365 Days of Climate Awareness 67 - Evapotranspiration

Evapotranspiration is the delivery of water vapor into the atmosphere via evaporation from plant leaves and its release from plants via their ordinary metabolic processes. It is a very important aspect of the global water cycle, but because of the Amazon River it is of particular importance in the western hemisphere.

Basic schematic of evapotranspiration.

The river of course transports a massive amount of water--209,000 cubic meters/sec, or 55 M gallons/sec--eastward to the Atlantic Ocean. It transports more water than any other river in the world, and has the largest drainage basin. It is disputed, depending on how length is calculated (from what point in the headwaters), whether the Nile or the Amazon is longest. Counter to the flow of the river itself, is an airborne transport of heat and water upstream, toward the west. This is the evapotranspirative river which has long-ranging influence over weather patterns.

The Amazon river basin.

But the process of the Amazon rainforest returning water to the atmosphere produces a monsoon effect, where the westward trade winds vary between north and southward flow.  Toward the end of the Amazon's dry season in the latter half of the year, plants and trees sprout new leaves, which increase the evapotranspiration throughout the region. The vapor condenses into water droplets  which become rain, and release heat into the atmosphere.

This released heat creates an upward convection which becomes the basis for the wind shift to the south, leading to increased storm activity throughout the central Amazon basin. Toward the end of the wet season, when leaves die off, the evapotranspirative cycle of the Amazon subsides, and the trade winds resume their northward motion, at the end of the dry season in Central America. It is a plant-driven monsoonal effect which straddles the equator. In this way, the forest itself creates the monsoon which brings warm, moist air into the upper reaches of the Amazon: the evapotranspirative river.

The tropical evapotranspirative monsoon, where vegetation drives wind patterns.

As the Amazon rainforest continues to shrink, this monsoonal effect is weakened, and threatens to disrupt weather patterns throughout the western hemisphere.

Tomorrow: boreal forests.

Be well!