ALABAMA A&M AND AUBURN UNIVERSITIES Rainwater Harvesting ... · ALABAMA A&M AND AUBURN UNIVERSITIES...

ALABAMA A&M AND AUBURN UNIVERSITIES

Rainwater Harvestingfor Irrigation Water

While worldwide waterconsumption is rising atdouble the rate of thepopulation, the amount offreshwater remains at only2.5 percent of the world'swater resources. Rainfallreplenishes much of thewater we use; however, it ispredicted that by 2025,eighteen countries will usemore water than can bereplenished (Credit Suisse,2007; Clark, 2007). Climatechanges occur annually,often, and in differentregions of the world. Variousregions will experiencefloods, droughts, earth-quakes, or tornadoes at anyone time. While little can bedone about these climaticevents, we can reduce theirimpact through planning andpreparedness. Builders inCalifornia are required toadhere to building codes andbuild structures that canwithstand low-levelearthquakes. Dams andlevees help prevent flooding.Rainwater collection, andstorage can also reduce theimpacts of drought, stormwater runoff, and peak flowlevels as well as reliance onground and surface water,also lower nonpoint sourcepollution, allow groundwaterto recharge, and promotewater conservation andsustainable practices(LaBranche et al., 2007;Hicks, 2008).

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Water can be conservedthrough proper xeriscaping(landscaping) and bychoosing the ideal nativeplants for desired locations.The collection of storm waterfrom parking lots and othersurfaces, storage in basins,swales or other watersheds,and distribution to plant bedsby predesigned Frenchdrains, berms, curbs,spillways, depressions,microbasins, and aprons willalso conserve water andreduce runoff and itsassociated problems(Mechell, 2008; Philips,2005; Waterfall, 2006).

Rainwater harvesting,whether it is from parking

lots or rooftops, is thecollection and storage ofrainwater. Collectedrainwater is used fordomestic purposes andirrigation. In some countries,it is the only source ofpotable water. Rainwater isusually collected fromrooftops, greenhouses, poolcovers and other relativelyclean surfaces. This storedwater can be used forirrigation, flushing toilets, orwashing cars; however, it isnonpotable and requiresextensive treatment for useas tap water. Potable waterpurification requirementsvary with state and localordinances and must beinspected by local authorities.

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Water harvesting systemscan range from a simplebarrel at the end of thedownspout to multiple tanksburied in the ground withnumerous pumps andcontrols. Regardless ofwhether you want a littlewater for your spring flowersor a lot for your crops, bothyou and your environmentwill benefit from harvestingrainwater. A small 10 X 10-foot surface will collect 40 to60 gallons of water from 1inch of rainfall. Simplenonpotable rainwaterharvesting systems for yourlandscape, garden, or evencommercial crops can bedesigned and installed by anyhomeowner or crop producer.

Components of aHarvestingSystemThe system is made up ofcomponents that catch,convey, clean, store, anddistribute water (Figure 1).The actual materials andmethods used will vary withthe catchment surface, thelocation and size of storagecontainers, and how thewater will be used.

Catchment

Surfaces that can be usedinclude roofs, greenhouses,clean and tight pool covers,and some patios and decks.Most roofing materials areacceptable for watercollection. Roofing materialsthat contain asphalt or thathave lead flashing (galvanizedmetal or sheet metal) shouldnot be used if you plan topurify your water for in-hometap water. Aluminum is the

Figure 1. The catchment system is made up of a watercatching surface, a conveyance system to get the water to

storage, a storage container, and a method of redistribution.

best for rooftop rainwaterharvesting.

Conveyance

Gutters typically collect thewater from your roof orelevated deck and move it todownspouts. Downspouts canbe used to deliver water to astorage tank. If your tank isabove ground, you may needto remove you originaldownspouts. There should bea least one downspout for

every 50 feet of gutter runand 1 square inch ofdownspout diameter per 100square feet of roof area (2 X3-inch downspout will support600 square feet of roof area).

If you only have one storagetank, all of the downspoutswill need to be connected to a4-inch line that runs to thetank. Sharp bends in the linethat lead to the storage tankshould be avoided. You mayjust prefer to put a rain barrel

Rainwater Harvesting for Irrigation Water 3

or other container under eachdownspout.

