Abstract: Based on the knowledge of the SPAC system, a conceptual model of water balance in winter wheat root zone was set up. The model assumed vertical flow of water only. The evapotranspiration was estimated by combining the reference evapotranspiration, crop coefficient and soil water stress coefficient. The crop coefficient is the function of crop LAI, and the soil water stress coefficient is the function of soil water content. The lower boundary flux was modeled as a function of soil water storage in the root zone empirically. Fundamental data required to run this model included the daily meteorological data, soil properties (such as soil type, wilting point and field capacity), the irrigation data, the initial values of soil water storage in root zone, LAI of crop, and the some other empirical parameters involved. The empirical parameters should be calibrated prior to the model application. Empirical parameters in the model were calibrated firstly and the model was validated independently with the winter wheat plot test data in Yucheng Station and Yongledian Station. The results showed that the simulated values by the model agreed well with the measured ones. This model contains only a few parameters and is simple in form. So it can be easily adopted in field water management. When it is used, prior calibration of the parameters in the model should be done with the field data.

Key words: the soil water storage of root zone, water balance model, winter wheat