Wherever you wander on Earth, from the windswept peaks of the Himalayas to the deepest trenches of the Pacific Ocean, you’ll find water. There might not be much, and it might not be easy to get to — dispersed as vapor in desert air or trickling along deep underground — but it’ll be there, driving weather and powering ecosystems.
The way water moves around the planet in all its forms is complex: it flows, falls, freezes, melts, evaporates, condenses, drizzles and pours. But understanding that complexity can help unravel all kinds of planetary processes, from snowstorms to the motion of seawater.
Breaking the ice
There’s no real “beginning” to the water cycle; it involves plenty of processes happening at the same time and feeding into one another. But we have to choose a spot to start our tour, and the top of a mountain seems as good a place to begin as any.
Let’s say it’s late spring or early summer. Snow from earlier in the year coats the mountaintops, but the weather is starting to get warmer.
“This accumulated water storage suddenly melts,” said Li Li, a hydro-biogeochemist at Pennsylvania State University. “You see some of the runoff go into streams, but a lot of it also infiltrates into the soil.”
Some places, like the western United States, rely heavily on meltwater from the mountain snowpack to fill waterways and moisten soils during the warmer part of the year. As climate change causes these snow reserves to dwindle, mountainous areas feel the effects.
“It doesn’t only affect skiing. It will also affect water supply in these regions a lot,” Li said. “It’s a huge problem.”
The water that does make it down the slopes has some options. It might evaporate and return to the atmosphere. It might flow along a creek, stream or river. Or it might continue its journey downward, percolating through the soil and collecting in layers of rock as part of the groundwater supply.
Groundwater is a crucial resource, accounting for almost a third of Earth’s freshwater. Humans dig wells to tap those deep rock layers, called aquifers, but the shallower soil zone still provides plenty of moisture, particularly for thirsty plants.
“Vegetation plays a key role in the hydrologic cycle,” said Serenity Montaño, a doctoral candidate in Environmental Science and Public Policy at George Mason University and a volunteer researcher in the Department of Botany at the Smithsonian National Museum of Natural History. “Healthy plants release water vapor in order to regulate heat,” they said.
When plants pull water up through their roots and allow some of it to evaporate from pores on their leaves — a process called transpiration — they’re shuttling water from the soil into the atmosphere.
Plants also participate in the water cycle in less direct ways. “Plants growing along waterways protect riverbanks from eroding,” Montaño said, “and they filter sediment and other contaminants, protecting the water quality downstream.”
Those rivers, flanked by vegetation and fed by groundwater, meltwater and rain, flow toward the next stop in the cycle: the ocean.
The ocean houses the vast majority — almost 97% — of the water on Earth. Glaciers and ice caps, containing less than 2%, are a distant second. But the ocean isn’t a giant, salty version of a stagnant swimming pool. It’s constantly in motion.
“If you’re a sailor, you know this very well,” said Brian Huber, geologist and curator of the tiny marine organisms known as foraminifera at the museum. Surface currents, like the Gulf Stream along the eastern U.S. or the Kuroshio Current off the coast of Japan, are driven partly by wind and bend according to the spin of the Earth.
They’re also powered by what’s going on beneath the surface. From pole to pole and down to the seafloor, seawater follows a “conveyor belt” known as thermohaline circulation.
“It’s all a matter of density,” Huber said. Cold, saltier water is denser than warm, fresher water — so in cold places like the poles, where some water freezes into ice and leaves a higher concentration of salt behind, ocean water sinks. Warm surface water flows from the tropics to take its place, the deep water heads toward the equator and rises as it warms — and the conveyor belt continues.
“We have one ocean in the world. It’s all connected,” Huber said. “And what we do on land affects the ocean.” Take climate change, for instance. As the poles warm and melting ice sends more freshwater into the ocean — lessening the differences in density — scientists believe that thermohaline circulation could slow down.
The ocean is the largest source of water evaporating to the atmosphere, which — even though it holds less than 0.01% of the water on Earth — is a site of major water-cycle action.
Water evaporates constantly from the surface of the ocean, entering the atmosphere as vapor. Eventually, the air will reach a point called saturation, when it can’t hold any more vapor, and water will begin condensing out as clouds.
“That point of saturation is completely temperature-dependent,” said Deanna Hence, an atmospheric scientist at the University of Illinois at Urbana-Champaign. “It takes less water to reach that saturation point at cold temperatures, and it takes quite a bit more at warm temperatures.”
Look up, and you’ll see that effect. “On a very cold day, you can have clouds, but if you bring that air inside and heat it, your skin is still so dry, because there’s not a lot of water in the atmosphere,” Hence said. “On the flipside, in the middle of summer, it can feel like you’re breathing in soup, but that air may still not be at saturation.”
Depending on how cold the saturated air is, water vapor will condense into liquid droplets or solidify into ice crystals.
“All those particles are also bumping into each other constantly,” Hence said. “As that happens, the particles will get bigger and bigger and bigger.” Eventually, they’ll get heavy enough that they fall toward the ground as precipitation.
The type of precipitation that reaches us — and, in fact, whether it reaches us at all — depends on the air it encounters on its way down. Water droplets might pass through air so unsaturated that they evaporate before they reach the ground. Ice crystals might melt into raindrops. Or snowflakes might drift down and settle onto a mountain snowpack, prepared to provide precious meltwater come spring.
There’s a reason scientists have long searched for liquid water on other planets: it fuels life as we know it. On a microscopic level, water dissolves molecules and allows them to interact, enabling the chemical reactions that sustain living things. But we can see how much life needs the water cycle with our own eyes, too. Rain feeds forests, marshlands and crops. Groundwater supplies our wells. The ocean, in all its varied, dynamic glory, supports countless species.
Understanding the water cycle also helps us untangle processes like harmful algal blooms, which can form when rivers carry nutrient-rich fertilizers to the ocean, or land subsidence, which happens when humans deplete groundwater reserves to the point that the ground buckles. And as climate change unfolds around us — shifting drought patterns, strengthening storms, changing the ocean’s chemistry — the water cycle is one of the many planetary systems in which we can see the effects up-close.
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