Effect of tensiometer cup size on field soil water tension variability

Reliable estimates of soil water tension in field soils using few tensiometers are often difficult to obtain because of the large spatial variability of soil water tensions. Therefore, the objective of this study was to evaluate the effect of tensiometer cup size on soil water tension variability. We installed three sets of tensiometers with different effective outside cup surface areas (4.8, 42.3, and 88.3 cm²) in a silty loam that was flood-irrigated for sorghum [Sorghum bicolor (L.) Moench] production. During a period of 10 mo, soil water tensions were measured with a tensiometer under fallow and cropped conditions on 71 nonconsecutive days. We found that an increase in tensiometer cup size reduced the variability of soil water tension measurements. This reduction in variability suggests that the medium cup size (42.3 cm²) extensively used for irrigation scheduling and environmental monitoring, is rather small and consequently causes a relatively large variability. Therefore, the medium cup should be replaced by a larger cup size. We also demonstrate that a logarithmic transformation of the water tensions is necessary to stabilize their variance and to allow pooling of the variances. The use of larger tensiometer cup sizes and the pooling of variances of log-transformed soil water tension measurements will lead to a lower number of tensiometers per irrigated field or environmental site.