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Multiple Tensiometer for Determining the Vertical Component of the Hydraulic Gradient in Soil¹

ABSTRACT

A description is given of an instrument for measuring the hydraulic head and hydraulic gradient of water in unsaturated field soil. Porous ceramic sections are mounted between plastic spacers to make a rod-shaped instrument for insertion in a hole made by the Standard Veihmeyer soil sampling tube. Five porous sections with a vertical spacing of 10 cm, are thus arranged with individual connections to mercury manometers mounted at the top of the assembly.

During infiltration, after 44 cm of water had entered a deep, uniform, fine sandy loam, the downward hydraulic gradient averaged 1.3 in the 10–30 cm depth interval. This corresponds to a downward water-moving force of 1.3 g. Six days later, a few hours after a 1-cm rain, the average downward water-moving force in the same depth interval was 5 g. Four days after the rain, there was a net upward water-moving force of 36 g in the 10–20 cm depth interval, due to the influence of surface evaporation. A much higher value existed in the 0–10 cm layer because of the greater moisture gradient near the soil surface.

The term static zone is used in connection with the soil-water system to designate the locus of points above which water movement is upward, and below which water movement is downward. A static zone passes downward through fallow soil following wetting. Over a four-day period of warm dry weather following a heavy irrigation, the static zone passed downward at an average rate of 6 cm per day in a fallow fine sandy loam.

¹ Contribution from the U. S. Salinity Laboratory, Riverside, Calif., BPISAE, U. S. Dept. of Agriculture, in cooperation with the 17 Western States and the Territory of Hawaii. Presented before Div. 1, Soil Science Society of America, Dallas, Tex., Nov. 17, 1953.

² Principal Soil Scientist. The author is pleased to acknowledge the skilled assistance of Henry Stumpf in connection with the construction and assembly of this instrument.

Received for publication June 25, 1953.