Introduction: Can Water Use Efficiency Be Modeled Well Enough to Impact Crop Management?

Crop water use efficiency (WUE, yield per unit of water use) is key for agricultural production with limited water resources. Policymakers and water resource managers working at all scales need to address the multitudinous scenarios in which cropping systems and amounts, timing and methods of irrigation, and fertilizer applications may be changed to improve WUE while meeting yield and harvest quality goals. Experimentation cannot address all scenarios, but accurate simulation models may fill in the gaps. The nine papers in this special section explore how four simulation models were used to simulate yield, water use, and WUE of cotton ( Gossypium hirsutum L.), maize ( Zea mays L.), quinoa ( Chenopodium quinoa Willd.), and sunflower ( Helianthus annuus L.) in North and South America, Europe, and the Middle East. All the models simulated WUE adequately under well‐watered conditions, but tended to misestimate WUE under conditions of water stress, which limits their use for exploration of deficit irrigation scenarios or rain‐fed or dryland situations with expected soil water deficits. None of the experimental conditions reported involved separate measurements of evaporation ( E ) and transpiration ( T ); so there was no opportunity to test the separation of E and T simulated in the newest of the models, AquaCrop. The lack of separate E measurements also limited the authors in exploring reasons why WUE was not simulated well under water stress conditions. Future studies exploring WUE simulation should include E or T measurements so that effects of management methods that reduce E can be studied.