Thus, WUE is key for agricultural production with limited water resources. Policymakers and water resource managers working at all scales need to evaluate the many ways in which cropping systems and the amounts, timing, and methods of both irrigation and fertilizer applications may be changed to improve WUE while meeting yield and harvest quality goals.
Although field experiments are too costly to address all scenarios, computer models of crop growth and yield may fill in the gaps if the models are shown to be accurate WUE predictors. An international team of experts led by the Food and Agriculture Organization of the United Nations (FAO) developed an agronomic model called AquaCrop to address the need for modeling WUE under widely varying conditions around the world and with limited data.
To look at the ability of this and other agronomic models to estimate WUE, a team member with USDA-ARS in Bushland, TX organized a symposium titled “Yield Response to Water: Examination of the Role of Crop Models in Predicting Water Use Efficiency” at the 2007 International Annual Meetings of ASA–CSSA–SSSA. Results using several agronomic models were discussed. Nine papers arising from the symposium have been published in a special section of the May–June 2009 issue of Agronomy Journal.
The papers explore how four of the simulation models were used to simulate yield, water use, and WUE of cotton, maize, quinoa, and sunflower in North and South America, Europe, and the Middle East. All the models simulated WUE adequately under well-watered conditions but tended to over- or underestimate WUE under conditions of water stress. This limits their usefulness for exploration of deficit irrigation scenarios or rainfed or dryland situations with less-than-adequate water.
None of the experimental conditions reported involved separate measurements of evaporation of water from soil (E) and transpiration of water by plants (T); so there was no opportunity to test the values of E and T calculated by the models, which is key to accurate water use simulation. 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 evaporation or transpiration measurements in addition to total crop water use measurements so that effects of management methods that reduce evaporation in favor of transpiration can be studied and models of WUE tested and improved,” says symposium organizer Steve Evett.
Evett says this examination of WUE estimation by multiple models helps close the gap that exists between what can be done using crop simulation models and what policymakers and managers need from these models in order to develop useful management alternatives for crop selection and timing, tillage systems, and irrigation and fertilization practices. Development of AquaCrop and several other models is continuing and will be guided by the findings of these studies.
Evett, S.R., and J.A. Tolk. 2009. Introduction: Can water use efficiency be modeled well enough to impact crop management? Agron. J. 101:423–425. View the abstract online at http://agron.scijournals.org/cgi/content/abstract/101/3/423