TEMPERATURE EFFECTS ON AIR-POCKET TENSIOMETERS

Abstract

Air-pocket tensiometers with pressure transducer sensors are used widely in routine monitoring of soil water potential and in studies of soil water movement. Air temperature fluctuations in the exposed head space of the tensiometer can cause large air pressure changes leading to misinterpretation of the soil water potential. Treating the tensiometer head space as an ideal gas with water vapor in dynamic equilibrium, a set of coupled equations is derived that describes the relationship between head space air pressure, the rate of temperature change, the conductance of the tensiometer cup, and the head space volume. The interaction of these factors is illustrated through a series of simulations and then demonstrated with laboratory and field experiments. Air pressure deviations as large as ±80% were observed in tensiometers of low cup conductance and large head space when exposed to rapid temperature change. When the temperature change is slow relative to the cup conductance, changes in air-pocket pressure are corrected by water flow across the cup, and interpretation errors are small. Air pressure deviations from both rapid and slow temperature change may be estimated with the simulation equations using an approximation for the water pressure gradient across the tensiometer cup. The most practical outcome of this research is the theoretical and experimental confirmation that air temperature effects are reduced by maintaining high water levels in the tensiometer (that is, minimizing the air-pocket volume) and by maximizing the tensiometer cup conductance. In the field, insulating or shading tensiometers reduces temperature-related errors in general but may aggravate the problem if removal of the insulation (as might be necessary to read the tensiometer) causes a sudden temperature change in the air-pocket.