[en] Ecohydrological coupling at the watershed scale is poorly characterized. While soil-water storage is dynamic and strongly influenced by plants, few integrated tools exist for quantifying the spatial and temporal dynamics and interactions among the major components of the terrestrial hydrologic cycle. We analyzed stable isotopes of oxygen (δ¹⁸O) and hydrogen (δD) in soil water to quantify spatial and temporal changes in evaporation, soil water, tree water and stream discharge isotopic signatures in a gauged watershed in the Pacific Northwest, a region with very dry summers and wet winters. At the end of the wet season, soil-water storage was at its maximum, and plant water uptake occurred primarily from the surface soils. As the dry season progressed, plants relied oil deeper soil water, and evaporation decreased. Evaporation resulted in a distinct isotopic signature on tightly bound soil water. Our isotope data indicate that most water taken up by plants was affected by evaporation at some point, including soil water deeper than 30 cm. In contrast, stream water did not show any evidence of an evaporation signature even though discharge rates showed distinct diurnal cycles driven by transpiration. We conceptualize these findings as two separate pools of water held within the soil: one a faster moving pool held at relatively weak soil tension, making it more subject to gravitational transport to streams when more water is added to the system. The other pool is water held under higher soil tensions and has a longer residence time within the soil, and a higher propensity to be taken up by plants. (author)

[en] Stream water quality and quantity depend on discharge rates of water and nutrients from soils. However, soil-water storage is very dynamic and strongly influenced by plants. Stable isotopes of oxygen and hydrogen were analyzed to quantify spatial and temporal changes in evaporation, transpiration and stream discharge in a gauged watershed with dry summers and wet winters. The isotope data indicate that plant and soil water have been affected by evaporation. In contrast, stream water is not evaporated, though discharge rates show diurnal cycles driven by transpiration. It is concluded that two separate pools of water are held within the soil. One is a faster moving pool held at relatively weak matric potentials, making it more subject to gravitational transport to streams. The other pool is held more tightly by matric forces, has a longer residence time within the soil, and will more likely be evaporated or taken up by plants. (author)