This project aimed to improve cotton irrigation WUE using dynamic deficits to (i) avoid plant stress and maximize yield and (ii) make the most effective use of in-crop rainfall. Our analysis of a large data set of soil water x plant stress (using as a measure) x climate experienced by the crop confirmed that atmospheric vapour pressure or evapotranspiration (ETo) can alter the plant stress response at the same soil moisture content. That is, if ETo is high a plant will may experience higher stress at higher soil moisture levels, and conversely if ETo is low a plant might not be stressed despite lower soil moisture availability. This analysis includes six experiments from three previous projects and three further experiments from the current project.

Two years of large scale field experiments have found that there is considerable utility in delaying irrigation timing and extending opportunities to capture rainfall when ETo

was low. This allows for more flexibility in cotton systems that require a significant number of fields to be irrigated at a point in time, and potential irrigation water savings. In both years there was no detrimental effect on yield or water use efficiency. In 2009/10 there was no difference despite considerable delays (up to 6 days) in one irrigation, and in 2010/11 the forecasted low ETo period also allowed an opportunity to capture rainfall event resulting in water savings of 0.8 ML over the season in one treatment. Periods of low ETo are often associated with a depression or low pressure weather front which may bring an opportunity to capture rainfall. Yeates found that delaying irrigations without taking into account ETo during flowering could have significant impacts on yield with a yield loss of 2.7% for every day that an irrigation was delayed.

Results from the past two experiments have indicated the need for a measure of plant stress used with soil water measurement to assist with a dynamic deficit irrigation approach. The results are showing that even when there are instances of high ETo, crops are not as stressed based on current understanding. We could continue to approach further analysis of the dynamic deficit approach without a measure of plant stress, changing the deficit accounting for crop stage, crop size, and boll load. This was similar to the approach used by HydroLOGIC to assist timing of irrigation.

The outcomes of the experiments in this project showed that there was considerable utility in delaying irrigation timing and extending opportunities to capture rainfall when ETo was low. This allows for more flexibility in cotton systems that require a significant number of fields to be irrigated at a point in time, and potential irrigation water savings.

The continuation of the research will involve determining a framework to provide a method to predict plant stress (based on a continuous measure) which couples current and future soil water with short term ETo forecast along with crop stage. This would allow the dynamic deficit approach to be used confidently and will accommodate local conditions. The approach used presently uses an average response of soil water, plant stress and ETo. There is also an opportunity to continually and directly measure plant stress directly using canopy temperature easily and being able to couple this with both soil water and forecast ETo would establish the value/risk of bringing forward and delaying irrigation.