1 Copyright © 2014. ASA, CSSA, SSSA, 5585 Guilford Rd., Madison, WI 53711-5801, USA. Pracical Applicaions of Agricultural System Models to Opimize the Use of Limited Water Lajpat R. Ahuja, Liwang Ma, and Robert J. Lascano, Editors Advances in Agricultural Systems Modeling, Volume 5. Lajpat R. Ahuja, Series Editor Quantifying Corn Yield and Water Use Eiciency under Growth Stage–Based Deicit Irrigation Conditions Quan X. Fang,* L. Ma, David C. Nielsen, Thomas J. Trout, and L.R. Ahuja Abstract An agricultural system model can help opimize limited irrigaion for higher grain yield and water use eiciency (WUE) across varied climaic condiions. In this study, the Root Zone Water Quality Model (RZWQM2) was irst cali- brated to simulate soil water, ET, and corn (Zea mays L.) yield under a range, 40 to 100% of crop ET under well-watered condiions (i.e., ET c , calculated by the reference ET and crop coeicient), of irrigaion treatments from 2008 to 2011 in Colorado. The model was then used to explore grain yield responses to irrigaion levels of 40, 60, 80, and 100% ET c (the Shutleworth– Wallace ET, ET sw , was used as a surrogate for ET c in the model) under 300, 400, and 500 mm of total irrigaion water and to provide guidelines to man- age limited irrigaion using weather data from 1992 to 2013. With 500 mm of irrigaion water, high grain yield and WUE were obtained from the 100% ET sw for the vegetaive stage and 80 to 100% ET sw for the reproducive stage. With 400 mm of irrigaion water, high grain yield and WUE were simulated at 80 to 100% ET sw irrigaion targets between the vegetaive and reproduc- ive stages. With 300 mm of irrigaion water, however, meeing 100% ET sw at the reproducive and 60% ET sw at the vegetaive stage achieved the high- est grain yield and WUE. Simulaions showed that meeing the crop water requirement during the reproducive stage is more important than during the vegetaive stage to achieve high grain yield and WUE under water-lim- ited condiions. Abbreviaions: CoAgMet, Colorado Agricultural Meteorological Network; CV, coeicient of variance; LAI, leaf area index; MD, mean diference; ME, model eiciency; RZWQM2, Root Zone Water Quality Model (version 2.6); UAN, urea–ammonium nitrate; WUE, water use eiciency. Q.X. Fang, Agronomy College, Qingdao Agricultural University, Changcheng Road 700, Chengyang District, Qingdao, Shandong, China, 266108. *Corresponding author (fangqx@igsnrr.ac.cn). L. Ma (Liwang.Ma@ARS.USDA.GOV) and L.R. Ahuja (laj.ahuja@ars.usda.gov), USDA-ARS-NPA-ASRU 2150 Centre Ave., Bldg. D, STE. 200 Fort Collins, CO 80526. D.C. Nielsen, USDA-ARS, Central Plains Resources Management Research, 40335 County Rd GG, Akron, CO 80720-0400 (david.nielsen@ars.usda.gov). T.J. Trout, USDA-ARS Water Management Research Unit, 2150 Centre Avenue, Bldg D, Suite 320 Fort Collins, CO 80526 (Thomas.Trout@ars.usda.gov). doi:10.2134/advagricsystmodel5.c1 Published December 5, 2014