Re: how do i calculate pressures from flow velocities?

Sandra,

When dealing with fluid flow, there are two type of pressure: static and dynamic. When you add the two together you get Total Pressure:
Ps+Pd = Pt

Pd = (1/2)*(density)x(velocity)^2

Total pressure is what you measure when the fluid is completely at rest, and this remains constant (assuming you aren't changing depth or altitude). When the fluid begins to move, the dynamic pressure term in the first equation above increases, so in order for Pt to remain constant, Ps (static pressure) needs to drop. You can think of this in terms of Energy... total energy remains constant, so as the flow starts to move, some of the static pressure is becoming kinetic energy.

This is the principle that allows airplanes to fly. As the wing moves through the air, the top surface of the wing is curved, so the air moving over the top of the wing has to move faster than the air going under the wing. So, the static pressure on top of the wing is lower than under the wing. The difference in pressure multiplied by the surface area of the wing is the lift.

So, back to your question. The equation I show above is called Bernoulli's Equation, and you can Google that term for lots of information on it. There are some limitations... in particular this equation assumes inviscid (frictionless) flow. However, for low velocities, this may not matter. I suspect this would be the case for you, but if you're having trouble getting your model to match reality, you may want to look into it.

So, since you know the water pressure at the depth the fish are eating, you would simply subtract the Pd term to get the static pressure along flow in your model.