This study goes deeply into the practical business affairs of management which is a theory matching with practical work conditions, use past rainfall data to modulate and deduce the daily or monthly effective rainfall, and use monthly effective rainfall occurance probability to derive the planned effective rainfall. The result shows that：(1).Use the irrigation operation method of which the Tao-Yuan Irrigation Association at present proceeding management method, the computation mode of the effective rainfall is to use the existing rainfall water famine days to seek for the effective rainfall on reverse. (2).as for the computation of planned monthly effective rainfall, it is too conservative to use the existing the fourth dry method, which should be modified to use the value of 75％ of the monthly effective rainfall based on the normally distributed occurance probability：and (3).one line of lateral use one rainfall gauge station to compute its effective rainfall method will obtain the result which is quite close to the value obtained from by using Thiessen's average rainfall method, therefore, it may use the above mentioned simplified method.
