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Stirzaker RJ (2003) When to turn the water off: scheduling micro-irrigation with a wetting front detector. Irrigation Science 22, 177-185. click here to open full article (PDF 340kb)

The science of irrigation scheduling is well advanced, but the field application of this knowledge among irrigators is limited. Case studies are presented to show why irrigators may fail to adopt or persevere with traditional irrigation scheduling methods. This paper describes a funnel-shaped wetting front detector that is buried at an appropriate depth in the root zone. As a wetting front moves into the funnel of the detector, the water content increases due to convergence, so that the water content at the base of the funnel reaches saturation. The free water produced is detected electronically and this provides the signal to stop irrigation. Since the philosophy of drip irrigation in most cases is to supply water little and often, the "when to turn the water on" question becomes redundant and knowing when to turn the water off is more useful. Two further case studies demonstrate the benefits of scheduling micro-irrigation using wetting front detectors. The detectors retain a water sample from each irrigation event and this was used to monitor nitrate movement in and below the root zone.

Stirzaker, R.J. and Hutchinson, P.A. 2005. Irrigation controlled by a Wetting Front Detector: field evaluation under sprinkler irrigation. Aust. J. of Soil Res. 43: 935-943. click here to open full article (PDF 212kb)

The accuracy of scheduling irrigation to turf by sprinkler was evaluated using a simple wetting front detector that automatically switched the water off after the wetting front had reached a prescribed depth in the soil. The detector consists of a funnel-shaped container that is buried in the soil. When a wetting front reaches the detector, the unsaturated flow lines are distorted so that the water content at the base of the funnel reaches saturation. The free water produced is detected electronically and this provides the signal to stop irrigation. The perfrmance of the detector was evaluated over 38 consecutive irrigation events to test the theory that the velocity of a wetting front depends on the difference in water content ahead of and behind the front. The experimental data plotting the irrigation amount permitted by the wetting front detectors as a function of the soil water content before and after irrigation yielded a linear relationship with a slope of 0.95 and a correlation coefficient of 0.73. Thus, if the soil is dry before irrigation the front will move slowly and an irrigation of long duration will be permitted. with the converse applying to wet soil. Independent monitoring of soil water content showed that irrigation was for the most part, scheduled accurately. Irrigation interval was the key variable to control. When the interval was too short then over irrigation occurred.

Stirzaker, R.J. (2005) Managing irrigation with a wetting front detector. UK Irrigation 33:22-24 click here to open full article (PDF 101kb)

WFD is not the Water Framework Directive in this instance but the Wetting Front Detector. This is a relatively low-cost device developed at CSIRO in Australia for aiding and supporting good irrigation scheduling. It is also a useful tool for monitoring nutrient losses and so in this way it could prove to be a very useful tool for compliance with the more traditional meaning of WFD.

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