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In this section you will find a basic overview of how to accurately and sustainably decide how much to irrigate your garden beds using evapotranspiration data from local weather stations provided through CIMIS.
Below you will find a detailed explanation on what evapotranspiration (ET) is, how you can access up to date local ET data, and how you can use that data to determine how much water your garden needs each day.
Feel free to use this spreadsheet template provided here to simplify calculating irrigation amounts but we recommend reading the process first.
What is CIMIS
The California Irrigation Management Information System is an integrated network of over 140 automated active weather stations located throughout California, ran by the California Department of Water Resources.
How to use ET data from CIMIS (easily)
Go to the CIMIS website here
click "Data" on the top of the screen
create a limited "daily" report, not an hourly or monthly report
click on weather station 104 in De Laveaga for Santa Cruz ET data
click "run report"
Theres a lot of numbers but we only care about ETo (in). In this column you can see the ET for each day of the past 7 days with the total amount of water lost in inches throughout the week in dark grey at the bottom.
Multiply the total ET by 0.623 gallons per square feet
Multiply that value by the square footage of your garden bed
Lastly multiply this number by the percentage growth stage of your crop. (next section)
This will give you how much water in gallons to irrigate your garden bed throughout the week.
Its helpful to set up an irrigation schedule so that if you know you want to irrigate 3x a week, just divide that total number by 3 so that you end up irrigating the full amount throughout those 3 days.
Its also helpful to keep this values and calculations recorded either on paper, a word doc, or a spreadsheet.
What is Evapotranspiration (ET)
Evapotranspiration is the amount of water transferred from the soil/land to the atmosphere through evaporation from the sun and transpiration from plants.
We can irrigate effectively by replacing the ET lost from the week before.
ET is measured in inches, to turn inches to gallons just multiply by 0.623
Growth stage
The simpilest way to find what to percentage to multiply your ET by based off growth stage is to find the days to maturity for the crop you're growing. This is often on the back of the seed pack and can also be found through searching online. Remember that this is variety dependent, not all lettuce crops grow at the same rate.
For example, Newhem lettuce take 52 days to maturity which we can round up to 8 weeks. The first 3 weeks after your lettuce seeds have sprouted should be at 25%, the next 2 weeks should be at 50%, the next 2 weeks should be at 75%, then irrigate at 100% until the lettuce seems mature. the days to maturity on seed packets are just an estimate and are often quicker then it might actually take for your crop to reach maturity.
There is so much information about this online and ways to calculate growth stage but it's helpful to use your instinct about where you think the plant's growth stage is.
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