"7000 ft" sounds wrong to me. That's over a mile of hose. Feels like that's unnecessarily long. I'd love to learn more about this. Anyone know when or what fire this was?
I wonder if maybe it can't even use hydrants that are too near each other in the plumbing graph.
There's a lot of variables in that equation. For example, say you have a "dead end" main that ends somewhere near the fire. If you connect to the last hydrant on the main and start flowing water, there's a good chance you won't get a lot of additional water by connecting to the next hydrant up the street. But if you connect to a hydrant that's on a main that is part of a loop, there's a better chance you'll be able to get more water by doing that.
And without getting into too much detail that would be boring to non-firefighters (probably)... there's actually two big variables for a given hydrant: the maximum volume of water it can supply (in GPM) and the pressure available at the hydrant. And those two things are related. Anyway, net-net, you can have a hydrant that is capable of - in principle - flowing, let's say 2000 GPM. But the pressure at the hydrant is only, say, 40 psi. That means you only have 20 psi (approximately) available[1] to overcome the friction loss in the supply hose between the hydrant and the engine. And that friction loss in turn is a function of the hose size and the flow rate.
Anyway, that results in a situation where you might have a hydrant that could supply you 2000GPM, but if your fire is, say, 1500 feet away, you might effectively only be able to take advantage of maybe 500GPM of that.
And that in turn leads into stuff like using a "four way" or "hydrant assist" valve, or having a relay engine sitting right on the hydrant (to minimize friction loss between the hydrant and the engine) and then using its pump to boost the pressure going to the attack engine. By using multiple engines like that, you can get closer to achieving that hypothetical 2000GPM (or whatever) flow.
It gets pretty complicated, but fortunately fires in urban areas where the municipal water system is the limiting factor seem to be relatively uncommon (but not unheard of!) in this day and age.
[1]: because you don't want to pull the residual pressure down too low or it can damage the water system, supply hose or your pump.