Notes
Chapter 7: Water
A water drop. Photo by José Manuel Suárez.
“Whiskey is for drinking. Water is for fighting” – Mark Twain
“No water, no life. No blue, no green.” – Sylvia Earle
Learning Objectives
By the end of this chapter, students will be able to:
Draw multiple interacting water molecules and identify the atoms and bonds.
Explain how the molecular structure of the water contributes to the unique properties of water.
Demonstrate an understanding of how much water is available on Earth and how it is distributed.
Name the major water reservoirs in the water cycle and explain their roles.
Explain how human modifications of natural water systems can be both beneficial and destructive.
Explain the components of groundwater.
Chapter Outline
7.2.2 The Physical State of Water on Earth
7.2.3 High Specific Heat Capacity
7.2.6 Water’s Cohesion and Adhesion
7.3 Global Water Distribution and Use
7.5 Components of the Hydrologic Cycle
7.5.1 Atmosphere and Precipitation
7.5.3 Lakes, Reservoirs, and Ponds
7.1 Introduction
Why do scientists spend a lot of time looking for water on other planets? Why is water so important that we have to keep looking for it? Water is one of the most abundant substances on Earth and also one of the molecules critical to life. Approximately 70 percent of the human body is made up of water. Without it, life as we know it simply would not exist as humbly put by Sylvia Earle’s quote above. The quotation above, which has been attributed to Mark Twain, suggests that water is extremely important and worth more than whiskey. In fact, some scholars claim that water is worth more than gold and silver. The value and importance of water is reflected in the frequent water related conflicts and disputes worldwide. Fortunately, most conflicts end up in the courts instead of the battlefields. This chapter is devoted to the properties, availability, distribution and utilization of this precious resource that sustains our planet and all known living things.
The importance of water to life is demonstrated by the numerous descriptions associated with water such as “the essence of life”, “blue gold” and “more precious than oil”. What makes water so unique and invaluable to life is its special properties that are attributed to its molecular structure. These special properties include a high specific heat capacity, high heat of vaporization, ability to dissolve numerous polar molecules, its cohesive and adhesive properties, and the ability to dissociate into ions (that leads to the generation of pH). Understanding these characteristics helps us comprehend and appreciate its importance in maintaining life on Earth. Before we discuss these properties, let’s review the molecular structure of water, which gives rise to these special properties.
A single water molecule is composed of two atoms of hydrogen attached to one atom of oxygen (H2O) by covalent bonds. Covalent means that atoms share electrons and must therefore stay together. This is not the same as ionic bonding in which atoms are attracted to one another due to opposite charges that result when each atom completely gives up electrons to or gains electrons from another. When the sharing of electrons among the atoms is equal, the bond is known as non-polar covalent bond. In the case of water, the oxygen atoms is much larger than the each of the hydrogen atoms so the electrons are not shared equally between oxygen and hydrogen resulting in polar covalent bonds. These polar covalent bonds (Figure 7.1), along with the molecular shape, cause the water molecule to have a slightly positive charge on the hydrogen end and a slightly negative charge on the oxygen side. Water’s charges are generated because oxygen is more electronegative than hydrogen, making it more likely that a shared electron would be found closer to the oxygen nucleus than the hydrogen nucleus, thus generating the partial negative charge near the oxygen and a partial positive charge near the hydrogen. This gives water molecules their properties of attraction.
Figure 7.1: Polarity of the water molecule due to the uneven distribution of electrons in its covalent bond. (From OpenStax Concepts of Biology text)
7.2 Properties of Water
7.2.1 Hydrogen Bonds
Due to water’s polarity, each water molecule attracts other water molecules as oppositely charged ends of the molecules attract each other. When this happens, a weak interaction occurs between the positive hydrogen end from one molecule and the negative oxygen end of another molecule. This interaction is called a hydrogen bond (Figure 7.2). As water molecules make hydrogen bonds with each other, water takes on some unique physical and chemical characteristics not found in other liquids. This hydrogen bonding contributes to the other water properties discussed below. It is important to note here that even if we are only focusing on water in this textbook, hydrogen bonding also occurs in other substances that have polar molecules.
