Abstract
One of the principal questions in hydrology is how and when water leaves the critical zone storage as either stream flow or evapotranspiration. We investigated subsurface water storage and storage selection of the Southern Sierra Critical Zone Observatory (California, USA) within the age-ranked storage selection framework, constrained by a novel combination of cosmogenic radioactive and stable isotopes: tritium, sodium-22, sulfur-35, and oxygen-18. We found a significant positive correlation between tritium and stream flow rate and between sulfur-35 and stream flow rate, indicating that the age distribution of stream flow varies with stream flow rate. Storage selection functions that vary with stream flow rate are better able to reproduce tritium concentrations in stream flow than functions that are constant in time. For the Southern Sierra Critical Zone, there is a strong preference to discharge the oldest water in storage during dry conditions but only a weak preference for younger water during wet conditions. The preference of evapotranspiration for younger water, constrained by oxygen-18 in stream water, is essential to parameterize subsurface storage but needs to be confirmed by isotopic or other investigations of evapotranspiration. This is the first study to illustrate how a combination of cosmogenic radioactive isotopes reveals the hydrochronology and water storage dynamics of catchments, constrains the subsurface architecture of the critical zone, and provides insight into landscape evolution.
| Original language | English |
|---|---|
| Pages (from-to) | 1429-1450 |
| Number of pages | 22 |
| Journal | Water Resources Research |
| Volume | 55 |
| Issue number | 2 |
| Early online date | 29 Jan 2019 |
| DOIs | |
| Publication status | Published - Feb 2019 |
Funding
We thank Mike Sharp for tritium sample analyses; the U.S. Forest Service Pacific Southwest Research Station, in particular Carolyn Hunsaker, for water balance data; Erik Oerter for insightful discussions on evapotranspiration; and Christie Egnatuk and Keenan Thomas for low\u2010level gamma counting expertise. All new measurements necessary to evaluate, replicate, and build upon this research is presented in tables or in the supporting information. Precipitation, snow depth, stream flow, and evapotranspiration data can be obtained from the Southern Sierra Critical Zone Observatory website (http://criticalzone.org/sierra/data/ datasets/). Prepared by LLNL under Contract DE\u2010AC52\u201007NA27344, with funding from Laboratory Directed Research and Development project 15\u2010ERD\u2010042 (LLNL\u2010JRNL\u2010739758). The authors declare no conflict of interest.
| Funders | Funder number |
|---|---|
| U.S. Forest Service Pacific Southwest Research Station | DE‐AC52‐07NA27344 |
| Laboratory Directed Research and Development | LLNL‐JRNL‐739758, 15‐ERD‐042 |
Keywords
- Hydrochronology
- Oxygen-18
- Sodium-22
- Storage Selection Functions
- Sulfur-35
- Tritium
