Vegetation water sources in California's Sierra Nevada (USA) are young and change over time,a multi-isotope (δ18O, δ2H, 3H) tracer approach. |
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| Authors: | Melissa Thaw Ate Visser Richard Bibby Amanda Deinhart Erik Oerter Martha Conklin |
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| Institution: | 1. School of Engineering, University of California, Merced, California, USA;2. Lawrence Livermore National Laboratory, Livermore, California, USA |
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| Abstract: | Sierra Nevada forests transpire a significant amount of California's water resources, sparking interest in applying forest management to improve California's water supply. Determining the source water of evapotranspiration enables forest managers to make informed decisions. To this end, a significant interest in critical zone science is to develop new methods to work across time scales to predict subsurface water storage and use. In this study, forest vegetation accessed young water and switched sources depending on availability, suggesting that forest drought vulnerability may depend on the range of water sources available (rain, snowmelt and deeply stored water). This finding also suggests that changes in transpiration rates may have immediate effects on water sources in close proximity to vegetation, and delayed effects on storage and runoff. New δ18O, δ2H and 3H data were used to track precipitation, runoff, evapotranspiration and storage through the critical zone seasonally, including seasons where evapotranspiration and snowmelt were in phase (winter snowmelt) and out of phase (seasonally dry summer). The main source of this headwater catchment's runoff is derived from its meadow saturated zone water, which was dominated by snowmelt. Water that originated as snowmelt contributed to transpiration, unless other sources, such as recent rain, became available. In cases where xylem δ18O and δ2H signatures matched those of deeper saturated zone water, 3H data showed that xylem water was distinctly younger than the deep saturated zone water. During 2016, which experienced relatively normal snowpack in winter and seasonally dry summer conditions, mean summer saturated zone water and vegetation water were similar in δ18O, ?12.4 ± 0.04 ‰ and ? 12.5 ± 0.3 ‰, respectively, but were distinctly different in 3H, 5.5 ± 0.2 pCi/L and 13.7 ± 1.1 pCi/L, respectively. While δ18O shows that vegetation and meadow saturated zone water have similar origins, 3H shows they have dissimilar ages. |
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| Keywords: | critical zone tritium (3H) vegetation water sources water age water stable isotopes δ18O (oxygen-18) δ2H (deuterium) |
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