Figure 7.2: Attraction between multiple water molecules when the slightly positive hydrogen in one molecule is attracted to the slightly negative oxygen in another water molecule forming a hydrogen bond. Image from OpenStax College - Anatomy & Physiology, Connexions Web site, June 19 2013.
7.2.2 The Physical State of Water on Earth
Water on Earth can naturally exist in three states of matter, solid, liquid or gas depending on the prevailing temperature and pressure conditions. Majority of water on Earth’s surface exists in liquid form which is one of the reasons why Earth is capable of supporting life. When heat is added to liquid water (increasing temperature), the kinetic energy of the molecules goes up and helps to break up the hydrogen bonds. As more heat is added to boiling water, the higher kinetic energy of the water molecules causes the hydrogen bonds to break completely and allow individual molecules to escape into the air as water vapor (gas). On the other hand, when the temperature of liquid water is reduced and water freezes, water molecules form a crystalline structure maintained by hydrogen bonding (since there isn’t enough energy to break the hydrogen bonds). The crystalline structure, ice, has a more open structure than the liquid form of water. The open structure of ice (Figure 7.3) makes ice less dense than liquid water. Water therefore is denser as a liquid and less dense as a solid, a phenomenon not seen in the solidification of other liquids.
The lower density of ice, illustrated in Figure 7.3, causes it to float on the surface of liquid water. This phenomenon is exemplified by icebergs in the ocean and ice cubes in a glass of ice water. When lakes and ponds experience freezing temperatures the top water molecules will cool first and form ice. Since ice is less dense than liquid water, the ice will stay on the surface of the pond creating an insulating barrier that keeps the water below in liquid form. This in turn protects animals and plants (that live in the water) from freezing over. Without this layer of insulating ice the entire water column would freeze killing most plant and animal life. The ice crystals that form upon freezing would rupture the delicate membranes essential for the function of living cells, irreversibly damaging them.
Figure 7.3: Hydrogen bonding makes ice less dense than liquid water. The lattice structure water is more condensed (left structure) than that of ice (right structure). The lattice structure of ice makes it less dense than freely flowing molecules of liquid water, enabling ice to float on liquid water. (Image credit: Lynn Yarris)
7.2.3 High Specific Heat Capacity
Water has the highest specific heat capacity of any substance that naturally exist as liquid at room temperature and pressure. Water’s high heat capacity is a property caused by hydrogen bonding among the molecules. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie. Other substances have lower heat capacities compared to water, for example: oil = 0.4 calorie, alcohol = 0.57 calorie, dry soil/sand = 0.19 calorie, wood = 0.14 calorie, iron = 0.11 calorie. It takes water a long time to heat up and a long time to cool down. For example, because the specific heat capacity of water is about five times more than that of sand, dry sand at the beach heats up faster than the water but is also cools down faster than the water. Due to its high heat capacity, warm blooded animals use water to disperse heat more evenly and maintain a constant temperature in their bodies: it acts in a similar manner to a car’s cooling system, transporting heat from warm places to cool places, causing the body to maintain a more even temperature.
7.2.4 Heat of Vaporization
Water also has a high heat of vaporization, the amount of energy required to change one gram of a liquid substance to a gas. A considerable amount of heat energy (586 calories) is required to accomplish this change in water. This process occurs on the surface of water. As liquid water heats up, hydrogen bonding makes it difficult to separate individual liquid water molecules from each other, which is required for water to enter the gas phase (steam). Thus, water acts as a heat sink and requires much more heat to boil than liquids such as ethanol, whose hydrogen bonds are weaker. Eventually, as water reaches its boiling point of 100° Celsius (212° Fahrenheit), there is sufficient heat to break the hydrogen bonds, and the kinetic energy between the water molecules allows them to escape from the liquid as a gas. Even when below its boiling point, water’s individual molecules acquire enough energy from other water molecules such that a few surface water molecules can escape and vaporize: this process is known as evaporation.