Other types of catchment,such as pool covers or othersurfaces, can be used but willrequire construction of acollection/conveyance systemor a small sump pump thatcan be used to transfer thewater to a tank. Structures,such as a quonset-shapedgreenhouse, will require agutter system at the hip boardor at the lower roofline if thegreenhouse has a peaked orrounded roof with verticalsides. You can purchaseflexible downspouts, gutterextensions, drainage pipe, andconnectors at most storescarrying plumbing andlandscaping materials.

Cleaning the Water

Dust, bird droppings, and treedroppings such as acorns,samaras, branches, or leavescan accumulate on the roofand other catchment surfacesbetween rain events. A firstflush diverter (Figure 2) is apipe that tees off the mainline, catches the first flush ofwater, and has a plug or trickledrain on the bottom. Once thispipe fills up with the initialdirty water, the remainingwater bypasses the pipe andruns directly to the storagecontainer. The plug at the endof the first flush divertershould be removed and thepipe drained after each rainfallevent or, if there is a trickletube, the water will slowlyclear from the tube.

Diverter tubes should redirect0.1 to 0.5 gallons of water per10 square feet of roof surface.For example, if you have 100square feet of catchment

Figure 2. A first flush diverter removes the initial flush ofwater from the catchment surface so it does not end up inthe storage tank. The size of the diverter tube should be

proportionate to the catchment surface.

Photo Permission from Rain Harvesting Pty Ltd.www.rainharvesting.com for both figures.

surface and want to redirect0.1 gallons of water per 10square feet of roof, you willneed to divert 10 gallons offlush water. This would requirea 6-inch PVC line 6.8 feetlong. The calculation is asshown in the box below.

Volume = π x r2 X length of tube

Length of tube = volume/(π X r2)

10 gallons = 2,310 cubic inches of water (1 cubic inch =.004329 gallons). You must convert gallons into cubic inchesto put all units in inches.

π = 3.142

r2 = the radius of the pipe squared (a 6-inch PVC pipe has aradius of 3 or 9 inches squared)

2,310 cubic inches = 3.142 X 9 square inches X length oftube (inches)

2,310/(3.142 X 9) = 81.7 inches or 6.8 feet of 6-inch PVCpipe

Other downspout filters calleda "leaf beater," "leaf eater," or"roof washer," depending onthe source, can be placed inthe downspout (Figure 3, nextpage) above the first flushdiverter. The purpose is toremove large objects like


leaves and acorns from thewater source prior toentering the first flushdiverter. If your gutters arecovered, a roof washer willprobably be unnecessary. Ifyou use trickle or dripirrigation, you should put inan inline filter after thepump (Figure 4). This willremove sediment that clogstrickle tubes and drip lines.The filter's screen sizeshould be at least a 200mesh. Remember, thelarger the mesh size, thefiner the screen.

If rainwater enters the tankat the manway or top hole,the end of the downspoutshould be covered with 1/4-inch or smaller hardwarecloth or a screened basket(Figure 5) should be placed inthe manway entrance toreduce the amount of organicmatter that enters the tank.The hardware cloth should beconnected in a way thatallows for easy removal forcleaning out debris. Otheroptions include roof washersthat screen the water prior toit entering the collection tank.

Water Use

Before discussing storage, itis appropriate to determinehow you will use your waterand how much you will needto store. This is a difficultconcept because everyone'slot size and landscapecontent is different. The sizeof your catchment area andyour irrigation methods willalso be important inestimating tank size. Yourwater needs may vary withweather conditions, plantwater requirements, soiltype, sun or shadeconditions, exposure to sun-

Figure 3. The leaf beater and first flush diverter (left)as a unit and installed into a home catchment system

(center). The leaf beater removes leaves and largeobjects from the water flow before the water enters the

first flush diverter (right).

Photo Permission from Rain Harvesting Pty Ltd.www.rainharvesting.com for above photos.

Figure 4. An inline filtercan reduce sedimentthat will clog trickleand drip irrigation

lines. The valve at thebottom allows you to

drain sediment and theplastic casing screwsoff to give you accessto the screen filter.


north, south, east, or westside of a structure, andvariety or cultivar. In droughtconditions, categorize yourplants into groups based onwater requirements. Plantssuch as gardenias, azaleas,rhododendrons, annuals,vegetables, and fruit treeshave high water require-ments for survival and/orfruit or vegetable produc-tion. You may also want togroup your most valuableplants. Priorities shouldinclude trees that defineyour landscape, heir-loomplants that cannot be re-placed, and mature plantsthat are massed and of thesame size (Waterfall, 2004).

