Unit 9 - Water and Soil Resources

Question 1

Describe the transfer of heat to and from water as it changes physical states.

Answer 1

When water evaporates, it changes from a liquid to a gaseous state. This process requires energy. The textbook states that evaporation requires 540 calories per gram, but the value actually changes with the temperature of the evaporating water (at 100 °C, the average value is usually given as 540 calories per gram, while at 0 °C it is given as 600 calories per gram). Because heat energy is subtracted from the surrounding air, evaporation is a cooling process. When condensation occurs, water is changed back from a gas to a liquid and the 540 calories per gram is released again. Condensation warms the surrounding air, so it is a heating process. Melting ice likewise requires energy input, but only eighty calories per gram. Freezing of water releases eighty calories per gram (an equal amount) into the air.
The heat taken up by the water during evaporation or melting is sensible heat, that is, heat you can feel, but it is incorporated in the water as latent heat or potential heat. It is considered latent because sensible heat will be released when the water condenses or freezes at a later time. The process of water changing physical states is an important global heat-transfer mechanism that helps to balance the uneven distribution of solar radiation on Earth.

Question 2

Define evapotranspiration, and explain its role in the hydrologic cycle.

Answer 2

Evapostranspiration, or ET, is the transfer of water vapour from the ground to the atmosphere by the combination of evaporation and transpiration from a vegetated surface. The main factor controlling the rate of ET is the amount of solar radiation available. Other influencing factors are the type of vegetation and the amount of water available. In returning water vapour to the air, ET reduces the amount of water available for agriculture, domestic use, and industrial use.

Question 3

What special problems does evapotranspiration create in desert areas?

Answer 3

In arid regions, abundant solar radiation creates high evaporation losses from the surface of reservoirs behind dams. This reduces the water available for irrigation and hydroelectric power generation. The intense ET also draws irrigation water applied to fields back to the surface by capillary action. Dissolved minerals precipitate on the surface and can potentially reach levels that are toxic for plants.

Question 4

List and describe the components of groundwater.

Answer 4

Immediately below the surface in the soil (and perhaps even farther down), surface attraction holds water as a thin film around particles. The pore spaces between particles contain water in transition, either percolating downward by gravity or upward by capillary action. This area is known as the vadose zone and is important as the source of moisture for most vegetation.
Below the vadose zone is the water table, the uppermost portion of the groundwater. The groundwater consists of water that has filled the pore paces between the particles and is continuously, slowly, moving downslope. This groundwater zone is the source of shallow dug wells. At depth, the groundwater zone may contain aquifers—layers of porous and permeable rock or sediments that act as conduits and storage zones for water under pressure. Aquifers provide water for drilled wells. Further down is the deep groundwater that is trapped and bound in geologically old sediments and sedimentary rocks. This zone has no practical use as a groundwater supply.

Question 5

Describe the three major problems with using water from aquifers.

Answer 5

One problem with aquifers is that many of them have recharge areas that receive less water than is being extracted from the aquifer. Coupled with a slow recharge, slow movement of water in many aquifers can make them an unreliable source of water. Even in humid areas, where natural recharge is not problematic, the rate of withdrawal is increasing. The result is the same: a valuable source of water—in some areas the only viable supply of water— is being taken out of production. A third problem is the contamination of groundwater supplies. As water slowly filters through rock, it naturally dissolves some of the more soluble minerals. As the water becomes more mineralized, its potential uses diminish. This problem is compounded by the increasing number of human-made pollutants such as chemical fertilizers, detergents, and liquid wastes placed on the ground by agricultural, domestic, and industrial practices.

Question 6

How has human activity increased the frequency and severity of floods?

Answer 6

The removal of natural vegetation from agriculture, mining, lumbering, and construction reduces the permeability of the Earth’s surface and increases the percentage of precipitation that will become surface runoff. Urbanization and the expansion of subdivisions into rural areas is a major contributor to this process. Increased runoff overloads natural drainage systems, and flooding occurs. Building on floodplains increases the potential damage that floods may cause.

Question 7

Describe how dams and channelization reduce flooding hazards. What are the drawbacks to creating dams and channels?

