Harvesting Your Roof: 

Even in areas where the rainfall averages only 12 inches a year, capturing rain from the roof can boast a garden’s productivity and beauty. Two overall ways exist. The first and cheapest is to direct the water to where you want it, with such techniques as a careful system of mulched and vegetated infiltration basins. These are tiered so that when the first is filled, the water flows naturally into the next and so on. Brad Lancaster's books describe these ideas very clearly. See Plant the Water First.

The second is to use tanks. Collecting roof catchment provides an important tool for saving water, plus helps erosion and heavy storm run-off. Tanks range in expense but provide water when and where it is needed. The more you first use direct systems, the smaller the tank can be. The goal in dry areas is to capture all the water from your roof and site, and not allow any run-off into storm sewers.
A water tank used for landscaping