Structure of the Hydrosphere
Water is Unique
Water is a unique resource for all living creatures. It is essential to life on Earth. And is one of the most common substances you will learn about. Just think about all the things we’ve already learned about water. We’ve seen how it changes physical forms, and about its chemical structure.
The physical properties of water molecules give it several unique characteristics that impact life. First, water is a solvent. This means it can dissolve other substances. Water dissolves nutrients from the rocks and soils. It then transfers these nutrients to the organisms on land and in water. It also dissolves gases in the atmosphere. As rain falls through the air toward the ground, raindrops dissolve gases. These gasses then reach the surface where they are available to organisms. In these ways, water makes nutrients and gasses more available to organisms on Earth. The figure below shows a single water molecule.
Notice it has a positive and a negative end. This unique structure allows water to stick together, and to other changed molecules. This property helps water dissolve many substances and move those substances inside living cells.
Water has a high specific heat. Specific heat refers to the amount of energy it takes to change 1 gram of water 1 degree Celsius. This means that it takes a lot of energy to change its temperature. This resistance to abrupt changes in temperature protects aquatic organisms from harsh temperature extremes. It also means that water is often at a different temperature than the surrounding air. Because of this temperature difference, water can cool or warm surrounding air masses. When water is heated, it changes from a liquid to a gas. This process is known as evaporation. Once evaporation begins, heat is transferred from the water to the air as water vapor. This, in turn, lowers the temperature of the liquid water left behind. Salt can be added to water to reduce its freezing point. If you add salt to water, it lowers the freezing point of the water. So to freeze salt water, it must be colder than 32oF or 0oC. This is why salt is added to icy roads to help melt the ice.
Bodies of Water
All water environments found on Earth make up the hydrosphere. The hydrosphere consists of all the oceans and water found above and below the ground. It also includes any water in the atmosphere. The water on Earth is divided into categories based on its salinity, or salt content. Saltwater areas have a high salinity, about 3 grams of salt per kilogram of water on average. Areas with fresh water have low salinity. These areas still have salt in the water, but the concentration is very low, less than 1 gram of salt per kilogram of water.
Salt Water
The vast majority of water on Earth is salt water and it can be found – you guessed it – in the world’s oceans. The ocean is a vast, continuous body of water that stretches across Earth. The salt in the oceans has been dissolved in the water a very long time. Oceans cover about most of the Earth’s surface. When pictures of Earth are taken from space, the oceans are the reason why most of the plant appears to be blue. The ocean is divided into 5 individual oceans: Atlantic, Pacific, Indian, Arctic, and Southern. There are no distinct physical boundaries between these bodies of water. They are actually all one ocean. Some classify the Southern and Arctic Oceans as regions of the Pacific and Atlantic Oceans, but most oceanographers agree that their currents mark them as distinct regions. These individual oceans can be further divided into small ocean basins.
Fresh Water
About 3% of the Earth’s surface is fresh water. Bodies of water such as glaciers, lakes and rivers are fresh water. Most of the fresh water exists in the form of glaciers and ice caps. Glaciers are slow-moving rivers of frozen ice. They are the largest source of fresh water on the plant. The largest glaciers in the world are Antarctica and Greenland. Ice caps are smaller masses of ice over land. Glaciers and ice caps are not really available for use by humans. They are frozen!
Other sources of fresh water are more accessible. A river is a natural waterway that comes from another source, such as a frozen mountaintop or a lake. A lake is a body of fresh water surrounded on all sides by land. Rivers and lakes make up the main sources of surface water for use by humans. The drainage area of a river is called a river basin. It is water that falls on the surface of the land and flows to a central river. River basins are divided based on the topography of the land where hills and valleys exist. The topography controls the way the water flows on land. All the water in a basin flows to the same place. The river carries the water out to an estuary and then on to the ocean.
