In the course of recent modernization and diversification of agricultural technique as well as of consolidation of terminal irrigation facilities in Japan, water requirement for irrigation has been considerably increased and, at the same time, its fluctuation has also been magnified. To alleviate these constraints, the facilities for effective use of water resources such as main canals have been consolidated under various irrigation projects. However, the present canal systems are still not adequate enough in utilizing water resources effectively, since fluctuation in water requirement is not properly taken into account. Flexible water conveyance is required in order to appropriately correspond to the fluctuating water requirement. In addition, in the case of irrigation canal system consisted of open channels, a time requirement for irrigation water to reach fields has also to be considered. To meet there requirements, an installment of reasonable buffer function becomes important in the system. The buffer function has been incorporated in the systems so far usually by constructing regulating reservoirs, the capacity of which has been designed on the basis of such conditions as topography of land and economic viability. However, the buffer function has to be met by not only reservoirs but also check-gates and other relevant facilities. This paper, first, presents a basic concept of buffer function and necessity of its enhancement, and second, discusses some problems pertaining to the fluctuation water requirement in an irrigation canal system with a limited buffer function. A method for calculating buffer capacity will be presented in the following paper.