Evaporation is an important process in the global water cycle. Solar radiation hits the surface of water or land and causes water to change state from a liquid to a gas. This is how water vapour enters the atmosphere: moisture in the atmosphere is linked to cloud formation and rainfall. Evaporation acts like an air conditioner for the surface because heat is used when water enters the atmosphere as moisture. But at the same time, water vapour acts as a green house gas by trapping radiation in the lower atmosphere.
- Evapotranspiration diagram
- Evaporation: liquid water held in the soil, lakes and rivers is heated by solar energy and becomes water vapour that rises into the atmosphere
- Transpiration: water enters plants via their roots and is carried to their leaves where it exits via small pours in the leaves
- Interception: rainfall water landing on a leaf or branch is evaporated straight back into the atmosphere
As temperature increases so does the process of evaporation. In addition the moisture holding capacity of the atmosphere increases with temperature. For every 1oC increase in global temperatures there is a 7% increase in the moisture holding capacity of the atmosphere. And more moisture in the atmosphere ultimately leads to changes in rainfall patterns.
Actual evaporation depends on availability of water, for example more water is evaporated from a lake than from dry soil. Moist areas like tropical rain forests have higher evaporation rates than arid regions. The amount of water that evaporates from the land surface depends on the amount that is contained in the soil (soil moisture).
Adding complexity is the fact that all terrestrial plants loose water through transpiration. Water is transported by the roots of a plant to its leaf, where it is lost through pores on the leaf surface. The surface of a leaf also captures or intercepts rain as it falls, and this water can be evaporated from the leaf surface before ever reaching the soil. In forests, where the area occupied by leaves is large, this “interception” can be an important mechanism to include when trying to model the process of evaporation.
Presented in these maps is modelled actual evaporation from the JULES model. JULES represents calculates the evaporation from soil, canopy and snow moisture stores as well as the surface of lakes, wet vegetation canopies and snow.