A river empties into an ocean at an estuary. In an estuary fresh water and salt water mix. Here, fresh water from the land flows and joins with the salt water from the ocean. This makes the water composition of an estuary blend of salt and freshwater called brackish water. The Outer Banks of North Carolina are the third largest estuary system in the world. The amount of salt in the water varies. Salt water is denser than fresh water. The difference in density helps circulate water in an estuary.
Large river basins are made up of many smaller watersheds. A watershed is a smaller area of land that drains into a small stream, lake or wetland. When the surface is saturated (full of water) and water cannot penetrate the ground, water flows into a watershed. It then drains into the ocean. Water that seeps into the soil travels as groundwater. Groundwater moves closely beneath the surface. It occurs due to gravity pulling water down towards the center of the Earth. There must be enough space between the rocks and soil particles for groundwater to be stored. The types of rocks determine how much water is stored and how fast it travels through the rocks. Groundwater gets stored in aquifers. Water from aquifers eventually seeps into riverbeds. In this way groundwater helps to replenish surface water by keeping rivers and lakes full.
Water is moved among the Earth’s surface, the atmosphere and the oceans. Condensation, evaporation, freezing and precipitation change the distribution of water on Earth. When the climate is cooler, more water is stored in its frozen form. It can become “trapped” in glaciers or polar ice caps. When this happens, ocean levels drop, as more water is stores on land as snow and ice. When the climate warms, frozen ice on land melts and flows into the oceans causing ocean levels to rise.
But, the total amount of water on Earth does not change. This means the water you are drinking has been around since the beginning of time! Since most of the water on Earth is in the oceans, there is a lot more salt water than fresh water. There must be a natural way to recycle the small amount of fresh water that exists or for that matter, change salt water into fresh water. Luckily there is, it is called the water cycle.
The Water Cycle
The water cycle circulates fresh water between the atmosphere and the Earth. The figure below shows a simple version of this cycling process. Fresh water is mostly in the form of glaciers, leaving a very small amount of fresh water available for land organisms. The water cycle ensures that the supply is replenished. How does it work?
Precipitation in the form of rain, ice or snow falls to the Earth. It collects in lakes, rivers and oceans through the precipitation itself or through runoff. The Sun provides energy in the form of heat. This drives evaporation that sends water into the atmosphere from bodies of water. The water vapor cools and condenses to form clouds. The clouds become saturated with water. The water eventually falls towards the Earth as precipitation. This starts the process all over again. Without this cycle of precipitation, runoff, evaporation and condensation, a freshwater supply would not be available.
Local Water Sources
Most of our drinking water comes from surface and groundwater sources. To access these water sources, large water supply systems were built to make water more readily available. Before these systems were built, people generally got their water by drawing it from wells or carrying buckets of water from nearby rivers and lakes. Entire communities were built based on where natural supplies of water existed. Nowadays, water comes to us. All you need to do to get water is turn a tap an out comes water. Large cities now thrive in the middle of a continent or desert far from any large body of water. This was not always possible. Water is transported great distances. Water that is delivered to a home or business is from a public-supply water system. The public-supply water system is your county or city water department. These systems use pipes and pumps to withdraw water from surface sources, like rivers, lakes and reservoirs. They also use groundwater. Most people today get their water supplied from a public-supply system. Water that people get themselves is self-supplied water. Self-supplied water generally comes from groundwater that is accessed through wells. A well is a structure dug into the ground to extract water from underground aquifers. Since we cannot see groundwater, wells can tell us a lot about the quality and amount of water in an aquifer. North Carolina has a good natural supply of water.
The state gets an average of 48 inches of rain per year. This rain help replenish aquifers. Most areas in the state have naturally potable (drinkable) groundwater. In fact, over half of the people in North Carolina get their drinking water from ground water, whether it is public or self-supplies. North Carolina also has many rivers and lakes. There are 17 major river basins. Of these, 5 are part of the Mississippi River Basin and drain into the Gulf of Mexico. All other river basins flow to the Atlantic Ocean.