The ETo is an abbreviation of Evapo-transpiration. The 'o' signifies that it refers to a grass crop.

Early attempts to determine water loss from plants used an evaporation pan. Plant water losses were calculated from a proportion of losses from a pan. eg grapes vines in full leaf may use 40% of a pan loss per day. This value was generically known as the crop factor.

It was soon realised that a living crop and a dead pan proved to have too many inconsistencies. A better comparison would be a living crop to replace the pan as a reference. So a new term was needed to describe the comparison or ratio of farm crop water loses to the reference crop. This was coined the crop coefficient. The reference crop was chosen as a pasture. It had to be healthy to lose water and as a standard it was 0.12 metres high.

The water loss from the reference crop consisted of two parts
1> Evaporation from the soil and foliage.
2> Transpiration from the foliage.

The two long words were shortened to the abbreviation ET. The subscript "o" was given to the reference grass crop to distinguish it from the general crop ET.

An example for grapes the water loss in mm = 0.4 x (pan loss) using crop factor
                                                                      OR = 0.5 x ETo using crop coefficient
                                                ETo in this case = 0.8 x (pan loss)