Help: calculating Water flow rate from water pressure

Look in a fluid dynamics book. You may have to turn it around, and look for the head loss in a pipe given the velocity, then solve for velocity from pressure.

Don't expect much precision -- fluid dynamics depends on a number of hard-to-control variables.

How is "resistivity" defined? If it's given as (head loss per unit length)/(flow rate), just plug in the numbers, but be aware it's a fiction. "Resistivity" so defined depends on flow rate, pipe diameter, boundary layer thickness, number of bends, bend radius, .... There's a lot of theory behind fluid dynamics, but a lot of empiricism too.

Jerry

Lots depends on whether you are considering a laminar or a turbulent problem (see the text books already referred to, on how to discover which flow regime you're in).

Very roughly (fully roughly for turbulent flow, in fact... fluid dynamics joke) relationship between pressure drop and flow (Q) is related by DP=[K*|Q|]*Q, where K is a constant.

For laminar flow DP=[k]*Q, where k is a different constant. You could take the [brackets] as analogous to resistance, if DP is analogous to V, and Q analogous to I.

Works for your old garden hose (which is turbulent at flows beyond a teaspoon-a-second...)

John

Indeed. Have you tried explaining to intuitive thinkers why roughening the pipe wall decreases the head loss at certain Reynolds numbers?

Jerry