CZO: Critical Zone Observatory--Snowline Processes in the Southern Sierra Nevada

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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.