Residential Water Harvesting - June 27, 2001
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County


Residential water harvesting methods vary from simple earthen berms designed catch surface runoff to complex systems using cisterns and pumps to store and apply captured water. To be effective in your water harvesting efforts you must consider seasonal rainfall patterns, the water requirements of the plants, and how well your soil can store water.

The amount of rainfall we receive and the season when it falls will satisfy the requirements of many desert adapted, non-native landscape plants. Wildflowers, perennials, and cool season grasses use the winter rainfall. Summer monsoons provide water to warm season grasses, shrubs, and trees. For the most part, local soils are course textured enough to allow adequate infiltration, but fine textured enough to store a substantial amount of water. To be successful, there are two other considerations: 1) plant roots are present and well distributed in the areas where the harvested water is being applied, and 2) the plant you are irrigating is actively growing and able to use the harvested water.

The potential volume of water available for harvest can be realized by knowing the relationship between inches of rainfall and volume of water: one inch of rainfall equates to 0.62 gallons per square foot of horizontal surface area. For instance, given the average city lot (1/4 acre or 10,890 sq ft) and average rainfall (12 inches/year for the Verde Valley), this yields 81,021 gallons of water. In reality, there is really no practical way to capture all that water for reuse, and if you tried, you may be challenged by the Salt River Project regarding surface water rights. However, this does provide you with some idea of the shear amounts of water that are falling across the landscape.

Most household water harvesting systems utilize the roof of the home and other buildings as a collection surface. Here, water is collected via rain gutters and downspouts and diverted into a storage container. The simplest and least expensive storage containers are plastic trashcans or barrels. Two or more can be connected with PVC pipe that allows water to flow from one barrel to the next. Water is released for irrigation from a valve connected to a hose at the base of the container. Rain gutters should also have screens above the downspouts to filter out leaves and other debris. Measures should also be taken to seal the barrel so that water is not accessible to mosquitoes and small animals that could fall in and drown.

Storage capacity required can be determined by calculating roof area and average monthly rainfall. Do not use the surface area of the sloped roof. Rather, use the area of the vertical projection of the roof. A 1,000 sq ft roof will yield 310 gallons during a inch rainfall event. Depending on your budget and needs, you may or may not be able to collect all that water. You would need at least six 50-gallon barrels to harvest this amount of rainfall. Also consider how quickly you would use the harvested water.

Earthen berms or swales can be sculpted from available soil to create areas where water is collected for infiltration. On flat ground, you would construct low berm surrounding the landscaped area and this could be irrigated by harvested water. Under mature trees, a basin can be sculpted that extends well beyond the tree's drip line. On a slope, berms should be crescent-shaped, higher in the center than the edges, and formed where they will catch surface runoff coming down the slope.

If you are considering paving and area, think about harvesting the water and directing it to another area or using a material that has some degree of permeability. Drains can be installed and water diverted to bermed landscape areas. Permeable pavements allow water to infiltrate through or in between the paving material. Paving materials such as brick or concrete pavers and flagstone can be set in sand rather than concrete to allow water to enter the soil if desired.

Complex water harvesting systems utilize large storage tanks to store harvested water. The tank can be above or below ground (cistern). The advantage to the above ground cistern is that gravity can be used to direct water areas where irrigation is needed. The disadvantages are that they take up space and may be unsightly or require screening to hide it. I have seen systems that used commercially purchased water tank and others that used home-built tanks constructed from corrugated steel culvert. Below ground cisterns are often installed at the time of home construction and built out of reinforced concrete. Advantages are large capacities and being hidden from view. Disadvantages are high cost and the need for a pumping system to distribute the water. Provisions should also be made for cleaning and inspection of the below ground cistern.

My advice is to start small: couple of barrels to collect roof runoff and some berms to harvest surface runoff should get you thinking about potential of your site. Observe how water flows during a rain. You can always expand the system. In the future, design and construction of new homes will likely incorporate these features as we face increasing scarcity of water. For more information on water harvesting, look at the University of Arizona Publication "Harvesting Rainwater for Landscape Use" by Patsy Waterfall on the web at: ag.arizona.edu/pubs/water/az1052/.

The University of Arizona Cooperative Extension has publications and information on gardening and irrigation. 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. Find past Backyard Gardener columns at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: March 15, 2001
Content Questions/Comments: jschalau@ag.arizona.edu
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