Chapter 7Water: Hydrologic Cycle and Human Use7.1 Water: A Vital Resource

I. 32.5 million cubic miles of water cover 71% of Earth’s surface. About 97.5% of this volume is the salt water of the oceans and seas. The remaining 2.5% is fresh water—water with a salt content of less than .1%.A. 2/3 of the fresh water is bound up in polar ice caps and glaciers.B. Fresh water is a continually renewable resource.

II. In the developing world, 1.1 billion people lack access to safe drinking water, 2.6 billion people do not have access to adequate sanitation services, and more than 1.7 million deaths each year are traced to waterborne diseases.A. Fresh water is a limiting resource in many parts of the world.

7.2 Hydrologic Cycle: Natural Cycle, Human ImpactsI. The hydrologic cycle consists of water rising to the atmosphere through evaporation and

transpiration and returning to the land and oceans and rivers through condensation and precipitation. Water vapor is referred to as green water, and liquid water is called blue water.

Evaporation, Condensation, and Purification I. As water molecules absorb energy from sunlight, the kinetic energy they gain may be

enough to allow them to break away from other water molecules entirely and enter the atmosphere. This process is known as evaporation, and the result it water vapor.A. Water vapor is a powerful greenhouse gas. It provides 2/3 of the total warming

from all greenhouse gases. B. The amount of water vapor in the air is humidity. Humidity is generally measured

as relative humidity, the amount of water vapor as a percentage of what the air can hold as a particular temperature. The amount of water vapor the air can hold increases and decreases with the temperature.

II. Condensation is the opposite of evaporation. It occurs when water molecules rejoin by hydrogen bonding to form liquid water. Condensation is greatly facilitated by the presence of aerosols in the atmosphere.A. Aerosols are microscopic liquid or solid particles originating from land and water

surfaces. They provide sites that attract water vapor and promote the formation of droplets of moisture. Aerosols may originate naturally and can have a significant impact on regional and global climates.

III. The processes of evaporation and condensation purify water naturally. Thus, evaporation and condensation are the source of all fresh water on Earth.

PrecipitationI. Warm air rises from the Earth’s surface because it is less dense than the cooler air

above. As it encounters the lower atmospheric pressure at increasing altitudes, the warm air gradually cools as it expands—a process called adiabatic cooling. When the

relative humidity reaches 100% and cooling continues, condensation occurs and clouds form.A. As condensation intensifies, water droplets become large enough to fall as

precipitation. Adiabatic warming occurs as the air descends and is compressed by the higher air pressure in the lower atmosphere.

B. As a cold front moves into an area, the warm, moist air already there is forced upward because the cold air of the advancing front is denser. The rising warm air cools, causing condensation and precipitation along the leading edge of the cold front.

II. Two factors—global convection currents and the rain shadow—may cause more or less continuously rising or falling air currents over particular regions, with major effects on precipitation.A. Global convection currents occur because the Sun heats the Earth most intensely

over the equator, where rays of sunlight are almost perpendicular to Earth’s surface. As the air at the equator is heated, it expands, rises, and cools; condensation and precipitation occur. The constant intense heat in these areas ensures high amounts of rainfall. The air, now dry, must come down again. Pushed from beneath by more rising air, it literally “spills over” to the north and south of the equator and descends over subtropical regions, resulting in subtropical deserts.

B. The two halves of the system composed of the rising and falling air make up a Hadley cell. Because of Earth’s rotation, winds are deflected from the strictly vertical and horizontal paths indicated by a Hadley cell and tend to flow in easterly and westerly directions—the trade winds which blow almost continuously from the same direction.

III. The second situation that causes continually rising and falling air occurs when moisture-laden trade winds encounter mountain ranges. The air is deflected upwards, causing cooling and high precipitation on the windward side of the range. As the air crosses the range and descends on the other side, it becomes warmer and increases in capacity to pick up moisture.A. Deserts occur on the leeward side of mountain ranges. The dry region downward

of a mountain range is referred to as a rain shadow.Groundwater

I. As precipitation hits the ground, it may either soak into the ground (infiltration) or run off the surface. The amount that soaks in compared with the amount that runs off is called the infiltration-runoff ratio.A. Runoff flows over the surface of the ground into streams and rivers, which make

their way to the ocean or to inland seas. All the land area that contributes water to a particular stream or river is referred to as the watershed for that stream or river. All ponds, lakes, streams, rivers, and other waters on the surface of Earth are called surface waters.

B. Water that infiltrates has two alternatives. The water may be held in the soil, in an amount that depends on the water-holding capacity of the soil. This water, called capillary water, returns to the atmosphere either by way of evaporation or transpiration. The combination of evaporation and transpiration is referred to as evapotranspiration.

C. The second alternative is percolation. Infiltrating water that is not held in the soil is called gravitational water because it trickles down through the pores under the pull of gravity. Eventually, gravitational water encounters an impervious layer of rock or dense clay. It accumulates there, completely filling the spaces above the impervious layer. This accumulated water is called groundwater, and its upper surface is the water table.

