Date of Award

12-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering

Advisor

Murdoch, Lawrence C.

Committee Member

Falta, Ronald

Committee Member

Ravichandran, Nadarajah

Abstract

Water storage in the soil is important for many hydrologic problems, but measuring how the storage changes at an appropriate scale remains a challenge. The objective of this thesis is to evaluate the feasibility of estimating change in water content in soil by measuring vertical displacement in underlying soils. This approach is appealing because the averaging volume of the displacement scales with the depth at which the measurements are taken. This should result in measurements of changes in soil moisture that are averaged over regions 10s to 100s of meters across, a measurement scale that is much larger than that of commercially available moisture sensors and that would be useful for model calibration and other applications. The approach of this research is to develop an instrument for measuring displacement and to evaluate the instrument in a field setting by monitoring displacement along with environmental variables. The instrument developed for this work is called a Sand Extensometer, or Sand-X, and it is designed for applications in sand or other unlithified materials. The Sand-X was evaluated in three installations at two field sites in the vicinity of Clemson, SC. One instrument was installed at 2 m and two were installed at 6 m depths in saprolite above the water table. The Sand-X was sensitive to precipitation events during continual sampling at 1-minute intervals for months, and periods between precipitation events indicated behavior suggestive of decreasing soil moisture. Increases in soil moisture due to typical precipitation events of 10 - 15 mm corresponded to displacements of about 1 micron of compression. The soil expanded at a typical rate of 0.5 µm da-1 between precipitation events. This expansion was interpreted as unloading of the soil due to decrease in soil moisture. The 6-m-deep installations were calibrated using precipitation events greater than 5 mm. The average response of one instrument was -0.16 µm displacement mm-1 rainfall with an R2 of 0.95 for one instrument and -0.66 µm displacement mm-1 rainfall with an R2 of 0.54 for another instrument. Using these calibrations, the periods between precipitation events were interpreted as periods of soil drying resulting from the combined effects of evapotranspiration and recharge. The net evapotranspiration and recharge estimated from land pan measurements and hydrograph data respectively was 43 cm with a standard deviation of 14.9 cm, and the net soil water loss estimated from displacement measurements was 27 cm with a 95% confidence interval of ±6.7 cm. These results indicate that the measurements of water loss from displacement are consistent with independent estimates of water loss. All of the installations were sensitive to fluctuations in barometric pressure. Typical diurnal pressure changes of about 400 Pa correspond to displacements on the order of 1 micron at depth. These effects were reduced slightly using an analytical solution for the soil response to barometric loading.

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