Chapter 7: Water Hydrologic Cycle and Human Use

16/10/2010 12:57


1. What are the lessons to be learned from the Aral Sea story?

The death of the Aral Sea was a result of wrong and thoughtless water use for irrigation. The Amu Darya and Syr Darya, the two large rivers flowing into the Aral, were tapped for irrigation water for millions of hectares of cotton fields. In 50 years (from 1930s to 1980s) the inflowing water was reduced to zero resulting in sea shrink.

The environmental disaster had multiple environmental, health and economic consequences. The increase of salinity devastated the lake’s ecology; commercial fishery collapsed; the exposed lake bed now contains accumulated salts and the dry winds of the area pick up the salt and dust, laden with fertilizers, herbicides, and pesticides, dropping them on the irrigated land and choking people for miles around; many health impacts have been reported from the area, including increased cancer rates and infant mortality; the local climate changed; the growing season shortened, forcing many farmers to switch from cotton to rice; many animal species unique to the Aral Sea have become extinct. [169]

The lesson to be learned from the Aral Sea story: the implementation of such a project (irrigation of a large area for agricultural use) requires a proper scientific base – preliminary investigation of the area, estimation of the supposed loading for environment, assessment of probable consequences for environment, economy and society. The complex analysis of probable impacts and effects should be done in order not to avoid a disaster. [opinion]


2. Give examples of the infrastructure that has been fashioned to manage the water resources.

During the last 2 centuries a huge infrastructure designed to bring water under control was constructed. We have built dams, canals, reservoirs, aqueducts, sewer systems, treatment plants, water towers, elaborate pipelines, irrigation systems, desalination plants. [170]

Dams are built across the rivers to create reservoirs, which hold water in times of excess flow and can be drawn down at times of lower flow. In addition, dams and reservoirs may offer power generation, recreation and flood control. Water for municipal use is piped from the reservoir top a treatment plant, where it is treated to kill pathogens and remove undesirable materials. After treatment water is distributed through the water system to homes, schools, and industries. Wastewater, collected by the sewage system, is carried to a sewage-treatment plant, where it is treated before being discharged into the same river from which it was withdrawn, but farther downstream. The reservoirs created by dams on rivers are also major sources of water for irrigation. [179]



3. What are two processes that result in natural water purification?  State the difference between them. Distinguish between green water and blue water.

The processes of evaporation and condensation purify water naturally. Evaporation: as water molecules absorb energy from sunlight or an artificial source, they kinetic energy they gain may be enough to allow them to break away from other water molecules entirely and enter the atmosphere. Condensation is the opposite of evaporation. It occurs when water molecules rejoin by hydrogen bonding to form liquid water.  Green water – is water in vapor form originating from the soil and organisms – the source of water precipitation. Blue water  (liquid water)– precipitation, renewable surface water runoff and groundwater recharge – the focus of management and the main source of water for human withdrawals and natural ecosystems.

When water in an ocean or a lake evaporates only the water molecules leave the surface; the dissolved salts and other solids remain behind in solution. When the water vapor condenses again, it is thus purified water – except for the pollutants and other aerosols ir may pick up from the air. The water in the atmosphere turns over every 10 days, so water is constantly being purified.[172]


4. Describe how the Hadley cell works and explain how the Earth’s rotation creates the trade winds.

Hadley cell is made up of two halves of the system composed of the rising and falling air.  As the air at the equator is heated, it expands, rises, and cools; condensation and precipitation occur. The constant intense heat in these equatorial areas ensures that this process is repeated often, thus causing high amounts of rainfall, which, along with continuous warmth, in turn supports tropical rainforests.

Rising air over the equator is just half of the convection current, however. 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 region, resulting in subtropical deserts. The two halves of the system composed by 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. [173-174]


5. Why do different regions receive different amounts of precipitation?

The general precipitation patterns for the land, general atmospheric circulation and the effects of mountain ranges combine to give great variation in the precipitation reaching the land. [174]

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, the warn air gradually cools as it expands - a process known as adiabatic cooling.  When the relative humidity reaches 100% and cooling continues, condensation occurs and clouds form. 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.

