PRESSURE PUMPING EFFECTS ON SOIL EFFLUX MEASUREMENTS OF CO2 Alan Joseph Ideris Kyaw Tha Paw U University of California at Davis Biomicrometeorology Laboratory, L.A.W.R.

Presentation Outline 1. Background information and significance of project 2. Method of measurements 3. Preliminary results and discussion

Background Information Climate change models have predicted a relatively large air temperature increase in the lower atmosphere mostly due to greenhouse gases such as CO 2 North American Terrestrial systems are major sink of carbon, while agricultural soils are major source

Significance of Project Understanding terrestrial ecosystems and soil sequestration Soil chambers may have to be modified is pressure pumping important

Past Evidence CO2 efflux from soils may be significantly modulated by turbulent and lower frequency pressure fluctuations (Auer et al., 1996; Massman et al., 1997) Relatively small gradients in total pressure can result in viscous gas fluxes which are much larger than diffusive fluxes (Alzaydi and Moore 1978; Thorstenson and Pollack 1990)

Mechanisms of Transport in Subsurface Soil Pressure pumping Knudson and molecular diffussion

Mechanisms of Transport in Subsurface Soil Pressure pumping is a result of a pressure gradient force within the subsurface. Pressure better linked with high or low frequency pressure fluctuations?

High Frequency Pressure Fluctuations as Opposed to Low Frequency Pressure Fluctuations

Takle et al. 2004

Mechanisms of Transport in Subsurface Soil Our project deals with high frequency pressure fluctuations caused by turbulence Colbeck (1989) Developed a relationship between pressure perturbations and turbulence as approximated by the mean wind speed: p′= exp0.383U Importance of permeability of soil

Methods of Measurements Initial measurements in February 2004 at Campbell tract Barometric pressure measured at 10 hertz at surface, 5 cm, 10 cm, and 20 cm (Omega Engineering, Inc., Model PX2670) Pressure Transducers

Secondary measurements in maize field in Yolo County during August Pressure was recorded at surface, 10 cm, 20 cm, and 40 cm. Turkovich Field

Eddy covariance measurements were recorded to measure CO 2 fluxes in addition to secondary measurements. (Campbell Scientific Inc. CSAT-3) (Licor-7500)

Results Following results are for initial measurements at Campbell tract Cross comparison of different depths Significant pressure attenuation with depth

Compared with Takle et al. (2004) Table 1: Standard deviation and range of pressure on the surface and at depths of 5 cm, 10cm, and 20 cm (Ideris and Paw U) Depths Standard Deviation (Volts) Surface cm cm cm Depth (cm) Standard Deviation of Natural Pumping (Pa) Standard Deviation of Artificial Pumping (Pa) Surface Table 2: Standard deviation of pressure at the surface and depths of 15 cm, 45 cm, and 60 cm (Takle et al. 2004)

Results Following results for secondary measurements done in maize field which include pressure data and turbulence data

Table 3: Standard deviation at depths of 10 cm, 20 cm, and 40 cm, at the Turkovich site DepthsStandard Deviation (Pa) Surface cm cm cm7.3

Also presented is a sample of the CO 2 fluxes measured by direct eddy-covariance during the month of August in 2004 while the corn is almost fully matured

There was a clear relationship between the standard deviation of pressure fluctuations and the variance and standard deviation of the vertical velocity

Standard Deviation (Pa) at 40 cm vs. Standard Deviation of vertical velocity (m/s)

Standard Deviation (Pa) at 40 cm vs. Variance of vertical velocity (m 2 /s 2 )

Further Implications Future will entail a possible relationship between pressure perturbations and soil CO 2 efflux Soil chambers may have to be modified is pressure pumping important This has implications both in terms of a basic understanding of CO 2 efflux and the measurement of CO 2 exchange.

Acknowledgments I would like to thank the Kearney Foundation for their continued support. I would also like to thank Dennis Rolston, Amy King, John Kochendorfer, and Liyi Xu for their contributions to the project