- showed a decrease. Trends were indicated for water-quality data from stations located in the following areas: the Atlantic coast above Florida; the Souris River, Red River of the North, and Missouri River basins; the Arkansas and Red River basins; the Texas Gulf and Rio Grande area; the Colorado River basin; and the Pacific coast area.

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