Answer 7

Dams are constructed for a number of interrelated purposes, not least of which is flood control. Water from a particularly intense rainstorm or from spring runoff can be temporarily stored in a reservoir and later released to prevent flooding downstream. The value of this ability is obvious, but there are some drawbacks to constructing dams and reservoirs. Several of the potential uses of reservoirs are incompatible. For example, dams have provided new lakes for recreational purposes, but periodic release of water in the fall to accommodate spring runoff interferes with recreational usage. Another problem is that reservoirs become sediment traps, and the sand, silt, and clay that accumulates can drastically reduce the lifespan of the reservoir. Other problems include increased evaporation and subsequent mineralization of the water, loss of usable land (often valuable agricultural land), interruption of river transportation and fish migration, and detrimental environmental effects on the river downstream of the dam.

Channelization is the artificial straightening and shortening of a meandering stream channel to improve the efficiency of drainage and thus reduce the risk of flooding. Through channelization, the water in the stream can move faster, because the distance it must travel is reduced and the gradient is increased. The negative effects of channelization include increased erosion, transfer of flooding to downstream areas where the increased discharge returns to the original channel, reduction in the natural filtration of groundwater, and loss of wetlands habitat.

Question 8

Differentiate between water withdrawal and water consumption.

Answer 8

Withdrawal is the temporary extraction of water from its source. Consumption is the water that has been withdrawn that is stored, consumed, or otherwise not returned to its source. Consumed water includes that which evaporates and transpires, that which is used by vegetation, livestock, or humans, and that which is incorporated into products.

Question 9

Identify which five commodities use the largest amount of water to produce.

Answer 9

The five most water-intensive commodities are, in descending order, rice, synthetic rubber, aluminum, sugar, and corn.

Question 10

How can more water be used (per year) for generating hydroelectric power than is available from annual stream runoff?

Answer 10

First, some hydroelectric power plants pump water that has already passed through the turbines back into the storage reservoir for re-use. Second, hydroelectric power plants located at different points along the same stream will use the same water as it flows downstream.

Question 11

What are the causes and effects of acid rain?

Answer 11

The burning of fossil fuels and industrial processing release gases and particulate matter into the atmosphere, where they form condensation nuclei for cloud droplets. The most harmful of these substances are sulphur dioxide (SO2) and nitrogen oxides (NOx). Any precipitation that forms from these cloud droplets will be a dilute acid. This acid rain has proven toxic to vegetation, fish, and some terrestrial organisms and is a powerful weathering agent of building stone.

Question 12

Differentiate between riparian law and law of prior appropriation.

Answer 12

Riparian law allows every landowner along a stream to use the water that reaches him or her, as long as the supply is not significantly diminished for landowners downstream. This law has been modified somewhat to allow cities bordering streams to withdraw a supply commensurate with their needs. The law of prior appropriation is basically a first come, first served principle. The landowner with the longest standing claim along a stream course may withdraw as much water as he or she needs.

Question 13

What are the differences between the English rules of “absolute ownership” and the American rule of “absolute use” of groundwater?

Answer 13

The English law allows a landowner to extract as much groundwater as desired for any use the landowner wishes. The American rule limits acceptable uses of water to personal use only and prohibits a landowner from transporting the groundwater to other areas for sale unless all farmers in the area are compensated if their wells go dry as a result.

Question 14

What determines the rate at which groundwater can be extracted from the ground?

Answer 14

The rate at which groundwater can be extracted from the ground will be influenced by the amount of water that is present and the rate at which the water can permeate the rock.

Question 15

Define specific yield and safe yield.

Answer 15

The specific yield of a well refers to the maximum rate at which water can be continuously extracted from an aquifer. The safe yield is the rate at which water can be extracted from an aquifer without lowering the water table.

Question 16

How is the process of reverse osmosis used in desalinizing water?

Answer 16

Reverse osmosis is used to remove salt from water (desalination). Salt water is forced through a semi-permeable membrane that allows water molecules through but traps the smaller salt ions (Na+ and Cl−1). About thirty per cent of the original salty water becomes fresh water while the remainder is discarded as a concentrated brine.

Question 17

Describe the water allocation schedule of the Colorado River (1992—Colorado River Compact) and the subsequent 1944 treaty with Mexico. Briefly discuss the three related problems that have occurred.

Answer 17

According to the Colorado River Compact, both the states sharing the Upper Basin of the Colorado River and those sharing the Lower Basin are guaranteed 7.5 million acre-feet of water per year, each. The 1944 treaty with Mexico guarantees an annual flow into Mexico of 1.5 million acre-feet of water containing no more than 900 parts per million of dissolved solids. The first problem is that the Colorado River does not ordinarily have 16.5 million acre-feet of water to distribute. A series of dams that have been constructed to store water have not entirely relieved the supply problem, and they have created other problems, such as increased mineralization caused by high evaporation rates. The quality of water passing into Mexico is also a problem. This water has greatly exceeded the agreed-upon dissolved solid content. In response, a large desalinization plant has been constructed at Yuma, Arizona that releases water with only 800 ppm dissolved solids. Finally, the Navaho Reservation, not considered in the Colorado River Compact, plans to withdraw much of the river water reaching the reserve for crop irrigation.

