water catchment.

I am new hear. and have searched the forum for water catchment. I am just starting mine, and would love to know your inputs. I just got my first 3 ibc totes. 275 gallons a pc.and got my rain gutters for one building. I have 4 buildings I wish to do this on. I live in the high desert of calif. and we don't get much rain hear. but when it does. it is a down poor. I am not sure how much water would come off one roof. but I am starting with 3 totes, off one building of 2 sides of 20 feet. connected together to fill those tanks. my other building are much bigger. and I have no clue how beg I should go with tanks. any relply would be great. and if you live in a low rain area, and done anything, that would be great. thanks Evie

Here's a link to help you figure out how much water will come off of your roof.

http://www.friendsoflittlehuntingcreek.org/description/roof.htm