Irrigation for approximately2,000 square feet will re-quire about 2,000 gallonsper month or more to main-tain when using conservativeirrigation methods (drip-and-trickle-type irrigation).Typical landscapes do notexceed 1,000 square feet ofplants (excluding bed spacewith no plants). However, ifyou plan to use the systemto maintain a lawn or turf,

Figure 5. A screened basket in the manwayhole will prevent a lot of organic matter and

mosquitoes from entering the tank.

you will have a much largerlandscape area and need alarger tank.

Commercial or gardencrops such as vegetables,fruits, or nursery plants aremost often watered withtrickle or drip hoses. Rowstotaling 2,000 feet in lengthwould require 2,000 gallonsor more per week if provid-ing about 1 inch of waterper week.

You can determine what theanticipated monthly rainfallshould be for your area bygoing online to http://www.city-data.com/city/Huntsville-Alabama.html andreplacing Huntsville-Alabamawith your city and state. Becareful not to remove thedash between city and state.Knowing how much rainfallis anticipated will help youcalculate how much it con-tributes to your water needs.For example, if you need towater a crop 1 inch per weekin June and the averagerainfall was 5 inches for thatmonth, you probably will notneed to water. Your storage

needs for that month will bezero. Of course, all 5 inchescould have been from asingle rain event, but that israre. Subtract the waterneeds (-gallons) from therainfall amounts (+gallons),and that will provide youwith the balance amount ofwater you will need to store(Table 1). Based on thisinformation, you will need aminimum 5,300-gallon tank.This is determined from the"Balance" figure shown forMay, after which the water inthe tank was depleted duringthe months of June, July,August, and September. Theminimum tank size is ob-tained by taking the largestbalance and subtracting thesmallest balance from it [ifthe smallest balance isnegative, that means thatyou add it]. For exampleusing Table 1 data, theformula would be:

5,241 - (-75) = 5,316therefore, a minimum 5,300-gallon tank would be needed.

Storage

Storage tanks can be madeof almost any nontoxicmaterial; however, becautious about what wasstored in them. You can use"never-used" lined fueltanks, precast fiberglass,polyethylene, or evenconcrete, such as septictanks (Figure 6). Largepolyethylene tanks with wireframes are used for sugarderivatives, are inexpensive,and can be rinsed andreused for water storage.

If you plan to bury the tank,be sure it is built for in-ground use. The tank inletand outlet should bedesigned so that minimal


Month Rain (inches) + Gallonsfrom Rain1,2

- Gallonsfrom Irrigation3

Balance

January 3.6 1,060 0 1,060February 3.3 972 0 2,032March 4.36 1,284 250 3,066April 3.98 1,172 1,000 3,238May 5.1 1,502 1,000 5,241June 1.61 474 2,000 3,715July 2.34 689 2,000 2,404August 4.03 1,187 2,000 1,591September 1.26 371 2,000 -38October 3.27 963 1,000 -75November 2.78 819 500 244December 1.74 512 0 756

Table 1.Water Collection From Rainfall and Water Use by Irrigation (based on a 500-square-foot collection surface and 2,000 square feet of vegetable crops grownfrom March through mid-November)

1Rainfall (inches) x catchment surface (square feet) x 0.62 x percent collectionefficiency=gallons collected. Collection efficiency is affected by leaks, wind, rainfall rate, etc.During a slow gentle rain, with no leaks in the system, collection efficiency is about 95percent. During a very fast, heavy rain, the efficiency would be closer to 60-75 percentbecause gutters overflow and gutter covers are overrun with water.

2The total collected has not had first flush diverter water deducted. Generally the first 5gallons/100 sq ft of catchment area, of each rain event, is roof wash water that is flushed.

3This takes into account that the actual rainfall events contribute to the irrigation needs ofthe plants. However, rainfall is rarely spaced evenly throughout a month and irrigation maybe needed all four weeks. Also, the late summer months have a higher evapotranspirationrate and require longer, more frequent watering.

sediment in the bottom ofthe tank is disturbed. Youcan purchase calming inletsthat disperse the water in anupward direction. Largeburied tanks should have amanhole, a vent, a cleanoutsump pump, and an overflowpipe. Aboveground tanksshould have a drain at thebottom and an overflow pipeif the water line coming intothe tank is tightly fit.

