The Earth’s water is always in motion—coursing through rivers, lakes, and oceans on the planet’s surface, flowing in aquifers beneath, and cycling through the air above. It’s the only chemical compound that naturally cycles through all three physical states—liquid, solid, and vapor—on the Earth’s surface. Despite the importance of the compound in biological and climate systems, there are still gaps in our knowledge about the mechanisms that drive these phase shifts.
Potential evaporation, for example, is generally an approximation of the maximum amount of water that would evaporate from a surface if water supply were not limited and there were no resistance, or “pull,” from the surface. However, the precise definition of the measure, its applications, and the relationship between potential evaporation and actual evaporation are still up for debate by scientists.
Researchers have long assumed a complementary relationship between the potential and actual evaporation. This relationship is created by an increase in the amount of heat transferred to the atmosphere from the Earth’s surface, driven by excess energy not used in evaporation as a surface dries. The increase in what is known as the sensible heat flux drives up the potential evaporation calculation, and by relying on the complementary relationship, scientists can estimate the actual evaporation from drying land surfaces without knowing much about the properties of the surface. However, research has revealed that the relationship between potential and actual evaporation is not symmetrical. In fact, as a surface dries, potential evaporation increases more than actual evaporation decreases.
To account for this discrepancy, Aminzadeh et al. present a new framework to better understand the relationship between actual and potential evaporation so that the relationship can be used to predict actual evaporation from drying surfaces. The framework includes a hypothetical steady state condition in which the temperatures of the air and the evaporating surface are equal. The researchers are then able to use the steady state temperature to quantify the so-called reference evaporation from a relatively small wet surface surrounded by drying land that takes into account the fact that measures of potential evaporation will vary as the amount of surface water changes while a surface dries.
The authors found that the predicted reference evaporation agreed with class A evaporation pan measurements—a standard cylinder used in many parts of the world to measure potential evaporation. However, in order for the framework to be more widely applicable for hydrological and climate science, scientists will need to determine how factors such as meteorological variability and vegetation cover influence evaporation. (Water Resources Research, doi:10.1002/2015WR017969, 2016)
—Kate Wheeling, Freelance Writer
Citation: Wheeling, K. (2016), Estimating evaporation, Eos, 97, doi:10.1029/2016EO047693. Published on 14 March 2016.