Sustainable Building Sourcebook
Chapter: Water
 
Rainwater Harvesting
 
CSI Numbers: 02516 Rainwater Harvesting Systems and Components
 
Introduction:
 

Water has long been a precious commodity in Texas. In the past, 50 percent of water used in Central Texas was from mineral rich groundwater. Our future growth will have a great impact on our water reserves. If the rapid population growth continues, our groundwater will be exhausted within twenty years. Another factor of growth is that there is more ground surface being covered over, and impervious surfaces prevent groundwater from being recharged. Surface water evaporates quickly in our climate and cannot keep up with the expected demand.

On-site rainwater collection is one means to augment our fresh water needs and can prevent rapid stormwater accumulation from roof areas. Harvested Rainwater is rainwater that is captured from the roofs of buildings. Harvested rainwater can be used both indoors and for irrigation.

Also see the Green Building Factsheets for introductions to this and other green building topics.

 
At-A-Glance Notes:
 
Technology:
Fairly well developed, with new products appearing often. Rainwater harvesting is an old tradition practiced in all parts of the world including Texas and is required by law in new construction in Bermuda, the U.S. Virgin Islands, Germany, and Israel.
 
Suppliers:
Suitable roof and gutter materials are common products in our region. Specialized products such as roof washers (pre-filters) are also available here. Storage tanks (cisterns) are available regionally and statewide. System designers and installers are present locally.
 
Cost:
A rainwater harvesting system is costly compared to a city hookup but may be less expensive than drilling a well in our area.
 
Public Acceptance:
In the Austin region, there are increasing numbers of rainwater harvesting systems. An excellent example can be found at the Lady Bird Johnson Wildflower Center. Austin Energy Green Building ™ has noted that rainwater harvesting presentations draw large crowds at conferences and trade shows.
 
Regulatory:
At present, there is no Texas regulation for rainwater for indoor or outdoor household use unless the system is backed up by publicly supplied waterlines. If there is a backup system, there must be an air gap between the public water and rainwater. This air gap must exceed two diameters of the city line in width. The Austin-Travis County Health Department does not allow uncovered cisterns because they can contribute to mosquito breeding. For a commercial building with less than 25 people, the regulations are the same as for a household. But for more than 25 people, the system must be chemically treated to Clean Water Act standards.
 
Considerations:
 

The Austin area receives an average of 32 inches of rain per year. A 2,000 square foot area can capture 36,000 gallons of water annually, which would meet 100 gallons per day in household water needs.

The quality of rainwater can vary with proximity to industrial pollution sources. In general, Austin's rainwater quality is quite good. The softness of rainwater is valued for its cleaning abilities and benign effects on water-using equipment such as water heaters and dishwashers, and cooling towers. As an irrigation source, its slight acidity is helpful in the high PH soils of our region and is the best water for plants.

Rainwater collection systems can supplement water for irrigation purposes with minimal equipment cost.

The primary expense for a rainwater collection system is in the storage tank (cistern). In our area, the cistern size for irrigation can be large because of high temperatures and extended dry periods in the summer. If the system is not counted upon as the only source of irrigating water, base the size of the cistern on affordability. If rainwater is for potable water, there will be additional expense and maintenance for filtration and treatment.

 
Guidelines:
 

Capacity

The capacity of a rainwater harvesting system depends on the amount of rainfall, size of collection area, storage capacity, and the household's level of demand for water.

Table 1 indicates the gallons of water produced annually for different size roof areas and rainfall amounts.

To determine the square footage of catchment area of a house, use only the roof footprint. The actual area of roof material will be greater because of the roof slope. However, the amount of rainfall on the roof is not affected by its slope. In Table 1, note that Austin's average rainfall is 32 inches.

For outdoor uses of rainwater, the types of plants, amount of exposure to direct summer sun, soil conditions, presence or lack of mulch, and size of the area will determine the necessary amount of irrigation water. Large landscapes with large water demands are not readily accommodated by rainwater catchment systems.

Indoors, a conserving household may use 25- 40 gallons of water per person per day. To determine the total amount of rainwater storage capacity required, multiply the number of persons in the household by the average water use. (See the Water Budget section to determine precise amounts.) Then ensure a safety factor for drought conditions. The Austin area's longest drought in 50 years lasted 75 days. Austin Energy Green Building recommends a 100-day safety factor to determine storage capacity.

Example: 3 people each use 40 gallons per day.

3 (persons) x 40 (gallons per day per person) x 100 (days) = 12,000 gallons of storage required.

