GIE 2003: Planning Makes Perfect

When it comes to determining a landscape’s irrigation requirements, it’s best not to guess, according to Jim Laiche of The Toro Co., during his presentation on Thursday, Nov. 6, at the Green Industry Expo in St. Louis, Mo.

“When you guess, you can’t go to the customer and accurately tell him what to expect,” Laiche said. “But by using some simple formulas to plan ahead, you can determine a project’s costs more accurately and whether or not that project is feasible for your company.”

To illustrate his point, Laiche took attendees through the basic scope of a typical irrigation project, including site information, such as:

Laiche’s example project consisted of one softball field, one soccer field, two football fields, a roadway with 30 feet of green space, an on-site 2.6-acre retention pond, three-phase electrical power, city water service availability and a maintenance window of seven hours a day, six days a week.

For this job, 6.69 acres need to be irrigated. Laiche arrived at this number by taking the total surface area of the job divided by the number of square feet in 1 acre, which is 43,560.

Then Laiche figured the estimated weekly evapotranspiration rate, adjusting this number for sprinkler inefficiencies. “Overall, overhead irrigation is typically 80-percent efficient, while drip irrigation is 90- to 95-percent efficient,” Laiche said, adding that he based this evapotranspiration rate on no rainfall because it’s better to plan for the worst-case scenario. “Breaking down overhead irrigation, rotors are 80-percent efficient, while spray heads are 85-percent efficient.”

Laiche said the next important step is deducing exactly how much water the site needs in gallons per minute (gpm), which for this sample site was 124 gpm. Laiche used another common conversion to arrive at this answer – the number of gallons per acre inch, which is 27,154.

The other formulas Laiche shared with the group included determining the suitability and coverage potential from a water source, determining coverage from an on-site supply (the 2.6-acre retention pond) and how quickly the pond needs refilling to avoid total drainage, determining the average watering demands for a season, and determining the cost to operate a system.

The author is Manager Editor of Lawn & Landscape magazine and can be reached at nwisniewski@lawnandlandscape.com.