Rain is nourishing, life giving, cleansing and renewing. And it’s valuable. Especially if you’re using it to supplement your irrigation. Rainwater harvesting is certainly a viable option for water conservation measures, but it takes planning, good infrastructure and a significant capital investment.
Take heed – make sure your state, county or municipality doesn’t prohibit rainwater harvesting. A few years ago, Colorado finally conceded and passed legislation allowing limited rainwater harvesting.
Depending on your area’s annual average rainfall, you may be able to use it as a supplement irrigation source or use it to irrigate 100 percent of your crops.
It doesn’t take much rainfall to accumulate a significant amount of stored irrigation water.
During a 1-inch rainfall event, a grower can expect to get .623 gallons of water for each square foot, explains John Smith, Texas A&M AgriLife Extension Service program specialist. Calculate an efficiency factor of approximately 90 percent.
Square foot of collection area x rainfall (in inches) x 0.623 x .90 = gallons
“From a 1-inch rainfall event on a 2,000-square-foot roof, you’re going to get about 1,200 gallons of water,” Smith says. “Nearly all greenhouses, shops or offices can be used as a catchment source. The steepest, slickest roof surface works best.”
Texas AgriLife Extension offers a rainwater supply calculator and a pressure loss calculator at http://rainwaterharvesting.tamu.edu/calculators.
Rainwater lacks hard minerals and salts, and it’s free of heavy metals, Smith says.
Let’s examine the basics of a harvesting system.
Steven Sweeney, chief technology officer at Rain Harvesting Supplies, explains the basic parts for an effective rainwater collection system.
- Collection surface (roof of a greenhouse, head house, office, etc.)
- Pre-tank filtration system
- Conveyance piping (which takes rain water from the roof to the tank)
- Storage tank
- Pre-pump filtration
- Pump system
- Pumping controls
- Post-pump filtration
Notice that Sweeney mentions more than one type of filtration.
“The general design of a large rainwater harvesting system must contain multiple levels of filtration,” he says. “Otherwise, you’re going to have problems with debris throughout the system.”
Depending on the catchment area, some growers may want to consider a first flush diverter, Smith says.
Sweeney also warns not to undersize the pump. In terms of piping, consider pressure-rated pipe for getting water from the roof.
“Pressure pipe and drainage pipe are the two main pipes. Drainage pipe is cheap. Instead, opt for the pressure-rated pipe, which is much more durable. You may spend twice as much for the pressure-rated pipe, but think about how much money and time you want to spend maintaining your system,” Sweeney adds.
For storage tanks, poly or plastic tanks are the cheapest, but they will crack and degrade in the sun, and they commonly have leaks at penetration points.
“A prefab stainless steel or galvanized steel tank with liner is a permanent piece of infrastructure that’s designed to last about 30 years,” he says. “Remember, you get what you pay for, especially in rainwater harvesting.”
Growers must also take into consideration that there is little regulation regarding rainwater harvesting, so do your homework on harvesting systems and contractors, Sweeney says.
Agri-Starts, a young plant grower in Apopka, Fla., completed its massive rainwater harvesting system last summer. Agri-Starts president Randy Strode and son Ty were looking for ways to conserve water. The USDA Farm Service Agency had been to the greenhouse proposing grants and design services for erosion control and water conservation projects, including rainwater harvesting, said Lee Goode, chief financial officer at Agri-Starts.
The first project that FSA presented was a system with a 200,000-gallon tank that collected only 35-40 percent of the rainwater. But Agri-Starts decided instead to go all out and designed a system with two 250,000-gallon tanks and a 50,000-gallon “day tank,” Goode says. Agri-Start sits on 15 acres and has 4 acres of greenhouses.
For every inch of rain, the grower gets 74 gallons of water from the greenhouse catchment roof.
Rainwater is collected from the roof of the greenhouses and stored in the two 250,000-gallon storage tanks until needed. The storage tanks are located at the back of the property and are 10 feet high and 65 feet in diameter. The storage tanks are made of steel and coated inside with a polymer.
The rainwater flows off the roof, through the gutters and down the 6-inch downspouts, and is connected to the twin 12-inch PVC pipes. These pipes are buried 5 feet in the ground and flow back to the large storage tanks at the back of the property. The 12-inch pipes total 2,400 feet and are also used for storage. The 12-inch pipes are level to help keep water in them at all times. They hold about 14,000 gallons of water, says Goode.
The two 12-inch pipes on each side of the greenhouse can handle any rain event with the exception of a 100-year flood rain, adds Goode.
“In May we had a 3-inch rainfall in two hours and the system held it all. We know it works,” he says.
The system includes an 18-inch PVC overflow pipe on each storage tank, which diverts overflow to the retention pond. There’s also a backup float and a timer on the 50,000-gallon tank to keep it from running over.
Water is pumped into the 50,000-gallon day tank from the two large storage tanks. Water in the day tank is treated overnight with chlorine for sanitation purposes. The treated water is used in the greenhouses, Goode explains. A 3-horsepower pump is used to fill the day tank with the rainwater. The water is not actually pumped from the large storage tanks directly, but from the 12-inch pipes that connect the gutters to the storage tanks.
“This keeps us from having to use a larger pump from the back of the property to the front of the greenhouse where the 50,000-gallon tank is located,” he explains.
Goode admits he was against the system at first. He was skeptical that the area’s average rainfall amounts would support their irrigation needs.
“I thought that for six or eight months of the year we wouldn’t have enough rain to pull water from this system. But we’ve only been out of water in the system once last September, and that’s because we didn’t have the system complete during last year’s rainy season. But we haven’t run out of water since,” adds Goode.
He calls it an “awesome” system, and one that’s over-engineered and costly. “We underestimated the cost of this system. Not necessarily the equipment needed, but other reasons. We ran into water while we were digging and the project took much longer than anticipated, for example. From an environmental standpoint, it was totally worth it. Since last July we’ve captured and reused more than 1.5 million gallons of water. Our return on investment will be a really long time. But I still encourage growers to look into this type of system,” he says.
For more: Texas AgriLife, http:/rainwaterharvesting.tamu.edu/publications; Rain Harvesting Supplies, www.rainharvestingsupplies.com; Agri-Starts, www.agristarts.com. Other resources: American Rainwater Catchment Systems Association; www.ARCSA.org; National Climate Data Center, www.ncdc.noaa.gov.