An oxygen isotope study of seasonal trends in soil water fluxes at two sites along a climate gradient in Washington state (USA)

Oxygen isotopes in precipitation and soil water were monitored at two sites along a climate gradient on the east side of the Cascade Mountains (Washington, USA). Precipitation at both sites is dominated by snowfall in the winter months. δ18O values of precipitation show a typical seasonal variation from around −17‰ in the winter to around −8‰ in the summer at the western site (Cle Elum), and slightly lower values at the eastern site (Ellensburg). Soil water isotope composition and moisture contents relate to hydrological season: (1) late summer to fall – cooling, increased soil moisture; (2) winter – snow cover; (3) late winter to early spring – snow melts, recharge of soils and groundwater; (4) late spring – initial drying of soils. A simple mass balance model that conserves isotopes and mass is used to constrain the rates of water loss due to evaporation and non-fractionating losses (transpiration and downward percolation) during these hydrological seasons. Non-fractionating losses at both sites are similar during the spring at approximately 0.8–1.0 mm/day. In contrast, during the fall, these non-fractionating losses are near zero at the drier site (Ellensburg) and remain at ≈0.8 mm/day at the wetter site (Cle Elum). The model yields evaporation rates between 0.01 and 0.2 mm/day for Ellensburg and between 0.2 and 0.9 mm/day for Cle Elum.