Cotton production in Australia is limited by the lack of water availability in most years. This project aimed to enable growers to adapt and tailor their irrigations to an uncertain future climate and water availability situations based on definitive data to manage risk. Earlier research supported by the CRDC enabled developing an irrigation scheduling method for furrow irrigated cotton based on canopy temperature monitoring. Through being strongly related to soil water availability, canopy temperature measurements enable continuous monitoring of a crop’s requirement for irrigation using a plant-based method that is practical to use on commercial farms. Through this project the canopy temperature method was further refined for fully irrigated systems and its use tested in partially irrigation situations. The three key areas of research for this project were:

1. Integrated Irrigation Agronomy for High Yielding Systems:

To achieve the highest yields the irrigation management of the crop needs optimizing through the entire crop development period. We conducted detailed research trials at the Australian Cotton Research Institute (ACRI) near Wee Waa (NSW) to optimize irrigation strategies at planting and during early, mid and late season which are explained below:

We conducted a comparative investigation of pre-watering and watering up at the ACRI to determine the best irrigation strategies for crop establishment. Watering up one day after planting resulted in slower germination and 25% less plants established compared with the crop pre-irrigated a week before planting. Soil temperature in the watered up treatment was up to 2.4 °C cooler than in pre-watered soil which most likely affected germination and establishment. These results are important in that there was only one cold shock (i.e. minimum air temperature <12 °C) during this trial in 2015-16 season compared with 20 and 10 cold shocks in the following two years, respectively, yet plant establishment was affected. It is important to consider these effects of watering up in cotton and, where watering up is unavoidable because of other farm factors, planting time may be adjusted to avoid cooler soil temperatures.

The timing of first in-season irrigation (excluding at planting) drives the establishment of a plant with sufficient vegetative growth to support high yields. Bollgard® varieties with high fruit retention may benefit from vigorous plant growth translating into high yields. A trial conducted at ACRI during 2015-16 to optimize the timing of first irrigation did not generate the expected treatment differences in plant development and yield because of wet weather conditions. It was identified that thermal cameras can be an effective tool for monitoring plant water stress during early season when canopy temperature infrared sensors cannot be used because of smaller canopy.

Timely application of mid-season irrigations is most important in terms of its effect on yield. In practical situations most cotton farmers may have to make an irrigation decision few days in advance. We investigated integrating canopy temperature and short-term weather forecast to make an irrigation decision five days in advance. Irrigations were planned in advance by either using an average daily stress time value based on historic data (treatment 1), or applied earlier or later than the predicted date based on short term weather forecast using dynamic deficit approach (treatment 2). Both the treatments underestimated the canopy temperature stress time compared with the measured observations. It was concluded that an irrigation decision

made in advance should be based on canopy temperature that is predicted from short term weather forecast rather than historical climate data.

Detailed trials were conducted at the ACRI to optimize the timing of last irrigation using canopy temperature sensors in a fully irrigated cotton system. The results showed canopy temperature sensors can be used for scheduling last irrigation with a need for further testing in different weather conditions to build confidence in the results.

2. Irrigation scheduling with limited water

We conducted detailed trials to develop irrigation decision frameworks in limited water situations to limit the risk and seasonal variability. The utility of canopy temperature sensors was investigated in partially irrigated systems with three different row configurations commonly used in cotton industry. There were strong relationships between yield and canopy temperature (and its derivatives) in all row configurations. As canopy temperature is affected by a plant’s access to (or lack of) soil water, regardless of location of water within soil profile, this method may help improve irrigation scheduling in partially irrigated systems where different row configurations are used. We tested applying a single irrigation during flowering at canopy temperature stress thresholds higher than that used in fully irrigated systems. In the years we conducted these studies we could extend/delay our irrigations with little impact on yield and yield gains in one instance. Further research will be needed to understand how we can best utilize our ability to better quantify stress (using canopy temperature sensors) in years that are dissimilar to those experienced in this study to manage risk in limited water situations.

3. Research Support for high impact delivery and adoption

Research support was provided in collaboration with the Cottoninfo team through on-farm trials and field days. On-farm trials were conducted in different cotton growing valleys in New South Wales (Wee Waa, Rowena, Walgett) and Queensland (Emerald, St. George), where farmers integrated the canopy temperature approach in their irrigation decision making. The overarching philosophy of these trials was to provide farmers the opportunity to integrate different tools for making an irrigation decision. Trials in Emerald and St. George resulted in saving at least one irrigation on both farms using canopy temperature sensors without impacting yield. This research enabled farmers to use an irrigation scheduling tool that is based on real-time monitoring of a crop’s need for water. A commercial partner has been identified to extend canopy temperature approach of irrigation scheduling to the Australian cotton industry.

Some important future research areas are: 1) Utility of plant-based method of irrigating such as canopy temperature for optimizing crop water use efficiency from the perspective of less water use; 2) capturing spatial variability on large farms with canopy temperature sensors, and/or how many canopy sensors per farm are required to make the best irrigation decisions? 3) Continuing research on utility of canopy temperature sensors in partially irrigated systems, and 4) Utility of thermal images in assessing crop stress during early cotton season.

This project was funded by the Department of Agriculture and Water Resources, and the Cotton Research and Development Corporation through Rural R & D for Profit program.