How Hydrology Works

The hydrologic cycle keeps the Earth's fresh water supply on the move.

Water -- We drink it, we bathe in it, and we play in it. Water gives us life, yet too much or too little can also take life away. It's the most common substance on Earth, but far too often there's not a drop to drink. Water's extraordinary attributes give life to this planet, which might otherwise be barren and lifeless. For example, water actually expands when it freezes, becoming less dense. If ice didn't float on water, our planet would be a very different place today. Ice would sink to the bottoms of lakes and rivers, where the sun would be unable to melt it in the spring, a necessary process for making clouds and rain, which continue the flow of the cycle. This would happen every year until, eventually, there was nothing left but ice and an uninhabitable wasteland.

Water exists in three states -- solid, liquid and gas -- which comprise the Earth's hydrologic cycle, better known as the water cycle. In this cycle, fresh water comes to Earth as rain, sleet, snow and hail, pouring down over oceans and land day and night. The water that hits land runs off into larger bodies of water, like lakes and rivers, or it seeps underground and becomes groundwater, nourishing our crops in the process. When the sun comes back out, it heats up the water and makes vapor, which rises up and condenses into clouds, which gather, become heavy with water and fall back to Earth as rain.


Thi­s cycle has gone on since the world began and has made life on Earth possible. In fact, there is no more and no less water in the world today than there was when the dinosaurs were around. Think of the Earth's water cycle as the ultimate recycling method. No wonder hydrologists are so fascinated by it.


Go to the next page to read about the different branches of science that hydrologists make use of to study the Earth's water cycle.



Leonard Malczynski, a hydrologist, software engineer and economist with Sandia National Laboratories, creates models using system dynamics to help water planners predict how different factors can alter use by consumers.
Andy Nelson/The Christian Science Monitor/­Getty Images

Hydrology is the study of the flow of water through the hydrologic cycle, and Hydrologists are the people conducting the research. While most people don't think much about water, hydrologists examine every aspect of it -- where it comes from and in what quantities, where it goes, how it gets there and what happens to it in the process.

To do this, they use a lot of math and some really cool gadgets, and they even get a little wet sometimes. The results are astounding. Hydrologists help designers and engineers construct dams and levees to keep towns safe from flooding, track pollution and find new water sources -- they even help track water


in space.

Because water is everywhere and is so critical to nearly everything we do, hydrologists span almost every branch of science, from physics and geology to astronomy and ecology. There are so many ways to study the Earth's waters; here are just a few.

Spatial hydrology is the study of the movement of the waters of the Earth through the hydrologic cycle using a spatial database developed in a geographic information system (GIS). This means that they try to connect the varying space that is the land with the varying time and flow that is the water

[source: Purdue University].

Hydrogeology, or groundwater hydrology, tracks the flow of water through the soil and into underground systems. Hydrogeologists help find underground sources of water (aquifers) that we can tap into for drinking water. They monitor these, as well as the flow of water into underground sites to determine how much is taken out. They also figure out how water flows through soil. This may sound boring, but if you're a farmer who needs the right amount of soil moisture to grow your crops, it's vital information. Hydrogeologists help with irrigation practices and monitor pollution that seeps down through the soil into our wells.

Vadose zone hydrology is a subset of hydrogeology that focuses on the layer of earth directly above an aquifer, called the vadose zone. This zone is important because it acts as a final filter and aeration device before rain becomes groundwater, which is sometimes taken straight from the ground through wells and used without purification methods. If the vadose zone is unhealthy, so is much of our drinking water.

Hydraulics has nothing to do with making your car do really cool tricks (well, it does, just not in this context). This is the study of the physical flow of water. Hydraulics professionals use mathematical modeling to track the flow of water from one source to another. The major purpose of this is to track not the water but contaminants in it. For example, if a factory is dumping industrial waste into a river, it will flow downstream and eventually get dispersed into the ocean. Who knows where it will end up? The people who study hydraulics do.

Hydrometeorology, also known as hydroclimatology, uses meteorological instruments to figure out how much rain will fall and in what specific areas. This is useful for many reasons -- not just so you know when to take your umbrella when leaving the house. Hydrometeorology can help determine how much water will reach one area in a given year. This helps with the building of flood-control devices and

irrigation systems.

In a form of science known as dendrohydrology, dendrohydrologists use tree rings to help determine historical rainfall and drought conditions, stream flow, runoff, and much more. Knowing this information can help regions without reliable historical records predict the hydrological future.

