Rainwater harvesting amount per season??

I put in a 5000 gallon koi pond in my backyard 4 years ago. I just put in a rain collection system last year to help with supplying water for the pond and for watering trees and shrubs that i put in at the same time as the pond. My collector only holds 120 gallons. I have found that with a half inch of rain my system fills up from empty. I live in Atlanta GA and we are in a horrible drought but I have found that I would be able to water my trees and shrubs along with filling water to my pond. The cistern has worked for centuries and can continue to be a great et if managed properly.