Irrigation Schedules for the Coachella Valley

To help you manage your water use at home, we’’ve created a unique irrigation schedule for each Evapotranspiration (ET) zone in Coachella Valley. Please note: The downloadable schedules are only available to paid Society members.

Why does ET matter?

Let’s take a closer look at Evapotranspiration and how it impacts your watering schedule.

ET is the combination water loss in plants from evaporation (from climate conditions) and transpiration (what the plants use and give up). It is measured in inches. It takes into account temperature, humidity, precipitation, wind, and additional data points. It is measured using a reference crop, often alfalfa, but in California we use pasture grass in the Central Valley as our reference crop.

ET is typically measured for geographical regions of varying size. California has 18 official ET Zones (see the full-size map here). Each location has a reference evapotranspiration rate (ETo) which is typically a 10-year average of measured evapotranspiration.

California ET Map

You’’ll see that Coachella Valley is mostly in ET Zone 18, which includes most of the Salton Sea Watershed and Imperial County. Our Historical Evapotranspiration (ETo) rate is 71.6 inches per year.

This ETo data makes up a significant portion of how we determine our irrigation schedules. The amount of water you need to apply is determined by a combination of ET and “plant factor.” Plant factor is a percentage of the water needs of the reference crop (pasture grass). For the purposes of our irrigation needs, pasture grass has a plant factor of 1.0. Bermuda grass has a plant factor of 0.6. Cactus has a plant factor of 0.1.

A plant’’s individual water needs are consistent regardless of location, but when evaluating supplemental water (your irrigation system), the plant factor percentage is multiplied by local ET data. Given the dependence of supplemental water on local climate conditions, you can see how ET might have a big impact on how much water you need to apply.

Supplemental Water Needed = Plant Factor X ET (or ETo)

If you live in Coachella Valley, you may not realize there are five very different ET zones that make up our overall ET profile. The Coachella Valley Water District has published an ETo Map for Coachella Valley</a.

CVWD ETo Zone Map

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