Rainwater for Irrigation

Since the largest need for irrigation water in our area occurs during the time of lowest rainfall and highest temperature, a rainwater system designed to meet this need will have to capture water prior to the summer. And since large or water-intensive landscapes would require prohibitively large storage systems, Austin Energy Green Building recommends using rainwater harvesting with low-water use landscaping.

Table 2 shows the gallons of rainwater that can be captured from roof areas and the gallons of water it takes to irrigate various landscape areas to equal a certain amount of rainfall. These are useful in calculating the storage size and roof area associated with various irrigation requirements.

Example of irrigation requirement estimation

The landscape to be irrigated for this example consists of 2,500 square feet. Landscape specialists have determined that the plants should receive a minimum of one inch of rain per week to be healthy from June through September. The roof area for collection in this example will be 1,500 square feet.

  1. Table 2 shows that 2,500 square feet of landscape area requires a little over 1,400 gallons of water to equal one inch of rain. (Find 2,500 in the landscape/roof size column and follow across to the one-inch rainfall column.)
  2. In 16 weeks (June - September), the water requirement is 22,400 gallons. (16 weeks x 1,400 gallons per week)
  3. For this example, we will estimate that only half of the average summer rainfall will occur. (June through September rainfall totals 10.79 inches. We will assume therefore only 5.25 inches will fall.)
  4. In Table 2, the 5-inch column for 2,500 feet of area equals 7,023 gallons and the 0.25-inch column for 2,500 feet equals 351 gallons. (The total is 7, 374 gallons. This is the amount of natural rainfall the landscape will receive at 5.25 inches for June-September.)
  5. Subtract the natural rainfall (7,374) from the required amount (22,400) for the net need of the landscape. This amount equals 15,026 gallons. This is the amount of water that will need to be collected for irrigating the landscape.
  6. The roof area during this period will similarly receive 5.25 inches of rain that can be collected for irrigation purposes. Locate the 5-inch column and the 0.25-inch column totals for 1,500 square feet of roof/landscape area. (The 5-inch total is 4,214 gallons and the 0.25-inch column is 211 gallons for a total of 4,425 gallons.)
  7. Subtract the amount the roof will collect in step #6 (4,425 gallons) from the required amount in step #5 (15,026 gallons). This equals 10,600 gallons . (This is the amount of rainwater that must be in storage prior to June for use as irrigating water for the landscape if rainfall is one half the average amount.)

By knowing the average amounts of rainfall that can fall in the period preceding the summer irrigation period, you can estimate the time needed to collect that amount of water. (Use the 1,500 square foot row on Table 2 and add each month's average rainfall until you reach the required amount.)

Some parts of the landscape may require different amounts of water throughout the entire year. Total the requirement for each month in the same manner as in the example above and follow the same procedure. When calculating water requirements for an entire year, it is best to use the average monthly rainfall figures rather than a conservative amount as in the above example.

System Components

A rainwater harvesting system consists of the following components: catchment area (roof), conveyance system (guttering, downspouts, and piping), storage (cistern), and filtration and distribution.

Catchment

Rainwater harvesting can be done with any roofing material if it is for non-drinking use only. For potable use of rainwater, the best roof materials are metal, clay, and cementitious, although all roof material types except asbestos have been used. Asbestos roof materials used in older homes should not be part of a system to provide drinking water. Asphalt shingles can contribute grit and leeched chemicals to the system and a pre-filter will be needed for the water before it enters the cistern. Lead materials in any form, including flashing, should not be used.

Conveyance

Gutters convey water from the roof to pipes to the cistern. If a straight run of gutter exceeds 60 feet, use an expansion joint.

Keep the front of the gutter 1/2 inch lower than the back. Provide a gutter slope of 1/16 inch per lineal foot minimum.

Provide gutter hangers at 3 feet O.C. (on center). Gutter should be a minimum of 26 gauge galvanized steel or 0.025 inch aluminum. (Galvanized steel, copper, or aluminum are preferred gutter materials.)

Downspouts should provide 1 square inch of downspout opening for every 100 square feet of roof area. The maximum run of gutter for one downspout is 50 feet.

The conveyance piping from the gutter system to the cistern or filter should be Schedule 40 PVC (or comparable) in a 4-inch diameter. Do not exceed 45-degree angle bends in horizontal pipe runs and provide 1/4-inch slope per lineal foot minimum. Use one- or two-way cleanouts in any horizontal pipe run exceeding 100 feet.