II. Groundwater will seep laterally as it seeks its lowest level. Layers of porous material through which groundwater moves are called aquifers.A. Groundwater aquifers may be found at various depths between layers of

impervious rock.B. The recharge area—the area where water enters an aquifer—may be many miles

away where the water leaves the aquifer.C. Groundwater may move through aquifers until in finds some opening to the

surface. These natural exits may be seeps or springs. In a seep, water flows out over a relatively wide area; in a spring, water exits the ground as a significant flow from a relatively small opening.

III. As water percolates through the soil, debris and bacteria from the surface are generally filtered out. However, water may dissolve and leach out certain minerals.

Pools and Fluxes in the CycleI. The hydrologic cycle consists of four physical processes: evaporation, condensation,

precipitation, and gravitational flow. There are three loops in the cycle: 1) in the evapotranspiration loop (consisting of green water), the water evaporates and is returned by precipitation. On land, this water is held as capillary water and then returns to the atmosphere by way of evapotranspiration. 2) In the surface runoff loop (containing blue water), the water runs across the ground surface and becomes part of the surface water system. 3) In the groundwater loop (containing blue water), the water infiltrates, percolates down to join the groundwater, and then moves through aquifers, finally exiting through seeps, springs, or wells, where it rejoins the surface water.

Human Impacts on the Hydrologic CycleI. A large share of the environmental problems we face today stem from direct or

indirect impacts on the water cycle. These impacts can be classified into four categories: 1) changes to Earth’s surface, 2) changes to Earth’s climate, 3) atmospheric pollution, and 4) withdrawals for human use.

II. As forests are cleared or land overgrazed, the pathway of the water cycle is shifted from infiltration and groundwater recharge to runoff, so the water runs into streams or

rivers almost immediately. This sudden influx of water into waterways may not only cause a flood, but also bring sediments and other pollutants via surface erosion.A. Increased runoff necessarily means less infiltration and therefore less

evapotranspiration and groundwater recharge. Lowered evapotranspiration means less moisture for local rainfall.

B. Wetlands function to store and release water in a manner similar to the way the groundwater reservoir does. Therefore, the destruction of wetlands has the same impact as deforestation: flooding is exacerbated, and waterways are polluted during wet periods and dry up during droughts.

III. A warmer climate means more evaporation from land surfaces, plants, and water bodies because evaporation increases exponentially with temperature. A wetter atmosphere means more precipitation and more flood events.A. A warmer climate will likely generate more hurricanes and more droughts.

IV. Aerosol particles form nuclei for condensation. The more such particles there are, the greater the tendency for clouds to form.A. Anthropogenic aerosols are on the increase, primarily in the form of sulfates,

carbon, and dust. They form a brownish haze that is associated with industrial areas, tropical burning, and dust storms. Their impact on cloud formation is substantial, and where these aerosols occur, solar radiation to the Earth’s surface is reduced. Their most significant impact is on the hydrologic cycle. Anthropogenic aerosols suppress rainfall where they occur in abundance, even though they encourage cloud formation.

B. With suppressed rainfall comes drier conditions, so more dust and smoke.

7.3 Water: A Resource to Manage, a Threat to ControlI. Most of the water used in homes and industries is for nonconsumptive uses. In contrast,

agriculture is a consumptive use, because the applied water does not return to the water resource.

II. Worldwide, the largest use of water is for irrigation, second for industry, and third is for direct human use.

III. In the United States, about 40% of domestic water comes from groundwater sources and 60% from surface waters.A. In developing countries, surface waters and shallow wells often receive runoff and are

frequently polluted with various wastes. This water is rarely treated.IV. Whenever possible, both water and sewage systems are laid out so that gravity maintains

the flow through the system. This arrangement minimizes pumping costs and increases reliability.

Surface WatersI. To trap and control flowing rivers, more than 45,000 large dams have been built

around the world. These dams have an enormous direct social impact, leading to the

displacement of 40 million people worldwide and preventing access by local people to the goods and services of now-buried ecosystems.A. Win a river is dammed, valuable freshwater habitats are lost. When the river’s

flow is diverted to cities or croplands, the waterway below the diversion is deprived of that much water. Wildlife that depends on the water or on food chains involving aquatic ecosystems is adversely affected.

B. Wetlands occupying floodplains along many rivers dry up, resulting in frequent die-offs of wildlife.

II. The problems extend to estuaries—bays in which fresh water from a river mixes with seawater. As river’s flow is diverted to irrigated fields, less fresh water enters and flushes the estuary. Consequently, the salt concentration increases, profoundly affecting the estuary’s ecology.

