Science Terms

Plant Uptake

Every tree, grass blade, and plant around you is performing a gravity-defying wonder: it is moving water, against the force of gravity, upwards from the ground into its roots, trunk, branches, and leaves (where it finally exits via transpiration). Plants mostly depend on water in the ground for their needs, not rainfall directly. Even when you sprinkle you garden with water to give it a drink, the plants want the water that soaks into the ground next to them, not from the water that lands on their leaves. In the ground, rain infiltrates downward into the ground, with much of it being held in tiny spaces between the dirt particles close to the surface. This is the same zone that plants put down their roots, roots that pull in the groundwater and send it up, via capillary action, to the rest of the plant.

Groundwater Flow

If you pour a glass of water onto the ground it usually sinks right in (infiltration), so you can imagine how much water sinks into the ground during a major storm (Find out here). Here in Atlanta, Ga, USA it rains about 50 inches/year. Over a 100 square mile (10 miles by 10 miles) area, that means 87,000 billion gallons of water falls, with much of it soaking into the ground to become groundwater. Yes, water below your feet is moving all the time, but, no, if you have heard there are rivers flowing below ground, that is not true. Water underground moves, due to gravity, downward and sideways. Groundwater serves many purposes in nature: keeping plants alive, filling aquifers, from which people can withdraw water, providing water to rivers and lakes, and eventually flowing into the oceans.

Vents

One of the more recent discoveries in the oceans are "thermal ocean vents", as you see in this picture. You are seeing very hot water pouring out of what looks like a mini-volcano at the bottom of the ocean, many miles below the surface. These vents are not large, so they don't really contribute much water to the global water cycle, but they are interesting because the water is coming from deep in the earth, which happens to be very hot (the rock material is molten). In an environment so hot you won't find liquid water, but you will find the components of water: hydrogen and oxygen molecules. The water coming out of these vents is full of dissolved minerals, even salt; this is one way that the oceans become salty. By the way, even miles below the surface, the areas around these vents are much warmer and support all kinds of unique ocean life that do not exist in other parts of the ocean.

Run Off

Runoff is nothing more than water "running off" the land surface. Just as the water you wash your car with runs off down the driveway as you work, the rain that Mother Nature covers the landscape with runs off downhill, too (due to gravity). Even though some rainfall soaks into the ground, most of it flows over the land surface, going downhill. This runoff water reaches rivers, lakes, and the oceans, keeping the water cycle going. Runoff is also important because as it flows over the land, some of it soaks into the ground, thus "recharging" groundwater, providing plants with water for their roots to take up so they can create the delicious beets and spinach you like so much, and keeps underground aquifers (areas underground full of water) full so people can drill a well and pull the water out for their own purposes.

Evapotranspiration

Take a breath and breathe out—you just participated in the global water cycle. Your breath contains water—breathe on a glass pane to see it appear. When a person breathes, the term is called "respiration". All the plants around you are "breathing" and releasing water, too. The term is called "transpiration", and although a Brussels sprout doesn't have a mouth, it has tiny holes in its leaves that allow water to leave the leaf, via evaporation, and go into the air. So, plants "transpire" water while humans "respire" water. A large tree can transpire many thousands of gallons during the growing season. If you click the picture to the right, you can see leaf transpiration in action. But the big word is "Evapotranspiration", which is just a combination of "evaporation" plus "transpiration". This just means that not only are plants transpiring water from their leaves, water is evaporating from the soil all around them, too.

Precipitation

The air is full of water, even if you can't see it. Higher in the sky where it is colder than at the land surface, invisible water vapor condenses into tiny liquid water droplets—clouds. When the cloud droplets combine to form heavier cloud drops which can no longer "float" in the surrounding air, it can start to rain, snow, and hail....all forms of precipitation. The amount of precipitation that falls is different all around the world. In deserts, such as in Chile, it may only rain one inch per year, while on some mountains in Hawaii and in India, it can rain more than 600 inches per year. That is almost 2 inches every day! Some locations get rain all year long, and many other places have rainy and dry seasons, and only get significant rainfall during certain months of the year. Some places, such as Antarctica, really don't get rain, but they sure get a lot of snow, which accumulates as icefields and glaciers. Precipitation is the "exit ramp" back to earth from the superhighway in the atmosphere that is moving water vapor and clouds all around the globe.

Ice Clouds

The clouds overhead are made of water, of course. Most of the clouds you see in the atmosphere are composed of tiny droplets of liquid water, but not all. Very high in the sky the air gets really cold, way below freezing. Clouds in this region are made of ice particles. You see ice clouds all the time as cirrus clouds. They are usually rushing by as if they are late for dinner, but that is only because the winds at high altitudes are often moving very fast. Actually, some of the cirrus clouds you see are created by humans—these are the "contrails", or condensation trails made by airplanes, as you see in this picture.

Groundwater Storage

The ground stores huge amounts of water and it exists to some degree no matter where on Earth you are. The water gets there by gravity. The top layer of the ground is the soil and below that is where true groundwater exists—sometimes called an aquifer. In these layers the rock has many small openings, cracks, and fissures and water occupies all the spaces. People can sometimes drill wells into this region and pull out water for irrigation and drinking. This picture gives you an idea of how an aquifer works. At a certain depth water saturates all the openings between the rock (sand, in this case) particles. That pool of water you see is a mix of dirt and water (the "aquifer" you see in this picture is more watery than a true aquifer). The deeper down you go, the rocks become more dense, thus "squeezing" out openings in the rock. Temperatures also get extremely hot further down, which prevents liquid water from existing. Thus, at a certain depth, miles into the Earth, no more liquid water will be found.

Sublimation

When rain falls where you live, much of it disappears from evaporation, right? But what about at the top of Mt. Everest? The elevation is over 29,000 feet and temperatures at the peak are never above freezing, so it never rains there. It does snow a lot. We all know that snow melts, but if it never rises above freezing, how does snow not build up forever on Mt. Everest? As this picture shows, winds, often at hurricane force, blow tons of snow off the peak. But a part of the water cycle called "sublimation" is also at work. Just as evaporation turns liquid water into water vapor gas, sublimation turns frozen water directly into water vapor gas, skipping the melting phase into a liquid. Also, as with evaporation, the sun's heat causes sublimation to work, and on top of Mt. Everest on a clear day, the sun is very strong and provides energy for sublimation, even though it is below freezing. Sublimation doesn't just occur on mountain peaks. If there is snow and ice buildup in your neighborhood then on a sunny and dry day sublimation is happening there, too.

Infiltration

You can't see it, but a large portion of the world's freshwater lies underground. It may all start as precipitation, but through infiltration and seepage, water soaks into the ground in vast amounts. Water in the ground keeps all plant life alive and serves peoples' needs, too. How much rainfall infiltrates the ground depends on many things and varies a lot all over the world. But infiltration works everywhere, and pretty much anywhere in the world you are, there is some water at some depth below your feet, courtesy of infiltration. Infiltration recharges groundwater: Just like a rechargeable battery, aquifers in the ground are "recharged" by water infiltrating from the surface. And because water underground often flows sideways, water in the ground can be recharged by rainfall hundreds of miles away.