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Soil Moisture Tension Transients Associated with Evapotranspiration from a Water Table¹

ABSTRACT

Soil moisture tensions were recorded continuously in lysimeters containing a loam soil and a controlled water table. Seven strain gauge-pressure transducer-tensiometers were spaced from 20 to 110 cm above the 158-cm water table. The velocity at which the morning and evening soil moisture tension minimum and maximum progressed downward through the root zone and capillary fringe proved inconsistent with soil moisture flow theory. This was interpreted as showing that even at low tensions a decrease in the moisture film continuity between the soil and tensiometer bulb relative to that between the few roots in contact with the bulb, caused the measurement to be biased toward the tensions associated with the root system. When the minimum daily tension exceeded 400 cm of H₂O, soil-bulb continuity was lost and, as a result, the characteristic wave form of daily evaporation (ET) demand was also lost. By placing the tensiometers in the capillary fringe where roots are absent, they could record the changing demands of ET on the capillary fringe. A method was tested for estimating water table uptake from a diffusivity based on the velocity of propagation of tension waves induced into the soil profile by daily ET and associated diurnal gradient changes.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA. Presented before Div. S-6, Soil Sci. Soc. Amer., Nov. 17, 1968. New Orleans, La.

² Research Soil Scientist, Fresno Field Station, and Agricultural Engineer, North Central Soil Conservation Research Center, Southwest and Corn Belt Branches, SWCRD, ARS, USDA.

Received for publication August 4, 1969. Accepted for publication January 9, 1970.