Take a Rain Check
Long gone are the days of telling your night water-man to plug the sprinkler into a quick-coupler, or to set the roller-base sprinkler out until the ground starts to puddle. For more than 25 years, state-of-the-art central control systems for irrigation on golf courses have been computerized, automating irrigation for more reliability and relieving crewmembers of a great deal of manual tasks.
Today, central controllers are sophisticated enough to give us the capability to set a specific run time for every single sprinkler — to water deeply, use frequent, short run times, or run cycles or repeats for repetitive run times. Turf managers have so much control that irrigation systems will do just about whatever you tell them to. If water applications are too heavy or too light, the problem frequently lies with the instructions that we're programming into the system, not its ability to carry them out.
The question, then, is, “How long should run times be and how do we determine them?”
ET HAS ARRIVED
You can use one of two basic methods to set run times for an irrigation system. The traditional method is to set run times, then decide from day to day whether to lengthen or shorten them, depending on weather conditions and your observations.
Another option, is to base run times on evapotranspiration (ET). ET is, in essence, the amount of water lost through evaporation (water lost directly from soil) and transpiration (water lost from the plant). Together, these account for the amount of water that must be replaced in order to maintain soil moisture at the desired level. The idea behind using ET to determine irrigation run times is that this enables you to replace the exact amount of water that the turf has lost — not more, not less.
Based on data collected at an onsite weather station, ET-based scheduling uses a mathematical equation to estimate water loss based on weather conditions and the specific “crop” (turf, in this case; agricultural crops routinely use ET as well, but with different equations). Sensors on the weather station collect data for minimum and maximum temperatures, relative humidity, wind speed, solar radiation and rainfall (see photo, right) for the past 24 hours. This data is then plugged into a formula — the Penman formula may be the most frequently used for turfgrass, though others are available — to derive an ET value for the turf. This is the amount of water (usually noted in inches of water) that needs to be replaced. Based on the known precipitation rates of the irrigation system, the central control system then is able to calculate the proper run time for each station.
The ET method is a better way to set run times because it is more efficient. For example, it would be difficult for a turf manager to notice the difference between a day with 0.16 inch ET and a day with 0.15 inch ET. Yet, that difference can mean thousands of gallons of water. This can reduce water costs if the course pays for its water. It also results in electrical savings because the pump station does not have to run as long.
Why would you want to irrigate less than ET? Perhaps reducing your watering could allow for a better playing golf course (no plugged shots, more consistency). It's also possible that you could reduce disease by drying the soil down a bit. On the other hand, agronomic reasons might dictate that you replace 100 percent of ET, or perhaps even slightly more. The point is, knowing how much to water to achieve your goals, rather than guessing at it, gives you the ability to control course conditions precisely.
DEALING WITH MICROCLIMATES
A question turf managers often ask about ET is, “How can one weather station in one location be representative of the whole golf course?” This is a valid question and the answer in many cases is, “It can't.” One solution is to position multiple weather stations throughout the golf course. Though expensive, this approach allows for very accurate determination of proper run times for the different microclimates on the course.
Another option is to account for varying microclimates by assigning a percentage, or multiplier, for various sprinkler stations around the course. Run times for any sprinkler can be adjusted by the pre-determined percentage, based on ET values from the one weather station. For example, a low area, or a pocket surrounded by trees, might experience less wind and higher humidity than at the location of the weather station. If you discover that the turf there uses only about 90 percent as much water, you could start with the ET values derived from the weather station, but you'd adjust run times downward by about 10 percent.
This is the “tweaking” a turf manager performs around the golf course to account for shady areas, unusually dry sites, slopes or even different turf types and heights. Once you determine the differing water needs of these areas, the adjustments to run times will rarely need to be changed from one day to the next because the ratio between the ET value generated from the weather station and each sprinkler station's microclimate should remain consistent.
IT'S RAINING … NOW WHAT?
Another factor affecting the use of ET with irrigation is rainfall. Obviously, you won't have to irrigate as much if it has rained, but there are various ways that a central control system can deal with rainfall, some better than others.
