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The LEAF Network is a community-based organization with the mission to link people with the benefits of edible trees and support edible trees with people’s stewardship.

Develop A Water Resources Strategy

​Click Here for a PDF of Develop A Water Resources Strategy​


​Providing multiple water sources to help support edible trees is good for the trees and helps conserve drinking water in Arizona. Anticipate the water needs of trees at their mature sizes. Start with on-site passive rainwater harvesting. Additional resources include harvesting rainwater in tanks and harvesting graywater and air conditioning condensate. Provide ways to use potable water to supplement on-site water sources, especially for nonnative trees. Stormwater can help support edible trees that are planted in stormwater management basins and on right-of-way lands where street runoff can be diverted to the trees. ​

​To help develop your water resources strategy, it can be helpful to show available water sources on your site plan. Water flows downhill (downslope). Even what appears to be “flat land” may have localized rises and falls. Look out your windows during rainstorms and walk around after the rains stop to see where rainwater runs off roofs, where is flows across the land, and where it collects—or “pools.” Note the location of gutters and downspouts. Note existing or potential graywater outlets and the locations of air conditioning units or condensate pipes. Note where water flows onto or off your site. ​

​A simple homemade water level measuring device (sometimes called a “bunyip”) can be made by attaching clear plastic tubing to two wooden stakes marked in inches, and partially filling the tubing with water. Using this device can help you find where land is level, where it slopes, and how much it slopes. For larger sites, a map showing contour lines—lines of equal elevation on the land surface—can help in determining water flow and water harvesting design. ​


Observing your site during and after rains will help you understand where water flows and pools.

A simple water level measuring device made with two stakes and clear plastic tubing that is partially filled with water can be used to mark lines of equal elevation and to measure land elevation differences.

Rainfall around Arizona. Even sites in Arizona’s deserts receive a surprising amount of rainfall over a year’s time. With a little over 8 inches of rain per year in Phoenix (Low Desert elevation area), the example 27,000 square foot site would receive 140,000 gallons of rain. If this site was located in Prescott (High Plateau/Mountain elevation area), which receives around 19 inches of rain per year, over 320,000 gallons of rain would fall on the site. You can figure out how many gallons of rain falls on your site using three simple steps. These steps work for figuring out both annual rainfall and monthly rainfall. See average rainfall amounts on the table below for selected Arizona cities and towns. Rainfall for other Arizona locations is available at the Western Regional Climate Center Online Reporting Stations Map

STEPS TO CALCULATE RAINFALL FOR YOUR SITE​

  • Step 1. Look up average rainfall in inches (in) for your area on the table Average Monthly and Annual Rainfall. Average rainfall for other Arizona locations is available at: http://www.wrcc.dri.edu/summary/Climsmaz.html. For example, average annual rainfall in Phoenix is 8.47 inches per year (in/yr) and average annual rainfall in Prescott is 19.05 in/yr. ​
  • ​Step 2. Calculate the area of your site by multiplying its length by its width. The example site is 180 feet (ft) X 150 ft = 27,000 square feet (sq ft).​
  • ​Step 3. Multiply rainfall in inches (in) by the site area in square feet (sq ft), and convert this to gallons using a conversion factor of 0.623 inches/square feet/gallon (in/sq ft/gal).

​If the example 27,000 square foot site is in: ​

Phoenix: 8.47 in/yr X 27,000 sq ft X 0.623 in/sq ft/gal = 142,474 gal/yr​

Prescott: 19.05 in/yr X 27,000 sq ft X 0.623 in/sq ft/gal = 320,440 gal/yr​

Click the image below to download a PDF of the table: Average Monthly Rainfall for selected Arizona cities and towns.

Tree water needs and rainfall rates vary. Both the water needs for trees and rainfall rates vary over a years time. Using monthly averages shows this pattern of change, as illustrated in graphs of Phoenix and Prescott Area Rainfall and Plant Water Needs. The basic patterns in both locations show that monthly tree water needs (yellow, light green and dark green lines) can be much higher than monthly rainfall (blue lines), especially in April, May and June. This same pattern occurs throughout Arizona. Be prepared to provide additional water to your trees—especially nonnative species—when it is hot and dry. ​

​Make the most of passive water harvesting. Keep the precious rain that falls by thinking ahead about where and how to harvest it. Direct rainfall can be harvested into passive water harvesting depressions. Additional water can be harvested off slopes. The rates of rainfall runoff from sloped lands that are vegetated or bare are about 30% to 60%. Much of that rain could evaporate unless it is infiltrated into depressions that are mulched to reduce evaporation loss. ​

​Prolong rainfall availability using a rainwater tank. Installing one or more tanks will increase the length of time rainwater is available. About 90% of the rain that falls on a smooth-surfaced roof could run off into a rainwater tank, while less runoff is be available for a rougher roof. If the example site was located in Phoenix, over 5,000 gallons of water could be harvested into a tank from just half of the roof. In Prescott, over 12,000 gallons could be harvested. Tanks are not sized to contain all the annual rainfall at one time. Instead, tanks are sized to fill with rain and be emptied multiple times a year. Any rainfall that exceeds the tank capacity is routed out an overflow pipe to an adjacent water harvesting basin. Determining the right sized tank for your site will depend on roof area, space for the tank, cost, plant water needs and other factors. Many resources are available to help determine appropriate tank sizing, including several listed at the end of this section.​

​Graywater supplies. Graywater supplies will typically be constant each month that a site is occupied. In the example site, a washing machine is located inside the back of the house and discharges water through a pipe that goes outside. Assuming laundry is done for three people, this house could produce 600 gallons of washing machine graywater a month, or 7,200 gallons a year. For information on graywater discharged from bathtubs, showers, bathroom sinks and washing machines see resources click here

