Evaporation Terminologies

Pan Evaporation (Pan loss)
http://en.wikipedia.org/wiki/Pan_evaporation
>=Free surface water evaporation (EVCAN in PRMS)

Lake Evaporation
Evaporation from a natural body of water is usually at a lower rate because the body of water does not have metal sides that get hot with the sun, and while light penetration in a pan is essentially uniform, light penetration in natural bodies of water will decrease as depth increases. Most textbooks suggest multiplying the pan evaporation by 0.75 to correct for this.



Potential Evaporation / Evaportranspiration (PET)
Potential Evaporation or Evaportranspiration with unlimited water supply (well-watered crop)
http://en.wikipedia.org/wiki/Potential_evaporation   
Also famous Penman and Penman-Monteith
Penman equation (1948) 
Penman-Monteith equation (1965)

In FAO webpage below, a definition for PET:
http://www.fao.org/docrep/W3094E/w3094e06.htm
Potential evapotranspiration (PET) has been defined as the volume of water per unit area of field evaporated and transpired by a dense stand of actively growing short grass that is well endowed with (never short of) water.

Different crops have their own PET
PET = Kc * RET

PET- Potential evapotranspiration (PET), is the evaporation and transpiration that potentially could occur if a field of the crop had an ideal unlimited water supply.
RET is the reference ET often denoted as ET_o. RET usually represents the PET of the reference crops most active growth. K_c, then becomes a function or series of values, specific to the crop of interest through its growing season. These can be quite elaborate in the case of certain corn varieties, but tend to use a trapezoidal or a leaf area index (LAI) curve for common crop or vegetation canopies.

http://en.wikipedia.org/wiki/Crop_coefficient