Chapter 9 Water Resources

Geosystems 5eAn Introduction to Physical Geography

Robert W. ChristophersonCharlie Thomsen

Web URL for GEOG 123b:

http://instruct.uwo.ca/geog/123b/http://instruct.uwo.ca/geog/123b/

-Assignment #2 can be printed from that location.

Note about previous discussion about Global Warming: Global DimmingScientists are now discovering that the amount of solar energy reaching the Earth's surface has been gradually falling in the last few decades. Paradoxically, the decline in sunlight may mean that global warming is a far greater threat to society than previously thought. Dimming appears to be caused by air pollution. Burning coal, oil and wood, whether in cars, power stations or cooking fires, produces not only invisible carbon dioxide - the principal greenhouse gas responsible for global warming - but also tiny airborne particles of soot, ash, sulfur compounds and other pollutants. Because the particles seed the formation of water droplets, polluted clouds contain a larger number of droplets than unpolluted clouds. Recent research shows that this makes them more reflective than they would otherwise be, reflecting the Sun's rays back into space. But perhaps the most alarming aspect of global dimming is that it may have led scientists to underestimate the true power of the greenhouse effect. They know how much extra energy is being trapped in the Earth's atmosphere by the extra carbon dioxide we have placed there. What has been surprising is that this extra energy has so far resulted in a temperature rise of just 0.6 degree Celsius. This has led many scientists to conclude that the present-day climate is less sensitive to the effects of carbon dioxide than it was, say, during the ice age, when a similar rise in CO2 led to a temperature rise of six degrees Celsius. But it now appears the warming from greenhouse gases has been offset by a strong cooling effect from dimming - in effect two of our pollutants have been canceling each other out. This means that the climate may in fact be more sensitive to the greenhouse effect than previously thought.

Lecture overview:In this lecture we will examine the Earth’s plumbing system—the hydrologic cycle. The text (Chapter 9) focuses in on the soil-moisture environment and the application of the hydrologic cycle to a specific site. Also, the water balance, which is an accounting of the hydrologic cycle for a specific area with emphasis on plants and soil moisture follows. We will also review the nature of groundwater and look at several examples of this generally abused resource. Groundwater resources are closely tied to surface-water budgets. We will also consider the daily water we withdraw and consume from available resources, in terms of both quantity and quality.

Some basic stats:Fortunately, water is a renewable resource, constantly cycling through the environment, endlessly renewed. Even so, some 80 countries face impending water shortages, either in quantity or quality, or both. One billion people lack access to safe water in 2001; some 1.8 billion lack adequate sanitary facilities. During the first half of the new century water availability per person will drop by 74%, as population increases and adequate quality water decreases.

After this lecture and reading the chapter you should be able to:

IllustrateIllustrate the hydrologic cycle with a simple sketch and labellabel it with definitions for each water pathway.RelateRelate the importance of the water-budget concept to your understanding of the hydrologic cycle, water resources, and soil moisture for a specific location.ConstructConstruct the water-balance equation as a way of accounting for the expenditures of water supply and definedefine each of the components in the equation and their specific operation.DescribeDescribe the nature of groundwater and definedefine the elements of the groundwater environment.IdentifyIdentify critical aspects of freshwater supplies for the future and cite specific issues related to sectors of use, regions and countries, and potential remedies for any shortfalls.

1. Explaining a simplified model of the complex flows of water on Earth—the hydrologic cycle.

Vast currents of water, water vapor, ice, and energy are flowing about us continuously in an elaborate open global plumbing system. A simplified model of this complex system is useful to our study of the hydrologic cycle (Figure 9-1). The ocean provides a starting point, where more than 97% of all water is located and most evaporation and precipitation occur.

Hydrologic Cycle Model: The model shows how water travels endlessly through the hydrosphere, atmosphere, lithosphere, and biosphere. The triangles show global average

values as percentages. Note that all evaporation equals all precipitation when all of the Earth is considered. Regionally, various parts of the cycle will vary, creating imbalances and,

depending on climate, surpluses in one region and shortages in another.

Figure 9.1Figure 9.1

Hydrologic cycle Cont’d If we assume that mean annual global evaporation equals 100 units, we can trace 86 of them to the ocean. The other 14 units come from the land, including water moving from the soil into plant roots and passing through their leaves. Of the ocean's evaporated 86 units, 66 combine with 12 advected (transported) from the land to produce the 78 units of precipitation that fall back into the ocean. The remaining 20 units of moisture evaporated from the ocean, plus 2 units of land-derived moisture, produce the 22 units of precipitation that fall over land. Clearly, the bulk of continental precipitation derives from the oceanic portion of the cycle.

Question: What are the possible routes that a raindrop may take on its way to and into the soil surface?

Answer: Precipitation that reaches Earth's surface follows a variety of pathways. The process of precipitation striking vegetation or other groundcover is called interception. Precipitation that falls directly to the ground, coupled with drips onto the ground from vegetation, constitutes throughfall. Intercepted water that drains across plant leaves and down plant stems is termed stem flow and can represent an important moisture route to the surface. Water reaches the subsurface through infiltration, or penetration of the soil surface. It then permeates soil or rock through vertical movement called percolation (Figure 9.3).

