Hi Emily! We will be installing a rainwater harvesting system for our home that will be built later this year. This system will service our entire household water needs. Here are a couple specifics you will need to know if you want to go ahead with this:
1. You will need 7500 gal per person. For my household of 4, we will be getting a 30,000 gal tank or several equaling that.
2. To figure out how much rain you'll be able to harvest: average rainfall X sq footage of roof X .623
We are having the system installed professionally, since this is for home use. For the land, I have no idea how to calculate what the watering needs would be. I'm sure it wouldn't be as complicated (and costly) a system as it would be for human use and consumption. I would go as big of a tank that you can afford AND that fits the formula (#2) above. For instance, no need to invest in a 5,000 gal tank when calculations only come up to 2,500 gallons of water catchment.