Abstract: The water justified system for construction projects provides an important basis for approving an application for water-intaking. In the first place, the scope of water resources demonstration should be determined. A method was presented to calculate the cone area of water level decrease, which was based on principle of water balance. The method formulates the water balance equation in the balance region, in which the area of cone of depression of water level was unknown, and then the unknown cone area was calculated inversely. The cone area provided scientific basis for the scope of water resources demonstration.