Assessing the maximum potential cooling benefits of irrigation in Australia during the Angry Summerof 2012/2013

Recent studies using global climate models have suggested that irrigation, assuming unlimited water supply and that all crops can be irrigated, may be an option for regional cooling in southeast Australia. In this paper, using idealised simulations, we aim to quantify the maximum possible benefits of irrigation in southeast Australia during a record breaking summer. By limiting the amount of irrigation applied to the actual amount used, and only to areas which are equipped for it, our study provides much needed practical context on the maximum potential benefits of irrigation for southeast Australia. We conduct simulations with the Weather Research and Forecasting regional atmospheric model to investigate the sensitivity to both land surface models and irrigation parameters (start hour, duration and frequency). All simulations used the same total amount of irrigation water. Our results show that irrigation could potentially reduce the mean seasonal maximum temperature during the Angry summer of 2012/2013 by -1.44 degrees C to -2.13 degrees C over irrigated regions. Maximum possible cooling was achieved by applying irrigation during the middle of the day, over 4 h, with daily application. During early hours of the morning, increasing the number of hours that irrigation is on has little to no effect on maximum cooling, but longer duration of irrigation later during the day leads to larger cooling. Similar results were found when decreasing irrigation frequency. The amount of cooling was strongly dependent on the land surface model, with use of the CLM model resulting in 2.32 to 2.66 times more cooling as compared to the Noah and Noah-MP land surface models, highlighting the importance of using multiple land surface models. Our results also highlight that considering realistic irrigation volumes and equipped-areas leads to cooling benefits that are largely local, and studies using global models that do not take this into consideration are likely over-estimating the potential cooling benefits of irrigation.

Automated summary

Recent studies show that irrigation may be an option for cooling southeast Australia, and this study aims to quantify its potential benefits. Using simulations, the study found that irrigation could reduce summer temperatures by -1.44 to -2.13 degrees C, with the most cooling achieved through midday irrigation over 4 hours. The choice of land surface model and consideration of realistic irrigation volumes and equipped areas were found to strongly influence the cooling effects. Global models that do not take these factors into account may overestimate the cooling benefits of irrigation.