Do we need ET for urban and suburban irrigation?

[Jim]

If you look at a chart of average daily high temperatures for Santa Clara County and compare that to a chart of average daily ETo the charts look remarkably similiar. They peak in hot summer months, and trough in cold winter months. Given that, do we really need to calulate ET every day?  Couldn't we just use the temperature curve.

The ET people claim to automatically adjust the application every day depending on ET but I wonder what the benefit really is on daily adjustment, especially if your irrigation area is less than 1/2 acre.

I suspect a monthly adjustment based on temperature would quite closely approximate ET.