For three and a half billion years, the Earth’s water has been moving from streams to lakes to oceans, flowing underground, sitting high up on mountain glaciers, freezing and melting on the edges of the polar ice caps and forming clouds in the atmosphere. The drop of water that falls on your head in a rain storm could have traveled from the Pacific Ocean, a mountain top wetland in Colorado, or from a melting glacier in Greenland. This never-ending water recycling is called the hydrologic cycle (sometimes called the water cycle) and it is the driving force behind our weather. (Fig 2.1)

Solar-powered water pump

The hydrologic cycle works like a huge water pump powered by solar energy and gravity. It is a global system, and every molecule of water on Earth travels through it. Because it is a cycle, it has no beginning or end. The sun warms water on the Earth’s surface and changes it into invisible water vapor. This process is called evaporation. Every time water evaporates, it leaves behind whatever salts, pollutants or other impurities were dissolved in it and becomes pure again. But as soon as the purified water is exposed to air or Earth it begins to pick-up pollutants again. This is why even rain can be polluted with contaminants found in Earth’s atmosphere. Living things, which make up the biosphere, also return water to the atmosphere. Every time we exhale, we release water vapor. Photosynthesis causes plants to release water vapor into the air in the process of transpiration. A one-acre cornfield (about the size of a football field) can give off as much as 4,000 gallons of water every day through the corn plants’ leaves, or as much water as you will use to take about 600 showers.

Rising air takes the water vapor up into the atmosphere where it cools. Cooling water vapor condenses into tiny suspended water droplets as fog, mist, or ice crystals which we often see as clouds. In fact a cloud is really just a huge group of tiny water drops held up by rising air. (Fig. 2.2) Raindrops and snowflakes condense around microscopic dust particles also suspended in the atmosphere. Water can pick up other contaminants from the air, too, such as smog (forming acid rain) or mercury vapor from trash incinerators and coal-burning power plants. Water returns to Earth as precipitation, either liquid (rain) or solid (snow, sleet or hail). This happens when the rising air can no longer hold up all the droplets of water. Water vapor can also condense on ground-level surfaces as dew or frost. About 85% of the world’s precipitation falls into the oceans. The rest falls on land.

Talk about the weather

The amount of water that falls in a local area changes with each season. Weather also increases or decreases the amount of available water. Seasonal weather patterns move water around the world and from the atmosphere back to the Earth’s surface. Average weather conditions over time is what we call climate. How much water a certain region will have in a given part of the hydrologic cycle depends on factors such as:

• • Amount of rainfall
  • Effect of temperature on evaporation
  • Amount of water that plants use during the growing season

Texas’ weather

Texas is so large it is affected by air drawn in from the Pacific Ocean as well as the humid topical air flowing in from the Gulf of Mexico. The state’s vast size also means climate varies greatly from north to south, and east to west. (Fig. 2.3) Rainfall increases going from west to east. The wet season does not happen at the same time everywhere. North, Central and East Texas receive the most rain in late spring, while the Panhandle and West Texas receive the most rain during the hottest months of the year. Coastal areas of Texas receive the most rain in late summer and early fall.

Most of Texas has a modified marine climate, which means it is influenced by the onshore flow of tropical air from the Gulf of Mexico. From October to June this tropical air which is full of moisture collides with colder dry air from the north, resulting in thunderstorms over Central Texas. Along the coast hurricanes can bring huge rain storms with large amounts of rain falling over short periods. This may cause flooding, water surges in bays and estuaries and high winds. Peak time for hurricanes is late summer and early fall. Texas is known for its extreme flooding. Flooding is natural, but can cause major damage to buildings, homes, roads, as well as damage to aquatic resources and habitat for wildlife.

While droughts do not happen as fast as floods, they may also cause severe economic damage and harm aquatic resources and wildlife habitat. (Fig. 2.4) Damage to property and habitat can be reduced by preserving wetlands and allowing natural sediment transport and adequate freshwater inflows into our estuaries and bays.