Aboveground tanks shouldbe purchased in dark greenor black to prevent lightpenetration, or they shouldbe painted a dark colorprior to installation. If in-ground tanks arecompletely covered, tankcolor is not an issue.

Figure 6. Large polyethylenetanks in various shapes and

sizes can be used for storageof water, as long as the

material and prior contentswere nontoxic. Above

ground tanks should bepainted black or green.


Tank placement depends onspace and resources. It isvery expensive to bury atank, and in many locationswhere it is rocky or theground water is high, it isnot feasible. There aremany tank sizes and shapesthat can be hidden byshrubbery or placed nearthe house wall.

Metal, fiberglass, andconcrete storage tanks willtolerate cold and warmtemperatures. However,plastic tanks come in severalgrades and polymers. Tanksconstructed usinghomopolymers should not beused for water catchmenttanks (Schultz, 2008). Checkwith your vendor to be surethe tank is made of high-density polyethylene. Thismaterial will tolerate greatertemperature extremes. Lessergrades of polymer tanks willbecome brittle and crack aswater freezes and thaws andif temperatures are extremelycold. All tanks, regardless oftype, should have a domedtop or an overflow pipebelow the top of the tank toprovide head space and toallow for the expansion ofwater as it freezes.

The size of your tank can bedetermined by consideringseveral factors:

• Space available• Resources/desire to bury

the tank• Roof size• Use/need

If space is not a limitingfactor, the primary concernshould be how much wateryou will need and when (Table1). If you collect water from a

large roof surface, 1,500 to2,000 square feet forexample, you will fill a 1,500-gallon tank with a 2-inchrainfall. If you are collectingwater for in-home use,outdoor use, or forcommercial crops, you willneed a 5,000-gallon or largertank. If you have beenpurchasing water to irrigateyour landscape or crops, youcan average your winter bills,typically December, January,and February to determinehousehold use. Since watermeters and bills report wateruse in cubic feet, multiply thisnumber by 748 to getgallons. Subtract the winteraverage from your monthlybills to determine monthlyoutdoor water use (Table 2,next page). Tank sizerequirements will depend onrainfall recharge rates.

For nonpotable, toiletflushing, determine thenumber of householdmembers and the flushvolume of your toilets.Generally, each personflushes five times a day, anda typical tank uses 1.6gallons. In a four-personhome, water use for flushingis about 32 gallons a day or960 gallons a month. UsingTable 1 as an example of howmuch raninwater can becollected monthly, put 960 ineach row of the -gallonscolumn. A 1,700 gallon tankwould provide enough waterfor year-round flushing. Ifyou have the resources tobury large tanks, they shouldbe placed close to the watersource. However, this mayrequire extensive plumbing tothe production site for cropproducers.

Distribution

Unless your collection tank is20 to 30 feet above whereyou will be watering, you willneed a pump to distributethe water from the tank toyour landscape or crops.There are several pumpsthat work well with thissystem. One-quarter to oneand one-half horsepowerirrigation pumps will provideenough pressure for mostdrip and trickle systems(Figures 7 and 8). "Irriga-tion," "lawn," or "shallowwell" pumps can be usedwithout bladder tanks. Atether switch can be placedin the tanks to turn off thepump when the water level islow (Figure 8). If your tankis not close to an electricalsource, a gasoline-poweredpump can be used.

When connecting the pumpto the tank, the plumbingcan include a floating filterlocated at the end of thepump's suction hose. Thiswill reduce the amount ofsoil sediment sucked into thepump and inline filters. Youcan also use a foot valve atthe bottom of the outflowline if it is placed a fewinches above the bottom ofthe tank; however, you willstill need an inline filter.

Pressure tanks hold a setvolume of water that ispumped into the bladder.When the valve is opened,pressure pushes out thewater. A pressure-sensitivepump should be used with apressure tank. A pressure-sensitive pump turns onwhen pressure decline isdetected. It shuts off when aspecific pressure is reached.Most pressure-sensitive


Table 2.Sample Monthly Demand for Water Use if You Pay to Irrigate Water

1 CCF=100 Cubic Feet of Water. Water bills usually report usage in 100 cubic feet units. Onehundred cubic feet of water equals 748 gallons.

