Abstract
Effective soil layer is one of the important factors to determine the amount of irrigation water and to evaluate evapotranspiration from the soil for efficiently scheduling irrigation in Japan. Here we propose a new definition of the effective soil layer based on soil water flux, which can be used to determine the thickness of the effective soil layer of the field under the non-equilibrated soil water conditions. In addition, we developed a new method to determine the depth of the effective soil layer from the field monitoring data of soil moisture content. This method is based on the new definition and uses a computer application of numerical simulation of soil water movement, HYDRUS-1D. To show the detailed calculation procedure of this method, the depth of the effective soil layer in lily field located in Tsunan, Niigata, Japan, was determined using the field observation data. The depth was estimated to be 35 cm, although lily roots mostly reached up to a depth of 20 cm. Downward soil water movement was considered to be not dominant a day after heavy rainfall, suggesting that upward soil water movement was the main factor that dried the soil at depths of 0-35 cm. These results support the availability to determine the depth of the effective soil layer using our new method.
