How Does Deforestation Affect the Water Cycle?

SubjectToClimate Team

Aug 19, 2021

Whether you’re teaching about the water cycle in school or simply curious to learn more about it, there’s a piece of the puzzle you might be missing: forests. Trees play a central role in bringing clean water to people around the world.

 

Unfortunately, the world’s forests are rapidly shrinking. Every year, people cut and burn down billions of trees to make room for agriculture, infrastructure, and urbanization and to supply wood for construction, manufacturing, and fuel. As of 2015, the total number of trees in the world had dropped by approximately 46 percent since human civilization began!

 

Deforestation presents a severe threat to human life. We need trees. At the most immediate level, deforestation endangers the communities that call forests home, many of them indigenous. The role of trees in carbon sequestration points to another danger. When we clear forests, we destroy a means of removing carbon from the atmosphere— while also releasing the carbon that had been stored in those trees. (The deforestation of the Amazon Rainforest has made the forest so vulnerable to wildfires that it recently began emitting more carbon than it absorbs).

 

A third, under-discussed role of trees is to provide us with clean water. How does deforestation affect the water cycle?

 

In elementary school, students often learn a simplified version of the water cycle: water evaporates from bodies of water into the atmosphere, forms clouds, and falls back down to Earth in the forms of rain, snow, and hail. (Here’s a quick Water Quality Backyard Activity you could do in your classroom!) This process repeats forever, making water a renewable resource. 

 

However, evaporation is not the only way that water enters the atmosphere. A second process, called transpiration, occurs when trees and other plants suck water from the ground through their roots, then release that water from their leaves into the air. This process is a crucial part of how we get water.

 

When people clear forests, this water-storing effect vanishes along with the trees. The loss of transpiration has many adverse impacts. Because forests are a primary source of rainfall, their destruction causes severe droughts. 

 

You might expect this problem to be concentrated in the areas where the forests in question used to grow. But in fact, deforestation’s effects on the water cycle may be global. Rainforests —the ecosystems that transpire the most— provide water to people far from the tropics. Wind currents pick up moisture from rainforest canopies and carry it across the globe. This “river in the sky” presents an especially crucial water supply to regions further inland, which often cannot rely on evaporation from the ocean as a source of precipitation. A working paper by the World Resources Institute found that the clearing of rainforests “could pose a substantial risk to agriculture in key breadbaskets halfway around the world in parts of the U.S., India, and China.”

 

Transpiration also has climatic effects. When trees absorb energy from the sun through their leaves, they use the biggest share of it for transpiration. This means energy that would have otherwise heated the air around the trees instead gets used for the process of transpiration-- and, as a result, that transpiration cools the air in forests. By halting this process, deforestation causes warming at the local level.

 

Oddly, the opposite is true of the atmosphere higher up above the rainforests. As water particles rise into the air from transpiration, they heat up, warming the air above the canopies. At a local level, deforestation therefore cools the air above tropical regions. This might seem like a good thing, but this local cooling sets off chain effects that cause warming in other places. For instance, one modeling study published by the American Meteorological Society found that the complete deforestation of the Amazon Rainforest would lead the snowpack in the Sierra Nevada mountains to shrink by up to 50%— depriving Californians of an essential water supply. You can get your students to explore data regarding deforestation with this tool.

 

Another answer to the question “how does deforestation affect the water cycle?” lies beneath our feet. In addition to providing us with the water we drink, trees also hold together the ground on which we stand. By forming a network of roots that run through the soil, they prevent erosion— that is, the wearing away of the earth’s surface by wind and water. The canopy also helps, slowing the fall of rain so that it hits the ground more gently. 

 

Trees’ root systems prevent the erosion of fertile soil and enable the filtering of harmful pollutants, stopping them from entering water supplies. Without trees’ root systems, the rain washes dirt and chemicals into nearby bodies of water, harming fish and makes clean drinking water hard to find. Deforested regions also become more vulnerable to floods and mudslides.



Trees play an essential role in the water cycle. Deforestation presents a serious threat to our environment and to our very existence. The struggle to save the world’s forests is a desperate fight for clean air to breathe, clean water to drink, and solid ground to stand on.

 

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Want more resources to answer the question “how does deforestation affect the water cycle?”-- and to learn about a wide range of other environmental topics? Check out SubjectToClimate’s lesson plan on deforestation or the wealth of other resources on our website!

 

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