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Water Entry and Downward Movement in Undisturbed Soil Cores¹

ABSTRACT

Water moves downward in a dry soil from a constant source at the surface according to Darcy's law which, combined with the law of conservation of mass, results in an equation for the downward flow of water in unsaturated soils. This equation has previously been discussed and solved for special conditions. This paper reports progress that has been made in measuring and evaluating the components of this equation for undisturbed soil cores.

Six undisturbed cores 10 cm in diameter and 120 cm long and containing different amounts of water were used. Water was allowed to enter the core under a constant head of 1.2 cm. The quantity of water that accumulated in the soil and the depth to the wetting front were measured as functions of time. The soil moisture potential was measured at 5 cm intervals of depth and at various time intervals. From these measurements infiltration rates, moisture potential gradients, and apparent moisture conductivities were calculated. Moisture conductivity was calculated from the moisture retention curve as suggested by Childs and Collis-George; this technique has not been experimentally verified sufficiently to justify the calculation of the conductivity gradient, hence the flow equation has not been completely solved.

The results indicate that the infiltration rate depends primarily on the gradient of the moisture potential and has secondary dependence on the capillary conductivity. Potential gradients in the wetting zone and across the wetting front may be much greater than in the transmission zone and they appear to be a principal factor in determining infiltration rates. Potential gradients were measured and used to calculate apparent capillary conductivity values which are always smaller than infiltration rates. As a result of large potential gradients, the infiltration rates exceeded the saturated permeability in five of the six cores studied.

¹ Research reported herein is supported by Regional Research Project W-9 with 11 Western States; B.P.I.S.A.E.; Soil Conservation Service (Research); Utah Idaho Sugar Co. and Amalgamated Sugar Co. cooperating. The material is taken from a thesis submitted by the junior author in partial fulfillment for the degree of Master of Science. Presented before Division I, Soil Science Society of America, Cincinnati, Ohio, Nov. 19, 1952.

² Assoc. Prof. of Agronomy and former Research Assistant, respectively.

Received for publication December 19, 1952.