ETo, 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.
Evapotranspiration is the loss of water to the atmosphere by the combined processes of evaporation (from soil and plant surfaces) and transpiration (from plant tissues).