In the course of modernization of agricultural techniques, especially consolidation of terminal irrigation facilities developed in Japan during the last decades, water requirement for irrigation has considerably increased, being accompanied by its magnified fluctuation. To rationalize water use in irrigation, attempts have been made to improve facilities. However, the present canal systems in Japan do not justify the increased fluctuation in water requirement. To resolve this problem, a buffer function has to be adequately incorporated to the canal systems so that water resources could be utilized effectively. With a large buffer function, the water management could be more easily operated. However, in view of limited size caused by socio-economic conditions in the country, an optimum size of buffer function has to be designed. The basic concept of buffer function and the necessity of its enhancement are described in the earlier part of the present paper. A method for deciding an adequate buffer capacity, taking into consideration changes of stored water in canals and delay time in operation, is proposed in the present part. A proposed formula is: N_v=△V+∫_T1^T2(Q₀(t)-Q_in(t))dt, where △V is changes of stored volume and the second term of right member means volume in delay time in operation. Availability of the proposed method is confirmed for various canals using an unsteady flow simulation model. Effects of various countermeasures for the enhancement of buffer function including cheek-gates are also examined.