I finally figured out how to automatically water with the rain water that I have collected!
Once you harvest rain water, the trick is easily watering your garden.
Regular dripline won't work: the holes are too big and need more pressure than a rainbarrel can supply. I made my own dripline which works with low pressure, and low volume. It is very reliable.
I used 1/2 inch polytubing. You can buy 20 feet or 50 feet or 100 feet at a time. It is available at Lowes or home depot. It is much cheaper online. The trick is to punch VERY tiny holes in the tubing. **More on this later**.
Then, I bought some programmable battery-operated timer valves (ie Hose Water Timer). These vary in how many zones they can water. More zones are better because if you try to water your entire garden with one zone, you will not have enough pressure to reach all the plants. Also, if you have different height-levels in your garden, you'll need a zone for each level. All the height-levels need to be lower than the bottom of your rain barrel.