Many systems only use rainfall to cancel out irrigation for the current or next irrigation cycle. But consider the following example. Suppose you got 1 inch of rain. In response, the irrigation system shuts down for one cycle, and then resumes the following night. If it was only scheduled to apply 0.5 inch of water, then you still will have gained 0.5 inch of water. Why not account of that extra water and save the course some water and electricity?
There are more sophisticated ways to handle rainfall, however. For example, let's say the system adds the amount of rainfall to the ET of the previous day, giving a net ET value with which run times for the next irrigation cycle are calculated. If there is still a value other than zero — that is, if the rainfall exceeded the ET — no irrigation will be allowed. By carrying the balance over to the next day, rainfall can be accounted for accurately, even if it takes several days without irrigating.
Another important factor is rainfall intensity. Occasional “cloudbursts” produce rain of such intensity that most of the water runs off before it can soak in. If the system counted all the rain from such an event, it would assume there is adequate soil moisture when there really isn't. A way to avoid this problem is for the system to monitor rainfall frequently, such as every hour, and then disregard any amount over a limit determined by the user. Only the amount of rainfall below the limit value will be counted. This limit should correspond to the infiltration rate of the site's soil.
With the capabilities of computerized central-control systems today, turf managers have more control at their fingertips than ever before. Rather than use “generic” runtimes such as 10, 15 or 20 minutes, state-of-the-art systems can apply the exact amount needed based on data derived from an onsite weather station generating ET values. ET provides a golf course with more consistent playability and much more efficient water usage.
Should you use ET? The answer is, “Yes.” A common objection to upgrading irrigation to an ET-based system is cost. Yet, such systems often save enough money to quickly pay for themselves. In addition, you'll also produce a better-quality golf course for your members or customers.
Alan Clark is the Great Lakes Golf Manager for Rain Bird Corporation - Golf Division.
ET IN THE NUMBERS
Environmental and biological factors both contribute to total ET. Solar radiation, temperature, humidity, wind and soil moisture are the most important environmental factors. Thus, a simple weather station can provide the necessary information for calculating ET. A mathematical model will use this weather data to determine an ET value. The Penman Equation may be the most frequently used model.
The type of vegetation is a critical biological factor — ET can vary a great deal depending on the plant species. A field of corn uses a different amount of water than an orchard of trees or a sward of turf. To deal with this variability, meteorologists calculate a potential or reference ET as a starting point. It is usually based on a particular kind of site (a pasture, for example). Some types of vegetation use more or less water than this, so a multiplier, or crop coefficient, is determined to convert potential ET into actual ET. With a crop coefficient, you can know that given “X” amount of potential ET, “Y” amount of actual ET has occurred for your crop (turf is a “crop”, as the term is used here). Turf has its own coefficient, just as do cotton, corn and various other crops.
Breaking it down further, the ET of turf will vary depending on turfgrass species, mowing height and level of quality being maintained. The University of Arizona has conducted research on turf ET and recommends the following turf crop coefficients:
|Type of grass||Cutting height||Quality||Crop coefficient|
|Warm-season||2 - 2.5 cm||Acceptable (park)||0.65|
|Warm-season||4 - 5 cm||High (golf course)||0.76|
|Cool-season||2 - 2.5 cm||Acceptable (park)||0.65|
|Cool-season||4 - 5 cm||High (golf course)||0.72|
Using information like this, you could calculate that if reference ET was 0.8 inches (reference ET is often tracked and made public by extension services), then your bermudagrass maintained at a 1-inch height will have lost 0.52 inches of water through ET. (0.8 [the reference ET] × 0.65 [the crop coefficient for this type of turf] = 0.52.) Fortunately, computer programs using weather data are available to perform all the math for you!
Because weather conditions vary so much depending on the microclimate, onsite weather stations are important for precise measurement of water loss. The ultimate point of all this, of course, is to irrigate with just the right amount of water to replace what's lost. That can mean money in the bank.
Technical credit: Paul W. Brown, Extension Biometeorologist, University of Arizona
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