If the world's water supply contains the same amount of water today as it did in the beginning, then how is it possible to waste water? Won't it just come back? Why do people fight over water? Find out on the

next page.


Water Supply Control and Pollution Control

Hydrologists continually search for ways to improve water treatment plants.
Flip Chalfant/Getty Images

While the Earth still has the same amount of water it has always had, the demand for water is greater than ever. The population keeps growing, and everyone needs to be fed and clothed with materials that have to be grown using water. The same people need to live in houses built with steel, lumber, or other various building materials -- again, these materials take a large amount of water to manufacture or grow. Of course, people also have to bathe and wash their clothes and go to the bathroom … all of this

takes water.


You want to know the real kicker? Water doesn't fall evenly over the entire Earth. One place may get too much rain, causing floods and devastation, while another might get very little, causing a drought. Once the water goes down the drain, it takes a while for it to be processed back into something that can be used again. If there is not enough rainfall for your area and the population uses too much water, the reserve levels go down faster than they can be replenished.

Hydrologists are constantly looking for ways to find more water to keep up with current demands. Water treatment facilities take the water that has been used for waste and turn it back into fresh, local water that can be consumed again. Hydrologists not only look for ways to improve these facilities, they also try to find new underground wells of water and, perhaps even more importantly, figure out how to maintain these supplies with the ever-growing threat of pollution.

Pollution is industrial waste, emissions from cars, runoff of pesticides and animal wastes from farms, and extra nutrients in the soils that cause an imbalance. These can run into lakes, rivers and streams; seep into the groundwater; or collect in the air and fall back to the ground as acid rain. Mapping the course of these contaminants through environmental hydraulics is one way hydrologists help locate and clean

up pollutants.

Don Moseley, Director of Experimental Projects for Wal-Mart, shows off the absorbent asphalt in the parking lot of a new, more eco-friendly experimental Wal-Mart Super Store in Aurora, Colo.
Thomas Cooper/Getty Images

Another way is through innovations like permeable pavement materials. When rain falls, it sometimes contains pollutants. In places with a lot of people, like most cities, there also tends to be a lot of concrete. This washes out all of the pollutants on the road and puts them into the nearest sewer, drain system or river, which overflows with water and has no place else to go. Concrete is impermeable -- it won't allow water to pass through it into the ground. This poses a problem, because filtering the pollutants out with rocks and sand is the first natural water filtration system for our groundwater.

One answer to this problem may be permeable concrete. This surface allows water to pass through it, sort of like soil. It takes the pressure off of the sewage systems in urban areas where all of the water and waste have to drain during a big rain. It also helps with flood control as well as pollution control. The costs, however, are sometimes prohibitively high. Digging up streets and replacing the concrete is no small task. The best way to begin the process of permeable planning is to start during the planning phase of a project.

This is just one of the many ways hydrologists work to keep our ground water safe. Other hydrologists research pollution levels in snow, soil and even glaciers.

On the next page, find out how hydrologists can predict, and sometimes even prevent, floods.


Flood Control

A business is surrounded by flood water from the Mississippi River in Foley, Mo.
Joe Raedle/Getty Images

Anyone who has ever experienced a flood or known someone who has understands that floods are serious business. They can take out entire communities and level hundreds of years of history in just seconds.

Some floods can arrive suddenly -- these are called flash floods or critical floods. Sometimes it doesn't even have to be raining anywhere nearby for a flash flood to occur. Flash floods can happen when a heavy rain occurs upstream and the water flows downstream, gathering more momentum as it passes. Another type of critical flooding happens when some sort of flood-retention device, like a dam or levee, breaks and water rushes out.


Non-critical floods are floods that happen slowly, over a relatively long period of time. These occur when heavy rains fall for days on end, and rivers, lakes and streams swell past their normal boundaries. Don't let the name fool you -- non-critical floods can be just as devastating as flash floods if they're not prepared for, and they can lead to critical flooding as well.

Hydrologists work constantly to prevent and lessen the severity of floods. Hydroclimatologists use rainfall data and meteorological technology to help determine the maximum rainfall for an area. They then try to determine when floods might occur and how much rain it would take to flood an area.

With this information, hydrologists work with organizations, like the United States Federal Emergency Management Agency (FEMA), to help plan city development and emergency response. This is based on the idea that a major flood happens once every one hundred years. A hydrologist's goal is to figure out what the worst flooding possibilities are for a span of one hundred years and design around that possibility to minimize the harm and the cost of rebuilding. Sometimes, if the cost to build a flood device to one-hundred-year flood strength is much more than the cost of rebuilding, engineers will build for the possibility of a 25- or 30-year flood -- the worst possible flood that might happen every 25 to 30 years. This usually happens in remote areas, like parks or forests, where there are not a lot of people who might

be affected.

