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Modeling

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

^a USDA-ARS, Agric. Syst. Res. Unit, Fort Collins, CO 80526
^b USDA-ARS Northwest Watershed Res. Cent., Boise, ID 83712
^c USDA-ARS Cent. Great Plains Res. Stn., Akron, CO 80720

^* Corresponding author (liwang.ma{at}ars.usda.gov)

Received for publication November 6, 2005. Better understanding of water stress calculation is needed to improve crop production simulation. In this study, the Root Zone Water Quality Model (RZWQM) and RZWQM-SHAW (Simultaneous Heat And Water) (RZ-SHAW) Hybrid Model were evaluated for simulating corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] growth and water use under a range of water conditions in the Central Great Plains. In both models, a water stress index (WSI) was calculated as a nonlinear form of the ratio between actual transpiration (AT) and potential transpiration (PT) [WSI = (AT/PT), 0 < < 1]. Evapotranspiration (ET) was calculated using the Shuttleworth–Wallace approach on a daily basis (ET_SW-DAY) in RZWQM and using either the Shuttleworth–Wallace approach on hourly basis (ET_SW-HR) or the SHAW approach (ET_SHAW) in RZ-SHAW. Results showed that RZWQM using ET_SW-DAY provided similar simulation for both corn and soybean production as RZ-SHAW using the ET_SW-HR option, given that the same plant parameters were used. However, RZ-SHAW with ET_SHAW provided less accurate simulations for corn and soybean growth. This study 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 value varied from crop to crop and among ET calculations. Based on the results, it is suggested to use = 0.75 for corn and = 0.5 for soybean in RZ-SHAW with the ET_SW-HR option.

Abbreviations: AT, actual transpiration • ET, evapotranspiration • ET_SHAW, potential evapotranspiration calculated using the SHAW approach • ET_SW-DAY, potential evapotranspiration calculated using an extended Shuttleworth–Wallace approach in RZWQM on a daily basis • ET_SW-HR, potential evapotranspiration calculated using an extended Shuttleworth–Wallace approach in RZ-SHAW on an hourly basis • LAI, leaf area index • PET, potential evapotranspiration • PT, potential transpiration • RMSE, root mean square error • WSI, water stress index