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Optimising spray efficacy in summer: Delta t red alert system

Kerry Bormann

John Stuchbery and Associates, PO Box 10, Vic 3480. Email


Failure to identify and utilise optimum climatic conditions for summer weed spraying operations can result in costly weed control failures, subsequent repeat applications and reduced operational efficiencies. The GRDC Southern Agribusiness Trials Extension Network allowed John Stuchbery and Associates to design the Delta T Red Alert System, which aimed to increase grower awareness of changing climatic conditions and to provide an innovative decision support tool to assist growers in optimising summer spray applications. The Delta T Red Alert System utilises data from the Bureau of Meteorology’s automatic weather station network coupled with SMS technology to alert spray operators to changing climatic conditions to allow maximum utilisation of optimal conditions and conversely minimise the occurrence of spraying activities during sub-optimal periods. The Delta T Red Alert System was evaluated by fourteen broad acre grain producers located in the Mallee, Wimmera and Western District of Victoria. Comparison of the accuracy of data from automatic weather stations was compared to in situ weather observations recorded by the participating farmers using hand held weather meters. The practical outcomes of utilising coupled information technologies have determined the systems value as a useful decision support tool for growers.

Key words

SMS alert; spraying conditions


During the summer months appropriate spraying conditions can rapidly deteriorate as the day progresses. These changes in weather conditions, particularly delta T, are not always easily identified in the field. Delta T is an important indicator of evaporation rate and droplet lifetime, calculated by subtracting the wet bulb temperature from dry bulb temperature. Regular monitoring of weather services is required to ensure spray applications are carried out in appropriate conditions to optimise spray efficacy. Growers have a vast range of weather information sources available to them and it is not unusual for growers to refer to more than one information source to monitor current and forecasted weather conditions. The potential benefits arising from the alert system will reduce this constant need to monitor conditions, whereby growers are notified automatically of changes in weather conditions and can act upon them accordingly. The project aims to evaluate the effectiveness of the alert system as a support tool in creating increased awareness of changing climatic conditions, in turn reducing the incidence of spraying in sub-optimal conditions.


The project focused on three climatic parameters that were identified as being critical to summer spraying conditions: delta T, wind speed and temperature. The Bureau of Meteorology’s network of automatic weather stations was utilised to capture and update this data at 30 minute intervals. For each parameter a range of acceptable limits was set. For conditions to be deemed optimal, all three climatic parameters must fall within their respective acceptable limits. Participating growers selected the automatic weather station nearest to their property to source localised data.

The alert system was in operation during January and February of 2008. A text message was sent to the grower as conditions in their area became favourable for spraying and again when they began to deteriorate and became unfavourable. A WAP website (accessible via mobile phone) was also made available to allow growers to check weather data at any time between messages. The system was in operation 24 hours a day, therefore not limiting the spraying window for growers, particularly those who prefer to spray at night.

Participants were asked to record weather information received in text messages and then take their own weather measurements in the paddock using a Kestrel (or other) weather meter. This was used to assist in comparing the accuracy of the message data, relative to the distance from the automatic weather station. Feedback was sought from participants through an evaluation questionnaire distributed to participating growers at the conclusion of the project.


Responses from the questionnaire show that the majority of participants found that the project increased their awareness of changing weather conditions. More than half of the participants had found the data provided via SMS influenced their decision to continue spraying or to stop spraying due to weather conditions. Growers agreed that the key variables they consider before and during spraying were wind speed and direction, temperature, humidity and delta T.

When comparing the accuracy of information received via SMS to the in situ weather observation recordings, several trends emerged. Of the three parameters, temperature was consistently more accurate and was less variable between the two readings. Temperature readings from the automatic weather station were consistently lower than paddock recordings (but usually within 0-3C). As for temperature, SMS readings for delta T were also similar to paddock observations, but consistently lower than those recorded in the paddock (within 0-2C). Wind speed proved to be highly variable, with automatic weather station readings consistently higher than paddock recordings and largely unreliable for decision making in most instances. These differences ranged from 0-12 km/hr.

Differences between automatic weather station readings and paddock observations were largely related to the relative distance between the reporting weather station and the farm location. Distances ranged from 3-100 km, averaging approximately 25 km. In general, paddock observations and automatic weather station observations were more consistent where the farm was located closer to the automatic weather station. Variability between readings was also likely to be due to differences in the height from which readings are taken. Also, automatic weather stations are situated in open areas to gain a representative reading and this may account for elevated wind speed readings. Recordings made by growers may have also lead to variability, particularly for wind speed, if the area was sheltered.

Most growers found that messages were received in a timely manner for decision making where phone coverage was adequate. On average, the time delay from data capture to message delivery was 5-10 minutes. Growers were divided on whether they could rely upon this tool for decision making. In many cases this uncertainty was masked by lapses in phone service reception and a lack of confidence that messages would be received in due time. In addition, the overall consensus was that wind speed data was not reliable enough, with large variations between automatic weather station readings and paddock observations.

Lapses in message delivery and the varying accuracy of some readings, left growers feeling unsure whether the tool would provide commercial value to their business. This uncertainty amongst growers created doubt whether they would be prepared to pay for such a service (as trialed) if it were commercially available. However they did agree that the tool added to their current sources of weather data and made the decision making process easier and aided work efficiencies.

In conducting the project, there were limitations to the capacity of parameters that could be trialed and the complexity of the system. Suggestion for improvements, including varying wind speed limits and the timing of when messages are received would be of most value to growers.


Results and feedback from the project have proven the technical feasibility of this coupled technology as a decision support tool. The systems potential as a commercial tool has been identified, but further development of the application is still required. If further developed, there would be scope for growers to set their own limits for parameters and even nominate days when the alert system would be in operation. This is technically possible via a website interface on the internet. There is also scope to include other parameters, including the likelihood of rain or frost (for in-crop application).

The use of this support tool would not only be limited to summer spraying applications, but could also extend to the wider industry. Its application could be well adapted to any practice where weather conditions and timing are fundamental to the operation, e.g. hay baling conditions.

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