If you could get 600 gallons of water at no cost, would you take it? It is possible to harvest that much water from rooftop runoff from only one inch of rainfall! Multiply that by the average number of inches that fall per year where you live and it can add up to a substantial amount of water.ven in arid climates, there are rainy seasons and the monsoon. When it rains water can be collected by simply putting out buckets and allowing them to fill up or by creating a more sophisticated water runoff and collection system. Simple water harvesting can be building up barriers around a plant to collect and hold water. A large garden area or grassy area can have a barrier around the edge to keep water from rainfall and runoff contained. Collection containers that are home made and as simple as a clean trash can with a screen to filter out debris can be used. Other options include manufactured rain barrels complete with spouts that fit hoses, and full sized cisterns that can hold 1000's of gallons of water. To harvest water the roof should have a gutter that will collect water runoff and direct it to the drain spout. A gutter screen will help eliminate debris in the water. A screen can be added to the top of the collection barrel to keep debris out also. The drainage spout should go directly into the collection barrel. A spout is handy at the bottom of the barrel. Depending on the size and height of the barrel, there will be varying degrees of water pressure. The video below shows how one man created a simple and effective water harvesting system.