Since hydrogen bonds need to be broken for water to evaporate, a substantial amount of energy is used in the evaporation process. As the water evaporates, energy is transferred from one source and taken up by the process, cooling the environment where the evaporation is taking place. In many living organisms, including humans, the evaporation of sweat (which is 90 percent water) allows the organism to cool when significant heat energy is transferred from the organism to the water in the sweat, helping to maintain a constant body temperature, a process known as homeostasis.
7.2.5 Universal Solvent
Since water is a polar molecule with slightly positive and negative charges, ions and polar molecules can readily dissolve in it. Water is, therefore, referred to as a universal solvent, because it is capable of dissolving more substances (polar substances) than any other liquid.
The charges associated with these substances will form bonds with water molecules (opposite charges attracting each other) causing the substance to be pulled apart and dissolve in the water. This is very important as it enables water to dissolve various chemicals and distribute them within living organisms, including taking toxic substances out of living things, and in the environment.
7.2.6 Water’s Cohesion and Adhesion
Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass (Figure 7.4a). This water can stay above the rim of the glass because of the property of water known as cohesion. In cohesion, water molecules are attracted to other water molecules (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air) interface allowing water molecules to protrude above the top surface of the glass. Cohesion allows for the development of surface tension, the capacity of a substance to resist rupture when placed under tension or stress. This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity (Figure 7.4b).
Figure 7.4. (left) Water in a glass forms a dome shape above the glass due to cohesive forces of attraction among water molecules (photo Credit: Sam Mutiti. (center) Beading up of water and (right) formation of droplets at the tip of pine needles due strong cohesive forces between water molecules (photo credit: J Schmidt; National Park Service).
When a steel needle or paper clip is placed carefully on the surface of liquid water it does not sink to the bottom even though steel is denser (heavier) than the water. Cohesion and surface tension resulting from the hydrogen bonds between water molecules create a surface that supports the item on the top. It is even possible for an insect to “float” on water if it sits gently without breaking the surface tension, as shown in Figure 7.5.
Figure 7.5 The weights of the needle (left image) and water strider (right image) are pushing the surface downward; at the same time, the surface tension is pushing it up, suspending them on the surface of the water and keeping them from sinking. (Credit: Cory Zanker (left) and Tim Vickers (right)
Another important property of water is adhesion, or the attraction between water molecules and molecules of other substances or objects. This attraction is sometimes stronger than water’s cohesive forces, especially when water is exposed to charged surfaces such as glass walls of capillary tubes (narrow glass tube). Adhesion is observed when water “climbs” up the tube placed in a glass of water: notice that the water appears to be higher on the sides of the tube than in the middle (Figure 7.6). This is because the water molecules are attracted to the charged glass walls of the tube more than they are to each other and, therefore, adhere to the tube. This type of adhesion is called capillary action. This process is also involved in the movement of water and nutrients from the root systems in the soil to other parts of plants above the ground, as well as the movement of blood through the veins in the human body.
Figure 7.6: Capillary attraction in a glass tube is caused by the adhesive forces exerted by the internal surface of the glass exceeding the cohesive forces between the water molecules themselves. (Credit:
public domain image from Wikimedia Commons, image by Pearson Scott Foresman)
7.3 Global Water Distribution and Use
About 71% of the Earth’s surface is covered by water, most of which is in oceans and unavailable for human consumption due to its high salinity (Figure 7.7). Approximately 97% of all water is saline and 2% is fresh water held in ice caps and glaciers. Therefore, at least 99% of all water is generally unsuitable for human use because of salinity (ocean water) and location (ice caps and glaciers), leaving less than 1% of total water as fresh water that is available for consumption. Of this available fresh water, approximately 97% is groundwater stored deep below the surface of the Earth and only 1.4% is surface water in rivers and lakes.
Figure 7.7: Graphical representation of available water: http://water.usgs.gov/edu/earthwherewater.html
There are three main sectors that use water – industrial, agricultural, and domestic. When water is removed from its source such as river or lake and returned to this source after use, this is referred to as non-consumptive use. An example is when water is used to in industrial cooling, it may be temporarily placed in cooling ponds and later returned back to the river or lake that it came from. Consumptive use is when water is taken out from a source and consumed by plants and animals or used in industrial processes. The water enters animal tissue or becomes part of industrial products or evaporates during use and is not returned to its source. Of the three sectors, the agricultural sector is by far the largest user of water that is never returned to its sources, consumptive use.
The largest percentage of water withdrawn in the US goes to thermoelectric cooling (Figure
7.8). In some countries, such as Egypt, irrigation accounts for over 70% of water withdrawn. Irrigation is water that is applied by a water system to sustain plant growth. Irrigation also includes water that is used for frost protection, application of chemicals, weed control, field preparation, crop cooling, harvesting, dust suppression, and leaching salts from the root zone.
Figure 7.8: Estimated 2015 water withdrawals in the US. Irrigation and thermoelectric power usages account for most water withdrawals. More water use terminology can be found here.
Water is being used at very high rates throughout worldwide due to human population growth and industrialization. As more countries become affluent (increase in industrialization and standard of living) they consume more water than they did when they were less industrialized.
To find out more about global water use checkout the world water use meter.
7.4 The Hydrologic Cycle
The major water reservoirs on Earth are oceans, ice caps and glaciers, groundwater, rivers, and lakes. Water spends different amounts of time in the various reservoirs. The main factors that control the amount of time water stays in a reservoir are the amount of water in the reservoir and how fast water moves in and out. The hydrologic cycle (water cycle) represents a continuous global cycling of water from one reservoir to another (Figure 7.9). This process is powered by two major forces - heat energy from the Sun that causes liquid water to change to water vapor and the gravitational pull of the Earth that brings water to the surface.
Figure 7.9: The water cycle at the global scale showing water moving through all the major reservoirs, including the ocean reservoir (source).
To gain a deeper appreciation of the water cycle, let us follow a water molecule through the water cycle. Starting in the ocean (an arbitrary starting point) the water molecule can become part of the water that is converted into vapor and enter the atmosphere. Evaporation is the process by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water takes from the liquid state back into the water cycle as atmospheric water vapor. Nearly 90% of moisture in the atmosphere comes from evaporation, with the remaining 10% coming from transpiration. Transpiration is the process by which moisture is carried through plants from roots to small pores (stoma) on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is a combination of water transpired from plants and that evaporated from the soil. The vapor rises into the air where cooler temperatures cause it to condense into clouds. Condensation is the process by which water vapor is converted from gaseous state back into liquid state. Clouds might eventually grow bigger and moist enough to release the water molecule in the form of precipitation. Precipitation is water falling from the clouds in the atmosphere in form of ice (snow, sleet, hail) or liquid (e.g. rain, drizzle). Precipitation that falls as snow can accumulate as ice caps and glaciers.
Did you know that the largest glacier on Earth is the Severny Island ice cap in the Russian Arctic?
Precipitation that falls as liquid usually ends up as surface flow and stream flow. Surface runoff is the portion of precipitation that travels over the soil surface to the nearest stream channel. Stream flow is the movement of water in a natural channel, such as a river. Most precipitation falls directly onto the ocean and returns the water molecule back to restart the journey. This is also true for surface runoff, most of the water eventually returns to the ocean via stream flow. This also returns the water molecule back the ocean to start the journey again.
A portion of the water that falls as precipitation can enter lakes where it can evaporate back into the atmosphere, condense into clouds, and fall back as precipitation again. Water in the lake can also be taken up by aquatic plants and transpired back into the atmosphere. Some of the water that falls as precipitation can infiltrate into the ground and become part of groundwater. Infiltration is the process by which water enters the subsurface by gravitation pull. Some of the water infiltrates into the ground and replenishes aquifers (saturated subsurface material), which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge, while some groundwater finds openings in the land surface and emerges as freshwater springs. Water that stays in the soil closer to the surface can be absorbed through plant roots and transpire from the leaves. Over time, though, all this water keeps moving and most of it ends up in the ocean.
