   Water Harvesting - November 17, 1999 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Plants have fairly simple requirements: they need growing space, light, 18 essential nutrients, and water. Of course these factors must occur at the proper times, places, and amounts. In our part of the world, space is available, light is not limiting, and nutrients are generally available (if not on site, then nearby). Conversely, water is likely the most limiting factor to plant growth in most of our gardens. At the risk of starting a feud with your neighbors (and possibly offending the Salt River Project), I will give you some ideas about capturing water that falls on your backyard garden. Several strategies can be employed to harvest precipitation. The simplest is to maintain the highest possible infiltration capacity on the soil surface. This can be done in many ways. Maintaining vegetation, mulch, or litter on the soil surface helps keep soil in place and slows overland flow of water offsite. Building earthen berms and swales also slows overland flow and diverts water to basins, terraces, or other planted areas. The overall idea is to keep the water onsite for use by forage plants, crops, or trees. Water in excess of that needed for plant growth is recharged to underground aquifers or released slowly through subsurface flow to a nearby creek or river. In urban environments, pavement and structures interrupt the infiltration process to accelerate overland flow and runoff. To capture some of this water, it can be harvested from rooftops into storage vessels for later use. Some homes are designed and built to do just this: roof areas, gutters, and down spouts are plumbed and channeled into an underground cistern. When the cistern is below ground level, a pump is used to distribute the stored water. A filtration system can be added to purify the water for consumption. The city of Austin, Texas is offering 30% rebates (up to $500) to homeowners that install rainwater collection systems for non-potable uses. We will probably see an increase in incentive programs across the west in the future. A home with a 2,500 square foot roof area can collect 18,700 gallons of water per year in an area that receives 12 inches of rain per year. Of course, this assumes that the cistern can store that amount of water and handle peak flows generated during storm events. For those that are interested, more simplistic systems can be designed with 55 gallon drums or plastic trash cans at the base of single down spouts. This water can simply be syphoned off into nearby garden areas. Several containers can be connected in unison to increase storage capacity. Water harvesting is not a new idea. In places where rainfall is plentiful but usable wells and/or freshwater bodies are nonexistent, water harvesting is the only source of potable water. System design gets much more complicated when designing water harvesting systems for potable water. Leaves, sediment, live organisms (such as mice, bacteria, mosquitoes, etc.), and filtration must be considered. For now, most of us are blessed with a supply of clean potable water. Luckily, it is a renewable resource. Yet, it is not an unlimited resource. In the arid southwest, we are and will continue to face water supply challenges in the future. Within the last decades, a system of sustainable farms and communities called Permaculture has emerged. The word comes from a combination of "permanent agriculture." Permaculturists utilize water harvesting strategies to increase productivity of the land. To learn more about permaculture and water harvesting, read Permaculture by Bill Mollison, 1990, Island Press, 579 pp. Also, the Internet has several sites that can be accessed by searching the topics "water harvesting" and "rainwater harvesting." The University of Arizona Cooperative Extension has publications and information on irrigation and soils. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |