Evaluating strategies for improved water use in spring wheat with CERES David B. Lobell a, *, J. Ivan Ortiz-Monasterio b a Energy and Environment Directorate, Lawrence Livermore National Laboratory, P.O. Box 808 L-103, Livermore, CA 94550, United States b International Maize and Wheat Improvement Center (CIMMYT), Wheat Program, Apdo. Postal 6-641, 06600 Mexico D.F., Mexico 1. Introduction Wheat is the most widely grown crop in the world, with irrigated wheat systems contributing over 40% of wheat production in the developing world (Pingali and Rajaram, 1999). Efficient use of water in these systems is increasingly important as demand for both food and freshwater resources is rising (Wallace, 2000). One component of improved water use in these regions is a better quantitative understanding of the relationship between irrigation practices and grain yield. With this knowledge, the value of each unit of water applied to a field can be estimated and compared with alternative uses within and beyond the agricultural sector. Experimental trials provide a well-established means to evaluate wheat yields under different irrigation scenarios. However, only a limited number of different management agricultural water management 84 (2006) 249–258 article info Article history: Accepted 21 February 2006 Published on line 5 April 2006 Keywords: Deficit irrigation Irrigation efficiency CERES Wheat Climate variability Soil heterogeneity abstract More efficient use of water in agricultural systems is widely needed. However, most irrigated systems are characterized by heterogeneous climate and soil conditions that interact strongly with irrigation management, making optimal irrigation decisions difficult to achieve. Here we investigated the impact of reduced irrigations on spring wheat yields in the Yaqui Valley of Mexico, a region experiencing increased water scarcity. Two years of field experiments containing three irrigation treatments each were used to evaluate the CERES-wheat crop model, with good agreement between observed and modeled yields. The model was then used in a sensitivity analysis whereby seven irrigation strategies were applied across a range of possible soil and climatic conditions. Results indicated that yield losses from reduced irrigations depend greatly on year, corresponding to large variations in rainfall between growing seasons. Estimates of the best timing strategy for a given number of irrigations were more robust with respect to climate variability. Soils also exhibited a strong interaction with irrigation, with the difference between initial soil moisture and wilting point deemed particularly important in this system. The optimal economic strategy was determined for each hypothetical soil based on the observed historical distribution of growing season climatic conditions. The results of this study demonstrate the need to consider soil and climate variability when interpreting experimental results, and the ability of the CERES model to serve this need by quantifying the relative importance of different heterogeneous factors. # 2006 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 925 422 4148; fax: +1 925 423 4908. E-mail address: dlobell@llnl.gov (D.B. Lobell). available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/agwat 0378-3774/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2006.02.007