Pan evaporation is used to estimate the evaporation from lakes. There is a correlation between lake evaporation and pan 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.