​Air Conditioning Condensate supplies. In Arizona, the amount of air conditioning (AC) condensate water produced at any given site varies with temperature, humidity, length of AC use and other factors. In the monsoon season, when temperatures and humidity are high and AC units are frequently operating, more condensate will be produced. In cold, dry winter months with no AC use, no condensate water is produced. AC condensate produced at one desert site in August yielded around 36 gallons a day, or over 1000 gallons a month. Find out where your AC unit discharges condensate water, and whether this could be diverted to trees. To measure its discharge, click here. ​


CALCULATING RAINWATER FOR TANKS
To determine how much water you want to catch in a tank, first calculate the area of roof you want to harvest from, then multiply that by rainfall. ​

​Using the example site, harvesting from the back half of the roof: ​

  • Roof area is 20 ft X 60 ft = 1,200 sq ft​
  • In Phoenix: 8.47 in/yr X 1,200 sq ft X 0.623 in/sq ft/gal = 6332 gal/yr​
  • In Prescott: 19.05 in/yr X 1,200 sq ft X 0.623 in/sq ft/gal = 14,242gal/yr​

​Assume 90% runoff from a metal roof:​

  • In Phoenix: 6332 gal/yr X 90% runoff = 5,669 gal flows through the tank in a year​
  • In Prescott: 14,242gal/yr X 90% runoff = 12,818 gal flows through the tank in a year​

Tanks can be filled and emptied multiple times. In one year in Phoenix, a 1,000 gallon tank could be filled and emptied five times and a 2,000 gallon tank 2.5 times. In one year in Prescott, a 1,000 gallon tank could be filled and emptied 12 times and a 2,000 gallon tank 6 times. The larger a tank is, the less chance of water overflowing but the higher the initial cost. Tank overflow water should be routed to an adjacent water harvesting basin where it is put to use growing a tree. ​

​Water needs of trees. One way to estimate a tree’s water needs is to look at how many inches of water should be applied under the canopy area of the tree each year to keep it healthy and productive. ​

  • Low water-use trees—including many native trees—need around 12 to 20 inches of water per year applied under the tree canopy area. ​
  • Medium water-use trees—including many nonnative fruit trees—need around 20 to 40 inches per year. ​
  • High water-use trees—including nonnative trees such as pecans and date palms—need around 30 to 50 inches per year. ​

A mature mesquite tree and a mature apple tree, each with a 20-foot diameter canopy, have very different water needs even though they both have about the same area below the canopy where water should be applied. ​

​A large-sized, low water-use mesquite tree could grow well in around 12 inches of water per year, for a total of around 2,300 gallons a year under a 20-foot canopy. Mesquites—as with many native trees growing in their natural habitats—grow to the size that direct or concentrated natural water sources (e.g. washes, shallow groundwater, valleys) will support. Less water results in smaller trees, more water results in larger trees—within the normal size range of the tree. In the built environment, mesquites can grow on direct rainfall alone, but will typically grow to a larger size if runoff from roofs, hardscapes, and sloped lands provide additional water sources. ​

A medium water-use apple tree might need around 35 inches of water applied per year under the canopy area, which totals around 6,800 gallons a year for a 20-foot canopy area. In most locations in Arizona, direct rainfall alone will not meet this requirement. Additional water will be needed from one or more sources including passive water harvesting, a rainwater tank, graywater, AC condensate and possibly from potable supplies, especially in desert elevations. ​

Information on general water use levels for edible trees is provided here. Water use levels for specific trees is provided in the PDF Elevation Area for Trees in Arizona. Information on edible tree canopy sizes is provided here.​


Develop Your Water Resources Strategy. Think about a water resources strategy that can support edible trees as they grow to maturity. Your water resources strategy should evolve along with your Tree Planting Plan. In determining water resources, take into account: ​

  • Site observations of where rainfall flows and pools and how to build on this to harvest and infiltrate water.​
  • Monthly and annual rainfall at your location.​
  • Whether and where to install one or more rainwater tanks.​
  • How graywater and AC condensate might help water trees.​
  • If supplemental potable water is needed and how to supply it.​
  • The number and type of edible trees you want to plant, their water use needs and mature sizes.​

A 3-dimensional view of a water resources strategy developed for the example site illustrates how to make good use of all on-site resources:​

  • Use existing patterns of water flow and pooling to harvest water in an array of passive water harvesting basins placed throughout the site.​
  • Space constructed passive basins around 30 feet or more apart to allow for the growth of full-sized trees.​
  • Construct linear swales to direct overflow water from one basin to the next lower basin. ​
  • Create a final path for water to safely flow off the site in the event of very large rainfalls.​
  • Install a rainwater tank to capture runoff water from the back side of the house roof, with overflow to a nearby passive basin. Use a gravity-fed hose to deliver water to multiple trees. ​
  • Extend the downspout from the front half of the house roof to a nearby basin.​
  • Pipe graywater from the washing machine to multiple tree basins that also receive tank water, so trees receive multiple water sources. ​
  • Pipe AC condensate water to a nearby basin that also receives tank water, so tree receives multiple water sources. Have the option to deliver AC water through a garden hose to provide it to other basins as well. ​



ADDITIONAL RESOURCES​

​For information on design and construction of on-site water resources strategies and calculations:​

​Brad Lancaster, Rainwater Harvesting for Drylands and Beyond website  ​


​Harvesting Rainwater Calculations 


​Texas A&M AgriLife Extension and tank calculators


Watershed Management Group

Instructions on making and using a water level device, Ecology Center

Copyright 2017

LEAF is under the fiduciary stewardship of the Arizona Community Tree Council, a 501(c)3 non-profit organization.

70 S Val Vista Drive, Suite A3-186, Gilbert, AZ  85296





 

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