This section hosted by Dick Peterson
Rainwater harvesting systems are required by law in new construction in Bermuda and the US Virgin Islands. California offers a tax credit for rainwater harvesting systems and financial incentives are offered in cities in Germany and Japan.
In this section, Harvested Rainwater is rainwater that is captured from the roofs of buildings on residential property. Harvested rainwater can be used for indoor needs at a residence, irrigation, or both, in whole or in part.
The Austin area receives an average of 32 inches of rain per year. A 2000 square foot area can capture 36,000 gallons of water, which would match up 100 gallons per day in water demand. This is a significant amount of water toward the needs of a water-conserving home.
The quality of rainwater can vary with proximity to highly polluting sources. However, in general, the quality is very good. The softness of rainwater is valued for its cleaning abilities and benign effects on water-using equipment. As an irrigation source, its acidity is helpful in the high PH soils of our region and, as one would expect, is the best water for plants.
Rainwater harvesting systems designed to fill all the water needs of a home can be similar in cost to the expense of putting in a well. Operating costs for a rainwater system can be less. Rainwater collection systems designed to supplement the water needs of a home already on the city system for irrigation purposes can be costly. The primary expense is in the storage tank (cistern). In our area, the cistern size for irrigation can be large due to the high temperatures and extended dry periods in the summer. If the system is not counted upon as the only source of irrigating water, building as large a cistern as one can afford is often the measuring gauge for cistern size.
|Satisfactory in most conditions|
|Satisfactory in Limited Conditions|
|Unsatisfactory or Difficult|
Fairly well-developed; new products are being developed. Rainwater harvesting is an old tradition practiced in all parts of the world including Texas.
Suitable roof and gutter materials are common products in our region. Specialized products such as roof washers (pre-filters) are also available in our region. Storage tanks (cisterns) are available regionally and statewide. System designers and installers are present locally.
Rainwater harvesting systems are costly compared to a city hookup. Compared to a well, they are equal or, likely, greater in cost.
Appraisers may not properly value a rainwater harvesting system and underwriters may not accept this system as the sole source of household water. If the owner provides a backup water source, such as an on-demand supply contract with a water hauler, lenders would be more favorably inclined to accept such systems. It has become more common for new homes with rainwater systems to receive conventional financing.
In the Austin region, there are a small but increasing number of rainwater harvesting systems. A small segment of the population desires rainwater catchment systems for indoor water use. A larger portion of the population feels there is an advantage of using captured rainwater for irrigation. Rainwater harvesting presentations draw large crowds.
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 a backup system is used, to avoid any cross-connection, an airgap must exist between the public water and rainwater. (An example is a city water line feeding into a rainwater cistern.) This airgap must exceed two diameters of the city line in width. The Health Department will require that the rainwater system does not contribute to mosquito breeding by having an uncovered cistern.
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 1.0 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 house’s footprint. (The actual area of roof material will be greater due to the roof slope. However, the amount of rainfall on the roof is not affected by the slope.) In Table 1.0, note that Austin’s average rainfall is 32 inches.
|Annual Rainfall Yield in Gallons for Various Roof Sizes and Rainfall Amounts|
|ROOF SIZE IN
|RAINFALL IN INCHES|
The average rainfall per month for Austin follows:
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 how much irrigation water is needed. Large landscapes with large water demands are not readily accommodated by rainwater catchment systems.
Storage capacity for indoor uses of rainwater can typically be more readily gauged; although this is not a precise science due to the vagaries of rainfall and personal habits.
A conserving household may use 25- 40 gallons of water per person per day. Multiply the number of persons in the household by the average use (40 gallons per person is a generous amount, 25 gallons is quite conservative. See the Water Budget section if more precise amounts are needed.) The longest drought in 50 years lasted 75 days in our area. Multiply the total of the number of persons times daily use times 100. This gives a safety factor of 25 days over the worst-case scenario of the last 50 years. The total is the amount of storage capacity required.
Example: 3 people use 40 gallons per day each. 3 (persons) x 40 (gallons per day per person) x 100 (days) = 12,000 gallons of storage required.
