This first picture is of our second generation rain collection setup off of the barn roof.
The plastic drum serves as a diverter to capture the initial water off the roof that has more dust and dirt and divert it from the primary collection tanks. The bottom of the 55 gal drum has a faucet connected to a hose to drain off for plants or stock tanks. Once the initial "rinse" of the roof is diverted the water fills up the drum and then rises to the level where it fill the big plastic water tanks. These black water tanks are 2500 gallons each and they are hooked together to equalize (level) as one is filled from the rain collection system.
That's my Son-in-Law who has got this thing rigged up!
Might a future business opportunity for him :)