Routine measurement of the soil water potential gradient near saturation using a pair of tube tensiometers

We describe a new tensiometer for routine measurements of the soil water po tential near saturation. The device is called the tube tensiometer because it is a long, open-topped, vertical tube (>1 m long) that is filled with po rous material. The tube tensiometer has advantages over other known tensiom eters as it does not require maintenance when the sensing tip dries beyond its air entry pressure and it is capable of being completely buried beyond the cultivation zone so that it does not foul tillage and harvesting equipm ent. The disadvantage of the tube tensiometer is that it only operates in t he range of soil water potentials from -L to 0 cm of water, where L is the length of the tube tensiometer. The output from the tube tensiometer was compared with mercury tensiometers in a 120-day controlled field drainage and evaporation experiment. The reg ression between instruments was high (r(2) = 0.99) and the accuracy of the tube tensiometer was <+/-0.5 cm of water. The soil water potential gradient near saturation can be measured by instal ling a pair of vertically separated tube tensiometers. If the soil hydrauli c conductivity is known then the soil water flux near saturation can be est imated using Darcy's Law. When the installation depth is below the active r ooting zone of a crop then deep drainage can be estimated. This application of the tube tensiometer was demonstrated by measuring the vertical soil wa ter potential gradient at a depth of 1 m beneath a wheat field near Harden, NSW, in response to winter rainfall. The major limitation to the use of Da rcy's Law for the routine monitoring of deep drainage remains the estimatio n of the soil hydraulic conductivity. Ongoing work is focussing on the use of tube tensiometers to provide simultaneous measurements of both hydraulic gradient and hydraulic conductivity.