The variation of the urban water demand and the amount of water in the distribution reservoir was studied with time for a day. The city of Kwangju in Korea was selected as a study area. The population of Kwngju in the end of 1993 was more than one million and two hundred thousand peoples. The average of daily water use in 1993 was about three hundred and fifty thousand peoples. The average of daily water use demand related to the amount of water in the distribution reservoir was observed for the given time interval of a day. The water use index was defind in percentage that indicates the ratio of the amount of water use for a time interval to the amount of water use for a day. The water use index with consideration of the water stored in the reservoir was found to be useful to manage ad to operate the water supply systems in real time.