CAUSES OF DIURNAL FLUCTUATIONS OF TENSIOMETERS

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Abstract
Large diurnal fluctuations of tensiometers were noted under soil and climatic conditions of the Yuma Mesa near Yuma, Arizona, during the 1949 season. Observations were made on 155 standard-type instruments with mercury manometers installed in alfalfa to depths of 6, 12, 18, 24, 36, 48, 72, 96, and 120 in. Min. tension values occurred between 6 and 8 a.m. and max. values between 7 and 9 p.m. Fluctuations were noted to depths of 48 in., the magnitude of the variations decreasing with depth. No appreciable change in tension occurred below 72 in. Daily variations in tension of 350-400 cm. of water at the 6-in. depth were common. Day-to-day changes in manometer readings indicated a progressive increase in the magnitude of daily fluctuations the first 2 or 3 days. The extent of these variations later decreased as the soil became drier and the upper tension limit of the instrument was approached. Laboratory studies under conditions of constant soil moisture tension have shown that diurnal fluctuations of tensiometers occur when a temp, gradient exists between the porous cup and the soil. Temp, changes at the cup were attributed to heat radiation from the metal support and the air trap to which the porous cup is attached. When the tensiometer was cooled, a temp, gradient from the cup to the soil was established. As water vapor diffused from the soil to the cooler porous cup surface, condensed vapor entered the tensiometer system and caused the mercury in the manometer to drop. As the temp, gradient approached 0, the mercury rose to the initial value within 12 to 24 hrs. Due to the complexity of the problem, it is advised that tensiometers be read in the morning at approx. the same time of day.

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