Critical Zone Observatory: Snowline processes in the southern Sierra Nevada
The rain-snow transition zone in California’s Sierra Nevada and other mountains of the Western U.S. is
particularly vulnerable to large and potentially rapid changes in climate and landcover. While this zone
has characteristically rapid seasonal changes, going from snowcover to wet soil to dry soil over a 1-2
month period, regional climate warming will make such changes occur earlier or eliminate them entirely
if snow is largely replaced by rain at the current rain-snow transition. This will lead to major waterrelated changes in seasonal-to-interannual critical-zone processes involving weathering, regolith
formation, nutrient cycling, and vegetation/ecosystem distribution and function. Forest density of the
largely mixed-conifer forest and the threat of catastrophic fire are very high, raising the chances of major
changes in longer-term critical-zone processes. Rapid seasonal changes and steep gradients in
precipitation make this zone an excellent natural laboratory for studying how critical-zone processes
respond to perturbations and how the water cycle drives critical-zone processes. In particular, differences
along locally pronounced altitudinal and aspect-related gradients in climate offer the opportunity to
substitute space for time in a rich cross-disciplinary array of process-oriented studies.
Intellectual merit. The Southern Sierra Critical Zone Observatory (CZO) is taking advantage of these
features, providing a community platform for research across disciplines. The CZO also promotes a
vigorous research program aimed at an improved understanding of interactions between water, the
atmosphere, ecosystems and landforms in the critical zone. The CZO is located at 37.053oN, 119.194oW,
spanning 1700-2100 m in elevation in the Kings River Experimental Watershed (KREW), a watershedlevel, integrated ecosystem project for long-term research in headwater catchments in the Sierra National
Forest. Hence, the CZO is able to take advantage of multiple, well-instrumented and characterized
catchments and long-term data sets. Four key findings from the CZO have helped shape the current
research agenda. First is closing the water budget at headwater catchment and larger scales using a rich
set of water-balance, flux-tower and sap-flow measurements of evapotranspiration (ET). Second,
multiple lines of evidence are consistent with a year-round growing season at mid elevation, with midmontane forests exhibiting cold tolerance in winter and avoiding late-summer drought stress through deep
rooting. Third, rates of ET at mid-elevation are high relative to nearby lower-elevation water-limited and
higher-elevation cold-limited ecosystems. Fourth, deep rooting and soil development are important for
sustaining high rates of net primary productivity (NPP), with over one-third of ET coming from depths
below 1m. Together these results provide a unifying theme for future research. Research for the next year
will address: i) how soil moisture and topographic variability interact to influence soil-formation and
weathering, ii) how the response of soil moisture, ET and streamflow, to snowmelt and rainfall is
controlled by landscape variability, iii) how and why vegetation and ecosystem distribution and function
vary with climate, iv) and how landscape heterogeneity and vegetation interact to influence cycling of
water, energy, nutrients, and NPP.
Broader impacts. There are four important broader impacts associated with this CZO. First, it provides
data and information that enhance the science of multiple individuals and research groups. Together with
the KREW data and infrastructure, it provides a data-rich community platform for process-level research.
Second, it provides a basis for the design and evaluation of a multi-disciplinary Earth science observatory,
which may be replicated elsewhere as the community builds observatory networks for both research and
applications needs. Third, new research infrastructure helps build both graduate and undergraduate
research at UC Merced and partner institutions. UC Merced is classified as a Hispanic-serving institution,
and diversity is part of programs across the campus. Fourth, the CZO team uses CZO data and
knowledge to enhance the science experience of thousands of middle and high school students and also
many undergraduate students at the participating universities. Work with educational institutions, such as
NatureBridge at Yosemite (formerly Yosemite Institute), will continue. Work will also continue with K12 schools in the Central Valley and Sierra foothills, technical colleges, local professional
organizations, and sponsors of community and visitor programs near the CZO to facilitate hands-on
activities that focus on how Sierra Nevada hydrology relates to California’s water resources.