Precipitation on Earth ranges from 0 in some areas to more than 100 inches (2.5 m) per year in others. The distribution depends basically on patterns of rising and falling air currents. As air rises, it cools, condensation occurs and precipitation results. As air descends, it tends to become warmer, causing evaporation to increase and dryness to result. A rain causing event is the movement of a cold front. A the 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. [173]

When moisture-laden trade winds encounter mountain ranges the air is deflected upward, causing cooling and high precipitation on the windward side of the range. As the air crosses the ranges and descends on the other side, it becomes warmer and increases its capacity to pick up moisture. Hence, deserts occur on the leeward sides of mountain ranges. The dry region downwind of a mountain range is referred to as a rain shadow. The severest deserts in the world are caused by the rain-shadow effect.  [174]


6. Definitions.

Precipitation – atmospheric water vapor condensation and its further deposition on the earth’s surface. [glossary of meteorology]

Infiltration – soak of the precipitation into the ground. [174]

Runoff – flow of precipitation over the surface of the ground into streams and rivers, which make their way to the ocean or to inland seas. [174]

Capillary water – water that is held in the soil in the result of infiltration and then returns back to the atmosphere either by of evaporation from the soil or by transpiration through plants.[174]

Transpiration -  the loss of water vapor as it moves from the soil through green plants and exits through leaf pores. [170]

Evapotranspiration – combination of vaporization and plant transpiration

Percolation – a blue water flow. [174]

Gravitational water – infiltrating water that is not held in the soil , because it trickles or percolates down through pores or cracks under the pull of gravity. [174]

Groundwater – gravitational water that encounters an impervious layer of rock or dense clay and accumulates there, filling completely all the spaces above the imperious layer. [174]

Water table – upper surface of the groundwater [174]

Aquifer – layers of porous material through which groundwater moves [175]

Recharge area – are where water enters the aquifer

Seep – natural exit of ground water, when water flows out over a relatively wide are. [175]

Spring – natural exit of ground water from the ground as a significant flow from a relatively small opening [175]


7. Give the full description of the hydrologic cycle. What is the water quality (purity) at different points in the cycle? Explain the reasons for the differences.

The hydrological cycle consists of 4 physical processes: evaporation, condensation, precipitation, and gravitational flow. There are 3 principal 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, the main sources for natural ecosystems and rain-fed agriculture, 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 (also containing blue water), the water infiltrates, percolates down to join the groundwater, and then moves though aquifers, finally exiting through seeps, springs, or wells, where it rejoins the surface water. [175]

PURITY - ???


8. How does changing Earth’s surface change the pathway of water? How does it affect streams and rivers? Humans? Natural ecology?

As forests are cleaned 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 flood, but also bring along sediments and other pollutants via surface erosion. Deforestation causes erosion and reduces infiltration. Increased runoff necessarily means less infiltration and therefore less evapotranspiration and groundwater recharge. Lowered evapotranspiration means less water for local rainfall. Groundwater may be insufficient to keep springs flowing during dry periods. Dry, barren and lifeless streambeds are typical of deforested regions – a tragedy for both the ecosystems and the humans who are dependent on the flow. 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. [176-177]


9. Explain how climate change and atmospheric pollution can affect the hydrological cycle?

A warmer climate means more evaporation from land surfaces, plants, and water bodies because evaporation increases exponentially with temperature. A wetter atmosphere means more and frequently heavier precipitation and more flood events.

A warmer climate will likely generate more hurricanes and more droughts.

Millennium Ecosystem Assessment: “A changing climate can modify all elements of the water cycle, including precipitation, evapotranspiration, soil moisture, groundwater recharge, and runoff. It can also change both the timing and intensity of precipitation, snowmelt and runoff. [177]

The air pollution with aerosols has a significant effect on hydrological cycle. The unique size spectrum of the aerosols causes them to suppress rainfall where they occur in abundance, even though they encourage cloud formation. [177]


10. What are the three major uses of water? What are the major sources of water to match these uses?

Worldwide, the largest use of water is for irrigation (70%); second is for industry (20%) and third is fro direct human use (10%). These percentages vary greatly from one region to another, depending on natural precipitation and the degree to which the region is developed.

The main sources for these needs are: surface water (rivers, lakes and reservoirs) and groundwater sources. [178]


11. How do dams facilitate the control of surface waters? What kinds of impacts do they have?
Dams are built across the rivers to create reservoirs, which hold water in times of excess flow and can be drawn down at times of lower flow. In addition, dams and reservoirs may offer power generation, recreation and flood control. The reservoirs created by dams on rivers are also major sources of water for irrigation. [179]


- social impact (displacement of at least 40 mln people worldwide and preventing access by local people to the goods and services of the now-buried ecosystem.

