Develop A Water Resources StrategyClick 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
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
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
Using the example site, harvesting from the back half of the roof:
Assume 90% runoff from a metal roof:
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.
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:
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:
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
LEAF is under the fiduciary stewardship of the Arizona Community Tree Council, a 501(c)3 non-profit organization.
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