Storage

The storage tank (cistern) can be located above or below ground. See the previous section regarding capacity for sizing information.

The best materials for cisterns include concrete, steel, ferro-cement, and fiberglass.

When ordering a cistern, specify whether the cistern will be placed above or below ground and if the cistern will be used to store potable water. Fiberglass cisterns are constructed differently to meet these criteria.

If using a manufactured tank designed to hold drinking water, the tank should conform to the published specifications of the American Waterworks Association. (See Resources .)

Cistern characteristics

A cistern should be durable and watertight, with a smooth, clean interior surface. Joints must be sealed with non-toxic waterproof material.

Manholes or risers should have a minimum opening of 24 inches and should extend at least 8 inches above grade with buried cisterns. Fittings and couplings that extend through the cistern wall should be cast-in-place.

Dissipate the pressure from the incoming water to minimize the stirring of any settled solids in the bottom of the cistern. This can be accomplished in a concrete cistern by placing concrete blocks (cavities facing upward) surrounding the base of the inlet pipe. The blocks can be 8"x 8"x16" blocks with the pipe exiting one inch above the bottom of the cistern. Baffles to accomplish the same result can be made as part of fiberglass cisterns. Settling problems do not occur in cisterns that maintain a large reserve of water.

The use of two or more cisterns permits servicing one of the units without losing the operation of the system.

Have a fill pipe on the cistern for adding purchased water as a backup and a cover to prevent mosquito breeding and algae growth from contact with sunlight.

Table 3 gives typical cistern capacities.

Filtration

Dirt, debris, and other materials from the roof surface may contaminate the rainwater. The best strategy is to filter and screen out the contaminants before they enter the cistern.

A primary strategy is to reject the first wash of water over the roof. A "roof washer" will use the first rainfall to clean away any contaminants.

The main function of the roof washer is to isolate and reject the first water that has fallen on the roof after rain has begun and then direct the rest of the water to the cistern. Ten gallons of rainfall per thousand square feet of roof area is considered an acceptable amount for washing. Roof washers are commercially available and afford reliability, durability, and minimal maintenance.

Roof washing is not needed for water used for irrigation purposes. However, prefiltering to keep out debris will reduce sediment buildup. A leaf screen over the gutter and at the top of the downspout is helpful for this purpose.

Distribution

Removing the water from the cistern can be achieved through gravity, if the cistern is high enough, or by pumping.

Most cases will require pumping the water into a pressure vessel similar to the method used to withdraw and pressurize water from a well. A smaller pump can be used to pump from a cistern.

A screened 1.25-inch foot valve inside the tank connected to a 1.25-inch outlet from the cistern approximately one foot above the bottom (to avoid any settled particles) will help maintain the prime on the pump.

A float switch should be used to turn off the pump if the water level is too low.

An alternative method uses a floating filter connected to a flexible water line inside the cistern. This approach withdraws the water from approximately one foot below the surface, considered to be the cleanest water in any body of water.

The water that will be used for potable purposes can pass through an inline purification system or point-of-use water purification system, such as an UV filter. Other uses for the water do not need additional purification.

Table 1: Annual Rainfall Yield in Gallons for Various Roof Sizes and Rainfall Amounts

 

Rainfall in Inches

Roof size (Square Feet) 20 24 28 32 38 40 44 48 52
1000 11236 13483 15730 17978 20225 22472 24719 26966 29214
1100 12360 14832 17303 19775 22247 24719 27191 29663 32135

1200

13483

16180

18876

21573

24270

26966

29663

32360

35056

1300

14607

17528

20450

23371

26292

29214

32135

35056

37978

1400

15730

18876

22023

25169

29315

314461

34607

37753

40899

1500

16854

20225

23596

26966

30337

33708

37079

40450

43820

1600

17978

21573

25169

28764

32360

35955

39551

43146

46742

1700

19101

22921

26742

30562

34382

38202

42023

45843

49663

1800

20225

24270

28315

32360

36405

40450

44495

48540

52584

1900

21348

25618

29888

34157

38427

42697

46966

51236

55506

2000

22472

26966

31461

35955

40450

44944

49438

53933

58427

2100

23596

28315

33034

37753

42472

47191

51910

56629

61349

2200

24719

29663

34607

39551

44495

49438

54382

59382

64270

2300

25843

31011

36180

41348

46517

51686

56854

62023

67191

2400

26966

32360

37753

43146

48540

53933

59326

64719

70113

2500

28090

33708

39326

44944

50582

56180

61798

67416

73034

 

Austin Average rainfall in inches per month

January: 1.60

February: 2.49

March: 1.68

April: 3.11

May: 4.19

June: 3.06

July: 1.89

August: 2.24

September: 3.60

October: 3.38

November: 2.20

December: 2.06

Determining Indoor Water Budget

A water budget establishes a baseline of how much the occupant should expect to use and where the water goes. This is helpful in determining the size of cistern you will need. The designer can also plan additional conservation strategies from the water budget information. Bear in mind that the figures derived from the water budget are estimates -- personal lifestyle will determine actual water use.