- ecological impacts. When a river is dammed, valuable freshwater habitats, such as waterfalls, rapids, and prime fish runs are lost. When the river’s flow is diverted to the cities or crop-lands, the waterway below the diversion is deprived of that much water. The impact on fish and other aquatic organisms is obvious, but the ecological ramifications go far beyond the river. Wildlife that depends on the water or on food chains involving aquatic organisms is also adversely affected.  Wetlands occupying floodplains along many rivers, no longer nourished by occasional overflows, dry up, resulting in frequent die-offs of waterfowls and other wild life that dependent on those habitats.[181]  Dams disrupt the integrity of the river system. [185]


12. Distinguish between renewable and nonrenewable groundwater sources. What are the consequences of overdrawing of these two kinds of groundwater?

Renewable groundwater is replenished by the percolation of precipitation water, so it is vulnerable to variations in precipitation.

Nonrenewable groundwater – that has recharge rate of centuries and more.


The simplest identification that groundwater withdrawals exceed recharge is a falling water table. As irrigation consumes the largest amount of fresh water, the most significant effect of that will be ultimately on crops production.

Surface waters are also affected by falling water tables. When water table drops, wetlands dry up, springs and seeps dry up, diminishing streams and rivers to the point of dryness.

Groundwater leaches 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 and land subsidence (gradual setting of the land) or sinkhole (sudden and dramatic collapse of an underground cavern) could occur.

Another problem resulting from dropping water tables is saltwater intrusion. 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. Thus, wells near ocean yield fresh water. 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 hence into wells. [179-183]


13. What are the four options for meeting existing water scarcity needs and growing demands?

The options for meeting the existing needs and growing demands of water are:

New dams construction to capture more of the runoff water. There are opportunities to capture more of the seasonal floodwaters that rush to the sea in a short period of time, and a combination of flood control, water storage and hydropower can be a powerful argument for dam construction.

Gain better access to existing groundwater aquifers. Exploiting renewable groundwater will continue to be an option, but it is unlikely to provide great increases in water supply because recharged by annual precipitation it is being increasingly polluted.

Desalination of seawater for domestic use is becoming more popular. Two technologies are usually used: microfiltration and distillation.

Conserve present suppliers by using less water: in agriculture (water-saving methods of irrigation), in municipal systems (reducing of water consumption, gray water recycling systems). [185-186]


14. Describe how water demands might be reduced in agriculture, industry and households.

To cut down the water wastes and losses in agriculture the following methods could be used:
- Surge flow method, in which computers control the periodic release of water in contrast to the continuous-flood method. Surge flow can cut water use in half.

- Drip irrigation system – a network of plastic pipes with pinholes that literally drip water at the base of each plant. Drip irrigation can reduce water consumption by 30 to 70%.

- Affordable irrigation technologies use, such as for example treadle pumps that work like a step exercise machine and allow irrigating small plots during dry periods.

To reduce water consumption and wastage in household:

- Programs whereby leaky faucets are repaired and low-flow showerheads and water-displacement devices in toilets are installed free of charge.

- Displacement of lawns with xeriscaping - landscaping with desert species that require no additional watering.

- Low-flow toilets save an estimated 600 mln gallons of water a day in US.

- Adoption of gray water recycling systems – slightly dirtied water from sinks, shower, bathtubs and laundry tubs used for flushing toilets, watering lawns, washing cars; using treated sewage water for irrigating golf courses and landscapes. [187-188]



15. What is the status of water policy in the United States? Cite some key issues that should be addressed by any new initiatives to establish a national water policy.

In the US water resources and needs are not addressed with a national policy. The Clean Wtare Act and subsequent amendments authorize the U.S. EPA to develop programs and rules to carry out its mandate for oversight of the nation’s water quality. The EPA, however, does not deal with water quality. There is no federal bureaucracy to provide similar oversight for the water quantity issues.

Key issues to be addressed:

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

- Water subsidies need to be reduced or eliminated.

- Polluters must be charged according to their effluents.

- Watershed management must be integrated into the pricing of water.

- Water authorities must regulate dam operations.

- The US must respond to the global water crisis with adequate levels of international development aid.

- Much more researches and monitoring are needed to provide the basic data for making informed policy decisions. [190]