Though Seattle receives 36 to 38 inches of rainfall each year, summers tend to be dry. So between September and May, Carlson collects over 4,000 gallons of rainwater off her roof, storing it in three cisterns lined up against the back fence. In this way, the 950-square-foot roof provides sufficient water to keep the garden going all summer, including the lawn and vegetable beds. When it's time to water, Carlson simply attaches a garden hose or hooks up a sprinkler to a small utility pump submerged in each of the 625-gallon polypropylene storage tanks. "I've never used it all up over the summer," she says proudly of her free irrigation supply. Siting the cisterns was key, since they inevitably overflow in winter. Here, excess water runs onto a gravel area, where it gradually filters down to recharge an underground creek bed.