Southern Sierra Critical Zone Observatory (CZO): hydrochemical characteristics, science & measurement strategy R. Bales, C. Hunsaker, M. Conklin, J. Kirchner, B. Boyer, P. Kirchner underlying hypothesis: The distribution of soil moisture throughout the catchments controls (bio)geochemical processes, including weathering & the extent of coupling among the carbon & nitrogen cycles.
CZO
Mixed conifer forest dominates the CZO, which crosses the rainsnow transition (1,500-2,000 m)
Rain-snow transition zone undergoes rapid seasonal changes, going from snowcover to wet soil to dry soil over 1-2 months Soil volumetric water content 0.2 0.1 0 drip edge SWE under canopy March April May 2007 snow depth 160 120 80 40 0 Snow & SWE depth, cm
Steep gradients in temperature & precipitation result in a lag in spring runoff in going from lower to higher elevation 2004
High frequency stream patterns during snowmelt 2004
Wolverton Creek, Sequoia NP: stream stage Base - flow August 14 - September 7, 2007 Base-flow period, 2150 m elevation stage cm 25 20 15 10 5 15 20 25 30 5 August September 0
Wolverton Creek, Sequoia NP: stage & precipitation Base - flow August 14 - September 7, 2007 Base-flow period, 2150 m elevation stage cm precip.001mm 25 20 15 10 As we add measurements and integrate them causal relationships become increasingly clear. 15 20 25 30 5 August September 5 0
Wolverton Creek, Sequoia NP: stage, precip & air temperature Base - flow August 14 - September 7, 2007 Base-flow period, 2150 m elevation stage cm air temp C precip.001mm 25 20 15 3 to 6 hour lag between daily low flow and Tmax or daily high flow and Tmin. 10 5 15 20 25 30 5 August September 0
Wolverton Creek, Sequoia NP: stage, precip, air temp & sap flow Base - flow August 14 - September 7, 2007 Base-flow period, 2150 m elevation stage cm sap-flow C air temp C precip.001mm 25 20 Peak sapflow occurs at solar noon about 1 hr before Tmax. 15 10 5 15 20 25 30 5 August September 0
Stream geochemistry Primarily Na +, Ca 2+, HCO 3 - Total ion conc. at higher elev. about half that at lower elev. Higher elev. streams exhibit wider average ph range (6.7-7.1) than do those at lower elev. (7.2-7.1) Deficiency in ion balance made up by organic acidity Stream nutrients mostly in organic phase (DOC) Inorganic N important in soils
Dissolved organic carbon & discharge Stream reactive nutrients mostly organic (DOC) Highest conc. during rain & snowmelt high water tables & flushing of DOC from landscape to stream Timing of DOC release controlled largely by timing of streamflow generation DOC, mg L -1 WY 2004 2004 J F M A M J J A S O Discharge, L s -1 Data courtesy M Solomon
Weathering & landscape features Stepped topography: a reflection of water-related critical zone processes? It is thought that the stepped topography evident in this landscape is an integration over time of the thinner soils, lower soil moisture & thus less weathering associated with steeper slopes. The distribution of soil moisture throughout the catchments controls (bio)geochemical processes, including weathering & the extent of coupling between carbon & nitrogen cycles.
Wahrhaftig's hypothesis, 1965: "Irregular steps characterize the topography on granitic terrane on the west slope of the southern Sierra Nevada... the stepped topography is confined to granitic rocks & is believed to result primarily from the much more rapid weathering of granitic rocks where buried than where exposed." 7200 5600 4000 0 5 10 15 miles 2400 feet
Explicit links between hydrology, geochemical processes & landform evolution Steeper slopes --> thinner soils --> less persistent moisture --> slower weathering Shallower slopes --> thicker soils --> more persistent moisture --> faster weathering Integrating through time yields stepped topography. Test the Wahrhaftig hypothesis using soil moisture, weathering rates, denudation rates across landscape
Spatial sampling strategy Use multiple tracers to characterize both longerterm processes (millennia) & short-term responses Capture physiographic variability: slope, aspect, elevation, soil depth, streams
CZO as a community resource CZO program supported through NSF-EAR CZO home: http://snri.ucmerced.edu/czo Data: http://eng.ucmerced.edu/snsjho USFS-PSW instrumented catchments in place CZO instrumentation in progress, completion summer 2008 Contact: rbales@ucmerced.edu