Abstract: From the relationships between horizontal and vertical wetting distances and the tunes of water application and the wetting patterns under isolated drippers, the restriction conditions of impiral cofefficients of the tune dependent functions of wetting distances were advanced and verified. A rational design method based on the time dependent functions was hence developed, which can be applied to determine both the wetted percentage while wetting front not exessing the planned irrigation depth and that as well as the wetted soilvolume by seepage water and the seepage water amount while wetting front exessing the planned irrigation depth.From experimental results of wetting front under isolated drippers, coefficients of the time dependent functions for wetting distanceswere thus obtained. And some design examples were given to demonstrate the procedures of applying this method to practic design.