The rain-snow transition zone in mountains of the Western United States 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, climate warming will shift this transition period earlier or eliminate it entirely. The result will be major changes in seasonal-to-interannual critical zone processes involving water, nutrients and ecosystem response of the largely mixed conifer forest found in the rain-snow transition zone. Steep gradients in precipitation patterns, along both elevation and aspect, plus rapid seasonal changes, make this zone an excellent natural laboratory for studying how critical zone processes respond to perturbations, and particularly how the water cycle drives critical zone processes.
The proposed Critical Zone Observatory (CZO) will take advantage of these features by establishing a snowline CZO as a community resource, providing both a platform for research by investigators from multiple disciplines and a vigorous research program aimed at yielding general knowledge and tools for understanding the interactions between water, atmosphere, ecosystems and landforms in the critical zone. CZO data and knowledge will also enhance the science experience of thousands of middle and high school students, and multiple university students. The CZO will be located in the Kings River Experimental Watershed (KREW), a watershed-level, integrated ecosystem project for long-term research in headwater catchments in the Sierra National Forest; it takes advantage of multiple well-instrumented and characterized catchments and long-term data sets at KREW (37.053oN, 119.194oW, 1,400-2,000 m elevation).
A primary, overarching goal is to understand how critical zone processes control fluxes and stores of water across the landscape, and how the water cycle modulates (bio)geochemical, biological, geomorphological and pedological processes in the critical zone. Five immediate research questions will define and focus the core measurement and research program: i) how do coupled hydrologic and biogeochemical fluxes vary across the rain-snow transition, ii) what is the role of extreme hydrologic events in hydrologic and biogeochemical balances, iii) to what extent does vegetation modulate or actively control the primary subsurface fluxes of water and nutrients, versus act as passive agents, iv) over what time and space scales, and during what seasons, are macropores and other short-circuit pathways dominant in the critical zone, and v) how does the presence of a seasonal snowpack affect the subsurface, critical zone, soils, geomorphology, biogeochemistry and hydrology in Sierra watersheds and hillslopes, and how will the relevant processes and reservoirs respond as the climate warms and snowpacks recede.
The Southern Sierra Critical Zone Observatory (SSCZO) is investigating how mountain soils and weathered bedrock develop over geologic time, and interact 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 close 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, both locally and as part of the broader 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 dominantly 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. The Providence catchments are the most heavily instrumented of the sites (Figure 3). 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. 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. The 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 nutrients are more variable than previously thought, impacting vegetation. Annual runoff is about 15-30% of precipitation in dry years, increasing to 30-50% in wet years. The ground is snow covered for 4-5 months each year, and may experience multiple melt events during the winter and spring. 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. The Sierra Nevada provides ecosystem services, ranging from water to biodiversity, to a large fraction of California’s and thus the nation’s population. 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. SSCZO data and lessons are used to enhance the science experience of thousands of middle- and high-school students, dozens of undergraduate students, and the public.
