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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.

Plain Language Summary Watersheds store water underground in soils and weathered bedrock. How long it takes for water to flow through the subsurface to feed streams is difficult to measure but important to understand how watersheds function. We used a novel combination of isotopic tracer methods to study the mixture of water ages in Providence Creek, a stream in the southern Sierra Nevada, California (USA). We studied naturally occurring radioactive isotopes of hydrogen (tritium), sodium-22, and sulfur-35. The abundance of these isotopes decreases because of radioactive decay as water spends more time underground. Each of these isotopes has a distinct half-life (12.3years, 2.6years, and 87days, respectively). By using this combination of isotopes we were able to tease out the mixture of water ages in the stream. This level of detail helps us understand how the subsurface selects water from storage to generate stream flow. We find that Providence Creek has a strong preference to remove the oldest water in storage during dry conditions but shows only a slight preference to remove younger water during wet conditions. Based on the age mixtures of stream water, we estimate that the Providence Creek watershed stores 3m of water in the subsurface.