Question 18

What problems are associated with the dams on the Colorado River?

Answer 18

There is conflict about reservoir use along the Colorado River. For flood control, the water levels in reservoirs must be lowered periodically, anticipating increased inflow resulting from large rainstorms in the basin. For irrigation, water levels must be kept fairly high to guarantee a constant supply to fields downstream. Obviously, these two uses conflict. Another big problem is that no restriction was placed on settlement of the flood plains below the dams, so they are now heavily populated. Therefore, there is a limit on the amount of water that can be released from the reservoir at one time.

Question 19

What is water mining?

Answer 19

Water mining is said to occur when quantities of water exceeding the safe yield are pumped from groundwater, resulting in a lowering of the water table and a decrease in available water.

Question 20

What is the Ogallala Aquifer, and what are the problems associated with it?

Answer 20

The Ogallala Aquifer is an especially large aquifer composed of Miocene gravels saturated with high quality groundwater. It is located under a vast area of the southern high plains. Since the 1930s, over 150 000 wells have been mining the water, and by the 1980s, the water table had dropped to an alarmingly low level. While water extraction from the aquifer has continued, improved conservation practices and the reduction of area under irrigation has resulted in a decline in water depletion levels from the aquifer.

Question 21

What causes salt water intrusion?

Answer 21

Water mining of aquifers in coastal areas allows salt from the ocean to migrate inland and contaminate the aquifer to the point where the water can no longer be used for human consumption.

Question 22

What causes the build-up of salts in irrigated lands? What is the solution to this problem? What problems are associated with this solution?

Answer 22

When arid land is irrigated in volumes that exceed drainage capacity, the water table rises to the point where water is drawn to the surface by capillary action. As water evaporates at the surface, its dissolved minerals precipitate, forming a saline or alkaline crust. This crust is toxic to many plants, and the waterlogged soil itself tends to drown most crops. Land in which salt has built up can be reclaimed by installing a water collection system below the root level of the plants to increase the drainage, but this process is expensive and creates other problems. It requires a large amount of water to flush the salts out of the soil, after which the flushed salts may contaminate groundwater supplies or be deposited into drainage systems.

Question 23

How does Los Angeles meet its growing water needs?

Answer 23

To supplement local supplies, Los Angeles receives water by aqueduct from the Owens Valley, located on the eastern flank of the Sierra Nevada Mountain range about 400 kilometres to the east, and from Lake Havasu, which contains dammed Colorado River water. Increased population and the loss of one million acre-feet of water annually from the Colorado River Compact have caused Los Angeles to consider bringing in additional water from the Sacramento River aqueduct.

Question 24

What is a soil?

Answer 24

The strict definition of a soil is a material composed of inorganic and organic fractions. The inorganic component comes from the breakdown of rock into smaller particles by weathering, while the organic component comprises dead and living matter introduced at or near the earth’s surface.

Question 25

What are the five main factors that influence soil formation?

Answer 25

The five main factors that influence the processes of soil formation are
the type of rock that weathers to produce the inorganic component of the soil,
climatic conditions,
the type of vegetation present,
the gradient of the terrain, which influences the rate at which precipitation will either infiltrate or wash away as runoff, and
time.

Question 26

What is weathering, and what are the processes involved in weathering?

Answer 26

Weathering is the progressive breakdown of rocks into smaller particles through mechanical disintegration (physical weathering) and chemical breakdown (chemical weathering). Weathering is the first step in the process of soil formation.

Question 27

Regarding chemical weathering of minerals and soil formation, describe the following processes without using chemical formulae or equations:
hydrolysis
hydration
carbonation
oxidation

Answer 27

During chemical weathering, rocks disintegrate as primary minerals break down to form secondary minerals. This mineral transformation can occur through processes that include
hydrolysis—a reaction that occurs with water to decompose minerals. Hydrolysis commonly occurs with silicates such as olivine.
hydration—involves incorporating water molecules into mineral structures. Hydration typically occurs with clay minerals, resulting in volumetric expansion or swelling.
carbonation—the reaction of a mineral with carbonic acid produced by the dissolving carbon dioxide in rainwater. Carbonation is particularly effective in weathering rocks containing calcite.
oxidation—occurs when oxygen reacts with metallic elements (e.g., potassium, calcium, magnesium, and iron) found in primary minerals. Oxygen is abundant in surface water.