7.5 Components of the Hydrologic Cycle
7.5.1 Atmosphere and Precipitation
Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Rain forms as water vapor in a rising air mass condenses in the cloud and form water drops. Condensation is the process in which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth's surface within the water cycle. Condensation is the opposite of evaporation.
Most precipitation falls in the form rain. There are three main kinds of rain; frontal, convective and orographic. Frontal rainfall is precipitation formed when two air fronts of different temperatures and moisture content converge. Convective rainfall is formed when intense localized heating causes hot moist air to raise and condense and form rain clouds. Intense rain would then fall as the clouds get supersaturated. Orographic rainfall is rain that form over mountains. When a moist air mass encounters a mountain, it rises and cools. As it cools water vapor condenses to form rain cloud that produces rain of the windward side of the mountain. Most of the rain ends up as surface water runoff. Surface water is a major component of the hydrological cycle and one that we interact with very regularly. It includes lakes, wetlands, stormwater runoff (overland flow), ponds, potholes, rivers and streams, and the ocean.
7.5.2 Streams and Rivers
A river forms from water moving from higher to lower altitude (elevation), under the force of gravity. When rain falls on the land, it can seep into the ground, become runoff (water running on the surface), or evaporate. Water that moves as runoff on the land surface usually converges as it moves towards lower elevation. The converging runoff can concentrate into single channels of conveyance called creeks, stream, or rivers. Usually these start as small rill and rivulets that would join up downhill into larger creeks, which then become streams later join up downstream to form even bigger channels referred to as rivers. The streams and small rivers that join up to form a larger river are called tributaries, Figure 7.10. The land area drained by a river and all its tributaries is called a watershed or catchment or river basin.
Figure 7.10: River systems. (left) A satellite image of a river system with multiple tributaries. (right) Two tributaries and their watersheds of one of Africa’s major rivers, the Zambezi River whose watershed covers Zambia, Zimbabwe, Angola, Namibia and Mozambique (Map created using Google Earth from Google Inc.)
The flat area adjacent to a river is a called a floodplain. Floodplains are characterized by frequent flooding, a means by which rivers temporarily store excess water, during storm events. Flooding delivers nutrients to the soil, making most floodplains very fertile areas. This has historically encouraged humans to move into floodplains (Figure 7.11) and use them for agriculture and other land uses, resulting in a reduction in the capacity of the floodplain to act as temporary storage for excess water during storm events, increasing the intensity of floods in downstream locations. Human structures such as buildings and roads can reduce infiltration and water-storage capacity of floodplain soils leading to increased flood frequency and intensity. Some agricultural practices, such as rice farming, are typically not associated with these negative impacts and are therefore not restricted in most floodplains. Properly functioning floodplains reduce the negative impacts of floods (by reducing severity of flood), and they assist in filtering stormwater and protecting the water quality of rivers. They also act as areas of recharge for groundwater.
Figure 7.11: The Neckar River in Germany. A and B) Meandering portion of the river with a mature flood plain showing substantial human development (buildings and agriculture) (Photo credit: Sam Mutiti). C) Portion of the Neckar River where the flood plain is agricultural fields that allow the plain to function effectively (Image credit: Google Inc.).
The United States of America (US) has numerous rivers that run throughout the nation’s landscape. It is estimated that the US has over 200,000 rivers with the Mississippi River being the largest by volume despite it only being the second longest. The Missouri River is the longest river in US. Most States have at least one important river. In Georgia, the main rivers are the Flint, Ochlockonee, Suwannee, Saint Marys, Satilla, Ogeechee, Altamaha, Oconee, Savannah, Chattahoochee, Tallapoosa, Coosa, Ocmulgee and the Tennessee rivers (Figure 7.12).
Figure 7.12: Watersheds (river basins) of Georgia representing the main rivers in the state.
These rivers are important sources of water for cities and their populations. The rivers also contain important biological communities and provide opportunities for recreational activities such as swimming, fishing, canoeing, and white-water rafting among others. Rivers largely control settlement patterns all over the world due to their widespread distribution and being an easily accessible source of water. Major cities, communities, factories, industries, and power stations are located along rivers. It is, therefore, very important to protect the quality and integrity of rivers all over the world.
