Stable isotope compositions of waters in the Great Basin, United States - 2. Modern precipitation

[1] Precipitation was collected between 1991 and 1997 at 41 locations within and adjacent to parts of the Great Basin lying in California, Oregon, Nevada, and Utah. These samples were analyzed for their deuterium (deltaD) and oxygen-18 (delta(18)O) contents. Separate collections were made of summer and winter season precipitation at stations ranging in elevation from -65 m to 3246 m. The deltaD per mil values of stations that were closely spaced but at different elevations showed an average deltaD decrease of approximately 10parts per thousand/km rise in elevation. Data for all samples representing winter precipitation, when plotted on a deltaD versus delta(18)O plot, fall close to the Meteoric Water Line (deltaD=8 delta(18)O + 10); samples representing summer precipitation define a line of slightly lower slope due to evaporation of the raindrops during their passage from cloud to ground. Comparison of our 1991-1997 deltaD data with those from the same three stations reported by an earlier study in the southeastern California shows seasonal differences ranging from 0 per mil to 19parts per thousand (average: 15) and annual differences ranging from 0 to 13 per mil (average: 2), illustrating the degree of annual and seasonal variability in this region. When contoured, the deltaD values display gradients indicating a north to northwest decrease in deuterium, with values ranging from -60 to -125parts per thousand in winter precipitation and from -40 to -110parts per thousand in summer precipitation. These gradient trends can be explained by the predominance of air mass trajectories originating in the tropical Pacific, the Gulf of California, and (in summer) the Gulf of Mexico.