Unit 9 - Water and Soil Resources Flashcards
Describe the transfer of heat to and from water as it changes physical states.
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.
Define evapotranspiration, and explain its role in the hydrologic cycle.
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.
What special problems does evapotranspiration create in desert areas?
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.
List and describe the components of groundwater.
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.
Describe the three major problems with using water from aquifers.
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.
How has human activity increased the frequency and severity of floods?
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.
Describe how dams and channelization reduce flooding hazards. What are the drawbacks to creating dams and channels?
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.
Differentiate between water withdrawal and water consumption.
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.
Identify which five commodities use the largest amount of water to produce.
The five most water-intensive commodities are, in descending order, rice, synthetic rubber, aluminum, sugar, and corn.
How can more water be used (per year) for generating hydroelectric power than is available from annual stream runoff?
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.
What are the causes and effects of acid rain?
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.
Differentiate between riparian law and law of prior appropriation.
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.
What are the differences between the English rules of “absolute ownership” and the American rule of “absolute use” of groundwater?
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.
What determines the rate at which groundwater can be extracted from the ground?
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.
Define specific yield and safe yield.
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.
How is the process of reverse osmosis used in desalinizing water?
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.
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.
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.
What problems are associated with the dams on the Colorado River?
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.
What is water mining?
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.
What is the Ogallala Aquifer, and what are the problems associated with it?
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.
What causes salt water intrusion?
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.
What causes the build-up of salts in irrigated lands? What is the solution to this problem? What problems are associated with this solution?
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.
How does Los Angeles meet its growing water needs?
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.
What is a soil?
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.
