Young and new water fractions in soil and hillslope waters

Abstract. The transport processes and corresponding times scales of water's infiltration into, and percolation through, the shallow subsurface are poorly understood. Here we characterize the transport of recent precipitation through a forested hillslope, using a continuous three-year record of O and H stable isotopes in precipitation, streamflow and soil waters from various depths. We found that the fractions of recent precipitation decreased with depth, both in waters extracted using suction-cup lysimeters and in waters extracted from bulk soil samples using cryogenic distillation. Not surprisingly, fractions of recent precipitation found in soils and streamflow were much larger with wet antecedent conditions, showing that wet landscapes can transmit recent precipitation quicker than dry landscapes. Approximately 18 % of streamflow was younger than 2–3 months, 11 % was younger than three weeks and 7 % was younger than one week; these new water fractions were similar to those seen in 20 to 80 cm deep soils. Mobile soil waters below 2 m depth contained much less recent precipitation (1.2±0.4 % younger than two weeks) than streamflow did (12.3±2.1 %), indicating that they are not the dominant source of streamflow. Instead, streamflow must be generated from a mixture of deep subsurface waters, with very little isotopic seasonality and short-term variability, and shallow soil waters, with more pronounced isotopic seasonality and short-term variability. This study illustrates how flow, storage, and mixing processes linking precipitation to streamflow and evapotranspiration can be constrained by measuring isotopic variability across different hillslope positions, subsurface depths, and time scales.