Incorporating Evapotranspiration Rates

[Kent Thompson]

Browsing the forum, I saw a lot of people talk about various ideas about how to incorporate weather data into the irrigation schedule.

I just wanted to throw in here that the best commercial controllers tackle this problem by using evapotranspiration (ET) rates. They’re a calculation of how much water is lost through evaporation and transpiration. Here’s a much better explanation: http://wwwcimis.water.ca.gov/cimis/infoEtoOverview.jsp

I know in CA there is a government service (CIMIS) that provides daily ET rates for all the counties, but this probably isn’t that valuable for developing wider functionality…I wonder if there is a national service or a simple calculation to turn weather data into an ET rate.

It’s a really cool idea that gets to the heart of the ‘how much water do the plants need’ question.

[Bob92553]

I’m in the irrigation field and the cimis Web site is good if you want a basic adjustment. But if you want an accurate adjustment you might want to look into a sensor. Hunter sells a wonderful et sensor with a rain Guage built in too. It’s called a solorsync I wonder if there is a way to connect it to this to get it to adjust for weather

[dbrook]

I am presently looking at the open sprinkler to replace my current one and have difficulty determining how the rain gauge sensor is used by the firmware and whether it can be used to take into account evaporation as well as rain.

As a background I incorporated a rain sensor in an old unit I have and used the sprinklers to water the rain gauge, the sprinklers were set to water every day and sense the rain gauge, the next cycle was activated after a few days when evaporation had taken effect, if it also rained in the mean time the evaporation was effectively negated. The only catch was that irrigation stopped as soon as rain (or being sprinkled) was detected, effectively turning off the last station early. I had to rig up a method of disconnecting the rain gauge during the last cycle that wet it whilst the valve was open so the watering did not turn itself off.

1. If it rains during a period that watering is underway, does this turn the watering off or is it only checked at the start of the cycle time?

This system worked very well in the hot dry summer days, achieving daily watering when very hot and reducing to every 2-3 days when cooler but still no rain. Rain would also reduce the frequency of watering as well depending on the quantity.

[Ray]

@dbrook: common off-the-shelf rain sensors are basically rain-activated switches. OpenSprinkler’s firmware handles rain sensor in the following way: if the ‘Use RainSensor’ option is turned on, and if the rain sensor is activated (this depends on the rain sensor type, either normally open or normally closed), then it disables all stations until the rain sensor is deactivated. In the situation you described, it will stop stations in the middle of a program, before the last station is completed.

Since the behavior of the rain sensor is implemented in software, you can modify the source code to implement any way you want.

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