Improved Evapotranspiration Simulation in the CERES-Maize Crop Model Under Limited Irrigation Management.

An increasingly considered alternative to full irrigation practices is limited irrigation, where the crop is intentionally stressed during specific growth stages in an effort to maximize yield per unit water consumed, or evapotranspiration (ET).  Recent studies have shown that CERES-Maize crop model performs well under full irrigation but overestimates ET of corn under limited irrigation management.  The primary objective of this study was to improve CERES-Maize ET simulation under limited irrigation without introducing detrimental effects in regard to other model output responses.  Field experiments with corn were performed in northern Colorado, USA from 2006-10, where full (100% of ET requirement for an entire season) and limited (100% of ET during reproductive stage only) irrigation treatments were analyzed, and the model was calibrated in a previous study. Local sensitivity of model input parameters affecting ET was evaluated, prompting model code adjustment to create a dynamic crop coefficient (K_CD) as a function of the crop leaf area index.  The modified CERES-Maize model more accurately represented expected ET under full and limited irrigation, for example reducing late-season ET potential from a plant with reduced canopy and more closely matched FAO-56 crop coefficient curves under full irrigation.  Using the limited irrigation data for evaluation, the new model showed significant decreases in model error for seasonal cumulative ET (root mean square deviation RMSD from 80.9 mm to 49.9 mm) and water use efficiency (RMSD from 5.97 kg ha-mm^-1 to 2.86 kg ha-mm^-1) as compared to the original model.  The modified model was subsequently applied using the same northern Colorado, USA datasets under several hypothetical irrigation management strategies, with limited irrigation results exhibiting a water production function (yield vs. ET) different from the other management strategies.  An additional hypothetical model application (using strictly limited irrigation management strategies) was conducted indicating that reducing weekly vegetative water applications may increase simulated water use efficiency.