OXYGEN ISOTOPE STUDIES OF PRECIPITATION AND SOIL MOISTURE ALONG A CLIMATE GRADIENT IN WASHINGTON STATE

Comparison of the stable isotopic composition of precipitation and soil moisture can be used to constrain evaporation rates from soils and to describe mechanisms of soil water infiltration. In this study, the oxygen isotopic composition of precipitation and soil moisture was monitored at two sites along a climate gradient in central Washington State. The sites lie in the rain shadow of the Cascade Mountains and receive 56 cm (Cle Elum, western site) and 23 cm (Ellensburg, eastern site) of precipitation annually, falling predominantly as snow during the months of November to February. There are strong seasonal trends in d¹⁸O values of precipitation at the two sites with higher values (-5 to –10) in the summer months and lower values (around –20) in the winter. The weighted average d¹⁸O of precipitation decreases from –14.2 in Cle Elum to –15.6 in Ellensburg. At both sites, the deep soil water d¹⁸O is relatively constant throughout the year and approximately 2 per mil heavier than the weighted mean of the precipitation at the site. These d¹⁸O values appear to reflect a mixture of spring snowmelt and water that has resided in the shallow soil and become ¹⁸O-enriched. d¹⁸O values of shallow soil water are more variable throughout the year, generally lighter in Spring and early summer when they are recharged with spring snowmelt and heavier after July when the shallow soils become drier, losing much of their moisture to evaporation. In addition, there is a strong inverse correlation between moisture content and d¹⁸O in the shallow soils, particularly at the drier (Ellensburg) site. Comparisons with simple evaporation models suggest that approximately 20% of the precipitation entering the soil in Ellensburg returns to the atmosphere as evaporation compared to 10-15% in Cle Elum.