# 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:

**Table 2.** Historical monthly average ET* _{o}* for two different climates in the watershed (footnote 5)

Note that Et* _{o}* values are available for many areas throughout the state at cimis.water.ca.gov. 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.