Evapotranspiration

As we have seen, many of the processes that occur in the boundary layer are dependent on the presence of moisture in the air. Earlier we mentioned how important moisture was as an influence in the PBL. From our discussion on water vapor in the atmosphere, we learned the amount of water vapor in the air varies from 0 to 4 percent, and this water vapor enters the atmosphere through evapotranspiration. Evapotranspiration is the combined process of evaporation and transpiration. It is used to describe the exhange of water vapor from the surface to the air via water reservoirs, soils, and plant/animal life. Evapotranspiration is, therefore, an important process within the planetary boundary layer.

In looking at the behavior of the PBL, it is observered that there are considerable differences between a dry surface and a wet (typically a vegetated) surface. The faster heating rate produced in dry land generates a vigorous turbulent mixing and an unstably stratified PBL. The height of the PBL over a dry surface is typically in the range of 2-4 km during the afternoon. The slower heating rate of a wet surface reduces the development of the PBL; in this case, the height of the PBL might only be one (1) km. However, evapotranspiration provides a supply of moisture which significantly increases the amount of water in this shallow PBL. An excess of evapotranspiration acts to destabilize the PBL to moist convection and to promote excess precipitation.