ET°, or Reference Evapotranspiration, is a standard measurement of environmental parameters that affect the water use of plants. ETo is expressed in inches per day, month or year and is an estimate of the evapotranspiration of a large field of four to seven inch tall, cool season grass that is well watered.

Coachella Valley Water District uses ETo to calculate water budgets for the tiered rate billing. This is the factor that allows for higher water budgets in the summer and lower budgets in the winter. CVWD’s daily ETo values are derived from the California Irrigation Management Information System (CIMIS) weather station in ETo zone 4. ETo zones 1, 2, 3 and 5 are calculated based on daily readings from the Indio CIMIS station #200.