Water Harvesting - July 16, 2008
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of 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, pressurize and redistribute rain water. To begin rainwater harvesting, you must consider seasonal rainfall patterns, topography of your property, the water requirements of the plants you are growing, how you can store water (in soil or tanks), and your budget.

For the most part, Verde Valley 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 plants you are irrigating are actively growing and able to use the harvested water.

Simple rainfall harvesting systems rely on topography and smart design. 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 to collect 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 an 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.

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 (7.48 gal./cubic ft). In reality, it is impractical to capture all this water for reuse. However, this does provide you with some idea of the shear amounts of water that are moving through the landscape.

Household water harvesting systems commonly utilize the roof of the home and other buildings as a collection surface. Here, water is collected via rain gutters, downspouts and collection pipes which are diverted into a storage container. The simplest and least expensive storage containers are plastic barrels. Two or more containers can be connected with PVC pipe that allows water to flow from one barrel to the next. Water is released for irrigation from a hose bib at the base of the containers. Steps 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. There are also specially designed downspout filters and other accessories to improve water quality and harvesting efficiency.

Complex water harvesting systems utilize larger storage tanks to store harvested water. The tank can be above or below ground (cistern). You can use commercially purchased water tanks or home-built tanks constructed from a corrugated steel culvert set upright in concrete. The advantage to the above ground cistern is that gravity can be used to direct water to areas where irrigation is needed and drip irrigation systems can easily be connected.

Start small: a couple of barrels to collect roof runoff and some berms to harvest surface runoff should get you thinking about potential of your site and structures. Observe and record how water flows during rain events of varying intensities and amounts using a rain gauge. You can always expand or modify the system. For more information on water harvesting on the web at: ag.arizona.edu/pubs/water/az1052/ and Texas A&Mís web site at: rainwaterharvesting.tamu.edu.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.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: July 5, 2008
Content Questions/Comments: jschalau@ag.arizona.edu
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