The Southern Sierra CZO is a platform for integrated, multi-disciplinary research that will provide a process-level understanding of the critical zone, establish a foundation for long-term hydrologic, (bio)geochemical and ecological studies, and improve the predictive ability of Earth system models. An underlying hypothesis is that the distribution of soil moisture throughout the catchments controls (bio)geochemical processes, including weathering and the extent of coupling among the carbon and nitrogen cycles. Mixed conifer forest dominate the CZO, which is located in the rain-snow transition zone (1,600-2,100 m), a zone that characteristically undergoes rapid seasonal changes, going from snowcover to wet soil to dry soil over a 1-2 month period. Steep gradients in temperature and precipitation patterns, along both elevation and aspect, result in a distinct lag in spring runoff in going from lower to higher elevation. Streams draining the catchments are primarily sodium and calcium bicarbonate waters. Stream total ion concentrations measured at the top of the catchments above are about half of those at lower elevation. The higher elevation streams exhibit a larger average pH range (6.7-7.1) than do those at lower elevation (7.2-7.1). The stepped topography is a landscape that provides links between soil formation and weathering rates to landform evolution Our spatial sampling strategy is to capture key topographic features (slope, aspect, elevation, soil depth, streams) and use multiple tracers to characterize both longer term processes (millennia) and short term responses to current conditions. Our specific geochemical measurement strategy includes: i) year-round sampling of stream dissolved and suspended material, ii) isotopic, geochemical sampling to infer water sources, flowpaths and residence times, iii) temperature and electrical conductivity measurements at high frequency and high spatial resolution in streams, iv) geochemical and cosmogenic radionuclide measurements to provide a longer-term context in which to assess extreme events and their associated material fluxes, v) seasonally integrated measurements of deposition, and vi) soil-water sampling in different landscapes and along flowpaths.
Bales, R.; Hunsaker, C.; Conklin, M.; Kirchner, J.; Bouer, B.; Kirchner, P. (2007): Southern Sierra Critical Zone Observatory (CZO): hydrochemical characteristics, science and measurement strategy. American Geophysical Union, Fall Meeting 2007, abstract #H51K-02.