I think it's because water moves on a curved surface with centripetal force. Because the gravity of water provides centripetal force, the pressure of water on the curved surface is reduced. When the velocity of water increases, the pressure of water on the curved surface will be more reduced. When the velocity of water is large enough, it separates from the curved surface, because gravity can no longer provide centripetal force. But some people think that viscosity reduces the pressure of water on curved surfaces. Are they right?
This surface is a convex surface.