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Water Vapor Movement Through Mulches Under Field Conditions¹

ABSTRACT

The loss of water vapor through 0.5-, 1-, and 2-cm depths of water repellent mulches was measured under field conditions with lysimeters. Concurrent measurements were made of wind velocities, air vapor pressures, and temperatures at the soil-mulch interfaces. Mulches used included 0.5-, 1-, and 2-mm glass beads and 1-mm glass bead aggregates.

By assuming that the relative humidity of the soil air at the soil-mulch interface was essentially 100%, effective diffusion coefficients for field conditions were calculated. The average effective diffusion coefficient for afternoon periods was 1.26 times greater than the molecular diffusion coefficient. An error analysis revealed that the value could have been as low as 0.90 or as high as 1.63, but these values would have been realized only if the effects of several possible systematic errors were completely additive.

Little correlattion was found between wind velocity and effective diffusion coefficient. However, the generally low range in wind velocities observed and the smoothing required for the reduction of the lysimeter data may have prevented detection of such a correlation.

¹ Contribution from the Agricultural Research Service, USDA.

² Soil Scientist, US Water Conservation Lab., Phoenix, Arizona 85040.

Received for publication January 2, 1973. Accepted for publication August 16, 1973.