Fourteenth International Water Technology Conference, IWTC 14 2010, Cairo, Egypt  SIMULATION OF THE EFFECT OF WATER STRESS ON SOYBEAN YIELD AND WATER USE EFFICIENCY Samiha A. Ouda 1* , Mouhamed A. Shreif 1 , and Rashad Abou Elenin 2 1 Water Requirements and Field Irrigation Research Department, Soil, Water, and Environment Research Institute, Agricultural Research Center, Egypt 2 IBS Coordinator, Agricultural Research Center, Egypt *Corresponding author E-mail: samihaouda@yahoo.com ABSTRACT Yield-Stress model was calibrated and validated for soybean yield and consumptive use with data of four-year field trial at Beni Sweif Governorate, Egypt under two treatments of irrigation water quality (fresh and drainage). The goodness of fit between measured and predicted values by the model was tested by calculated percent difference between measured and predicted values of yield and consumptive use, in addition to root mean squared error and Willmott index of agreement. Then, the model was used to predict the effect of four deficit irrigation treatments (95, 90, 85, and 80% of full irrigation). Water use efficiency was calculated under all cases. The results showed that the model performance was highly acceptable in predicting soybean yield and consumptive use. Low yield losses occurred under both fresh and drainage deficit irrigation water as a result of saving of 15% of irrigation water. Water use efficiency was the highest under irrigation with 80% of full fresh or drainage water irrigation, which suggested that there is a high potentiality to save an ample amount up to 20% of the irrigation water to be used in cultivating more lands and increase soybean production. Keywords: Yield-Stress model, Consumptive use, Fresh and drainage water irrigation INTRODUCTION Soybean occupies a unique position in science and agriculture, in addition of being a crop with enormous uses. Soybean is grown in almost all parts of the world for human consumption, industry and animal feed (Boydak et al. [1]). In Egypt, soybean growth period ranges usually between 100 and 120 days and requires 325-436 mm of irrigation water depending on the location (Ainer et al. [2]). The most important phenological growth stages of soybean, which need adequate water, are during pod development and seed fill (Kranz et al. [3]). These are the stages when water stress can lead to a significant decrease in yield. Stressful conditions, such as moisture deficiency reduces soybean yield. As the soybean plant ages from R1 (beginning bloom) through R5 (seed enlargement), its ability to compensate under stressful conditions decreases and yield losses could increase (Foroud et al. [4]).