June 06, 2013

Evaporation Versus Precipitation: Which Wins in the Northwest?

On one hand, there is precipitation reaching the ground.

On the other, there is evaporation from the ground and transpiration of water vapor from plants.  

(Note: Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves.)

Which wins here in the Northwest and elsewhere:  evaporation or precipitation?

The sum of evaporation and transpiration is given the fancy name of evapotranspiration.  This is the kind of term that will impress your friends.

It is interesting to look at the ratio of evapotranspiration to precipitation.   Where it is greater than one there is more evaporation than precipitation.  Take a look at the climatological ratio of these two quantities for the U.S. over the year:
Western Washington has real bragging rights here...we have the LOWEST ratio in the U.S., with evaporation being only about 25% of our precipitation.  So we accumulate lots of water that ends up in our rivers and streams, before heading out into the Sound and the Pacific.  Northwest Oregon also does pretty well as does parts of the upper Northeast U.S.   But the Northwest leads in this measure before for sure. 

In contrast, there is eastern Washington, where some portions have ratios greater than one.  They evaporate more water than falls as rain.  There are variety of ways this can occur naturally (e.g., water coming in from rivers, groundwater rising to the surface), but certainly in this case the extensive irrigation using Columbia River water must be important.

Most of the eastern U.S. and Midwest have ratios below 1 (they receive more rain than evapotranspiration), with large sections of the southwest U.S. having ratios above one (no surprise there).

As temperatures increase, the amount of evaporation (evapotranspiration) increases on both sides of the Cascades.  Below is the evapotranspiration at Corvallis, in western Oregon, and George, in eastern WA (in inches per day).  You go from essentially zero in mid-winter to about .25 inches recently.  With very little precipitation falling, there is way more evaporation than precipitation and the ground is drying up.  It shows in my garden.


  1. Stomates occur on both leaf surfaces, top and bottom. Rougly equal stomate density on top and bottom of grasses and other monocots; higher density of stomates on the top of dicots.

  2. How is evapotranspiration measured?

  3. Somewhat surprising with this perspective, that you haven't mentioned humidity's impact on both evaporation and transpiration. .. http://ga.water.usgs.gov/edu/watercycletranspiration.html


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