It is reported that Korea is a county of water shortage. The government make plans to construct dam to cope with this problem. But, it become difficult to construct dam, because of public opinion of residents and objection of environmentalist. In the sense, the government set up new policy of demand-management which means water saving. It is known that the efficiency of agricultural water is so poor that the first target to save water is focussed on agricultural water. In the study, the function of agricultural water was evaluated and it is found that it is not a waste, but a good ability on eco-system by return flow.