DEEP DRAINAGE ESTIMATES FOR IRRIGATED CROP ROTATIONS IN SUBHUMID, SUBTROPICAL AUSTRALIA

Deep drainage rates were modelled for irrigated crop rotations in terms of typical cropping sequences that differ in crop characteristics and irrigation demand. A forage–grain–pasture sequence (sorghum, wheat and lucerne) with supplementary irrigation had an average deep drainage of 11 mm yr−1, transpiration of 703 mm yr−1 and irrigation demand of 326 mm yr−1. In contrast, a vegetable sequence (broccoli, lettuce, bean and sweetcorn) with adequate irrigation had a deep drainage of 121 mm yr−1, transpiration of 602 mm yr−1 and irrigation demand of 598 mm yr−1. Under similar growing conditions, deep drainage decreased as rooting depth and cropping duration increased, and as irrigation and rainfall decreased. Deep‐rooted, stress‐ tolerant crops such as lucerne and sorghum maximised water use and minimised the deep drainage risks through the use of moisture from preceding well‐irrigated vegetable crops and extraction of moisture from deeper soil layers. The study suggests that deep drainage risks under vegetable rotations can be minimised through the selective incorporation of these crops and improved irrigation timing and application rates. These understandings will help to develop water use efficient and economic management strategies under crop rotations. Copyright © 2014 John Wiley & Sons, Ltd.