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Soil Moisture Content Calculations from Capillary Tension Records¹

ABSTRACT

A method is outlined whereby the quantity of water, or the change in the quantity of water, held in a column of soil is calculated from capillary tension records. The calculations require a knowledge of the functional relation between tension and the moisture content for the particular soil in question.

Experimental curves relating moisture content and tension are given for four soil types. They all show a measurable hysteresis loop. Effects of the compaction of the soil on this relation were observed. With enough data available it was assumed that a single-valued relation between tension and moisture content could be chosen which would be valid during a given period of wetting or drying.

A pressure cell employing a porous ceramic wall is described for obtaining curves relating tension and moisture content over a tension range greater than one atmosphere.

Continuous records of capillary tension were obtained at four depths in a one-foot column of sand for various conditions of capillary flow in the sand. These records and the data from the hysteresis curves for a sample of the same sand were used in the indicated calculations. One computation was made for the total quantity of water present in the sand column. It was accurate within five per cent. The change in the water content over three different periods were calculated and were in error by 250, 150, and 73 per cent respectively.

¹ Experimental results taken from a thesis presented to the faculty of the Graduate School of Cornell University in partial fulfillment of the requirements for a Ph. D. degree, February 1938.

² Assistant Soil Physicist, New Jersey Agricultural Experiment Station.