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Title: Evaluating Various Water Stress Calculations in RZWQM and RZ-SHAW for Corn and Soybean Production

Author
item Kozak, Joseph
item Ma, Liwang
item Ahuja, Lajpat
item Flerchinger, Gerald
item Nielsen, David

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2006
Publication Date: 6/27/2006
Citation: Kozak, J.A., Ma, L., Ahuja, L.R., Flerchinger, G.N., Nielsen, D.C. 2006. Evaluating Various Water Stress Calculations in RZWQM and RZ-SHAW for Corn and Soybean Production. Agronomy Journal 98:1146-1155.

Interpretive Summary: Better understanding of water stress definition is needed to improve crop production simulation. In this study, two simulation models, the Root Zone Water Quality Model (RZWQM) and RZWQM-SHAW (Simultaneous Heat and Water) (RZ-SHAW) Hybrid Model, were evaluated for their ability to simulate corn and soybean growth and water use under a range of water conditions in the Central Great Plains. In both models, water stress index (WSI) was calculated based on a nonlinear form of the ratio of actual (AT) and potential transpiration (PT). Results showed that in general RZWQM using daily evapo-transpiration (ET) provided similar simulation for both corn and soybean production as RZ-SHAW using the hourly ET option, given that the same plant parameters were used. However, RZ-SHAW with SHAW ET provided the worst predictions for corn and soybean growth. The reason for the ill-behavior of SHAW ET was due to differences in AT and PT calculations as that in RZWQM, which might warrant recalibration of the plant parameters. The results also showed that, when two models were linked to develop a hybrid model, it was essential for variables to communicate correctly between components of the two models, not only in unit but also in magnitude. This study further demonstrated that, when RZ-SHAW is used for plant simulation, AT and PT should be calculated the same way as in RZWQM to preserve the plant parameters. Otherwise, recalibration of plant growth parameters is needed. This study also showed that the nonlinear form of WSI was applicable to both crops and all the ET calculations, although the alpha value varied among the two crops and various ET calculations.

Technical Abstract: Water stress calculations in agricultural systems models are empirical and yet control all biological processes under sub-optimum water conditions. Better understanding of water stress definition is needed to improve crop production simulation. In this study, two simulation models, the Root Zone Water Quality Model (RZWQM) and RZWQM-SHAW (Simultaneous Heat and Water) (RZ-SHAW) Hybrid Model, were evaluated for their ability to simulate corn and soybean growth and water use under a range of water conditions in the Central Great Plains. In both models, water stress index (WSI) was calculated based on a nonlinear form of the ratio of actual and potential transpiration [WSI=(AT/PT)alpha, 0