So if you have a closed container some will evaporate until equilibration - the saturation vapor pressure. The standard method is to measure it using a Class A land pan. This has to be placed off the ground and away from anything that would shade the pan. Evaporation can also be estimated from other meteorological conditions. Then you can use the nomographs developed by the Weather Bureau to get lake evaporation.
Soil evaporation is more complex.
Water vapor has to diffuse from the saturated or capillary zone and is thus dependent on depth to water, hydraulic properties of the soil in addition to the other factors such as temp. The combined process of transpiration and evaporation. Transpiration is defined as the loss of water from plants in the form of vapor. Transpiration is dependant on plant type, density of vegetation, growth stage. Penmans definition of ET is water transpired in a unit time by a short green crop covering soil with adequate moisture.