The Math

(Don’t be scared!)

If you are not afraid to try a formula or two, you can use ET to figure out how many inches of water or gallons of water your plants need. You’ll need to know the following:

math copy.jpg

Table 2. Historical monthly average ETo for two different climates in the watershed (footnote 5)

Note that Eto values are available for many areas throughout the state at Values will differ slightly year-to-year.


By knowing the answers to the formulas above, and the rate of water that is delivered through your sprinklers, you can calculate how to program your irrigation controller.

This sounds like too much work!

Not to worry. We've done the work for you. See the IRRIGATION SCHEDULES near the end of this chapter for sample watering schedules based on ET.