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. 6180 values of precipitation show a typical seasonal variation from around -17 parts per thousand in the winter to around -8 parts per thousand in the summer at the western site (Cle Elum), and slightly tower 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 tosses (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 approximate to 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. (c) 2006 Elsevier B.V. All rights reserved.