By Chris Harbison (WEST Fellow 2005-7)

How thirsty are plants?  Did you know that plants use a large percentage of the water that falls as precipitation?  Similarly, evaporation causes much of the precipitation and snowmelt to go right back into the atmosphere.  For example, when snow melts in the spring, anywhere from 30-95% of the snowmelt can be lost to evapotranspiration in forests.  In this section, we’re going to learn about evapotranspiration in the alpine zone, and explore how much evapotranspiration occurs as water travels through the alpine ecosystem.

First, a few quick definitions:

Transpiration is the plant process by which water is lost to the air.  Plants need carbon dioxide to make sugar in a process known as photosynthesis.  To get this carbon dioxide, plants have “holes” on their leaves called stomata that they can open or close to let air carbon dioxide inside.  The one major problem with this system is that every time stomata are open, water vapor escapes into the atmosphere.  This process of water loss is known as transpiration.

Evapotranspiration, often abbreviated as ET, is the total amount of water lost to the air through transpiration from plants and through evaporation from the soil. 

ET = Soil Water Loss + Plant Water Loss

Alpine zones in the Wasatch Mountains are special in that the majority of water comes in the form of snow.  As you might expect, soil water is plentiful in the spring during snowmelt, and gradually dries out throughout the dry summer.  Not surprisingly, evapotranspiration follows this pattern as well.  However there are many factors besides seasonality that influence the amount of evapotranspiration.

Click here to learn about Factors that Influence the Amount of Evapotranspiration.

Click here to find out How Scientists Measure the Amount of Evapotranspiration.