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Pressure Dependence of the Relative Vapor Pressure of Water in Soil¹

ABSTRACT

The equation ln (p/p_o)/P = ( – v^o)/RT, expressing pressure dependence of the relative vapor pressure in terms of the difference between the partial molar volume of the water in the system and the molar volume of pure water v^o, was derived and was used in interpreting the experiments. A thermocouple psychrometer (TP) was calibrated at 25°C. using 0.10 and 0.20 molal KCl solutions at several air pressures in the sample chamber, ranging up to 15 bar. The TP reading for a sample of soil, when referred to the measurement for an appropriate sample of KCl solution, did not change significantly as the air pressure in the soil chamber was increased by 5-bar steps up to 15 bar. The total suction of water in soil is directly related to the relative vapor pressure p/p_o of the soil water and for our test sample the total suction was 6.44 bar at atmospheric pressure. The values of the standard deviation of total suction corresponding to the standard deviation of the TP readings were calculated to be 0.053, 0.084, and 0.109 bar for pressure increases of 5, 10, and 15 bar. Decreases in total suction greater than these amounts were not found.

¹ Contribution from the U. S. Salinity Laboratory, Soil and Water Conserv. Res. Div., ARS, USDA, Riverside, Calif. This work was supported in part by the Meteorology Department, U. S. Army Electronics Research & Development Activity.

² Physicists, U. S. Salinity Laboratory, Riverside, Calif.

³ Professor, Department of Agronomy, Purdue University, Lafayette, Ind. Formerly Soil Scientist Consultant to U. S. Salinity Laboratory.

Received for publication April 5, 1963. Accepted for publication May 16, 1963.