The following information is based on the average per capita interior water use rates developed by the U.S. Department of Housing and Urban Development [HUD]. Actual use rates for water fixtures will vary.

HUD has determined the following daily average use and flow rates:

Item

Use Rate

Flow Rate

Toilet

4.0 flushes/person

1.6

Shower

4.8 minutes/person

2.5

Washer

0.30 loads/person

40 gallons/load

Dishwasher

0.17 loads/person

8.5

Faucets

8.5 gallons (total)

8.5

Bath

0.14 bath/person

50 gallons/bath

To calculate usage, multiply Use Rate times Flow Rate times # people for each type of fixture or appliance. Then add the subtotals. The total can be multiplied by 365 to show the amount of water that would be consumed in one year.

Determining Outdoor Water Budget

The outdoor water budget is for sunny turf/lawn areas only. Shaded lawn areas have reduced water requirements. Soil type can also affect water demand. The method below gives a general indication of water demand and is based on the five types of grass that are common to the Austin area. Austin Energy Green Building staff has assigned a "grass factor" to each type of grass, depending on its water requirements. The higher the factor number, the more water required.

To minimize water needs for turf areas, do not remove grass clippings, keep grass at the recommended cutting height with frequent mowings, and use natural soil amendments such as "Dillo Dirt. Note that watering rates will vary during the year, with most watering occurring in the summer.

Key For Calculations

Type of Grass

Grass Type Factor

St. Augustine

1.0

St. Augustine/Bermuda Mix

0.9

Zoysia

0.9

Bermuda

0.8

Buffalograss

0.5


CF/YR: cubic feet per year CFNR/YR: cubic feet of natural rainfall per year

G/YR: gallons per year GNR: gallons of natural rainfall

GTF: grass type factor NR: natural rainfall

SQFT: square feet of turf

Calculation Sequence to determine outdoor water needs:

Step 1: Cubic feet of water demand per year

Multiply 50 (inches of turf water demand for one year not including natural rainfall) times the square feet of the turf area and divide by 12.

[50 x SQFT/12 = CF/YR]

Step 2: Gallons per year required by grass type

Multiply the cubic feet of water demand per year times 7.48 (conversion factor) times the grass type factor.

[CF/YR x 7.48 x GTF = G/YR]

Step 3: Cubic feet of water supplied by rainfall per year

Multiply 32 (inches of natural rainfall) times the square feet of turf area divided by 12.

[32 x SQFT/ 12 = CFNR/YR]

Step 4: Gallons of natural rainfall

Multiply the cubic feet of natural rainfall per year times 7.48 (conversion factor)

[CFNR/YR x 7.48 = GNR]

Step 5: Amount of additional irrigation water required

Subtract the gallons of natural rainfall in Step 4 from the required water for the selected grass type in Step 2.

G/YR - GNR = water requirement in gallons

Note: if the number is negative, natural rainfall is adequate to keep the grass alive


 
Resources:
 
Professional Assistance:
See "systems" suppliers below
Components/Materials/Systems:
Atlantis Tank Modules
Local Distributor:
PolySteel of Austin
Joe Bailey
109 Fenway Court
Austin, TX 78734
(512) 415-9460
joe@masterbuilderhomes.com
http://www.atlantiscorp.com.au/

Austin Pump & Supply
3803 Todd Ln.
Austin, TX 78744
(512) 442-2348
Polyethylene tanks

Barrel City USA
8401 South 1 st St.
Austin, TX 78748
(512) 282-1328
Recycled 55-gallon drums

Bowerbird Construction
Keith Miller, Owner
P. O. Box 698
Dripping Springs, TX 78620
(512) 858-5395
(512) 858-2061 (fax)
mailto:bowrbird@texas.net
Ferrocement tanks, rainwater systems

