Abstract

To gain insight into the hydrological functioning of high elevation catchments, we used a suite of radioactive and stable isotopes to trace water through the Southern Sierra Critical Zone Observatory, including water stable isotopes, d18O and d2H, tritium (t½ = 12.3 years), sodium-22 (t½= 2.6 years) sulfur-35 (t½=87 days). Water stable isotopes are typically used to track water through vegetation to determine its source. Combining stables isotopes with natural levels of tritium in vegetation and soil water, the age and source of water in vegetation and plant water strategies at the CZO are studied. Preliminary results show plants prefer recent rain or melt over water with longer storage times. The precipitation input signatures and variation was also measured, determining the effects of forest structure and storm source. Snowmelt samples collected from passive capillary samplers show a more evaporative signal in the isotopic signature of water underneath canopy than in open areas. Variations in tracer input precipitation for stable isotopes, tritium and sulfur-35 complicate the interpretation of stream water signatures, but also allow for specific event-based conclusions. In addition to monthly tracer sampling, high frequency sampling during a March storm revealed the catchment functioning under high flow conditions. In general, tritium concentrations correlate with stream flow. Tritium activities in stream flow correlate with sulfur-35, intercepting at approximately 7 pCi/L at zero sulfur-35. Compared to the 2-year mass weighted average tritium activity in precipitation of 12 pCi/L, the mean residence time of the old base flow component needs to be approximately 10 years. Our results point to distinctly different residence times for water leaving the catchment through evapotranspiration and stream flow.

Citation

Thaw, M.; Visser, A.; Deinhart, A.L.; Sharp, M.; Everhart, A.; Bibby, R.K.; Conklin, M.H. (2016): Tracking water through the Southern Sierra Critical Zone Observatory using radioactive and stable isotopes . Fall Meeting, American Geophysical Union, December 2016. Abstract H31K-07..