Increasing Water Use and Water Use Efficiency in Dryland Wheat

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Abstract
Water use efficiency (WUE), the ratio of grain yield to crop water use, provides a simple means of assessing whether yield is limited by water supply or other factors. Based on this assessment, yields of commercial dryland wheat (Triticum aestivum L.) crops in southeastern Australia are usually not limited by water. Transpiration efficiency (TE), the ratio of yield to transpiration, is relatively stable for well‐managed crops, but the amount of water used is strongly affected by crop management. In a review of 13 comparisons of water use and wheat yield, providing optimum N fertilizer or suppressing cryptic root diseases with break crops increased water use by 23 mm and yields by 378 kg ha−1, equivalent to 10% of the control yields. The additional soil water was extracted to levels of water potential as low as ‐5 MPa. A possible means of increasing yield potential of dryland crops is to manage transpiration so that relatively more water is used during the vegetative phase when vapor pressure deficit (VPD) is low, and hence TE is high. However, based on budgets of soil water and soluble carbohydrates stored in the vegetative organs and available for retranslocation, this option provides lower TE than conserving soil water for transpiration until grain filling when assimilates are directed to grain. Increasing the proportion of water transpired during the vegetative phase with N fertilizer can lead to particularly inefficient water use because increasing N status generally reduces the soluble carbohydrate reserves available for retranslocation to grain.