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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.
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.
- 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.)
- In 16 weeks (June - September), the water requirement is 22,400 gallons. (16 weeks x 1,400 gallons per week)
- 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.)
- 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.)
- 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.
- 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.)
- 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.
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
| 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 |
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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 |
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:
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 |
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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.
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.
St. Augustine |
1.0 |
St. Augustine/Bermuda Mix |
0.9 |
Zoysia |
0.9 |
Bermuda |
0.8 |
Buffalograss |
0.5 |
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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
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