Programming Irrigation Controllers using the Evapotranspiration Rate

'Evapotranspiration' is a big word that means the amount of water lost from the soil due to evaporation and the loss of moisture from plants due to transpiration. Think of it as the amount of water lost from the reach of the plant. The reason this is handy is that if you know what the ET rate is, you know exactly how much water the plant needs on that day to keep thriving. We now have the technology to trigger sprinkler systems to run for only the length of time required to replenish the amount of moisture lost by using the evapotranspiration rate.

Plant needs


Plant material uses moisture at differing rates. Even among turf grasses, there are different rates of transpiration. Once the needs are determined, the controller can be set and the time for each station measured to match the evapotranspiration rate based on the weather conditions.

ET changes

The ET rate changes based on other factors, too. A 110-degree day with 35% humidity will result in a lower evapotranspiration rate than a 110-degree day with 5% humidity. The atmosphere already has so much moisture in it that slows the evapotranspiration process. So, while we humans suffer as a result of high humidity, the plants love it.

Understanding how the ET can change without a corresponding change in temperature is a cornerstone of understanding how ET works. For example, wind speed will also affect the evapotranspiration rate, as will specific physical characteristics for the plant.

Armed with the knowledge of the factors which affect the evapotranspiration rate and with the ability to change the irrigation rate, wasted water becomes a thing of the past. ET is a tool which was first used on golf courses in the eighties. By combining it with the technology of wifi and Smart options, there is almost unlimited control over water consumption. Plant material remains beautiful and healthy while water waste is eliminated.

ET system sensor

An ET system sensor negates the need to depend on weather reports to measure climatic conditions--weather reports which, incidentally, include conditions within a hundred-mile radius. As we Arizonians are all too acutely aware, one part of the Valley could be hot and sunny while ten or twenty miles away can be the site of a gullywasher. Is there anything more annoying than seeing the sprinklers go off in the midst or aftermath of a colossal rainstorm?

An onsite ET system sensor measures the climatic conditions at the local microclimate (aka- your home or business) which signals the irrigation controller to program for those conditions.

As water bills climb, people enact more sustainable habits and are sensitive about wasting precious natural resources. As a result, products like electric cars and ET system sensors have become more available and prevalent.

Living Water Irrigation is a Phoenix-area irrigation company specializing in professional sprinkler and drip irrigation and landscape lighting installation and repair. Call 623-299-2996 today!