The work of the hydrologists determines where and how to build not just flood devices like levees, dams and culverts, but also where to put entire cities and neighborhoods. Safety is what's most important. Successful flood control is not always apparent, but unsuccessful attempts are far too obvious.

Some hydrologists go to the ends of the Earth to study the water system -- some go even farther. Find out how far some are willing to go on the next page.


Mapping and Tools

This digital map shows the risk of flooding in certain areas of the United Kingdom. Norwich Union produces flood maps using digital technology, enabling it to assess the risk of individual houses near flood plains.
Norwich Union/Getty Images

Water is everywhere, but it's not always reachable. Hydrologists use models, maps and figures to help them distinguish where the water is -- underground aquifers and remote glaciers are no match for a well-informed hydrologist.

Spatial hydrologists use GIS (global interface systems) and GPS (global positioning systems) to help them map streamflow, pollution and various environmental problems. These can look like anything from a line graph to a full-scale 3-dimensional model of a river or underground water system. These high-tech systems make use of coordinates, measurements and data to generate detailed pictures of the area.


Hydrologists will go pretty far to get information on the movement of water. Remote sensing satellites orbit the Earth, taking pictures of the oceans and what's happening in them. The satellites produce topographical maps of the ocean, which help hydrologists study the oceans and their movements as a whole. There's even a coral-reef remote sensing tool. Reefs are a great ecological resource, helping to produce oxygen and keep an environmental balance for the entire planet. Knowing where these reefs are and how to study them is very important.

This image from a NASA satellite shows the coastline of the Bahamas, Florida and Cuba. The banks are shallow coral reefs that reflect light through the ocean, giving it a bright blue color.
Stocktrek Images/Getty Images

Parametric hydrology is a more theoretical branch of hydrology. It uses computer modeling to come up with relationships between events in the hydrological cycle. It then uses these relationships to generate possible new events.


Overall, hydrologists are a large and diverse group. They are dedicated to the preservation of the Earth's water by any means necessary. Hydrologists help maintain the livelihood of mankind by helping to regulate floods, pollution and sewage. Not only that, they get to make some really fascinating models. Hydrology is a branch of science that will always be in demand, especially as our population continues to grow and the demand for fresh water increases.

To learn more about the water that's all around us, follow the links on the next page. They'll provide you with more information.


Lots More Information

Related HowStuffWorks Articles

More Great Links

  • A­ccess Science. "Parametric Hydrology." "Dendrohydrology." (July 6, 2008)
  • Australian Government - Bureau of Meteorology. "Hydrometeorology." March 22, 2000. (July 6, 2008)
  • Baird Software. "Modeling, GIS, Visualization." 2004. (July 10, 2008)
  • Coastal and Environmental Hydraulics Laboratory, "Water Resources." (July 3, 2008)
  • Culvert BC. "Estimating Design Floods for Forest Roads in British Columbia." (July 6, 2008)
  • Feldman, Gene. Kuring, Norman. Franz, Bryan. Stumpf, Richard P. Robinson, Julie A. Green, Edmund P. Noordeloos, Marco. "Remote Sensing of Coral Reefs." NASA. December 4, 2003. (July 8, 2008)
  • Feldman, Gene. "Monitoring the Earth from Space with SeaWiFS: Why study the oceans from space?" NASA. (July 9, 2008)
  • Intro to Hydrology: Spatial Hydrology lecture notes. (July 8, 2008)
  • Keinath, Thomas M. "Water." World Book Encyclopedia 2007 edition. Vol. 21. Chicago: World Book, Inc, 2007.
  • Likens, Gene E. "Water Pollution." World Book Encyclopedia 2007 edition. Vol. 21. Chicago: World Book, Inc, 2007.
  • Minard, Anne. "Moon Water Found, raises question about origin theory." National Geographic. July 9, 2008. (July 10, 2008)
  • Purdue University. "Hydrogeology." May 8, 1998. (July 6, 2008)
  • The American Heritage Science Dictionary. "Vadose Zone." (July 3, 2008)
  • The University of Arizona. "Vadose Zone Hydrology." (July 6, 2008)
  • Young, Samantha. "Mount Shasta glaciers growing, despite warming." Forecast Earth. July 9, 2008. (July 9, 2008)