Watering based on Historical ET will save water. But it will damage plants.

The chart to the right plots Actual ET measurements versus Historical ET. The data are taken from CIMIS station #78(Pomona) in Southern California. CIMIS is the California Irrigation Management Information Service.

It can be easily seen that for most of the month the actual needs of the plants as measured by Actual ET was significantly above Historical ET. During this month, Actual ET was 32% above Historical ET.

If plants are watered at a 32% deficit (historical level) for one month, plants will be damaged and some plants will die.

Historical ET vs. Actual ET
Actual ET was 32% higher than Historical ET for one month.