Young and new water fractions in soil and hillslope waters

Abstract

The transport processes and corresponding timescales 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 3-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. 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 3 weeks, and 7 % was younger than 1 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 2 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 timescales.