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Evaporation from a Simulated Soil Shrinkage Crack¹

ABSTRACT

A simulated soil shrinkage crack, 46 cm wide and 62 cm deep, was constructed in such a manner that depth could be varied by 15.2-cm increments and width could be adjusted between 10 and 70 mm by 20-mm intervals. The walls of the crack were lined with porous ceramic plates connected to a water supply maintained at –5 mb potential. The simulated crack was installed in the floor of a laboratory wind tunnel located in a controlled environment room maintained at 24.0 ± 1.5C air temperature and 43 ± 3% relative humidity. Evaporation from the simulated crack walls (ceramic plates) was determined for all combinations of four crack widths (10, 30, 50, and 70 mm), three crack depths (30, 45, and 60 cm) and five windspeeds (0, 2.2, 4.5, 6.7, and 8.9 m sec^-1).

Total water loss (evaporation) from the walls of the simulated crack increased as any one or all of the three variables (windspeed, crack depth, or crack width) increased. These three variables accounted for 99.1% of the variability in total evaporation from the crack walls. Turbulent air movement within the simulated crack was the major factor affecting the evaporation.

Both straw and gravel mulches reduced evaporation from the crack walls 85 to 90% with no wind and about 60% at 8.9 m sec^-1 windspeed. The results suggest that evaporation from soil shrinkage cracks may be reduced by closer plant or row spacing or by surface residues such as trash and clod mulches.

Total evaporation from a simulated crack 60 cm deep varied from 0.5 cm³ hr^-1 (10 mm wide, 0 m sec^-1 windspeed) to 18 cm³ hr^-1 (70 mm wide, 8.9 m sec^-1 windspeed). In simulated cracks 60 cm deep and 30–70 mm wide, 50–60% of the total evaporation occurred below 15 cm and 30–40% occurred below 30 cm.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA, and the Texas Agr. Exp. Sta.

² Research Soil Scientists, ARS, USDA, Temple, Texas 76501, and Research Agricultural Engineer, ARS, USDA, Big Spring, Texas 79720, respectively.

Received for publication January 15, 1969. Accepted for publication March 27, 1969.