ET is the loss of water from a vegetated surface through the combined processes of soil evaporation and plant transpiration. In short, this is the amount of water that plants lose that needs to be replaced. It is expressed in inches of water needed to replace what was lost to the atmosphere in a given time period.
Rancho Water tracks ET for 319 micro-zones within the District on a daily basis as a factor in determining an outdoor water budget specific to your property.