Exapotranspiration Data (ET)

The Casitas Municipal Water District provides up to date information on Evapotranspiration (ET) to our customers from two stations that we operate.

Click here for ET (Ojai)

What is ET?

ET is defined as the water lost from an area through the combined effects of evaporation from the ground surface and transpiration from the vegetation. Sun, wind, temperature, and humidity are some of the factors that influence how much water various plants need for any given day. This information is helpful to our customers that have agriculture or large landscapes. It allows them to adjust their watering practices so they are the more efficient in the application of water.

How to Use ET Data:

ET indicates how many inches of water is required to water your lawn. For example, if yesterday’s ET equaled .5 your water requirement today would be .5” inches of water for your lawn. Place a bucket or cup in a representative area of your lawn and turn on the sprinklers until the bucket or cup is filled to .5” inches. If you know how many minutes it takes to place 1” of water on your lawn you should be able to calculate how long to turn your sprinklers on for each day or week. It is not necessary that you water each day but to make sure the accumulative amount of water required for each week is approximately applied to your lawn. Most grass is near equal to ET but other plants require a percentage amount less than ET. Plant specific ET references can be found at your local nursery or on the internet. Sometimes it may be necessary to have two watering cycles to avoid runoff or water drainage going below the root zone.

Location of ET Stations:

The District operates two ET stations. One station is in the Rincon area near the coast and the other station is in the Ojai Valley. The weather conditions can vary significantly further inland, which creates different water needs for those customers. The data collected from these stations is updated to our website daily.

Water Budget:

This is the amount of water in gallons that is required to water a landscape based on the water needs of your plants, land area, and irrigation efficiency. This formula works best by categorizing each hydrozone (plants grouped by similar water needs that our watered on the same irrigation valve within a specific land area) and adding all the zone totals to get a final water demand figure.


ET = Reference evapotranpiration (inches per year) for bluegrass in your area.

K = Crop coefficient for plant type. Grass is usually .8 and low watering plants and shrubs .3.

LA = Landscaped area in square feet.

.623 = Conversion Factor (to gallons per square foot).

IE = Irrigation Efficiency (varies based on irrigation system). If your irrigation system distributes water evenly on a lawn (place several cups on a lawn and determine if they fill up after one minute at the same level). Most efficient lawn irrigation systems are at about 80%.

Water Budget = (ET)(K)(LA)(.623)/IE

If you are interested in this program or have any other questions please contact
Ron Merckling, Water Conservation Manager at (805) 649-2251 Ext. 118.

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