DIURNAL FLUCTUATIONS OF TENSIOMETRIC READINGS DUE TO SURFACE-TEMPERATURE CHANGES

Pressure fluctuations in tensiometers in response to temperature chang es are examined. Mechanisms considered include temperature variation w ithin an air gap at the top of the tensiometer, the air gap size, satu rated water vapor pressure, and hydraulic conductivity of the soil. Pr essures measured in a tensiometer generally fall between two simplifie d, limiting cases. The first limiting case assumes that the tensiomete r cup is impermeable for water. This leads to very high fluctuations a s air and soil temperatures change. For a cyclical temperature of 35 /- 15 degrees 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 read ings, within +/-1 cm for the above 15 degrees C fluctuation. While cup impedance was found to be a negligible factor for all cases considere d, the analysis presented here suggests that conductivity of the soil immediately around the cup is the main factor governing temperature-in duced pressure fluctuations inside the cup.