Sample Monthly Demand

Large Garden/Small commercial Crop ProductionAverage Winter Use = 8.9 CCF1, Household Size = 4

Month Monthly Use Average Winter Use LandscapeUse

January 6,732 6,679 0February 5,986 6,679 0March 7,945 6,679 1,266

April 8,361 6,679 1,682May 8,520 6,679 1,841June 10,620 6,679 3,941July 14,560 6,679 7,881August 16,521 6,679 9,842September 12,330 6,679 5,651October 10,253 6,679 3,574

November 8,694 6,679 2,015December 7,320 6,679 0

Figure 8. A 1 hp pump (left) will provide enough water toirrigate a home landscape and garden if trickle and dripirrigation are used. The pipe on the right has a foot valve atthe bottom (closest end to you)and the black cylindrical-shapedobject is a tether float switch.The black cord (attached to thetether float switch) at the farend of the pipe plugs into anoutlet and the pump plugs intothis cord. When the black floatdrops vertically due to lack ofwater, it will turn off the powerto the pump.

Figure 7. This 1.5 hp pump isused to irrigate shiitake mush-rooms and 1/2 acre of organic

vegetables using a mist systemand soaker hoses.


pumps are not submersible,and they are more expen-sive. However, pressuretanks must be protectedfrom freezing temperaturesand placed in a structure.Pressure or bladder tanksare more commonly usedunder the house when catch-ment water supplementstoilet water.

An inline pump controller isan optional part that isscrewed directly onto theexisting pump and waterline. It will detect pressurechanges and turns on thepump when pressure is low.Some units can even detectif the tank is out of waterand will turn off the pumpso it will not burn out(Pushard, 2004).

Pump size requirements arebased on volume of waterneeded at any one time,pressure requirements,distance the water musttravel (run), and the riserequired to get the water outof tank and to the irrigationsite. Most manufacturersprovide this information onthe box or in the pumpinstruction book.

Irrigation

Your method of irrigation willdetermine pump size andother distribution equipmentneeded. If you plan to handwater with a hose, a smallerpump can be used. Irrigationof lawns and large areas withsprinklers will require largerpumps, depending on thesprinkler head and distanceand slope the water musttraverse. Trickle and dripirrigation are the most effi-cient systems for beds andcrop production (Figure 9).

Figure 9. These small mist heads reduce the water pressure and volume needed to keep the shiitake

mushroom logs moist throughout the year.

These systems will require theinstallation of a pressureregulator and an inline filter(Figure 4). The shiitake mush-room logs shown in Figure 9have a mister head at the topof each riser; this provides afine mist to wet the burlap andincrease the humidity underthe burlap.

Cost of theSystemWe have constructed a small,tank system for under $400(see Figure 1) and 1,500-gallon systems forcommercial crops that costabout $3,000, not includingthe irrigation lines. Yoursystem costs will depend onwhether you need some orall of the following:


• Gutters• Downspouts• Tank

- Above ground or in-ground- Size- Tank material (polyethylene, fiberglass, metal, concrete)

• Pump- Rise of land to irrigation site- Lift (if water must be raised out of the tank)- Run (greater distances reduce pressure)- Pressure tank

• Filtration- Leaf collection- First flush diverter- Insect screens- Inline filters- Floating filter

• Plumbing-pipes and fittings• Irrigation supplies

System DesignThere are several steps involved in creating a suitable rainwater collection system. There arealso numerous solutions and methods you can use to create a system.

For best results, consider the following steps:

1. Ensure that the catchment surface is suitable for collecting quality rainwater.2. Ensure roof gutters are installed to the appropriate standards according to the regulation

fall and number of outlets.3. Install a fireproof mesh or gutter cover system to prevent leaves and debris from blocking

outlets.4. Fit gutter outlets under the roof gutter to prevent sludge buildup.5. Fit rain head to gutters to divert leaves and debris. Use a screen with less than 1

millimeter apertures to reduce mosquito infestations.6. Fit insect proof screens to all pipes that hold water and all pipes and openings to and from

the tank.7. Fit appropriately sized first flush water diverters. Diverter tubes should redirect 0.1 to 0.5

gallons per 10 square feet of roof surface.8. Select an appropriate tank based on collection area, rainfall, water use, and climate.9. Choose a pump that provides sufficient pressure for desired lift, rise, and run of the water

lines and type of irrigation system.10. Draw water from the highest point in the tank possible. Floating outflow filtration systems

will do this best.11. Ensure the system is maintained. Service gutters, rain heads, water diverters, tanks, and

pumps regularly.12. Insulate and protect all water lines and pumps from freezing temperatures.13. Check local ordinances and tax laws to see if you can get tax credits or rebates for

installation of your system.

