Watering By ET

How many Superintendents have done some soil probing and general plant appearance observations and decided to not water their greens, tees, and/or fairways that night, only to come in the next day and begin turning on sprinklers on all over the golf course in order to irrigate a stressed plant?

Over the past two seasons, I’ve had the pleasure of working with a couple golf courses in Southern Ontario whom are interested in watering by volume or Evapotranspiration opposed to the traditional form of watering by minutes.

Let me get something straight though. I’m a firm believer in the “Art of Greenkeeping”. Nothing can replace the local knowledge and feel a Superintendent has for his or her property. ET data is yet another tool in a Superintendents tool box to better predict when he/she should water and takes some time to get used to.

For those who have begun watering by ET, the overall feedback is that there is a more consistent wetting pattern throughout the property, less localized drying, and less of playing irrigation catch-up.

Therefore, this will be the first of two articles addressing the principles of watering by Evapotranspiration or ET.

To effectively water by ET, there are three critical components that need to be identified:
• Soil-plant relationships
• Sprinkler uniformity
• Precipitation rates

First and foremost, the predominant soil texture must be identified on all greens, tees, fairways, and roughs. A specific soil texture will provide the available water and basic infiltration rates for that soil (See table 1.1) When the available water is obtained, the plant available water and allowable depletion of water in the root zone can be determined.

For example:

Plant Available Water = Available Water (AW) x Root Zone Depth (RZ)

Allowabale Depletion = PAW x MAD

Soil Textural Class – Silty Clay Loam
Available Water – 0.20 inch per inch of soil
Root zone depth – 4”

Plant Available Water = 0.20 x 4' - .80 inches of PAW

Allowable Depletion = 0.80 x 50% = 0.40 inches

Table 1.1
Second, good sprinkler head uniformity is crucial. If there are nozzle, spacing and pressure inconsistencies, these issues need to be addressed before proceeding or watering by ET will be ineffective. An irrigation audit is an effective tool to improve uniformity.

Lastly, but not least, accurate precipitation data is required on each irrigated area in order specifically replace what is lost based on the amount of water that actually reaches the turfgrass area. This is why watering by minutes is not effective. If you water all your greens for 10 minutes, but the sprinkler pattern is different on each green, and therefore, the precipitation rate is different, one is watering some greens too much and others not enough.

Let’s take a closer look at how to calculate precipitation rates:

PR = Precipitation Rate
96.3 = a constant to convert cubic inches of water per gallon to inches per square foot per hour
GPM = gallons per minute
S = spacing in feet between sprinklers
L = spacing in feet between rows
D = diameter of a sprinkler

Square Sprinkler Spacing:

PR = 96.3 x GPM/S x L

Equilateral Triangular Spacing:

PR = 96.3 x GPM/S x (L x 0.866)

Single Row Spacing:

PR = 96.3 x GPM/S x D x 0.8

Sprinkler pattern – Square Spacing
Sprinkler flow – 25 gpm
S = 65 feet
L = 65 feet

PR = 96.3 x 25 GPM/65 = 0.57 inches/hour

Now, to effectively begin scheduling and replacing the plant available water, the acquisition of a reference ET value is needed. Reference ET is a useful reference point for irrigation water use calculations because it represents a specific rate of use in response to local weather conditions. There are several ways of obtaining estimates for ET:
• Onsite weather Stations
• Historical Data
• Universities
• Websites
• Lysiometers

Once a reference ET source has been established, it must be converted to represent the vegetation type we intend to irrigate. This modifier is called a crop co-efficient.

For example, a local weather station shows a reference ET value of 0.69 inches over 3 days. Table 1.2 shows what typical crop coefficients are for golf course greens, tees, fairways, and rough

Table 1.2
Now look at the ET loss on greens in Table 1.2. There was a loss of 0.35 inches over 3 days. Remember when we calculated the allowable depletion (AW). It was 0.40 inches.

Here’s the exact reason that I like working with ET! If it was Friday night, and the next day was supposed to be really hot and windy, the ET loss would exceed the allowable depletion and this is where the catch up game would begin.

Because the Superintendent could better predict their plant water requirements, they could irrigate when the turf needed it most!

With all that said, I’m going to stop there and allow this information to sink in. The next article will address how to calculate run times, adjusting for microclimates, using your central control to help out, and keeping record of your progress.

For Part 2 of this post, go here.