Evapotranspiration: What I Study

Nov 20, 2015 By Susana Alvarado
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I study the process by which water returns from the surface of the Earth to the atmosphere in the form of vapor. This process is called evapotranspiration. Without it, water would not cycle around the globe!

It includes the evaporation of water from all surfaces, for example a lake, ocean or the top of the soil. It also includes transpiration, which is the process by which plants return the water they take from the soil to the atmosphere through tiny pores in their leaves.

Because water vapor is a gas, it is harder to measure it compared to water in its liquid form. For example, if you wanted to know how much rain fell in your town during a storm, you can use a container to hold all the rain and later measure the volume of water using a calibrated cylinder.

But how can we know how much water vapor returns to the atmosphere from a single plant, an ecosystem, or the ocean? Figuring ways to do this as accurately as possible is part of my job, which makes it very challenging but exciting.

How Thirsty Is A Tree?

To know how much water a tree consumes, it is impractical to measure all the water that enters its roots or how much vapor escapes from the stomata (pores) on all of its leaves.

Instead, one can measure the amount of water that flows inside the main trunk because this water (called sap) will be transpired. I used this “sap flow” technique to know how thirsty pine trees are. It involved inserting three small needles in the trunk and sending a short heat pulse to measure the velocity at which sap moves inside. It also involved a lot of wires connected to small computers to collect data in the field.

This study helped solve the puzzle: a young Mexican weeping pine drinks on average 5 liters of water during a sunny day.

How much water returns to the atmosphere from an ecosystem?

A large part of the energy that comes from the sun to the surface of our planet is used to evaporate water.

Learning about how much evapotranspiration there is from different land surfaces helps us predict what would happen to the water cycle, weather and climate under different land uses. For example, what would happen if a forest is cut and sugarcane is planted instead? The different plant species have different color and height, so they reflect the sun’s energy differently.

Also, trees and grasses are not equally thirsty. So evapotranspiration from different land surfaces may not be the same. Using equipment that measures wind speed and air humidity really fast (for example, 20 times per second) we can directly measure evapotranspiration over a field or forest.

Recently, I started to work at a Mexican station that will collect earth observations from satellites (ERIS – ECOSUR Chetumal) and hope to develop ways to monitor evaporanspiration using these observations.

You can check this video out to learn more about the interaction between the land surface and the atmosphere: