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Evaporation from Soil Shrinkage Cracks¹

ABSTRACT

Field and laboratory studies have shown that evaporation from shriukage cracks may equal or exceed that from surface soil. Evaporation from moist soil samples (soil atmometers), suspended at various depths in naturally occurring shrinkage cracks in Houston Black clay, was determined. During the first 19 hours, the moisture evaporated at the 24-inch depth was 55% of that evaporated at the surface. Soil atmometers suspended at depths of 12 inches or less were reduced from 55% soil moisture to < 17% in 44 hours. Crop cover affects evaporation in shrinkage cracks. Atmometers at the 12-inch depth in grain sorghum lost half as much water as atmometers at the same depth in corn or permanent pasture. Field measurements showed from 2.9 to 4.6 times more surface area exposed on the shrinkage crack walls than as surface soil per square yard.

Laboratory studies indicate that evaporation from the side walls of shallow shrinkage cracks may vary from 35 to 91% of that from a comparable area of surface soil depending on soil moisture content. Wind tunnel studies with a simulated shrinkage crack 20 inches deep showed that evaporation increased at all depths as surface wind velocity increased. Increasing the wind velocity from calm to 25 mph. increased evaporation 570% at 2 inches below the surface and 100% at 18 inches. Evaporation increased as crack width decreased from 2.75 inches to 1.75 inches.

¹ Contribution from Soil and Water Conservation Research Division, ARS, USDA, in cooperation with the Texas Agr. Exp. Sta. and the Kansas Agr. Exp. Sta. Technical article No. 3920, Texas Agr. Exp. Sta., and Paper No. 756, Kansas Agr. Exp. Sta.

² Research Soil Scientists, Agricultural Research Service, USDA, Temple, Tex., and Fort Collins, Colo., respectively.

Received for publication June 19, 1963. Accepted for publication August 23, 1963.