ET? How Does It Work

With all its calculations and considerations, landscape irrigation is a technical affair. Even trickier is efficient landscape irrigation, also known as water management. Efficient management means that twice-a-year schedule changes, overwatering to achieve green grass and “educated” guesses are out of the picture, especially with the drought conditions many areas of the country are currently facing. And, there to take a starring role in efficient water management is ET.

Let us clarify that while ET is indeed alien to many of us, we are not referring to the extraterrestrial who needs to phone home. Evapotranspiration or ET is the loss of moisture in the landscape caused by evaporation from the soil and transpiration of moisture from plant leaves. It is affected by the length of the daylight hours (solar), the temperature, wind and humidity. ET is usually expressed in inches or fractions of inches of water loss per day, week, or month.

Generally speaking, whatever water is lost to ET is the amount that must be replaced by rainfall and irrigation. That’s why keeping track of this amount is the single most important factor to consider in scheduling your irrigation. It allows you to replace only the water that is lost and makes you the good steward of our precious resource that you were meant to be. Apply water beyond ET and you waste it, either through evaporation, runoff or percolation through the soil. Bad, bad, bad.

Water has become a limited resource. To assure continued access to this resource it is imperative that we conserve and manage it efficiently. Irrigation is a major user of water. At best, landscape irrigation is fourth in priority of water use behind drinking water, water used for washing and cleaning and water used for growing food and industrial use.

ET is a tool that helps us manage our water use by providing us with information about the quantity of water required to maintain a healthy landscape. One-hundred percent of ET for a given region is the maximum amount of water necessary to grow and maintain cool season turf. So, it is reasonable to assume that shrubs, trees and other plant material require less. By not exceeding ET, we help prevent potential water waste. Using an ET-based, self-adjusting controller provides a labor-saving tool to monitor our water use by paralleling ET requirements.

Getting back to the tricky part, calculations and formulas, such as Penman-Monteith or Hargraeves, are required to finally arrive at ET. The good news is that this time you don’t have to be the one behind the calculator. In fact, you can retrieve accurate ET data from very reliable outside sources for use in setting your irrigation schedule. Or, you can use the latest in irrigation controller technology to calculate it for you and adjust your irrigation schedules automatically. A number of controllers with built-in ET and ET-measuring devices are available from many manufacturers, allowing every landscape professional to remove the guess work and realize 10 to 50 percent water savings.

A Brief Look at ET
Even though you don’t have to be the one measuring ET, it’s good to know a little about it. Measuring ET is very similar to measuring rainfall, except where rainfall is a water gain, ET is a water loss. ET is measured in inches over a period of time, as in inches per day, per week, per month or per year. The measurement refers to a depth of water and should not be confused with flow rates like gallons per minute or acre inches per hour, which are a given amount of water over a certain area.

ET is affected by a number of factors, mainly wind, temperature, solar radiation and humidity. If humidity increases, ET will decrease, or if the temperature increases, ET will increase. But measuring these factors alone doesn’t equal ET. The final measurement is achieved with those highfalutin-sounding calculations and formulas mentioned earlier.

Again, the only way the water lost through ET can be replaced is by rainfall and irrigation. For example, if ET is 0.25 inches per day, then 0.25 inches per day is what must be replaced. It should be noted that most irrigation systems are not 100 percent efficient and also lose water to evaporation, wind and runoff. To counteract this, even more water has to be applied, over and above ET alone.

Low-Tech ET
For the beginner, using ET can be as simple as obtaining the data for your area from an agency. Typically, what you’ll see will be historical information, a monthly average over a period of years. For example, you may see 5.5" for the month of August and 4.5" for the month of September. What that tells you is that historically, in your area, there has been a roughly 20 percent decrease in water lost to ET between those two months.
After obtaining the data, you can adjust watering times globally with a controller that has a water-budgeting feature. With most manufacturers, water budgeting simply means to increase or decrease watering by a percentage, usually in 10 percent increments.

For the water-budgeting method to work, you need to have a good watering schedule already in place. Ideally, your station run times are already proportional, meaning that the sunny areas get more water than the shady areas, the top of the slope gets more water than the bottom, the grass gets more water than the shrubs and the flowers get more water than the drought-tolerant plants, etc. If your watering schedules are not proportional and you’re not sure how to do this, you should hire a water-management professional or a certified water auditor to help get your schedules into shape. Doing this will make water management that much easier.

Assuming you already have a good watering schedule, simply use your controller’s water budgeting feature up or down according to ET. In our example above, you would decrease your water budget by 20 percent come September. Just to cover the opposite, let’s say it’s May and you check your ET source. Your source says six inches for May and seven inches for June, a roughly 15 percent increase for your water budget. Of course, if your controller only allows 10-percent increment changes, you’d round up to a 20-percent increase to be safe.
Okay, so we lied, you do have to do some calculating, that being the percentage difference of ET lost between months. But this little bit of work easily takes the guesswork out of the schedule. By doing this kind of ET gathering at the minimum, and adjusting your schedule to this data, you’re taking a big step toward efficient irrigation. Congratulations!

High-Tech ET
For those who would rather let technology do the work, new advances in ET-based and ET-compatible controllers, and ET-sensing devices make the job much easier. You can become an expert water manager in no time with the new array of sensors, ET gauges/trackers and controllers with built-in ET or with the ability to use ET input.

ET-based and ET-compatible controllers

The biggest advance in ET technology has been in ET-based controllers. These controllers are programmed with historic average ET data that can take over irrigation scheduling once a baseline program has been set. Typically, you set the baseline program for a worst-case scenario, such as the hottest week of the summer, and then let the ET controller make automatic adjustments monthly or bi-monthly, depending on the manufacturer.

As discussed earlier, you should already have a good proportional watering schedule in place. Most ET-based controllers allow for manual override in the event of an unusual hot or cold spell, and you can even collect and input your own ET data into them. ET-compatible controllers don’t have ET data pre-programmed into them, but they can use ET or other environmental data such as changes in temperature or solar radiation from an external sensor to automatically change watering schedules.

Adding a sensor
The next level of high-tech automation takes you from pre-programmed historic ET and automatic monthly adjustments to up-to-the-minute, real-time adjustments. This involves connecting an external sensor such as an ET gauge, ET tracker, solar radiation or temperature sensor to your ET-based or ET-compatible controller. Each manufacturer has a different twist on which environmental factor is measured and how, be it solar radiation, temperature or actual ET, but any one of the sensors will keep you in the efficient water-management game. These devices automatically modify the watering schedule, based on one of those environmental conditions.

Depending on the controller, the sensor can either completely take over the schedule adjustments permanently or, during extreme conditions, temporarily override the schedule with the built-in ET data acting as a short-term backup schedule. Keep in mind that some (but not all) sensors are proprietary, meaning that they only work with the same manufacturer’s ET controller.

Creating a controller network for large sites
Finally, for the ultimate in advanced ET tracking of large or multiple sites, you can rely on a network of controllers or satellites that are adjusted and monitored automatically from a single location. A network can receive real-time ET data from your choice of three different sources: a remote off-site weather station, your own on-site weather station, or an ET web site.

A weather station is a device that measures wind, humidity, solar radiation and temperature — all of which affect ET — with very high accuracy. It then communicates this information back to the entire network of controllers, which then adjust the irrigation. Your own on-site weather station would be the most accurate because it is providing information directly for you, as opposed to an off-site station that may be in a different microclimate and may require adjustments. However, an on-site station is also a more expensive investment.
Web-based ET control is rather new and is different than weather-station control. It is a service that some controller manufacturers offer, to download current ET information directly to your network. This data may need some adjusting to your particular area.

August 2002