Diurnal fluctuations of tensiometric readings due to surface temperature changes

Abstract

Pressure fluctuations in tensiometers in response to temperature changes are examined. Mechanisms considered include temperature variation within an air gap at the top of the tensiometer, the air gap size, saturated water vapor pressure, and hydraulic conductivity of the soil. Pressures measured in a tensiometer generally fall between two simplified, limiting cases. The first limiting case assumes that the tensiometer cup is impermeable for water. This leads to very high fluctuations as air and soil temperatures change. For a cyclical temperature of 35 ± 15°C, variations in water pressure inside the cup can be ±70 cm water head. For the second limiting case the water moves freely between the tensiometer and the soil; which leads to more stable readings, within ±1 cm for the above 15°C fluctuation. While cup impedance was found to be a negligible factor for all cases considered, the analysis presented here suggests that conductivity of the soil immediately around the cup is the main factor governing temperature-induced pressure fluctuations inside the cup.