Evaporation is simply the process of liquid water being converted to water vapor.  If this process involves plants, then it is referred to as transpiration.  The amount of water evaporated into the atmosphere varies not only from season to season, but also on a daily basis.  Last year, North Dakota State Climatologist Dr. Akyuz acquired an Evapotranspiration Simulator (ETGage) which measures the amount of evaporation taking place each day.

This past Monday and Tuesday when the temperatures were in the 80s with dew points in the 20s the ETGage measured an evaporation loss of 0.65 inches over those two days.  The dry air in combination with nearly 100% sunshine and a good wind made for an ideal evaporation environment.  On the opposite extreme was May 8 when only 0.02 inches of evaporation loss was measured because of the combination of a light rain, cloudy sky and high relative humidity.

Dr. Akyuz reports his data on cocorahs.org if you want a better sense of water loss during the growing season.