Unfortunately, most of the rivers in the world are too polluted to support some human activities, especially swimming, fishing, and drinking. Close to half of the rivers in the US have been deemed too polluted for swimming and fishing. A lot of the rivers have also been dredged, channelized and restricted in width (Figure 7. 13) or impounded by dams (Figure 7.14) which may impair their ability to support a lot of human and other biological activities.
Figure 7.13: channelized Rivers in Paris (City Center), France (left) and Zurich (City Center) Switzerland
(right). (Photo credit: Sam Mutiti)
Figure 7.14: Two large dams used for hydroelectric power: (photo on left) Hoover Dam Between Nevada and Arizona in the eastern United States of America. (photo on right) Kariba Dam on the Zambezi River between Zambia and Zimbabwe). Image credit: (Google Earth)
The impoundments can trap stream sediments resulting in reduced sediment supply downstream, as well as increased deposition behind the dam. This shift in sediments flow can disrupt and damage aquatic habitats and can increase downstream stream erosion due to lack of sediment supply. The impoundments can also prevent certain aquatic organisms from migrating either upstream or downstream, therefore, reducing their range and ability to survive environmental changes as well cutting them off from spawning areas (e.g. salmon spawning). Construction of dams can also result in displacement of the local people and loss of traditional lands and cultural history to the reservoirs and ponds that usually form behind these impoundments.
It is estimated that over 600,000 river miles have been dammed in the US. Benefits of dams to humans include providing a source of water (reservoirs and farms ponds), recreation waters and controlling local flooding. On the flip side, dams can also have negative impacts on people and the environment. They can lead to increased severe flooding downstream of the dam, especially during high rain events or when they break. A recent example of such failure of Saddle Dam was experienced in the southern part of Laos (near the Cambodian border) where a dam with an estimated project cost over 1 billion dollars collapsed on the 23rd of July 2018.
7.5.3 Lakes, Reservoirs, and Ponds
If water flows to a place that is surrounded by higher land on all sides, a lake will form (Figure 7.15). A lake, pond or reservoir is a body of standing water on the land surface. When people build dams to stop rivers from flowing, the lakes that form are called reservoirs. It is estimated that over 300 million water bodies in the world are lakes, reservoirs, and ponds. Most of the Earth’s lakes (about 60%) are found in Canada. Even though lakes and rivers contain less than 1% of the Earths water, the US gets over two thirds (70%) of its water (for drinking, industry, irrigation, and hydroelectric power generation) from lakes and reservoirs. Lakes are also the cornerstone of the US’s freshwater fishing industry and are the backbone of the nation’s State tourism industries and inland water recreational activities.
Figure 7.15: Lake Sinclair in Baldwin and Putnam counties (Photo Credits: GCSU Hydro-Research lab)
7.5.4 Wetlands
A wetland is an area which is home to standing water for notable parts of the year, has saturated soils for a large part of the year and has plants that are adapted to surviving under flooded conditions or in saturated soils. They are transitional areas between the terrestrial land and the aquatic environments such as rivers, lakes, and oceans. Some major wetland types include swamps (dominated by trees), marshes (dominated by non-woody plants), and bogs (dominated by moss). Wetlands are identified using three characteristics: soils (water-saturated soils are present), hydrology (shallow water table) and vegetation (wetland plants that are adapted to areas that are saturated with water for long periods of time). Wetlands are very important areas of biological diversity and productivity. These are also important areas where geochemical and biological cycles/ processes are constantly taking place. For instance, wetlands are considered areas of significant carbon sequestration (storage), which impacts global climate change. They also act as filters for storm-water runoff before it enters rivers and lakes.
7.5.5 Oceans
As you have probably already guessed, oceans are an important component of the hydrologic cycle because they store majority of all water on Earth (about 97%). Most of the major rivers drain into them. The five oceans covering the surface of the Earth are the Atlantic, Indian, Pacific, Arctic and the Southern Ocean (Figure 7.16).
