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Some Aspects of the Kinetics of Moisture Flow into Unsaturated Soils¹

ABSTRACT

The movement of water into an air-dry silt loam soil was studied as a function of temperature. The results are interpreted with the use of the Arrhenius equation in terms of energy barriers, and the activation energies necessary to overcome the barriers were calculated from the experimental measurements. Activation energies of 1 to 3 kilocalories per mole of water taken up were found.

Qualitative relationships were observed between the energy barriers found in the system studied and the soil particle size, bulk density and wetting suction.

The rate of water entry into soil and the rate of advance of the wetting front was faster in soils at lower bulk density. This effect is about the same at all temperatures; consequently, the activation energies are not significantly different. Activation energies appear to be high for the finer size fraction of Millville silt loam. The barriers opposing water movement into soil appear to operate at the air-water-interface associated with the advancing water front.

Theoretical consequences of these findings provide a physical explanation of the wetting front, an explanation for hysteresis, and a basis for including shrinking and swelling as contributing factors in modifying the rate of flow of water through soil.

¹ Research reported here was done as part of Western Regional Research Project W-30 with 12 western states, and the Agricultural Research Service cooperating. Presented at the meetings of the Western Society of Soil Science, Aug. 27, 1957, at Palo Alto, Calif. Approved for publication by the director of the Utah Agr. Exp. Sta. as Journal Paper No. 24.

² Formerly graduate assistant, Utah Agr. Exp. Sta., now Assistant Irrigationist, University of California, Davis; and Professor of Soil Physics, Department of Agronomy, Utah State University, Logan, respectively.

Received for publication September 18, 1959. Accepted for publication November 19, 1959.