TRENDS IN THE CHEMISTRY OF PRECIPITATION AND SURFACE-WATER IN A NATIONAL NETWORK OF SMALL WATERSHEDS

Trends in precipitation and surface water chemistry at a network of 15 small watersheds (<10 km(2)) in the USA were evaluated using a statis tical test for monotonic trends (the seasonal Kendall test) and a grap hical smoothing technique for the visual identification of trends. Com posite precipitation samples were collected weekly and surface water s amples were collected at least monthly. Concentrations were adjusted b efore trend analysis, by volume for precipitation samples and by flow for surface water samples. A relation between precipitation and surfac e water trends was not evident either for individual inorganic solutes or for solute combinations, such as ionic strength, at most sites. Th e only exception was chloride, for which there was a similar trend at 60% of the sites. The smoothing technique indicated that short-term pa tterns in precipitation chemistry were not reflected in surface waters . The magnitude of the short-term variations in surface water concentr ation was generally larger than the overall long-term trend, possibly because flow adjustment did not adequately correct for climatic variab ility. Detecting the relation between precipitation and surface water chemistry trends may be improved by using a more powerful sampling str ategy and by developing better methods of concentration adjustment to remove the effects of natural variation in surface waters.