Use of the stable isotopes of water (dD, d 18 O) to determine vegetative water sources, runoff paths, and residence times generally assumes that, other than shallow evaporative enrichment, the isotopic composition of precipitation is conserved as it travels through the subsurface to the stream channel. Here we follow rainfall through a thick (up to 25m) vadose zone of soil, saprolite, and weathered bedrock mostly composed of argillite, and underlying a steep (328) forested hillslope. We discover a persistent structured heterogeneity of water isotopes inside the hillslope. Summer dry season causes evaporative enrichment of the soil, but not in the saprolite and weathered bedrock. In winter, the mobile water, generated by successive rainstorms with widely varying isotopic composition, mixes in the vadose zone, elevating soil and rock moisture content, and eventually recharging the groundwater with isotopically invariant water similar to the seasonally averaged rainfall. Yet throughout the winter the less mobile winter soil and rock moisture remains relatively light, and water extracted from the interior of argillite lies well to the left of the local meteoric water line. This persistently light composition of soil and rock moisture and the deviation from average meteoric values suggest that subsurface fractionation, or the inheritance of paleo-meteoric rock moisture associated with rock uplift may lead to large enduring isotopic differences between high and low mobility water. These differences suggest that the use of water isotopes as tracers must consider the possibility of subsurface isotopic evolution and the influence of exchange with more tightly held water.

In Mediterranean climates, the season of water availability (winter) is out of phase with the season of light availability and atmospheric moisture demand (summer). We investigate the seasonality of evergreen tree transpiration in a Mediterranean climate, using observations from a small (4000 m 2 ), forested, steep (32 ) hillslope, in the northern California Coast Range. We analyze 3 years of half-hourly measurements from 39 sap flow sensors in 26 trees, six depth profiles of soil moisture measured by TDR, and spatially distributed measurements of micrometeorology from five locations. The sap flow measurements show that two common evergreen tree species have different seasons of peak transpiration. Douglas-firs (Pseudotsuga menziesii) maintain significant transpiration through the winter rainy season and transpire maximally in the spring, followed by a sharp decline in transpiration in the summer dry season. Pacific madrones (Arbutus menziesii), and to a lesser extent other broadleaf evergreen species (Quercus wislizeni, Notholithocarpus densiflorus, Umbellularia californica), in contrast, transpire maximally in the summer dry season. The seasonal patterns are quantified using principal component analysis. Markov chain Monte Carlo estimation of response to environmental variables shows that the difference in transpiration seasonality arises from different sensitivities to atmospheric evaporative demand and root-zone moisture. The different sensitivities to atmospheric evaporative demand also create species differences in transpiration variability at synoptic time scales. Using the sap flow measurements and a regional forest inventory, a bottom-up regional transpiration estimate is constructed. The estimate suggests that sensitivity of Douglas-fir transpiration to water stress suppresses dry season evapotranspiration at the regional scale.

Obtaining the d‐excess parameter from oxygen and hydrogen stable isotope composition of meteoric waters has the potential power to reconstruct changes in atmospheric water pools (e,g. sources, origins and overall balance) and the climatic conditions that prevail during surface evaporation. Recently, plant and ecosystem scientists turned their attention using d‐excess information to inform questions at these scales. Here, we use the d‐excess parameter to evaluate the influence of forest canopies on atmospheric humidity within a mixed evergreen forest in coastal California. We found that during the day, when transpiration was at a maximum, the d‐excess of atmospheric water vapour exceeded model predictions for the background atmosphere into which the ecosystem evapotranspiration mixes. At night when transpiration was minor, the d‐excess of atmospheric water vapour was on average less than model predictions for an ocean derived water vapour source. The observed diurnal fluctuations around the d‐excess of the modelled background water vapour provided a strong evidence that during the day as the land surface warms and the boundary layer grows plants alter the isotope composition of atmospheric humidity via non‐steady state isotope effects. In contrast, at night equilibrium isotope effects dominate as the atmosphere stabilizes. These day and nighttime fluctuations around the d‐excess of ocean derived water vapour highlight the importance of plant transpiration for the isotope hydrology of near surface humidity and subsequently for the isotope composition of condensate like dew, an important water input to this ecosystem. Copyright © 2013 John Wiley & Sons, Ltd.