THE DISTRIBUTION OF OXYGEN ISOTOPES IN THE WALKER RIVER-WALKER LAKE SURFACE WATER SYSTEM: CLUES FROM A HYDROLOGIC AND ISOTOPIC BALANCE MODEL

Over the past century, the Walker River-Walker Lake surface water system has experienced big changes in lake water d¹⁸O (> 4 per mil) and lake surface level (40 meters) due to reduced stream discharge of the Walker River, the only large water input into Walker Lake. Historical stream gauge readings, Walker Lake surface level measurements, and d¹⁸O of stream and lake water have been well documented by USGS. Also, a high-resolution (~~1 year per sample) d¹⁸O record has been produced from a box core collected from one of the deepest portions in Walker Lake. These data serve as a basic input dataset of a hydrologic and isotopic balance model.

The model consists of three components, a hydrologic mass balance model (HydroModel), an oxygen isotopic mass balance model (IsotopicModel), and a paleolake surface level recovery model (PaleoLakeModel). The model assumes a constant precipitation and evaporation rate and uses historical stream gauge readings as an input variable for the HydroModel. The annual mean evaporation of 140 cm has been estimated by fitting the modeled lake elevation curve with that observed. Then, two key model parameters, fraction of advected air (f_ad) and relative humidity (RH), are determined by applying d¹⁸O of stream and lake water and historical stream discharge readings as inputs for the IsotopicModel. Finally, using these model parameters, the model determined a predicted d¹⁸O record of bulk carbonate sediments and changes in lake surface level over the last 80 years. The results are very close to measured values. This approach validates the notion that the d¹⁸O of bulk carbonate sediments in Walker Lake is linked with the lake hydrologic budget. This points directly to changes of stream discharge, a climate related index in prehistoric times, as the source of variability in downcore d¹⁸O of lake carbonate.

[http://www.albany.edu/~~fy7247/talk2002spring.pdf for details]