STABLE ISOTOPE COMPOSITION OF WATERS IN SOUTHEASTERN CALIFORNIA .1. MODERN PRECIPITATION

Over a 7-year period from April 1982 to April 1989, integrated samples of rain and snow were collected at 32 sites by oil-sealed storage gage stations in (and adjoining) the southeast California desert; station elevations ranged from -65 m to 2280 m, and the collection network covered an area measuring about 400 km in each dimension. Deuterium (delta-D) analysis of 406 samples shows that the average delta-D of summer precipitation was -56 per mil (parts per thousand), whereas winter values averaged -78 parts per thousand; averaged annual values were close to -69 parts per thousand because most of the area is in a winter-dominated precipitation regime. We found no correlation between wetness or dryness of a season and the delta-D of its precipitation. The delta-O-18 versus delta-D plots show that rain samples define a tine of slope 6.5, less than the 8 of the Meteoric Water Line, whereas snow samples define a tine of slope 9.2. These differences in slope are the result of isotopic fractionation which occurred during evaporation of raindrops but not during sublimation of snow. Trajectory plots of 68 of the major storm events show that all of the winter storms originated in the Pacific, and passed over high mountains before reaching our collection stations. However, 21 of the 30 summer storms had trajectories that originated either over the Gulf of Mexico or the subtropical Pacific and traveled either west or north to reach our stations, without traversing high mountains. The difference in delta-D between winter and summer precipitation is due to different air flow patterns during those seasons.