Question 28

Briefly describe how climate controls the process of soil formation.

Answer 28

The importance of climate in soil formation cannot be exaggerated. Variables such as precipitation, temperature, and vegetation are important factors in soil formation; these variables are controlled by climate.

Question 29

What are the five most important characteristics used to classify soils? What features distinguish soil horizons from one another?

Answer 29

The five most important characteristics used to classify soils are
colour—this property usually comes from the soil formation process but it sometimes comes from the original rock.
texture—the proportions of sand, silt, and clay within a soil create texture.
consistence—consistence is a measure of the stickiness or plasticity of a soil.
soil structure—determined by the character (shape, size, and durability) of lumps or peds made up of clusters of soil particles.
soil horizons—horizontal layers, usually identifiable within a soil profile vary in chemical structure and composition.
Soil horizons can be distinguished based on chemical composition and structure. Soil horizons are categorized into two major groups based on composition: organic horizons and mineral horizons. Organic horizons are prefixed with the letter O and comprise debris from plant and animal remains. Mineral horizons are made up of products of rock weathering. The A horizon at the top contains significant organic material and is darker in colour than the B horizon. The B horizon, underlying the A horizon, contains less organic material. The C horizon, below the B horizon, comprises weathered material from which the soil is formed. The C horizon directly overlies the parent rock.

Question 30

What factors influence agricultural soil productivity? Which soils are the most productive? Why are tropical and subtropical soils usually infertile?

Answer 30

Agricultural soil productivity depends on soil fertility, consistency, and structure. The latter two factors facilitate tilling.
The most productive soils include mollisols, alfisols, and spodosols, which are found in temperate and humid regions of the world.

Tropical and subtropical soils are usually infertile because their nutrients often leach out, leaving hardened residual products not amenable to tilling (such as laterites).

Question 31

Which three elements from the soil are required in substantial amounts for plant growth? Which nine elements are needed in trace amounts?

Answer 31

Plants require substantial quantities of phosphorus, potassium, and calcium from the soil in order to grow. Plants require magnesium, sulphur, boron, copper, iron, manganese, zinc, molybdenum, and chlorine in trace amounts.

Question 32

How does extreme acidity or alkalinity affect soil fertility?

Answer 32

Extreme acidity or alkalinity affects soil fertility by destroying nutrients or promoting conditions under which nutrients can leach out, leaving behind barren soils.

Question 33

The total amount of cropland in the USA has remained constant since the 1920’s while the population has nearly tripled. How is this possible?

Answer 33

The total amount of cropland in the USA has remained constant since the 1920s even though the population has nearly trebled. A constant volume of USA cropland has continued to feed a growing population largely by increased productivity. Intensive agricultural methods accompanied by increased fertilizer use, improved hybrid seeds, and wide-application irrigation techniques have all contributed to better crop yields.

Question 34

What role does vegetation play in preventing soil erosion?

Answer 34

Vegetation prevents erosion by binding the soil, which stabilizes it so that water and wind cannot easily wash it away.

Question 35

Why does soil in farmed areas experience more nutrient loss than does soil under natural vegetation?

Answer 35

Areas under natural vegetation do not experience significant nutrient loss because when the plants lose their leaves, or die, the organic remains return to the soil. With farmed crops, harvesting removes the yields from the fields so nutrients in the plants do not return to the soil they came from.

Question 36

What is desertification, and what causes it?

Answer 36

Desertification is the inadvertent conversion of agriculturally productive land into desert. Humans are the main agents of desertification, which they achieve through deleterious activities such as overgrazing, clearing of forests, and inappropriate soil and water management.

Question 37

What does soil conservation involve?

Answer 37

Soil conservation seeks to minimize soil erosion, reduce leaching of nutrients, and guard against the build-up of salts or alkalis by controlling drainage. At the same time, nutrients stripped from soil through cultivation must be restored.

Question 38

How does seed drilling help reduce soil erosion? What are its drawbacks?

Answer 38

Seed drilling involves punching a suitably deep hole into the ground in which to place a seed. Seed drilling replaces ploughing, reducing the number of times a tractor needs to pass over the land. This effectively reduces erosion.
The major disadvantage of seed drilling is that ploughing assists in weed control by turning weeds under. When a field isn’t ploughed, more herbicides may be needed to control weeds.