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Heat and Water Movement Under Surface Rocks in a Field Soil: II. Moisture Effects¹

ABSTRACT

Results are presented from a field experiment conducted to determine the effect of surface rocks on soil water content changes under bare soil. First, stones were placed at intervals over an initially-dry field (gravimetric water content = 0.021 g/g) and left for 6 weeks. Subsequent sampling showed a small but detectable excess of water stored under the rock compared to adjacent bare soil. Following an irrigation, buried thermal conductivity probes were used to monitor water content changes under and adjacent to surface rocks. After 24 days, the soil under the rock contained significantly more water than did the soil region adjacent to the rock, a finding confirmed by gravimetric sampling. Following this, the stones and probes were relocated for a further 24 days of observation, with similar results obtained.

In a separate laboratory experiment using a large sealed soil column with a rock covering part of the surface it was demonstrated that a significant amount of water moved to the cylinder of soil under the rock from the soil region under the bare surface due to horizontal temperature gradients induced by the rock covering part of the surface.

A two-dimensional computer program calculating water vapor movement under thermal gradients was used with measured field temperature boundary conditions to estimate the amount of water vapor movement expected to occur due to a rock cover on the soil surface. Results qualitatively confirmed the observations of the sealed laboratory column experiment.

¹ Contribution of the Dep. of Soil Science & Agricultural Engineering, University of California, Riverside, CA 92502. Research was supported in part by funds from the National Science Foundation under IBP Desert Biome, Utah State Univ., Ecology Center, Subcontract No. 539.

² Assistant Professor of Soil Physics and Laboratory Technician, respectively.

Received for publication December 8, 1975. Accepted for publication April 12, 1976.