ET0 (Reference ET) using Tempest weather data and Penman-Monteith equation

I’ve written (with help from ChatGPT) a rudimentary Python script for calculating ET0 using Tempest weather data and the Penman-Monteith equation. The script calculates ET0 every hour starting at midnight and tallies the 24 hourly values the following midnight to get a daily ET0. It’s working as expected and I’m in the midst of validating the calculated values using ET0 values obtained from CIMIS (California Irrigation Management and Information System, see chart below). I’m not a professional programmer/developer so there may be errors in my logic that have yet to present themselves. The script can be found on Github at:

The file explains the script in detail.

The ET0-Tempest value is consistently ~0.02 inches/day higher than the value from CIMIS. The nearest CIMIS station, from where the CIMIS data was obtained, is 16 miles (straight line) from the Tempest weather station which may account for the discrepancy. As I collect more data, a “fudge factor” may be needed to bring the two estimates into closer agreement. It’s also possible the ET0-Tempest value is correct and the discrepancy is due to actual differences in microclimate between the two locations.


wow - chatgpt is kinda scary isn’t it.

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