(Adapted with permission from Rain Harvesting Pty Ltd. www.rainharvesting.com)


References

Clark, E. H. Water prices rising worldwide. (2007, March 7) Earth Policy Institute. Retrieved October 27, 2008,from http://www.earth-policy.org/Updates/2007/Update64_printable.htm

Credit Suisse. (2007, June 7) Water. Global Equity Research. Retrieved October 27, 2008, from http://www.ewatertek.ca/PDF/Credit%20Suisse-Investment%20Water%20Research%20Report-June%202007.pdf

Hicks, Bill. (2008). A cost-benefit analysis of rainwater harvesting at commercial facilities in Arlington County,Virginia. Duke University. Retrieved October 27, 2008, from http://dukespace.lib.duke.edu:8080/dspace/bitstream/10161/512/1/MP_wdh11_a_200805.pdf

LaBranche, A., Wack, H. O., Crawford, D., Crawford, E., & Brand, C. (2007). Virginia rainwater harvestingmanual. Retrieved October 27, 2008, from http://www.cabellbrandcenter.org/Virginia%20Rainwater%20Harvesting%20Manual.pdf

Mechell, J. (2008, August). Rainwater harvesting: Raingardens. Texas A&M University, Publication No. L-5482.Retrieved October 27, 2008, from http://agrilifebookstore.org/tmppdfs/viewpdf_2396.pdf?CFID=2279462&CFTOKEN=1a888dbdd322c69c-94C0B7EB-7E93-35CB-8B90D249DA1AA58C&jsessionid=8e301ef0c6a454797856

Philips, A.A. (2005). City of Tucson water harvesting guidance manual. http://dot.ci.tucson.az.us/stormwater/downloads/2006WaterHarvesting.pdf

Pushard, D. (2004). Rainwater harvesting: Pumps or pressure tanks. HarvestH20.com. Retrieved October 27,2008, from http://www.harvesth2o.com/pumps_or_tanks.shtml

Schultz, R. H. (2008, November 2). Personal communication.

Waterfall, P. H. (2004). Harvesting rainwater for landscape use. University of Arizona Cooperative Extension.Retrieved October 27, 2008, from http://ag.arizona.edu/pubs/water/az1052/harvest.html

Waterfall P. H. and Bickelmann, C. (2006).Harvesting rainwater for landscape use. University of ArizonaCooperative Extension. Retrieved October 27, 2008, from http://cals.arizona.edu/pubs/water/az1344.pdf

Resources

The following publications provide extensive details about the information presented here and can be obtainedonline:

Virginia Rainwater Harvesting Manualhttp://www.cabellbrandcenter.org/Virginia%20Rainwater%20Harvesting%20Manual.pdf

Rainwater Harvestinghttp://rainwaterharvesting.tamu.edu/publications.html

The Texas Manual on Rainwater Harvesting (3rd ed.)http://www.twdb.state.tx.us/publications/reports/RainwaterHarvestingManual_3rdedition.pdf

Harvesting Rainwater For Landscape Usehttp://cals.arizona.edu/pubs/water/az1344.pdf

Product Suppliers

There are numerous suppliers available via the Internet forconstruction materials. Note that local suppliers can also bea good resource for construction materials.

Catherine Sabota, PhD, Extension Horticulturist, Alabama A&M University

Trade names are used only to give specific information. The Alabama Cooperative Extension System does notendorse or guarantee any product and does not recommend one product instead of another that might be similar.

Questions regarding rainwater collection should be directed to your county Extension agent.

Published by the Alabama Cooperative Extension System (Alabama A&M and Auburn Universities) in cooperationwith the U.S. Department of Agriculture. An Equal Opportunity Educator and Employer.

New March 2009

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