When scientists search other planets for life, the first thing they look for is evidence of water cycle steps. That’s because without water, there can be no life. Fundamental to the normal, biological functions of everything from bacteria living in deep-sea hydrothermal vents to human beings bicycling around Beijing, water is the one need all life has in common.
What makes water so important? It comes down to the basic structure of life: the cell. Everything alive has a membrane, or membranes in the case of multi-cellular life, that separates it from its environment. In order to transport nutrients across that membrane, cells need water. As a powerful solvent able to dissolve molecules out of a solid and transport them from place to place, water makes the movement of materials between cells and their environments possible.
Water has other important properties that shape Earth’s landscapes and affect global climate. Water can exist as liquid, solid or gas in a relatively narrow band of temperatures, which allows it to cycle around the globe through a series of processes that erode mountains, carve out valleys and deposit sediments that build new rock.
It’s incredible capacity to hold heat, known as its specific heat capacity, means that water transports warmth from Earth’s hot equatorial regions to the north as it cycles around the globe. This keeps places like London and New York free of ice and comfortable for humans to live.
Overview of the Water Cycle
Earth’s water cycle, or hydrologic cycle, describes the main internal workings of Earth’s hydrosphere, which includes all the water in, on and around the Earth. The water cycle steps encompass how water changes from liquid to water to solid, how it moves on the planet, and all the places it spends time along the way. Some Earth scientists are focused entirely on the processes taking place within the water cycle. Others are interested in how those processes interact with Earth’s other systems.
A water molecule’s journey
To get a clear picture of Earth’s entire water cycle, it’s helpful to imagine the journey of a single water molecule as it traverses the globe. It begins in the ocean where the molecule comes to the surface. The heat of the sun causes the molecule to leave the ocean and rise up into the atmosphere.
As it rises, wind currents carry it over land to a high mountain peak. As the molecule rises, it loses heat, condenses into a droplet with other water molecules and falls back to Earth. Here it joins a mountain glacier, where it exists as ice for a long time. Eventually, on a warm summer day, it melts away from the glacier to join a river flowing down the mountain. In time, the water molecule reaches the sea and begins the cycle again.
Let’s consider another journey. The same molecule rises into the atmosphere from the ocean and is carried overland. Instead of falling high on a mountain, it rains down over a plain where it soaks into the soil, eventually reaching an underground pool, or aquifer. It reemerges at the surface through a spring and flows back to the sea.
Perhaps instead of reaching the aquifer, the molecule soaks into the soil where it is absorbed by the roots of a tree. It rises up through the trunk to the leaves of the tree, where it is eventually released back into the atmosphere. Maybe it will rain back down into the sea, or fall on soil and join another aquifer. All of these journeys are possibilities in the water cycle.
Earth’s Reservoirs
The places where water spends time are called reservoirs. The ocean is the largest reservoir. It holds over 97 percent of Earth’s water, and a water molecule may spend more than 3000 years circulating around the ocean before it enters the atmosphere. Freshwater, the kind usable by humans, is just 2.5 percent of the water on Earth. Freshwater reservoirs include rivers and lakes, groundwater, the atmosphere, glaciers and permafrost, which is water frozen in the soil.
The largest freshwater reservoir by far is glaciers. More than 68 percent of all freshwater is locked up in ice. A water molecule may spend hundreds of thousands of years in a glacier and the oldest ice on Earth, found in Antarctica, is over 2.7 million years old. Water spends the shortest amount of time in the atmosphere: an average of 9 days.
Evaporation, Transpiration and Sublimation
Evaporation, transpiration and sublimation are the processes by which water enters the atmosphere. Evaporation is the transformation of water from a liquid state into gas, and that is how water gets from the ocean, lakes and rivers to the atmosphere. Sublimation is the direct conversion of water from a solid to gaseous state without becoming liquid first. This is how water molecules move from glaciers and permafrost into the atmosphere. Transpiration is the release of water molecules from plant leaves into the surrounding air.
Condensation and Precipitation
Condensation and precipitation are how water leaves the atmosphere to return to Earth’s surface. When water vapor loses heat, it contracts and turns from vapor into liquid, a process known as condensation. When condensation happens in the sky, water falls to the earth as precipitation.
Infiltration and Runoff
Infiltration and runoff are the ways that water interacts with the solid Earth. When water soaks into the soil, it’s called infiltration. How fast it soaks depends on how much water is already in the soil, how tightly packed the grains are in the soil, and gravity. Runoff is the flow of water over the surface of the Earth, either because the ground is already saturated or it’s impermeable. Runoff is the main way that water erodes rock and soil, shaping and changing the landscape over time.
Water is constantly moving within and between reservoirs, carrying heat and transporting nutrients, minerals and sediments as it goes. These water cycle steps not only determine where life can exist, but they also shape the environments life occupies.