Figure 7.16: The five oceans found on planet Earth. The Pacific Ocean is the largest. Source
Approximately 90 % of the water that is evaporated into the hydrologic cycle comes from the ocean. Oceans are an important and large part of the hydrologic cycle, with lots biological diversity and many landforms. They face threats from human activities including pollution and overfishing.
Did you know that the average depth of the oceans is about 3.6 km with a maximum depth that can exceed 10 kilometers in areas known as ocean trenches?
To learn more, watch the video from the Habitable Planet: Oceans Video
7.5.6 Groundwater
About 97% of the available fresh water is found below the surface as groundwater. Groundwater is not created by some mysterious processes below ground but is part of the recycled water in the hydrologic cycle. When precipitation falls, some of the water runs off on the surface while some infiltrates into the ground. Groundwater is replenished when water moves from the surface, through unsaturated rocks or sediment (unsaturated zone), all the way down to the saturated parts (saturated zone) and becomes groundwater (Figure 7.17). The top of the saturated portion is called the water table, which is the boundary between the saturated and unsaturated zones.
Large quantities of groundwater are found in aquifers, which are rock formations or sediments that store (and yield) large amounts of usable water in their pore space. Aquifer productivity is controlled by porosity and permeability. Porosity is the percentage of open space in a rock or sediment body. Permeability is the ability of subsurface material to transmit fluids. Groundwater is found in the saturated zone of a rock body where all pores are filled with water. Water found in the unsaturated zone is NOT called groundwater, it is called soil moisture. An important concept is that surface water always moves from higher elevation to lower elevation while groundwater always moves from higher energy (hydraulic head) to lower energy.
Figure 7.17: Model of groundwater system showing the different components of an unconfined groundwater system.
Groundwater will continue to flow until it emerges as a spring, or discharges into surface water bodies on the land or in the ocean. Only a negligible amount of groundwater is stagnant at any given time in the subsurface. To utilize groundwater, we drill holes (wells) into the ground and pump the water out. While majority of the freshwater that is accessible for human use is located in groundwater, surface water (specifically rivers) are the most widely used sources of water both in the United States and around the world because they are easier to access and more widely distributed. Using groundwater requires special equipment to drill into the subsurface to extract the water and, aquifers are not as widely distributed as rivers. _____________________________________________________________________________
Parts of this chapter have been modified from the OpenStax textbooks.
OpenStax Biology 2nd Edition, Biology 2e. OpenStax CNX. Nov 26, 2018 http://cnx.org/contents/8d50a0af-948b-4204-a71d-4826cba765b8@15.1.
OpenStax, Concepts of Biology. OpenStax CNX. Nov 26, 2018 http://cnx.org/contents/b3c1e1d2-839c42b0-a314-e119a8aafbdd@14.1.
USGS Water Resources, the EPA, and The Encyclopedia of Earth
Test your understanding
What are hydrogen bonds and what impact do they have on water properties?
What is the maximum number of hydrogen bonds that a single water molecule can have with other water molecules?
What biological function/process in humans depends on water’s high specific heat capacity and what would happen if say water had the same heat capacity as sand?
Why does ice float on liquid water?
Discuss what would happen to planet Earth and its aquatic ecosystems if ice was denser that liquid water?
Where is majority of the Earth’s freshwater located?
How does consumptive use of water differ from non-consumptive use?
Industrial cooling and agriculture are both heavy users of water. Explain how these two differ in the way that they use water.
The hydrologic cycle is driven by which two major forces?
Based on your understanding of what the hydrological cycle involves, which special property of water makes this process possible and why?
In the hydrologic cycle, what is the largest and most important reservoir?
Which reservoir in the hydrologic cycle is the most important source of water for human use worldwide and why?
What is the difference between evaporation and transpiration?
What hydrologic cycle process is responsible for replenishing groundwater?
Define groundwater and explain how it is different from surface water and other water found below the surface
What are the benefits and disadvantages of channelization and damming rivers?
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