Urban Affairs and New Nontraditional Programs

Rainwater Harvesting:
Using, Storing, & Distributing the Collected Water

Water Use

Before discussing storage, it is appropriate to determine how you will use your water and how much you will need to store. This is a difficult concept because everyone's lot size and landscape content is different. The size of your catchment area and your irrigation methods will also be important in estimating tank size. Your water needs may vary with weather conditions, plant water requirements, soil type, sun or shade conditions, exposure to sun-north, south, east, or west side of a structure, and variety or cultivar. In drought conditions, categorize your plants into groups based on water requirements.

Plants such as gardenias, azaleas, rhododendrons, annuals, vegetables, and fruit trees have high water requirements for survival and/or fruit or vegetable production. You may also want to group your most valuable plants. Priorities should include trees that define your landscape, heir-loom plants that cannot be replaced, and mature plants that are massed and of the same size (Waterfall, 2004).

Irrigation for approximately 2,000 square feet will require about 2,000 gallons per month or more to maintain when using conservative irrigation methods (drip-and-trickle-type irrigation). Typical landscapes do not exceed 1,000 square feet of plants (excluding bed space with no plants). However, if you plan to use the system to maintain a lawn or turf, you will have a much larger landscape area and need a larger tank. Commercial or garden crops such as vegetables, fruits, or nursery plants are most often watered with trickle or drip hoses. Rows totaling 2,000 feet in length would require 2,000 gallons or more per week if providing about 1 inch of water per week.

You can determine what the anticipated monthly rainfall should be for your area by going online to http://www.city-data.com/city/Huntsville-Alabama.html and replacing Huntsville-Alabama with your city and state. Be careful not to remove the dash between city and state. Knowing how much rainfall is anticipated will help you calculate how much it contributes to your water needs. For example, if you need to water a crop 1 inch per week in June and the average rainfall was 5 inches for that month, you probably will not need to water. Your storage needs for that month will be zero. Of course, all 5 inches could have been from a single rain event, but that is rare. Subtract the water needs (-gallons) from the rainfall amounts (+gallons), and that will provide you with the balance amount of water you will need to store (Table 1). Based on this information, you will need a minimum 5,300-gallon tank. This is determined from the "Balance" figure shown for May, after which the water in the tank was depleted during the months of June, July, August, and September. The minimum tank size is obtained by taking the largest balance and subtracting the smallest balance from it [if the smallest balance is negative, that means that you add it]. For example using Table 1 data, the formula would be: 5,241 - (-75) = 5,316; therefore, a minimum 5,300-gallon tank would be needed.


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 grown from March through mid-November)

Month

Rain (inches)

+ Gallons from Rain1,2

- Gallons from Irrigation3

Balance

January

3.6

1,060

0

1,060

February

3.3

972

0

2,032

March

4.36

1,284

250

3,066

April

3.98

1,172

1,000

3,238

May

5.1

1,502

1,000

5,241

June

1.61

474

2,000

3,715

July

2.34

689

2,000

2,404

August

4.03

1,187

2,000

1,591

September

1.26

371

2,000

-38

October

3.27

963

1,000

-75

November

2.78

819

500

244

December

1.74

512

0

756

[1] Rainfall (inches) x catchment surface (square feet) x 0.62 x percent collection efficiency=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 95 percent. During a very fast, heavy rain, the efficiency would be closer to 60-75 percent because gutters overflow and gutter covers are overrun with water.

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

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

Storage

Storage tanks can be made of almost any nontoxic material; however, be cautious about what was stored in them. You can use "never-used" lined fuel tanks, precast fiberglass, polyethylene, or even concrete, such as septic tanks (Figure 6). Large polyethylene tanks with wire frames are used for sugar derivatives, are inexpensive, and can be rinsed and reused for water storage.

Figure 6. Large polyethylene tanks. Figure 6. Large polyethylene tanks. Figure 6. Large polyethylene tanks.
Large polyethylene tanks in various shapes and sizes can be used for storage of water, as long as the material and prior contents were nontoxic. Above ground tanks should be painted black or green.

If you plan to bury the tank, be sure it is built for in-ground use. The tank inlet and outlet should be designed so that minimal sediment in the bottom of the tank is disturbed. You can purchase calming inlets that disperse the water in an upward direction. Large buried tanks should have a manhole, a vent, a cleanout sump pump, and an overflow pipe. Above ground tanks should have a drain at the bottom and an overflow pipe if the water line coming into the tank is tightly fit.

Above ground tanks should be purchased in dark green or black to prevent light penetration, or they should be painted a dark color prior to installation. If in-ground tanks are completely covered, tank color is not an issue. Tank placement depends on space and resources. It is very expensive to bury a tank, and in many locations where it is rocky or the ground water is high, it is not feasible. There are many tank sizes and shapes that can be hidden by shrubbery or placed near the house wall.

Metal, fiberglass, and concrete storage tanks will tolerate cold and warm temperatures. However, plastic tanks come in several grades and polymers. Tanks constructed using homopolymers should not be used for water catchment tanks (Schultz, 2008). Check with your vendor to be sure the tank is made of high-density polyethylene. This material will tolerate greater temperature extremes. Lesser grades of polymer tanks will become brittle and crack as water freezes and thaws and if temperatures are extremely cold. All tanks, regardless of type, should have a domed top or an overflow pipe below the top of the tank to provide head space and to allow for the expansion of water as it freezes.