GroundwaterI. 99% of all liquid freshwater is in underground aquifers. Of this groundwater, more

than ¾ has a recharge rate of centuries or more. Such groundwater is considered nonrenewable.A. Renewable groundwater is replenished by the percolation of precipitation water,

so it is vulnerable to variations in precipitation.B. The sustainability of groundwater depends on balancing withdrawals with rates of

recharge. II. The simplest indication that groundwater withdrawals are exceeding recharge is a

falling water table, a situation that is common throughout the world.A. Depleting water resources will ultimately have its most significant impact on crop

production.III. Surface waters are also affected by falling water tables. When water tables drop,

wetlands dry up. Springs and seeps dry up as well, diminishing streams and rivers.IV. Over time, groundwater has leached cavities in the ground. Where these spaces are

filled with water, the water helps support the overlying rock and soil. As the water table drops, this support is lost. Then there may be a gradual settling of the land, a phenomenon known as land subsidence.A. Landing subsidence causes building foundations, roadways, and water and sewer

line to crack. In coastal areas, it causes flooding.B. Another kind of land subsidence, a sinkhole, may develop suddenly and

dramatically. A sinkhole results when an underground cavern, drained of its supporting groundwater, suddenly collapses.

V. Another problem resulting from dropping water tables is saltwater intrusion.A. In coastal regions, springs of outflowing groundwater may lie under the ocean. As

long as a high water table maintains a sufficient head of pressure in the aquifer, fresh water will flow into the ocean. However, lowering the water table or removing groundwater at a rapid rate may reduce the pressure in the aquifer, permitting salt water to flow back into the aquifer and into wells.

7.4 Water Stewardship: Public Policy ChangesObtaining More Water

I. The hydrologic cycle is entirely adequate to meet human needs for fresh water, because it possesses several times as much water as we require. However, the water is often not distributed where it is most needed, and the result is a persistent scarcity of water in many parts of the world.A. In the developing world, there is still a deficit of infrastructure for capturing and

distributing safe drinking water.B. There are four ways of meeting existing needs: 1) capture more of the runoff

water, 2) gain better access to existing groundwater aquifers, 3) desalt water, and 4) conserve present supplies by using less water.

II. Dams disrupt the integrity of river systems, breaking them up into fragments of river interrupted by standing water. Increasingly, existing dams are being challenged.A. Often, the reservoir behind a dam has received massive amounts of sediment from

upriver, and removing the dam exposes the sediments and washes them downstream.

B. Protection has been accorded some rivers with the passage of the Wild and Scenic Rivers Act of 1968, which keeps rivers designated as “wild and scenic” from being damaged or affected by other harmful operations.

III. Already, 2 billion people depend on groundwater supplies. In many areas, groundwater use exceeds aquifer recharge, leading to shortages as the water table drops below pump levels. Groundwater depletion is considered the single greatest threat to irrigated agriculture.A. Exploiting renewable groundwater will continue to be an option, but it is unlikely

to provide great increases in water supply because it is the same sustainable supplies, recharged by annual precipitation, that are being increasingly polluted. Agricultural chemicals and industrial wastes readily enter groundwater, making groundwater pollution as much of a threat to domestic water use as depletion.

IV. With increasing water shortages and most of the world’s population living near coasts, there is a growing trend towards desalinization.

Using Less WaterI. Agriculture is the largest consumer of fresh water. Most present-day irrigation wastes

huge amounts of water.A. One way of combating this waste is the surge flow method, in which computers

control the periodic release of water.B. Another method is the drip irrigation system, a network of plastic pipes with

pinholes that drip water at the base of each plant. These system waste less water and reduce salinization, while increasing crop yields.

C. In developing countries, irrigation often bypasses the rural poor. Affordable irrigation technologies can be remarkably successful, such as treadle pumps.

D. Xeriscaping—landscaping with desert species that require no additional watering—is a popular way to reduce water usage in arid environments.

E. Gray-water recycling systems are being adopted in some water-short areas.Public Policy Challenges

I. The hydrologic cycle provides a finite flow of water through each region of Earth. Maintaining a supply of safe drinking water for people is a high-priority issue. In addition, water for irrigation is vital to food production across much of the world.A. Often, these two demands come into conflict. Water for people can often mean

less water for natural ecosystems.II. There is no federal bureaucracy to provide oversight for water quantity issues. Most

water decisions are made at the local and regional levels. The following issues need to be addressed by a water resources policy commission:

1. Water efficiency must be promoted as the primary strategy for meeting future needs.

2. Water subsidies need to be reduced or eliminated. Caps must be placed on withdrawals from groundwater and surface waters in order to protect ecosystem services.

3. Polluters must be charged according to their effluents. 4. Watershed management must be integrated into the pricing of water. 5. Water authorities must regulate dam operations so that river flow is

maintained in a way that simulates natural flow regimes.6. The US must respond to the global water crisis with adequate levels of

international development aid.7. Much more research and monitoring are needed to provide the basic data for

making informed policy decisions. Essential data on recharge and withdrawal, of water from groundwater aquifers, on stream flow, on river runoff, and on water uses are poorly collected in most developing countries.