Water exerts elevation pressure by virtue of it's height above a given datum e.g. sea level so that a 50 foot deep tank filled with water and located on a 50 foot hill or tower would exert the same pressure as the 100 foot column of water i.e. 43.3 psi relative to sea level. This makes hill tops a favorite for locating water tanks. The pitot pressure from a hydrant flow test is a direct measure of the velocity pressure. Using a formula derived from Toricelli's Law, the velocity pressure is converted into the flow from the hydrant. The flow from a discharging sprinkler is estimated by multiplying the K-Factor with the square root of the pressure at the sprinkler.
Friction losses occur as water flows through fire sprinkler piping.
Part of the friction loss is from direct contact with the interior of the pipe, fittings and devices such as valves. Part of it is due to turbulence within the fluid. The smoother the pipe the lower the friction loss. The Hazen-Williams equation is simple and works quite well in estimating friction loss under conditions of smooth or laminar flow. The Darcy-Weisbach equation is more complicated but it is much better at estimating friction losses when flow is not smooth i.e. turbulent flow.