The size of your tank can be determined by considering several factors:

  • Space available
  • Resources/desire to bury the tank
  • Roof size
  • Use/need

If space is not a limiting factor, the primary concern should be how much water you will need and when (Table 1). If you collect water from a large roof surface, 1,500 to 2,000 square feet for example, you will fill a 1,500-gallon tank with a 2-inch rainfall. If you are collecting water for in-home use, outdoor use, or for commercial crops, you will need a 5,000-gallon or larger tank. If you have been purchasing water to irrigate your landscape or crops, you can average your winter bills, typically December, January, and February to determine household use. Since water meters and bills report water use in cubic feet, multiply this number by 748 to get gallons. Subtract the winter average from your monthly bills to determine monthly outdoor water use (Table 2). Tank size requirements will depend on rainfall recharge rates.


Sample Monthly Demand for Water Use if You Pay to Irrigate Water

Sample Monthly Demand

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

Month

Monthly Use

Average Winter Use

Landscape Use

January

6,732

6,679

0

February

5,986

6,679

0

March

7,945

6,679

1,266

April

8,361

6,679

1,682

May

8,520

6,679

1,841

June

10,620

6,679

3,941

July

14,560

6,679

7,881

August

16,521

6,679

9,842

September

12,330

6,679

5,651

October

10,253

6,679

3,574

November

8,694

6,679

2,015

December

7,320

6,679

0

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

For nonpotable, toilet flushing, determine the number of household members and the flush volume of your toilets. Generally, each person flushes five times a day, and a typical tank uses 1.6 gallons. In a four-person home, water use for flushing is about 32 gallons a day or 960 gallons a month. Using Table 1 as an example of how much raninwater can be collected monthly, put 960 in each row of the -gallons column. A 1,700 gallon tank would provide enough water for year-round flushing. If you have the resources to bury large tanks, they should be placed close to the water source. However, this may require extensive plumbing to the production site for crop producers.

Distribution

Unless your collection tank is 20 to 30 feet above where you will be watering, you will need a pump to distribute the water from the tank to your landscape or crops. There are several pumps that work well with this system. One-quarter to one and one-half horsepower irrigation pumps will provide enough pressure for most drip and trickle systems (Figures 7 and 8). "Irrigation," "lawn," or "shallow well" pumps can be used without bladder tanks. A tether switch can be placed in the tanks to turn off the pump when the water level is low (Figure 8). If your tank is not close to an electrical source, a gasoline-powered pump can be used.

Figure 7. This 1.5 hp pump. This 1.5 hp pump is used to irrigate shiitake mushrooms and 1/2 acre of organic vegetables using a mist system and soaker hoses.

Figure 8. A 1 hp pump. A 1 hp pump (left) will provide enough water to irrigate a home landscape and garden if trickle and drip irrigation are used. The pipe on the right has a foot valve at the bottom (closest end to you) and the black cylindrical-shaped object is a tether float switch. The black cord (attached to the tether float switch) at the far end of the pipe plugs into an outlet and the pump plugs into this cord. When the black float drops vertically due to lack of water, it will turn off the power to the pump.

When connecting the pump to the tank, the plumbing can include a floating filter located at the end of the pump's suction hose. This will reduce the amount of soil sediment sucked into the pump and inline filters. You can also use a foot valve at the bottom of the outflow line if it is placed a few inches above the bottom of the tank; however, you will still need an inline filter.

Pressure tanks hold a set volume of water that is pumped into the bladder. When the valve is opened, pressure pushes out the water. A pressure-sensitive pump should be used with a pressure tank. A pressure-sensitive pump turns on when pressure decline is detected. It shuts off when a specific pressure is reached. Most pressure-sensitive pumps are not submersible, and they are more expensive. However, pressure tanks must be protected from freezing temperatures and placed in a structure. Pressure or bladder tanks are more commonly used under the house when catchment water supplements toilet water.

An inline pump controller is an optional part that is screwed directly onto the existing pump and water line. It will detect pressure changes and turns on the pump when pressure is low. Some units can even detect if the tank is out of water and will turn off the pump so it will not burn out (Pushard, 2004). Pump size requirements are based on volume of water needed at any one time, pressure requirements, distance the water must travel (run), and the rise required to get the water out of tank and to the irrigation site. Most manufacturers provide this information on the box or in the pump instruction book.

Irrigation

Your method of irrigation will determine pump size and other distribution equipment needed. If you plan to hand water with a hose, a smaller pump can be used. Irrigation of lawns and large areas with sprinklers will require larger pumps, depending on the sprinkler head and distance and slope the water must traverse. Trickle and drip irrigation are the most efficient systems for beds and crop production (Figure 9). These systems will require the installation of a pressure regulator and an inline filter (Figure 4). The shiitake mushroom logs shown in Figure 9 have a mister head at the top of each riser; this provides a fine mist to wet the burlap and increase the humidity under the burlap.

Figure        9. Small mist heads.
These small mist heads reduce the water pressure and volume

needed to keep the shiitake mushroom logs moist throughout the year.




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