15 Feb 2024
 | 15 Feb 2024

Young and new water fractions in soil and hillslope waters

Marius G. Floriancic, Scott T. Allen, and James W. Kirchner

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.

Marius G. Floriancic, Scott T. Allen, and James W. Kirchner

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-437', Anonymous Referee #1, 19 Mar 2024
  • RC2: 'Comment on egusphere-2024-437', Anonymous Referee #2, 25 Mar 2024
Marius G. Floriancic, Scott T. Allen, and James W. Kirchner
Marius G. Floriancic, Scott T. Allen, and James W. Kirchner


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Short summary
We use a 3-year timeseries of tracer data in streamflow and soils to illustrate how water moves through the subsurface to become streamflow. Less than 50% of soil water consists of rainfall from the last 3 weeks. Most annual streamflow is older than 3 months, waters in deep subsurface layers are even older, thus deep layers are not the only source of streamflow. After wet periods more rainfall was found in the subsurface and the stream, suggesting that water moves quicker through wet landscapes.