EFFECT OF TENSIOMETER CUP SIZE ON FIELD SOIL-WATER TENSION VARIABILITY

Reliable estimates of soil water tension in field soils using few tens iometers are often difficult to obtain because of the large spatial va riability of soil water tensions. Therefore, the objective of this stu dy was to evaluate the effect of tensiometer cup size on soil water te nsion variability. We installed three sets of tensiometers with differ ent effective outside cup surface areas (4.8, 42.3, and 88.3 cm2) in a silty clay loam that was flood-irrigated for sorghum [Sorghum bicolor (L.) Moench] production. During a period of 10 mo, soil water tension s were measured with a tensiometer under fallow and cropped conditions on 71 nonconsecutive days. We found that an increase in tensiometer c up size reduced the variability of soil water tension measurements. Th is reduction in variability suggests that the medium cup size (42.3 cm 2) extensively used for irrigation scheduling and environmental monito ring, is rather small and consequently causes a relatively large varia bility. Therefore, the medium cup should be replaced by a larger cup s ize. We also demonstrate that a logarithmic transformation of the wate r tensions is necessary to stabilize their variance and to allow pooli ng of the variances. The use of larger tensiometer cup sizes and the p ooling of variances of log-transformed soil water tension measurements will lead to a lower number of tensiometers per irrigated field or en vironmental site.