For a reservoir-type simulation, the lowest elevation in the reservoir is found by subtracting the largest hydraulic height among any of its associated impounding structures from the reservoir's maximum water surface elevation. The volume at this lowest elevation is assumed to be zero. However, the reservoir's given normal volume is also zero, but the normal elevation is above the lowest elevation. A reservoir cannot have the same volume at two different elevations. This error is usually caused by incorrect reservoir normal or maximum water surface elevations or volumes, or incorrect impounding structure hydraulic height(s).