Let's say you have a tank of water open to the air and poke a hole at the bottom of the tank, where the water is flowing out of the hole at some velocity, which we will call V2. According to Bernoulli's principle equation, P + 1/2*rho*v1^2 + rho*g*h1 = P + 1/2*rho*v2^2 + rho*g*h2, we are referring to two points involving the tank: at the top of the tank water level, where h1= some number (since h is relative to the datum set at the hole) and the point at the hole, where h1 = 0. We also assume that velocity is insignificant at the top, so v1 = 0. We then set up an equation that looks like this and then just solves for the velocity at the point of the hole, or v2.
My question is, what is v1, or the velocity at the top of the tank? Is it referring to the change in position of water molecules, as the water level drops (velocity = displacement/time)? Is it referring to the velocity of the individual water molecules at the top of the tank water level? I just need someone to explain what makes v1 approximately 0.