With some reasonable approximations, the kinetic energy of water leaving the the container is proportional to the pressure difference inside<->outside, which is proportional to the water level above the hole. The velocity of the water is then proportional to the square root of this value: v^{2}= 2 h g with the height difference h and the gravitational acceleration g=9.81m/s^{2}.

The flow through the hole is then given by A*v with the area A.