This paper estimates the probability distribution of the excess demand of water on the paddy field in Goseong Gun. Very severe drought in 1992 had urged Korean government to make a fresh water lake named Madong for suppling agricultural water by building a see wall in Danghangman of south coast in Korea. However, water demand as well as the capacity of water supply of existing reservoir had not been explored before the planning of Mandong lake. This caused a campaign against the project of making fresh water lake. For evaluating the validity of the project, this paper examines the daily volume of water during 1992~2008 in Gaega reservoir which is the largest water source in the Goseong Gun. Daily agricultural water demand for the same period is estimated using Rural Agricultural Water Resource Information System. Based on the calculated excess demand of agricultural water, we estimate Kernel density of water shortage. The estimated probability of water shortage in Daega-area is between 10 and 29 percents.