If you really want to get serious about collecting and using rain water, you can install a larger tank, or cistern, that holds several hundred gallons of water and uses a small pump to distribute the water in the yard and garden. You will be able to store more water during the rainy season and it will be available during periods of low rainfall.
Here are some resources to help you get started:
Rainwater Cisterns: Design, Construction and Water Treatment. Penn State Cooperative Extension.
Cisterns To Collect Non-Potable Water For Domestic Use. University of Florida Extension.
National Sanitation Foundation information on rainwater collection.