Rainwater is a superior water source for irrigation. The pH is slightly acid to almost neutral, so plants love it. 

Rainwater has neither dissolved minerals from the soil, nor chemicals from water treatment plants. An essential ingredient to successful gardening is water. The best water for the garden is rainwater. Ever notice how the lawn and garden seems to have a sudden growth spurt after it rains and also looks much greener? This is because rainwater does not contain salts and minerals that are found in tap and well water. Plants absorb rainwater irrigation more efficiently compared to tap and well water. Thus rainwater irrigation can be stretched much farther because plants do not need as much rainwater as tap or well water.