Temperatures also vary throughout the state with coldest temperatures in the north and warmest in the southwest (an area known as the Trans-Pecos). Water also plays a role in temperature. In dry areas the difference between the highest and lowest temperatures is larger than in more humid areas. Water in the atmosphere, called humidity, buffers large changes in daily temperature. This is why even if Houston and El Paso have the same daytime temperature, Houston will typically be warmer than El Paso at night because the higher humidity in Houston will keep the air warm.

Weather forecasters sometimes refer to the effects of El Niño and La Niña. These are changes in ocean-atmosphere conditions that can happen every few years and affect weather around the globe. El Niño occurs when the area of warmer waters in the tropical Pacific Ocean expands, causing warmer, wetter, and more severe weather conditions in Texas. La Niño occurs when the area of warmer waters shrinks in the Pacific Ocean, causing cooler, drier conditions in Texas.

The National Oceanic and Atmospheric Administration (NOAA) notes that El Niño is occurring more frequently. Is there a connection between this frequency and global warming? Will we have more severe weather because of this? According to NOAA, these are the important research questions facing scientists today. Many scientists believe we are now encountering a period of rapid climate change. Overall, Texas is known for extremes in weather, from droughts to floods, to freezing temperatures and burning heat. All are present and relatively common in our state.

Surface water runs off

On land, plants catch much of the rainfall before it reaches the Earth’s surface. In a wooded area, for example, rain slowly drips off leaves of bushes and trees and trickles down branches. Roots and the leaf-covered forest floor act like a sponge, soaking up water and slowly releasing water into the ground or waterways. About 66% of the precipitation that fall on the continental United States each year returns to the atmosphere right away. Half of the rest runs off the surface of the land. This water is called surface water. It collects in streams and flows to the ocean. People and some wildlife species, such as beaver, may build dams to slow and hold water for use later. (Fig. 2.5)

When precipitation falls as snow, it can build up as snowpack, ice caps, and glaciers. Ice caps and glaciers can store frozen water for thousands of years. Snow in warmer places melts when spring arrives. The melted water that does not soak into the ground flows overland and is called snowmelt.

If rain is hitting the ground faster than it can soak in, it becomes runoff. The slope of the ground also affects runoff. On steep slopes, water moves quickly and very little of it soaks in the ground. Hard surfaces reduce the amount of water that soaks into the soil even further.

In urban areas, there is less porous ground for the rain to soak into. Paved roads, rooftops, and parking lots block water from soaking in, so all of it becomes runoff. Heavy rains run off streets, sidewalks, and other paved surfaces up to 10 times faster than on unpaved land. Some people use rainwater harvest barrels or tanks to collect water as it falls from building roofs. The water can then be used to water plants during dry periods/when rain is scarce. This is a good way to conserve water.

The faster water flows, the more power it has to wash away soil or to cause flooding. (Fig. 2.6) Storm water that runs off paved roads, rooftops and parking lots flows into ditches and storm drains. This water then drains directly into streams, lakes and wetlands without any filtration or treatment. Any excess fertilizer, pesticides, mud, motor oil and antifreeze, trash, even lawn clippings and pet waste on the pavement or roads wash into waterways during heavy rains. This type of “runoff” is called non-point source pollution and is the leading cause of pollution of our aquatic resources in the United States.

Let it soak in—groundwater

Only about 3% of the rain soaks into the ground. When water soaks into the ground it fills the empty spaces between soil particles. The water may remain as soil moisture, evaporate back into the atmosphere, be taken up into the roots of plants or trickle slowly through the soil. The solid part of the Earth is called the geosphere.

Below the Earth’s surface, layers of spongy soil, sand, and rock act as filters to help clean the water. If the water is badly polluted or contains certain contaminants, the soil can’t remove all of the pollutants. In some cases, water moving through the geosphere can even pick up pollutants already present in the soil. Eventually the water reaches a layer where all the spaces in the soil or rock are already filled up. This area is called the saturated zone, and the water it holds—over half of the freshwater on Earth—is called groundwater. (Fig. 2.7) The boundary between the spongy layer and the saturated zone is known as the water table. The water table rises or falls as the amount of groundwater in the saturated zone increases or decreases.

