Limited water supply?

As greenhouse operations expand, water can be a limiting factor as demand exceeds the yield rate of a well. Zoning the sprinkler irrigation system so that sections get watered in sequence is one solution. A second option for growers is installing drip irrigation, which lowers water needs and distributes water over a longer period of time.

A third, better option is to install an intermediate storage tank between the well and the pressurized distribution system. This reservoir can be a non-pressurized corrugated steel, polyethylene or fiberglass tank that serves as a primary source of supply for the pressure pump and tank.

The system operates such that the pump in the well supplies water to the storage tank at a rate slightly less than the well yield rate. A larger pump in the reservoir transfers the water to the pressure tank and into the distribution system. The depth of water stored in the non-pressurized reservoir is regulated either by a float switch or water level sensor that controls the on-off operation of the well pump.

Meeting peak water demands

At minimum, the intermediate storage tank should hold enough water to meet peak use on the warmest day of the year, minus the amount of water that can be supplied by the well during the watering period. Let’s use the following example:

For most crops, the peak use during the summer is about 0.4 gallons per square foot of growing space per day. In a greenhouse having 20,000 square feet of growing space, the maximum daily water requirement is 8,000 gallons (20,000 sq ft x 0.4 gal/sq ft = 8,000 gal). Next, calculate the amount of water that can be pumped from the well into the intermediate storage during the watering period. Assume that the yield from the well is 15 gallons per minute (gpm) and the irrigation system will water all the zones in four hours. This make-up water amounts to 3,600 gallons (15 gpm x 60 minutes/hour x 4 hours = 3,600 gal).

The size of the intermediate storage is the difference between these amounts (8,000 gal – 3,600 gal = 4,400 gal). The well pump supplying 15 gpm would have to operate 8.9 hours to supply the total water needs (8,000 gal ÷ (15 gpm x 60 min/hr) = 8.9 hours). The pump in the intermediate storage tank would have to supply at least 34 gpm (8,000 gal ÷ (4 hr x 60 min/hr) = 34 gpm) to meet the demand of the irrigation system if all watering was to be done in four hours.

Intermediate reservoir advantages

If the tank is located above ground in the greenhouse or headhouse, the water can be tempered before irrigating the plants. If the tank is filled the night before irrigating, by morning the water will be several degrees warmer. Supplemental heat could be supplied from a hot water heater or boiler to raise water temperature.

The tank could also serve to chemically treat the water. For example, chlorine could be added to precipitate iron from the water and allow it to settle to the bottom of the tank.

The tank might also serve to collect rainwater from the roof gutters, saving well water. A one-inch rainfall on a 20,000-square-foot roof will supply about 13,000 gallons. Besides the collection gutter, the system consists of PVC piping to direct the water to a roof washer where trash, leaves and other debris re removed by a screen mesh. All rainwater storages need an overflow to handle excess water once the tank is full.

Polyethylene and fiberglass tanks up to 20,000 gallons are available. These are the largest that can be transported over the highway. Corrugated steel tanks can be larger, as they are assembled on site.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com