Isotopic (?18O) characteristics of weekly precipitation collected across the USA: an initial analysis with application to water source studies

A portion of the precipitation samples collected and stored by the National Atmospheric Deposition Program (NADP) are shown to be useful for analysis of isotopes in precipitation. The potential problems with evaporation are small based on deuterium excess analyses and comparisons with the Global Meteroic Water Line. Presented here are the δ18O values of precipitation collected from nine NADP sites during 1989, 1990 and 1991. The trends in the isotopic (δ18O) characteristics of recent precipitation are in agreement with findings from previous International Atomic Energy Agency (IAEA) sites in the USA. The findings are also in agreement with several major isotope–environment relationships, further supporting the use of these samples for a modern global data base on the isotopes in precipitation being developed by IAEA, called GNIP (Global Network for Isotopes in Precipitation) and for use by research groups in the hydrological modelling, palaeoclimate and ecological communities. As expected, the average δ18O values of precipitation that is derived from the Gulf of Mexico (−3‰) and from the Pacific North-west are isotopically distinct (−7‰). In addition, using the NADP network, isotopic depletion in the δ18O values of precipitation in the range of 8‰ was observed from coastal to inland locations either in the Pacific North-west or along the east side of the Rocky Mountains, from Texas to Eastern Montana. In central USA, especially at high elevation, there is a strong seasonal variation in the δ18O values of precipitation, differing by almost 25‰ between January and August, whereas at coastal locations the seasonal variation in the δ18O values of precipitation was minimal. Comparisons between the average δ18O values of precipitation from several NADP sites and those of the IAEA collected in the 1960s and 1970s are exceptionally comparable, differing in some cases by only 0\1‰, further substantiating the quality and utility of precipitation from this network. The δ18O–temperature relationships for two of our sites had slopes of 0\55 (Colorado) and 0\46 (Nebraska) that are within the range of those reported earlier (0\29–0\7), although they are slightly lower, possibly owing to the large temperature variation between winter and summer in central USA. Further, the seasonal variation in the δ18O values of precipitation are used in a pair of ecological studies to articulate water sources of different plant life forms in riparian and prairie ecosystems, with grasses typically using summer rain to a much greater extent than woody species, such as willow or saltbush.