Watering By ET - Part 2

Yesterday, (for part 1 click here) I introduced watering by Evapotranspiration. I discussed how to determine your plant available water and allowable depletion. I also discussed how to effectively determine precipitation rates, a reference ET and crop co-efficient.

In this article, I would like to discuss how to take all that information into the field, begin scheduling by ET and using it to your advantage.

But before I begin, I want to discuss the theory of deep and infrequent watering. This theory is practical during spring and fall, but not so much in the summer.

Why? There is less daily evapotranspiration and deeper roots in the earlier and later months of the season. Therefore, there is more plant available water and the Superintendent can wait for longer periods before watering in an effort to grow a healthier plant.

In the summer months, the rate of evapotranspiration is high, and the roots are so shallow due to mowing heights. Therefore, there is less plant available water and the turf can reach its allowable depletion in only one day!

With that said, let’s continue with the example from last week:

· Precipitation rate (PR) = 0.57 inches/hour
· Reference ET = 0.59 inches over 3 days
· ET for greens (60% crop co-efficient) = 0.35 inches over 3 days
· Plant Available Water (PAW) = 0.80 inches
· Allowable Depletion = 50% = 0.40 inches of ET loss allowed

Note: the allowable depletion has almost been reached and the Superintendent needs to start thinking about watering.

Once again, this is why I like watering by ET. Knowing your PAW, and creating a bench mark value from allowable depletion, the Superintendent has excellent data to better manage each area and irrigate at the precise time!

To determine the appropriate run-time, the following calculation is used:

Run-Time = Actual ET loss/PR x 60 minutes/hour

Run-Time (RT) = 0.35/0.57 x 60 = 36 minutes

36 minutes! My greens can’t be watered for 36 minutes. This is a common reaction!

If that’s the case, there are three options available:
1 - Make adjustments to your plant available water (PAW). Maybe the root system isn’t as deep as believed or there were errors in the soil texture. Note: remember PAW will also change through the season as root depths change

2 - Change nozzles in the sprinklers to achieve a higher precipitation rate and reduce the run-time

3 - Use the repeat or cycle soak feature in your central control so water isn’t applied all at once

Now, let’s compare watering by minutes and using no data such as a weather station with the same data above.

The greens were watered for 10 minutes. The irrigation cycle did not bring the soil back to field capacity and the turf is likely showing stress by mid-morning.

Consequently, water will have to be applied through other means than the nightly cycle. Normally, it’s turning on sprinklers in the morning or hand-watering all day.

Ultimately, the turf will likely get more water than by watering by ET in the first place, golfers ticked off by all the sprinklers, and the turf is more susceptible to other pest or mechanical problems.

So, how can a Superintendent use ET data to their advantage?

First, evapotranspiration data is a great resource. Much like soil testing, it’s amazing when one can begin to evaluate their management decisions.

Second, watering by ET is considered a Best Management Process (BMP) and environmental bodies that govern water usage really like to see the reports you produce to evaluate your watering.

In conclusion, watering by Evapotranspiration is a detailed process. I always recommend that this is done through the course of an entire season. Adjustments need to be made as the season progresses and watering patterns are noted. It’s impossible to begin watering by ET overnight!

I also recommend hiring a professional irrigation consultant to help with the weather station data and to accurately enter precipitation data into the central control.