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Improving Irrigation with Wetting Front Detectors by Richard Stirzaker, Richard Etherington, Ping Lu, Tony Thomson and Joyce Wilkie
February 2005
RIRDC Publication No 04/176 RIRDC Project No CSL-14A
Low cost and simplicity are essential to breach the impasse of poor adoption. However simplicity cannot be at the expense of accuracy, so we need to find the balance between simplicity, accuracy and cost for improving water and nutrient management from a low base.
This project introduces the FullStop wetting front detector to farmers and evaluates its performance on a range of farms under surface drip, buried drip, fixed sprinkler, centre pivot and mini-sprinkler irrigation on a variety of annual and perennial crops. The wetting front detector is a funnel-shaped instrument that is buried in the soil. The funnel concentrates the downward movement of water so that saturation occurs at the base of the funnel. The free (liquid) water produced from the unsaturated soil activates an electronic or mechanical float, alerting the farmer that water has penetrated to the desired depth. The detectors retain a sample of soil water that is used for nutrient and salt monitoring.
Irrigation scheduling is often portrayed by scientists as an exercise in accuracy - the idea that there is a defined refill point and upper drained limit and a precise amount of water can be added to satisfy the crop without wastage. Things look different on the farm; irrigators are aware of non-uniformity in their irrigation systems and variability in soils and plant growth. Moreover they often cannot irrigate exactly on cue because water is being used elsewhere on the farm or some other cultural operation requires the irrigation to be withheld. Since many other tasks compete for their attention, the key issue from the farmer’s point of view is the value of information relative to the time and expense involved in getting the information.
The case studies showed that the wetting front detector helped irrigators to evaluate their own practice and challenged their perceptions of what was happening in the root zone. In several cases the detectors quickly honed in on the most important issues to be addressed by the farmer, which is the art of troubleshooting. Detectors do not provide quantitative data, but help irrigators to move in the right direction; after all the soil is a buffer and it is not important to be right every time – just important not to be consistently wrong.
Perhaps the most tantalising aspect of this research project was the ability of the detector to provide information on the electrical conductivity and soil nitrate in the root zone from the water sampled from the wetting front. Wherever we monitored EC of nitrate in the case studies above, it proved to be highly instructive. The use of simple colour test strips for nitrate and portable EC meter means that a water sample can be tested in-field in less than two minutes for a cost of under $1. The management of water, salt and nitrate are inextricably linked and it is not possible to be on the “clean and green” road without monitoring all three.
In most of the case studies water was independently monitored by tensiometer, gypsum block, capacitance probe or time domain reflectometry to evaluate the accuracy of the detector. Weak redistributing fronts can pass the detector without activating it – and this was observed - but on the whole the evidence was that the wetting front detector was sufficiently accurate to improve irrigation management.
The wetting front detector is best seen as a learning tool. Wherever it was deployed it aroused curiosity and opened up a dialogue between farmer and scientist. Because the dynamics of water, salt and nitrate in the soil are complex, this dialogue needs to be facilitated if we want to see sustained change. Social researchers know that change is a complex process consisting of many steps, including pressure for change, the vision for change, capacity to change, actionable first steps, roles models and the like. The wetting front detector can start the process. It proved to be a simple way of showing irrigators how deep wetting fronts penetrated into the soil and the solutes moving with them. We conclude that a simple tool can stimulate irrigators to re-evaluate their practices and help them to take another step along the difficult road of managing water and the solutes it contains.
 
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