Honestly I don't know why the flow is so terrible. I'm guessing either the pipes are too small, bent, or otherwise plugged up, if not all three. It's eventually hooked up to a 1" city water main, and builds up to 100PSI when it's turned off. Technically a well pressure tank would work here but I want several hundred gallons of capacity.
I could run a new line from the water meter to the barn but that would be around 300 feet and it's all covered in ashpalt or concrete, plus a whole lot of city fees. I also like to add rain water to my barrels when I can so I'm not wasting so much water....
not really knowing any thing about the situation, but I would suggest you talk to some plumbers or "NO DIG" OR "TRENCH LESS" or horizontal boring contractors, that either have pipe busting or pulling, or horizontal drilling equipment and see if they can pull a new pipe in or do a few short horizontal boars and slip a new pipe in, you may only need a few working holes,
(a local town near me replaced a lot of there water mains here a few years ago, and in the process they replaced the lines going to the homes, in 90% of them they hooked the back hoe on the old pipe and pulled a new pipe copper line in that was attached to the old steel line, and never dug a trench many of these were over 60 feet,
I my self have a small attachment that I can put on a good 1/2 drill, and bore up to a 7" hole 50' long,
Borzit Home (and there are other companies making a similar items,
Borit :: Hand-Held Earth Boring Tools
and consider replacing the line going to the barn,
I am all for the water storage, but having a good supply,it would be great thing to have the storage, but,
if you have a static 100 psi, and 1" line, one could have up to 40 gallons a min, and even if the pressure drops considerable one should easily still get 20 gallons a min,
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In doing some looking for the borit, tool ran across a few web sites that looked like some of the horizontal boring by a contractor may not be any or much more expensive than having it trenched,
It may be well worth some calls to get a real answers, on the cost of replacement,
having a good supply would be worth a lot,