Ecos, Inc.
PO Box 1313
Concord, MA 01742
(978) 369-3951
http://www.ecotechusa.com/
Small rainwater systems & components

Farm & Ranch Service Supply Co.
P. O. Box 10165
San Antonio, TX 78210
(800) 292-0007
Concrete tanks, roof washers, floating filters, more

John Dorn Tank Building, Inc.
P.O. Box 150
Vidor, TX 77662
(409) 769-5129
Bolted, galvanized tanks

L & F Manufacturing
P.O. Box 578, Highway 290 East
Giddings, TX 78942
(800) 237-5791
Fiberglass tanks

Matrix D-Raintank®
Atlantis Corporation
401/781 Pacific Hwy Chatswood
NSW 2067
Australia
Corey Simonpietri
(804) 271-2363
csimonpietri@acfenvironmental.com
http://www.atlantiscorp.com.au/

Midessa Membranes
Midessa Industrial Vinyl Company
Rt. 4, 5203 W. 42 nd
Odessa, TX 79764
(915) 333-3055
PVC bladders

Preload, Inc.
5710 LBJ Freeway, Ste. 140
Dallas, TX 75240
(800) 645-3195
Concrete tanks

RainMan Waterworks
P. O. Box 972
Dripping Springs, TX 78620
(512) 858-7020
http://www.rainharvester.com/
Design & installation, systems

Rainwater Collection Over Texas
P.O. Box 953
Dripping Springs, TX 78620
(800) 222-3614, (512) 288-7151
mailto:raincollect@earthlink.net
http://www.rainco.net/
Rainwater systems, parts, and maintenance

Red Ewald, Inc
P.O. Box 519
Karnes City, TX 78118
(800) 242-3524
Fiberglass reinforced tanks

Sustainable Homesteads
P. O. Box 2669
Wimberley, TX 78676
(512) 832-0737
Systems, consultation

Sweetwater Filtration
1321 Rutherford Ln., Ste. 180
Austin, TX 78753
(512) 837-2488
(512) 459-3131
Rainsoft water treatment systems

Tank Town
1212 Quail Ridge
Dripping Springs, TX 78620
(512) 894-0861
Rainwater systems

Triple S Feed
2111 Highway 290 West
Dripping Springs, TX 78620
(512) 894-0344
Polyethylene tanks

Water Filtration Company
1205 Gilman
Marietta, OH 45750
(740) 373-6953
Roof washers, floating filters, more

Water Works of Texas
2206 Matterhorn Ln.
Austin, TX 78704
(512) 326-4636
mailto:rainfall@swbell.net
Rainwater systems

 
General Assistance:

American Rainwater Catchment Systems Assoc.
c/o Kate Houser, Sec.
Water Works of Texas
2206 Matterhorn Ln.
Austin, Texas 78704
(512) 326-4636
http://www.pondscapes.com/page145.html

Barley & Pfeiffer Architects
1800 West 6 th St.
Austin, TX 78703
(512) 476-8580
System design, design/build, consulting

Stephen Bell Landscape & Irrigation
P.O. Box 16159
Austin, TX 78716
(512) 899-8888
Design, installation, systems, consultation

Bracken Designers
916 Remschel Ave.
Kerrville, TX 78028
(830) 257-7400
Consultation and design

Center for Maximum Potential Building Systems
8604 FM 969
Austin, TX 78724
(512) 928-4786
http://www.cmpbs.org/
Consulting and design

National Small Flows Clearinghouse
West Virginia University
NRCCE
P. O. Box 6064
Morgantown, WV 26506-6064
(800) 624-8301
http://www.estd.wvu.edu/nsfc/nsfc_homepage.html
Information, "Small Flows" newsletter

Publications

Rainwater Collection for the Mechanically Challenged
By Suzy Banks and Richard Heinichen, 1997
Dripping Springs, TX
Available at Book People, Gardenville and Eco-Wise in Austin, TX. Nationally available from RealGoods.

Rainwater Collection Systems
Iris Communications
(800) 346-0104
http://www.oikos.com/
$30 (video and 45 page instructional book)

Texas Guide to Rainwater Harvesting
Texas Water Development Board
Conservation Division
P.O. Box 13231
Austin, TX 78711
Patsy Waters
(512) 463-7955
http://www.twdb.state.tx.us/assistance/conservation/Rain.htm
Free, easy to understand introduction to rainwater harvesting, can be downloaded from their web site. Also available from the City of Austin Water Conservation division and for rebate information: http://www.ci.austin.tx.us/watercon/