Abstract
Reliable water supply for farming is a global concern, including the Lockyer Valley region, Australia. Knowledge of rainfall variation and resulting irrigation demand and deep drainage can improve crop water use and minimize salinity. Therefore, we modelled irrigation demand and deep drainage for different land‐use types of the Lockyer Valley and classified them according to rainfall. During the periods of extremely low rainfall (mean = 468 mm yr ‐1 ), deep drainage fell by 35–239 mm yr ‐1 , whereas irrigation demand rose by 160–310 mm yr ‐1 , relative to extremely high rainfall (1138 mm yr ‐1 ). With all land‐use types, deep drainage rose with water input, but it was consistent in the order (mean, mm yr ‐1 ): lucerne (8–11), native grass (13), sorghum–wheat sequence (37–64), bare fallow (169) and sweetcorn–broccoli–bean sequence (225). Decadal trends in irrigation demand and deep drainage signalled a raising future irrigation demand; and in turn potential deep drainage risks if irrigation is not managed wisely. These findings will have broader implications for crop and environment management under rainfall variation. Copyright © 2015 John Wiley & Sons, Ltd.