FLUCTUATIONS OF THE PHREATIC SURFACE

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Abstract
In a study of diurnal drain-flow fluctuations, theoretical relations were reviewed for entrapped air. Colonie [fine sand], which possesses a capillary fringe of 50 to 60 cm., was utilized in a laboratory study. Cooling the soil surface from 75[degree]F to 35[degree]F for a period of 2 hours or more resulted in a 15-to 20-cm depression of the phreatic surface. This depression took place when the capillary zone contained 5% or more entrapped air. When, however, entrapped air was excluded from the capillary zone, the phreatic surface was not lowered under cooling. Nor was it appreciably lowered under cooling with entrapped air when the phreatic surface stood at a dpeth of 50 cm or below. There was agreement between calculated and observed values for the lowering of the water table. These laboratory results support the view that entrapped air is a major consideration in determining the position of the phreatic surface. Under very moderate cooling of the soil surface to slightly above freezing, capillary-saturated Colonie containing 5% entrapped air gave a marked reduction in the elevation of the phreatic surface. This reduction was over 300% greater than when no confined air was present.

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