Areas of underground rock that hold water in pores or crevices are called aquifers. To use the water in aquifers, people dig wells to bring it to the surface. Unfortunately, digging a well and pumping out too much groundwater can lower the water table and can cause lakes, streams and wetlands to dry up. Sometimes the Earth’s surface can sink or even collapse when we pump out too much groundwater. In most places, groundwater moves so slowly that aquifers can take thousands of years to fill back up. In the Texas Hill Country, however, the rate of flow is much faster and in some places can be measured in miles per hour. Places where water soaks into aquifers are called recharge areas. Some streams lose water to the soil or rock around them, helping recharge aquifers. Streams also receive groundwater with the most visible examples called springs. Lands near springs have long been inhabited by people due to the ready availability of a constant supply of freshwater. (Fig. 2.8)

Clean water for us

Texans depend on both surface water and groundwater sources for drinking and other uses. In general, people in the western portion of the state use groundwater while in wetter parts of the state large reservoirs hold surface water that can be added to groundwater sources for use by people. Some well water is safe to drink right out of the ground. In other cases, it must be treated first. For community water supplies, water from wells or lakes is piped to drinking water treatment plants. At the plants, workers may filter the water or let it settle to remove suspended material. They also may add chemicals to the water to kill bacteria and other organisms. Pipes buried in the ground carry the water to homes and businesses.

Clean water for the environment

A different set of underground pipes carry used water (sewage) to wastewater treatment facilities. Sewage from homes and business that are not located in towns or cities usually goes to underground septic systems that store wastewater until it can soak into the ground. The solid material is disinfected and often recycled into fertilizer. Wastewater treatment systems use bacteria to break down much of the organic waste in the water, and make the wastewater safer to return to the environment. Water from sewage treatment facilities is piped back into surface waters such as rivers, lakes and wetlands after it is treated. Wastewater treatment is important to the reduction of water-borne diseases that can sicken humans and even cause death.

Learn About Aquatic Science Careers

Click on photos below for enlarged view.

Fig. 2.1 — Water is constantly recycled, moving from ocean, to mountain top, into the ground, to the atmosphere, to the ocean, and back again, cleansing it of pollutants as it changes from liquid to vapor. Credit: U.S. Geological Survey

Fig. 2.2 — Clouds form out of condensed water vapor. Tiny water droplets and ice crystals from clouds fall to Earth filling our lakes and rivers with clean water. Credit: Rudolph Rosen

Fig. 2.3 — The eastern part of Texas is among the wettest places in America. The western part of the state is among the driest and is part of the Chihuahuan Desert. The figure shows average annual rainfall in inches in Texas. Credit: Texas Parks and Wildlife Department

Fig. 2.4 — Texas is at the same latitude as the Sahara Desert. As in the Sahara, large high pressure cells in the atmosphere may set over Texas for weeks or months blocking storms and moisture from reaching the state from any direction, increasing drought conditions. Credit: Texas Parks and Wildlife Department

Fig. 2.5 — Beaver and some other wildlife species have learned ways to save water for their needs, much as people build dams and reservoirs to store water for drinking, agriculture, power production, and recreation. Credit: Texas Parks and Wildlife Department

Fig. 2.6 — Flooding occurs when there is more runoff than can be absorbed into the ground or fit into streams, rivers, lakes, and reservoirs. The water has nowhere to go except spill out into adjacent areas. Credit: Corpus Christi Caller Times

Fig. 2.7 — Groundwater is recycled slowly and can be depleted if more is pumped from the ground by people than soaks into the ground each year. Many people depend on groundwater for drinking water and irrigating lawns and crops. Credit: Missouri Department of Conservation

Fig. 2-8 — The San Marcos Springs, on the campus of Texas State University in San Marcos, is the second largest spring in America west of the Mississippi River. The Springs are home to eight endangered aquatic species and are thought to be the longest continually inhabited site by humans in North America. Visitors can see the springs bubbling from an underground aquifer and many species of aquatic life by taking glass bottom boat rides. Credit: Rudolph Rosen