One inch of rain over a small sized 1000 square feet roof will actually collect 600 gallons of water. We had this catchment discussion on a previous thread. It's true it might be more cost effective to install a drip system than a catchment to lower water costs. But if you are trying to grow delicate tropical plants it's very good to have the rain water. Best is to do both: drip and catchment if possible.
"Installing a water catchment system takes some time and planning. Do you have kids? If so they will love you for installing a doughboy type pool. During the summer they can use it to frolic around in. During rainy winter you use it for catching water and watering your plants. It's only a matter of installing a connecting 3 inch pvc pipe to the pool from your rain gutter downspout, and getting a small pump on the doughboy to pump out the water.
Here is some info on how much water you can catch off your roof. Probably a lot more than you think!
If you have 1,000 square feet of roof on your house, and it rains just 1 inch, you can collect 600 gallons of water to be used in your garden, for washing your car, or just for drop irrigation around your property. That’s 600 less gallons you have to pay for and use from your town water supply! So how can you do the math for your own roof? Just multiple the square footage of roof space you have available X 0.6 gallons per square foot per inch of rain, and you can see how much water you can collect from each inch of rain that falls.
So if you have 2,500 square feet of roof available for water catchment, and a single inch of rain falls one day, we see that:
2,500 X 0.6 = 1,500 gallons of water can be harvested for future use…from only one inch of rainfall!"
from:
http://thegoodhuman.com/2008/08/25/just-how-much-rainwater-can-you-collect-off-your-roof/Even with only 3.5 inches of rain in S. Cal. you can see that with a 2500 sq. ft. roof you missed out on catching a whopping 4,500 gallons! Not a small amount at all."
http://tropicalfruitforum.com/index.php?topic=8830.msg114046#msg114046Right now unfortunately most of the rain falling in California goes straight into the ocean. Very little of it is caught and stored. See related news story:
http://news.yahoo.com/rain-pummels-california-see-way-fight-drought-070830101.html