Estimation of Evapotranspiration and Crop Coefficients of Summer Soybean in Guanzhong Region

The crop coefficient-reference crop evapotranspiration method is the popular method to calculate water require of crop.As an important parameter for this method,the determination of crop coefficient has become a key question to study water requirement of crop.According to the field experiments from 2005 to 2007,reference crop evapotranspiration in Guanzhong were calculated by using Penman-Monteith equation and actual crop evapotranspiration were achieved by water balance equation and adjustment coefficient of soil water.Crop coefficients of soybean in different stages were calculated based on the above results.Variations of crop coefficient were also analyzed.The results indicated that average reference crop evapotranspiration during the growth stage in Guanzhong was 524.6mm.Average crop coefficient for the whole stage was 0.82.The highest value was 1.22,occurring at the stage from flowering to podding,the second is the stage from podding to maturity with the value of 1.05 and the lowest value is at the stage of seeding to seedling of 0.26.There was a good relation ship among crop coefficient of soybean and the cumulative temperature above 10℃.