‘Dynamic Deficits' Improving Irrigation Decision Making in Australian Cotton Furrow Irrigation.

Current irrigation strategies for furrow-irrigated cotton in Australia use schedules that are aligned to fixed irrigation points (a soil-water deficit) based on measurements or previous experiences for particular fields. In many instances the irrigation point is based on average climatic conditions and soil water status to prevent plant stress and does not take into account the actual or future level of plant stress in response to these variables.  To improve irrigation water use efficiency (IWUE) a flexible or ‘dynamic’ soil water deficit may need to be employed in furrow irrigation scheduling to more effectively match irrigations with potential crop stress and short-term forecasted climatic conditions. The recent availability of more accurate short-term (up to 7days) forecast of evapotranspiration (ETo) could enable the prediction of crop stress to guide decisions on timeliness (either earlier or later) of irrigation events. This is important as most of Australia’s cotton production experiences significant in-crop climatic variability. Yield and crop water use were measured in four large-scale field experiments with treatments that compared a fixed deficit approach to a ‘dynamic deficit’ approach varied according to the short-term ETo forecast. There were two dynamic deficit treatments, one where irrigations were brought forward when the forecasted 72 hr ETo > 7mm/day (smaller deficit), and the other where irrigations were delayed when the forecasted ETo was for < 5mm/day (larger deficit). Yield was not significantly different across the different irrigation treatments in any of the experiments. In two experiments rainfall occurred prior to irrigating the larger deficit treatment, resulting in an increase in IWUE. In this study there was no benefit in irrigating earlier in response to high ETo, however there was utility in delaying irrigation timing when ETo was low, extending opportunities to capture rainfall with potential for irrigation water savings in furrow-irrigated cotton systems.