About 70% of the earth’s surface is covered with water. This water evaporates and condenses into clouds. The clouds cause precipitation and this water falls back to the earth’s surface, ready to start the cycle again.
Why do I care? The water cycle is critical not only to weather, but to life on earth. Rain is necessary for the survival of plants and humans. Condensation is necessary for cloud formation. Evaporation is necessary to cooling and keeping a good balance of water vapor in the air.
Figure A: The Water Cycle
The water on Earth now is the same water that’s been on Earth since the beginning. The rain that falls on us is the same water that rained on the dinosaurs, King Tut, and George Washington. What makes that awesome feat possible? The water cycle. The water cycle is the process that re-circulates water so we can have bodies of water as well as clouds and precipitation.
The first step of the water cycle is evaporation. About 85% of the water vapor in the air comes from water that evaporated from the oceans. The other 15% comes from evapotranspiration, which is a catch-all term for water that evaporates from over land. This includes water vapor produced by plants during transpiration, water from lakes, streams, puddles and soil moisture, direct evaporation of snow and even water vapor from the breath of animals.
The second step of the water cycle is condensation. Now that the atmosphere is full of water vapor, that water vapor condenses into water droplets. Sometimes, like early in the morning, the water vapor condenses on the grass as dew and seeps back into the soil, ready to be evaporated again. But most of the water vapor condenses higher up in the air and forms clouds. Once the water droplets are in a cloud, two things can happen. Either the cloud will dissipate and the water droplets will become vapor again, or the cloud will grow and it will begin to precipitate.
The third and final step of the water cycle is precipitation. Precipitation includes all water that falls from the sky, both in liquid and frozen form, which reaches the ground. Once the precipitation makes its way to the ground it can end up soaking into the ground, run off into streams and lakes, become snow cover, be used by plants, be inhaled by animals or fall directly back into the ocean. Then the water cycle can begin again and continue for millions of years to come.
Figure B: Infiltration and runoff
Aside from the above steps of the water cycle, there are also ways that water can be stored on Earth that play a role in the water cycle at various times throughout the year. Water can be stored in lakes, streams, glaciers, icebergs, and the ground.
As precipitation falls towards the earth, some of the water seeps into the ground, a process known as infiltration. The amount of water that seeps into the ground varies with several factors such as duration, type, and strength of precipitation, type of soil, slope of the land, and land cover. Duration and strength of precipitation play a role in the amount of water that the ground can hold and whether the water will seep into the ground or travel across the ground surface. Certain types of soils hold water better than others and can absorb more water. As the slope of the land varies, so does the amount of water that will be able to seep into the ground. If there is a steep slope, the water will be more likely to just run down the hill rather than get absorbed by the ground. If the ground has no slope, the water will be more likely to seep into the ground or remain above the ground (as over a flat road) and potentially cause flooding. The amount of water that the ground absorbs also will depend on the land cover. Vegetation impacts the speed of water that will move across a surface. More vegetation leads to slower flowing water.
The factors that impact infiltration also impact surface runoff. Runoff occurs when water flows across the ground surface. If no water is able to seep into the ground, the water will flow across the ground surface. This occurs many times near mountains as water flowing quickly down a mountainside is unable to be absorbed by the ground. If a certain type of soil does not absorb water very well or if the soil is already saturated, then the water remains on the surface. Clay is an example of a soil that absorbs water slowly in comparison to sandy soils. More runoff will occur over land with clay soil rather than land with sandy soil. Runoff can also occur during snowmelts from mountainsides.
How does this relate to forestry?
Figure C: Woodfin Watershed in Western North Carolina is a forested watershed which helps to protect water resources from pollution and is protected by conservation efforts. (Image from the Southern Appalachian Highlands Conservancy www.southwings.org)
Forests and trees represent a crucial part of the water cycle. The soil absorbs precipitation that falls from the clouds, and trees draw water from the soil into their roots to support all of their life major processes such as growth, reproduction, and maintenance. As water travels from the roots out to the leaves, water is lost through tiny pores, or stomata, in a process called transpiration. Transpiration and evaporation together comprise total evapotranspiration, the amount of water returned to the atmosphere as vapor to continue the water cycle. Forests use more water than lower-growing types of vegetation, and also produce lower surface runoff, groundwater recharge, and water yield. Tree species and age, forest structure, and harvest patterns influence the amount of water a forest requires. For example, evergreen conifer trees such as pines demand more water than deciduous trees. Young trees require more water than older trees. In the Southeast US particularly water-inefficient tree species include black cherry, dogwood, yellow poplar, basswood, birch, buckeye, and sycamore. Thinning out a forest can help to reduce the water demand by the trees, but increases erosion and produces holes in the canopy which removes shade and shelter for other forest species. Maintaining a developed understory layer to protect soil moisture reduces the water requirements of forests, even if many trees are cutdown to reduce water demands by trees.
As climate change causes greater precipitation extremes and higher temperatures, trees and forests will play an increasingly vital role in the Earth's water cycle. Forests increase water quality by minimizing erosion and intercepting polluted runoff, which may become more important if climate change threatens local water supplies. Forests also produce less chemical and nutrient pollution than equivalent, more intensively managed agricultural operations. Forests can reduce the impacts of floods by absorbing water during periods of abundance and slowly releasing it during dry spells. If planting trees to sequester CO2 to reduce carbon dioxide concentrations in the atmosphere, managers must be sure the trees’ water demand will not worsen regional water shortages during drought—particularly for fast-growing, short-rotation forest crops such as poplar.
Want to learn more?
Links to National Science Education Standards:
7th grade science: 7.E.1.2 : Explain how the cycling of water in and out of the atmosphere and atmospheric conditions relate to the weather patterns on earth.
Earth Science: EEn.2.3.2 : Explain how ground water and surface water interact.
Activities to accompany the information above:
Activity: The Water Cycle (Link to original activity.)
Description: This activity focuses on building a model representative of the water cycle. Students will develop their own models of the water cycle using the recommended supplies and will be able to explain all of the elements of the water cycle with their models.
Relationships to topics: The Water Cycle