Revealing the origin, age and seasonality of streamflow, soil waters and transpiration

The forest water cycle is dominated by vegetation-mediated processes, such as interception, infiltration, and transpiration, that greatly impact the redistribution of waters between the atmosphere and subsurface. Based on a three-year time series of water stable isotopes in precipitation, soils of various depths, groundwater, streams and xylem from the WaldLab Forest Experimental Site in Zurich, Switzerland, we estimated seasonal signals and the fractions of more recent and older waters across the different compartments of the forest water cycle. These findings yield new understanding of water transport in forest ecosystems. Seasonal variation in streamflow isotopic signatures was small, indicating that annual streamflow was dominated by old waters draining from subsurface storages. Mobile and bulk soil waters all showed a distinct seasonal signature, with the seasonal amplitude decreasing with depth and mobile soil waters varying less than bulk soil waters. Young water fractions and new water fractions in forest soils decreased with increasing depth, indicating different degrees of subsurface mixing with waters from previous events and seasons. The fractions of recent precipitation in soil waters were generally smaller in summer than in winter, revealing the effects of interception and evaporation. Xylem water signatures in beech and spruce trees largely matched the bulk soil water signatures. The relative lack of soil water recharge in summer led to both species predominantly transpiring winter precipitation. Canopy interception did not substantially alter the isotopic signal of precipitation, but where it is more significant it could bias interpretations of transit times and seasonal precipitation partitioning.