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Hydraulic Properties of a Clay Loam Soil and the Field Measurement of Water Uptake by Roots: II. The Water Balance of the Root Zone¹

ABSTRACT

Soil water depletion rates in a 115-cm and a 170-cm deep profile of Adelanto clay loam were compared with lysimetrically obtained consumptive use rates for periods of many days after measured water applications. When the soil was bare, the depletion rates were always higher than the rate of loss to the atmosphere, and the inferred flux at the 170-cm depth was as high as 2 mm/day 8 days after irrigation. When the test plot was planted to sorghum (Sorghum vulgare Pers.) an initially strong downward flux at the 170-cm depth reversed itself after about 10 days and became as high as 4 mm/day, representing upward flow of water from wet soil into the root zone above. The data imply that indiscriminate use of soil water depletion rates as representing consumptive use rates can be highly misleading at any time in an irrigation cycle. Further analysis shows that a rational and satisfactory correction of depletion data is not likely feasible, and, at any rate, unworkable for the condition of the experiment.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA.

² Chief Physicist, Agricultural Engineer, and Visiting Scientist, respectively, U. S. Water Conservation Lab., Phoenix, Ariz. Permanent affiliation of third author: Division of Soils, CSIRO, Brisbane, Queensland, Australia. Present address of first author: Inst. of Life Science, Texas A & M Univ., College Station.

Received for publication May 12, 1967. Accepted for publication January 29, 1968.