Abstract

Rain and snow were collected during major storms at 26 stations in California and Nevada during the exceptionally wet 1968–69 season. The deuterium/hydrogen ratios (δD, in per mil, VSMOW) of the samples varied according to the elevation of the sample site and its position relative to the Sierra Nevada. Eighteen storms were sampled at one or more stations. Eleven of them were sampled at almost all of 13 stations that lie east and southeast of the Sierra Nevada which allowed a storm-by-storm comparison of the influence of the topographic setting of the station and the synoptic meteorology of the storm on the δD characteristics of the precipitation.

A model is presented that illustrates possible changes in δD values as a storm moves inland. It is calculated using probable starting values for moist air masses arriving from the Pacific and follows the meteorological and isotopic changes that should occur during adiabatic uplift, cooling and condensation. The model agrees satisfactorily with data from areas where the storms were mostly caused by orographic or convective uplift.

The δD values of sequential samples collected at a station during the same storm differed by as much as 114‰; the mean values of δD in samples collected at the 13 stations during individual storms ranged from −89 to −126, with the more negative values generally coming during the middle of the winter season. Variation during storms and between storms is attributed to different altitudes and temperatures at which condensation occurred, and to the different trajectories of air masses (relative to the high mountains) prior to reaching the sample station. In some areas, the δD values also varied systematically according to the elevation of the station. Samples from the Mojave Desert and adjacent areas suggest that condensation occurred ∼2000 m above the station (snow core data from the west slope of the Sierra Nevada suggest that condensation occurred on the average at levels 1000–1500 m above those sampled areas). However, there is little relation between the deuterium content and the elevation of stations east of the Sierra Nevada, apparently because much of the precipitation condensed over the range and drifted eastward.

Pleistocene precipitation is inferred from this study to have had δD values as much as 50–85‰ more negative than normal values for present precipitation. The 1968–69 season appears isotopically intermediate between the present and full glacial climates of the Pleistocene.

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