Sierra Nevada isotope hydrology, tracking links between forest water sources, subsurface storage and major river runoff
- Author(s): Thaw, Melissa Nicole
- Advisor(s): Conklin, Martha H.
- et al.
The work presented here was motivated by the importance of the Sierra Nevada to California’s water resources and the need for understanding fluxes of water across expanded spatial and temporal scales through the critical zone, the area from bedrock to atmosphere, in which fluxes of energy and water support life on Earth. A new set of isotopic data is presented, characterizing each component of the hydrologic cycle (precipitation, evapotranspiration, runoff and storage), on two scales: headwater catchment scale and mountain range scale. These analyses are preceded by analyses of factors affecting precipitation signatures. From initial precipitation, links between headwater catchment evapotranspiration sources, subsurface storage and runoff, were tracked and, finally, the source-elevation of river water was determined across the entire Sierra Nevada using isotopic data from major rivers.
Precipitation input stable isotope signatures represent the starting point to track water through the critical zone’s atmosphere – vegetation – subsurface continuum and allow interpretation of streams, deeply stored saturated zone water, soil water and xylem water signatures. Elevation, season and canopy interception are factors affecting precipitation stable isotope signatures in the Sierra Nevada. Precipitation ?18O signatures decreased with elevation at a rate of -3.3 (± 1.8) ‰/km in the northern central Sierra Nevada and -2.8 (± 1.8) ‰/km in the southern central Sierra Nevada. By measuring a total of 31 isotopic lapse rates along two elevation transects, temporal and spatial variability was observed. Individual isotopic lapse rates were compared to mean amount weighted lapse rates to determine that individual lapse rates do not represent the aggregate input into the system, concluding that aggregate lapse rates should be measured. Similarly, the Local Meteoric Water Line (LMWL) in the Sierra Nevada changed seasonally, but lacked interannual variability. The LMWL for the entire Sierra Nevada had a slope of 7.2 (± 0.1) and an offset of 3.1 (± 1.2). Seasonality in the Central Sierra Nevada was modeled with a sinusoidal function, which varied with elevation. Snowmelt signatures beneath canopy and in open areas were measured every ten days throughout two snowmelt seasons, showing that canopy interaction significantly affected snowmelt stable isotope signatures.
By measuring both water stable isotopes and tritium (3H) in each component of the hydrologic cycle on the catchment scale, it was determined that although evapotranspiration and deeper subsurface saturated zone water originated as snowmelt, vegetation in all seasons used a younger component of water compared to deeper subsurface saturated zone water. Even when stable isotope signatures of xylem water and saturated zone water matched, 3H data showed that the age of xylem water and saturated zone water were distinct, demonstrating the utility of using multiple tracers to track water through vegetation. Likewise, water stable isotopes ?18O and ?2H showed that vegetation responded to new water inputs over time, while saturated zone water did not, further confirming that vegetation accessed younger water compared to saturated zone water. These findings contribute to current debate in recent literature regarding the concept of ecohydrological separation, in which it is argued that vegetation uses a separate source water compared to runoff. Additionally, by understanding how vegetation source water is connected to saturated zone water on this expanded temporal scale, the implication is that older water provides drought resilience to runoff from the saturated zone and new precipitation provides drought resilience to vegetation.
Previous studies have shown that water from upper elevations in the Sierra Nevada contributed a disproportionate amount of water to runoff, partially due to additional subsurface stores of water and lower evapotranspiration at upper elevations. This dissertation uses isotopic data to confirm these previous findings through new Sierra Nevada river ?18O and ?2H data applied to isotopic lapse rates. Even across geologically heterogeneous catchments, river source waters originated above their mean catchment elevation, with higher elevation source waters correlating to higher elevation catchments. Isotopic results agree with results and findings derived from a spatially distributed mass balance approach.