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Understanding Local Weather Data and Using it for Irrigation Decisions and Planning: How to Access CIMIS Data

Simarjeet Singh, CCA

Water is essential for crops, so understanding the process of water getting in and out of the crop root zone is necessary. Precipitation (rainfall, snowfall) and irrigation are primary water sources for crops. The decisions about the precipitation are not in the hands of human, but irrigation is all decided by the farm management i.e., how much water to apply and when to use.

Irrigation volume and timing is driven by the term “crop evapotranspiration.” When solar radiation hits the soil surface, it converts the water available in the soil into vapor form, which is blown away by the wind; this process is called evaporation. Transpiration is known as the water lost through the plant surface (leaves) to the atmosphere, due to plant needs and various atmospheric factors. This water lost to the atmosphere by combining evaporation and transpiration is described as “evapotranspiration (ET).” Accurate estimates of evapotranspiration for your local area are crucial for making farm decisions such as irrigation system designs and scheduling. Inches of water lost from the agricultural systems due to ET in any specific week or month are critical to know. Both over-irrigation and under-irrigation adversely effect our natural resources and plants.

California Irrigation Management Information System (CIMIS) provides local weather and reference evapotranspiration (ETo) data for agricultural communities with their automated weather stations throughout the state. The reference evapotranspiration is the maximum evapotranspiration from the well-watered actively growing, closely clipped grass surface. You can get your actual crop evapotranspiration (ETc) by multiplying the ETo with crop coefficient (kc). Each crop’s coefficient (0.1 – 1.2) varies by growth stage and cultural practices.

ETc = ETo X kc

Irrigation volumes should be driven by the amount of water lost by any specific crop i.e., crop evapotranspiration (ETc). We have two CIMIS stations collecting continuous weather data, one in Alturas and the other in McArthur. I am happy to share that one more CIMIS station has been installed near Nubieber and active since November 2023.

Check out the historical monthly average reference evapotranspiration and precipitation for the McArthur and Alturas regions (Figures 3 & 4). You can access hourly, daily, and monthly weather data from around your area. If the Irrigated pasture is 4-6 inches tall, it requires water at the ETo rate i.e., there is no need to adjust for crop coefficient (kc). See the Kc values for some major local crops in the table below.







Rotated Grazing Pasture


*this Kc value is an overall averaged Kc coefficient that averages Kc for both before and following cuttings.

Above Fig 3&4: Monthly reference evapotranspiration (ETo) and precipitation for McArthur and Alturas CIMIS stations.

Step-by-step guide to access the CIMIS data:

Step 1: Open your browser and go to           

              You will be directed to the webpage similar to the picture below.

Step 2: Click on the Login option at the top right corner. You will see the option to “Register.” Click on register button and you will be directed to the webpage below.

 Step 3: Register on this page by providing your name, email address and the code you will see right there.


Step 4: Once you hit register, you will see the screen as in picture below (Picture 3).

Step 5: Click on the link from your email to complete your registration. You will be directed to the CIMIS website to create your password and complete the registration.


Step 6: Once you complete that, you should come to the webpage similar to picture 4. Click “click here” to come back to the home screen. 

Step 7: Click on the blue tab “STATIONS” in the top section of the screen and then click “Station Location Map” in the navigation bar that will open below the blue bar. See picture 5. 

Step 8: All the purple numbers on the California map in the picture represents the CIMIS weather station. Scroll onto the map, and find the closest station to your field and note down the station number. (Alturas station is #90, McArthur #43, and Nubieber #268)


Step 9: Once you have your station number, click on the blue tab “DATA” in the top section of the screen. See the picture 6 below.

Step 10: Once you are on the webpage as in above picture, find your station in the station list and select the station by clicking on it.


Step 11: Choose the report style you want to run; hourly, daily, or monthly. (Picture 6)


Step 12: Choose the report type you would like. You will see the below options. I would recommend generating a CSV or PDF report. (Picture 7)

Step 13: Select the units and date range for the data (whether you want to see the data for ongoing week, past week, a month, a year or multi-year).


Step 14: Once you have selected all the options in the bar, hit the “Run Report” button at the bottom right corner of the webpage.

Step 15: Once you hit “Run Report” you will get the report in your browser. Depending on your file type, it would either open in the browser or automatically download. Below is the example of 2023 monthly PDF report of McArthur station.

CONGRATULATIONS!! You have done it all.

Have fun with your data and managing irrigation.


Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. Fao, Rome, 300(9), D05109.

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