The Southern Sierra Critical Zone Observatory (CZO), located at 1700-2100 m elevation, crosses the rain-snow transition, an area that is particularly vulnerable to large changes in climate and landcover. 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. Collocated with the USFS Kings River Experimental Watersheds, the CZO takes advantage of multiple, well-instrumented and characterized catchments and long-term data sets to provide a community platform for research. 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 mid-montane 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 the current research, which addresses: 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. The foundation of CZO research is a rich field measurement program, with geochemical analyses and a well-integrated modeling effort that will provide a fundamental understanding of the links among climatic, hydrologic, (bio)geochemical, ecosystem and landscape processes.

The Southern Sierra CZO is a data-rich community platform for process-level research that enhances the science of multiple individuals and research groups. It is located in an elevation range that 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 water- related 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. Research at the CZO is providing an unprecedented predictive ability for how the linked hydrologic, (bio)geochemical, ecosystem and landscape processes will respond to perturbations, i.e. changes in temperature, precipitation patterns, and human management of forests. It is providing a solid foundation for science-based vegetation and water management in the Sierra Nevada and other seasonally snow-covered mountain forests. Further, the CZO provides a basis for the design and evaluation of broader observation networks for both research and applications. Finally, the research 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.

Project Report

The Southern Sierra Critical Zone Observatory (SSCZO) is investigating how mountain soils and weathered bedrock develop over geologic time, and interactions with shorter-term climate variability and ecosystem behavior. This understanding provides the foundation for predicting how environmental change, including human disturbances, fire, pests and changes in climate, influence water resources, material flows and forest health. The SSCZO is pioneering accurate measurement systems for snow accumulation and melt, soil moisture, climate and evapotranspiration; and the use of the measurements to drive advanced models for forecasting future conditions. Through partnerships with regional stakeholders SSCZO results help to assess options available to resource managers to enhance management of forests, water and other ecosystem services, given environmental change. The SSCZO is also a community platform for research on critical-zone processes, locally and as part of the CZO national network. Located in the Southern Sierra Nevada near Fresno (Figure 1), it lies along a steep elevation transect where precipitation grades from dominantly rain to snow and ecosystems range from oak savannah biomes to subalpine forests (Figure 2). Spatial gradients in critical-zone properties and processes permit substitution of space for time, making the SSCZO an excellent natural laboratory for studying how the critical zone responds to disturbance and how the water cycle drives critical-zone processes. Combined with the Forest Service Kings River Experimental Watershed project, there are 2 meteorological stations, a 50-m flux tower, a 60-node wireless embedded sensor network, 215 EC-TM sensors for volumetric water content, over 110 MPS sensors for matric potential, 60 snow-depth sensors, meadow piezometers and wells, sap-flow sensors, stream gauges and water-quality measurements (Figure 3 and 4). SSCZO research involves a core SSCZO team from 6 campuses, plus collaborators and cooperators from other institutions who use SSCZO data and other resources in their research. The conceptual science model for the SSCZO is built around links between water/material fluxes and landscape/climate variability across the rain-snow transition (Figure 4). Investigations link drivers of change to impacts on the water cycle, ecosystems and biogeochemical cycles, and ultimately to impacts on ecosystem services. Ongoing research focuses on water balance, nutrient cycling and weathering across the rain-snow transition, with soil moisture as the integrating variable. Distributed snow and soil-moisture measurements show a close coupling between snowmelt and soil drying in spring/summer, with systematic variability across elevation, aspect and canopy cover. Runoff increases with elevation, corresponding to decreasing temperature, more precipitation falling as snow, decreasing vegetation density and coarser soils. Evapotranspiration decreases proportionally as soils dry, going from about 1 to 0.1 mm d-1 over the summer. However, evapotranspiration is high in mid-elevation vegetation despite dry summers and freezing winter temperatures (Figure 5). That is, photosynthesis persists through the winter, and soils and regolith store enough water for photosynthesis to occur all summer. As soils dry out, trees apparently extract water from the deeper soils. Bedrock indicates weathering as deep as 40 m in some locations (Figure 6), providing a source of water for continued summer evapotranspiration. Higher elevations experience winter cold shutdown and lower elevations summer shutdown due to moisture stress; however, in between the broad mixed-conifer elevation zone of the Sierra Nevada experiences neither limit. Soil-mantle patterns at a larger scale indicate bedrock-vegetation interactions are more complex than previously thought. Annual runoff is about 15-30% of precipitation in dry years, increasing to 30-50% in wet years. The snow covered ground may experience multiple melt events during the winter and spring. Snowmelt timing influences soil water accessibility during summer, but little was known about the depth and persistence of the effects. Through snowmelt manipulations, we altered the timing of seasonal snowmelt at the SSCZO site during consecutive wet and dry years. We discovered that deeper soil water did not show a statistically signicant change. Drying effects postponed shallow water availability by 2 months, the deeper soils indicated 4 or more months. The fate of snowmelt timing on soil moisture can delay recovery of drought, and windblown dust could decrease water availability to plants and soil biota. The SSCZO provides a platform for research in a landscape with vital importance to society, yet poorly understood in its potential response to climate warming. The twin threats of a changing climate and land-use practices raise fundamental questions about the sustainability of critical-zone services in the semi-arid U.S. West, which depends heavily on seasonally snow-covered mountains for many of these services. SSCZO partnerships with federal, state, and local resource-management agencies show the interest that decision makers have in using both research results and SSCZO technology to improve predictive capabilities. One product decision makers can access is snow depth data (1 m2 resolution) derived from LiDAR and accurate to within 30 cm compared to field snow depth measurements (Figure 3). SSCZO data and lessons enhance the science experience of thousands of middle- and high-school students, several undergraduate students and the public.

Agency
National Science Foundation (NSF)
Institute
Division of Earth Sciences (EAR)
Type
Standard Grant (Standard)
Application #
1239521
Program Officer
Thomas Torgersen
Project Start
Project End
Budget Start
2012-09-01
Budget End
2014-08-31
Support Year
Fiscal Year
2012
Total Cost
$1,000,000
Indirect Cost
Name
University of California - Merced
Department
Type
DUNS #
City
Merced
State
CA
Country
United States
Zip Code
95343