2.0 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.
The size of the storage system may be prohibitive for using rainfall for the sole source of irrigation water in large or water-intensive landscapes. A low water demanding landscape is required.
The average rainfall will not indicate the actual amount that will fall in any particular year.
Table 3.0 shows the amount of gallons of rainwater that can be captured from rain for various roof areas in smaller increments than Table 1.0 (these are termed rainfall “events”) and the gallons of water it takes to irrigate various landscape areas to equal a certain amount of rainfall.
Table 3.0 and the average rainfall amounts are useful in calculating the storage size and roof area associated with various irrigation requirements.
2.1 Example of Irrigation Requirement Estimation
The landscape to be irrigated for this example consists of 2,500 square feet. It is determined through consultation with landscape specialists 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 3.0 shows that 2,500 square feet of landscape area requires a little over 1400 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 1400 gallons per week)
- 3. Choose if you wish to assume average rainfall or a lesser amount will fall during this period. 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. Select the amount of gallons from Table 3.0 that this amount of rainfall will equal. The five inch column for 2500 feet of area equals 7,023 gallons. The 0.25 inch column for 2,500 feet gives 351 gallons. (The total is 7, 374 gallons. This is the amount of natural rainfall the landscape will receive.)
- 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 when rainfall is half the average amount.
- 6. The roof area during this period will similarly receive 5.25 inches of rain which 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 gives 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). (15,026 minus 4,425 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.)
Landscape / Roof Size
Rainfall in Inches
By knowing the average amounts of rainfall that can fall in the period preceding the summer irrigation period, the time needed to collect that amount of water can be estimated. (Use the 1,500 square foot row on Table 3.0 and add each month’s average rainfall until you reach the required amount.)
Some parts of the landscape may require water throughout the entire year in various amounts. 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.
3.0 Subsystem Components
3.1 Catchment Subsystem
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 have been used(except asbestos). Asbestos roof materials used in older homes should not be part of a system to provide drinking water. Asphalt shingles can contribute grit to the system and need a pre-filter for the water before it enters the cistern. Lead materials in any form should not be used in the system (i.e. lead flashing).
3.2 Conveyance Subsystem
Gutters are used to 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 one half inch lower than the back.
Provide a gutter slope of 1/16 inch per foot minimum.
Provide gutter hangars at 3 feet O.C.(on center).
Gutter should be a minimum of 26 gauge galvanized steel or 0.025 inch aluminum.
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 foot minimum. Use one or two-way cleanouts in any horizontal pipe run exceeding 100 feet.
3.3 Storage Subsystem
The storage tank (cistern) must be sized properly to ensure that the rainwater potential is optimized. See the previous section regarding capacity for sizing information.
Cisterns can be located above or below ground.
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 the various 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.)
- A cistern should be durable and watertight.
- A smooth clean interior surface is needed.
- 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. This is not a concern for cisterns that always have a large reserve.
- 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.
- Have a cover to prevent mosquito breeding and algae growth from contact with sunlight.
Diameter of Round Type
Length of Sides of Square Type
3.4 Filtering Subsystem
The rainwater may become contaminated by dirt, debris, and other materials from the roof surface. The best strategy is to filter and screen out the contaminants before they enter the cistern.
A leaf screen over the gutter and at the top of the downspout is helpful.
A primary strategy is to reject the first wash of water over the roof. The first rainfall will clean away any contaminants and is achieved by using a “roof washer.”
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 to this function.
Roof washing is not needed for water used for irrigation purposes. However, prefiltering to keep out debris will reduce sediment buildup. A sand filter can also be used.
Removing the water from the cistern can be achieved through gravity, if the cistern is sufficiently 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 (except a smaller pump can be used to pump from a cistern).
A screened 1.25 inch foot valve inside the tank connected to an 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.
Another alternative is the use of a floating filter inside the cistern connected to a flexible water line. This approach withdraws the water from approximately one foot below the surface which is considered to be the most clear 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. Other uses for the water do not need additional purification. (Water purification options are not discussed in the Sourcebook.)