Effect of Gas-Phessure on the Distribution of Mositure in a Loam Soil

Abstract

To study the effect of total gas-phase pressure on the distribution pattern of moisture in soil, 7.6cm of de-aired water were admitted at zero suction at one end of a horizontal column of uniformly packed Columbia silt loam soil and alloewed to distribute in the soil column for 24 hours while maintaining a constant total gas-phase pressure within the soil column and a room temperature of 25 +0.5 C. Three levels of total gas-phase pressure were used, namely 0, 0.5 and I atmosphere. At the end of the 24 hour period following the entrance of the water into the soil column, the soil column was cut into uniform slices parallel toits cross section and moisture content of each slice determined gravimetrically. Horizontal distance from the water-entrance end to the visible wetting front was also recorded with time during the period of moisture distribution. The experiment was repeayed 5 times for each levedl of total gas-phase pressure and means of the 5 replicates were used in plotting curves in each case. The results indicate that the admitted water spread more readily within the soil at lower total gas-phase pressure showing the decrease in the effects of entrapped air and of the adsorbed films of gasses and vapors on the spreading of the water with the decrease in total gas-phase pressure. The change in moisture content across the visible wet front becomes more gradual and the rate of advancement of the visible wet front is slower at lower total gas-phase pressure. The proportion of moisture the from of water vapor appears to increase with decreasing total gas-phase pressure. The differences in the moisture distribution pattern are more pronounced between 0 and either 0.5 atmosphere or 1 atmosphere than between 0.5 atmosphere and I atmosphere of total gas-phase pressure,suggesting that magnitude of the effects of the gas phase in reducing the spreading of moisture in soil may depend both on the magnitude of the increase in total pressure of the confined air in the soil as water tends to more into the soil and on the initial level of the total pressure of the air. None of the total gas-phase pressure tested gave a constant soil moisture content behind the wet front and a straight- line relationship between distance to the wet front and square root of time during moisture distribution.