Fig. 9.3Fig. 9.3: The soil-moisture environment: Precipitation

supplies the soil-moisture environment. The principal pathways for water include

interception by plants; throughfall to the ground; collection on the surface, forming overland flow to

streams; transpiration (water moving from the soil into plant

roots and passing through their leaves) and evaporation from plant; evaporation from

land and water; and gravitational water moving to

subsurface groundwater. Water moves from the surface into the soil by infiltration and

percolation.

How do precipitation and evaporation volumes from the ocean compare with those over land?

More than 97% of Earth's water is in the ocean, and here most evaporation and precipitation occur. 86% of all evaporation can be traced to the ocean. The other 14% comes from the land, including water moving from the soil into plant roots and passing through their leaves by transpiration. Of the ocean's evaporated 86%, 66% combines with 12% advected from the land to produce the 78% of all precipitation that falls back into the ocean. The remaining 20% of moisture evaporated from the ocean, plus 2% of land-derived moisture, produces the 22% of all precipitation that falls over land.

How might an understanding of the hydrologic cycle in a particular locale, or a soil-moisture budget of a site, assist you

in assessing water resources? Some specific examples.

A soil-moisture budget can be established for any area of Earth's surface by measuring the precipitation input and its distribution to satisfy the "demands" of plants, evaporation, and soil moisture storage in the area considered. A budget can be constructed for any time frame, from minutes to years. See Figures 9-11 in next slide.

Figure 9.11: Sample water budget. Annual average water-balance components graphed for Kingsport, Tennessee. The comparison of plots for precipitation inputs (PERCIP), and potential evapotranspiration outputs (POTET) determines the condition of the soil-moisture environment. A typical pattern of spring surplus, summer soil-moisture utilization, a small summer deficit, autumn soil-moisture recharge, and ending surplus highlights the year.

What does “The soil-water budget is an assessment of the hydrologic cycle at a specific site” means?

A water balance can be established for any area of Earth's surface by calculating the total precipitation input and the total of various outputs. The water-balance approach allows an examination of the hydrologic cycle, including estimation of streamflow at a specific site or area, for any period of time. The purpose of the water balance is to describe the various ways in which the water supply is expended. The water balance is a method by which we can account for the hydrologic cycle of a specific area, with emphasis on plants and soil moisture.

What are the components of the water-balance equation? (Fig. 9.4)What are the components of the water-balance equation? (Fig. 9.4)

Explain how to derive actual evapotranspiration (ACTET) in the water-balance equation.

The actual amount of evaporation and transpiration that occurs is derived by subtracting DEFIC, or water demand, from POTET. Under ideal conditions, POTET and ACTET are about the same, so that plants do not experience a water shortage. Droughts result from deficit conditions, where ACTET is greater than the available moisture.

What is potential evapotranspiration (POTET)? How do we go about estimating this potential rate?

POTET is the amount of moisture that would evaporate and transpire if the moisture were available; the amount lost under optimum moisture conditions—the moisture demand. Both evaporation and transpiration directly respond to climatic conditions of temperature and humidity. For the empirical measurement of POTET, probably the easiest method employs an evaporation pan, or evaporimeter. As evaporation occurs, water in measured amounts is automatically replaced in the pan. Screens of various sized mesh are used to protect against overmeasurements created by wind. A lysimeter is a relatively elaborate device for measuring POTET, for an actual portion of a field is isolated so that the moisture moving through it can be measured. See next slide (Figure 9-7) for a sketch of such a device.

Fig 9.7: Lysimeter Drawn is a weighing

lysimeter for measuring evaporation and transpiration. The various pathways of water are tracked: Some water remains as soil moisture, some is incorporated into plant tissues, some drains from the bottom of the lysimeter, and the remainder is credited to evapotranspiration. Given natural conditions, the lysimeter measures actual evapotranspiration.

Explaining the operation of soil-moisture storage, soil-moisture utilization, and soil-moisture recharge.

Soil moisture storage (STRGE) refers to the amount of water that is stored in the soil and is accessible to plant roots, or the effective rooting depth of plants in a specific soil. This water is held in the soil against the pull of gravity. Soil is said to be at the wilting point when plant roots are unable to extract water; in other words, plants will wilt and eventually die after prolonged moisture deficit stress.

The soil moisture that is generally accessible to plant roots is capillary water, held in the soil by surface tension and cohesive forces between the water and the soil. Almost all capillary water is available water in soil moisture storage and is removable for POTET demands through the action of plant roots and surface evaporation; some capillary water remains adhered to soil particles along with hygroscopic water. When capillary water is full in a particular soil, that soil is said to be at field capacity, an amount determined by actual soil surveys.

Cont’dWhen soil moisture is at field capacity, plant roots are able to obtain water with less effort, and water is thus rapidly available to them. As the soil water is reduced by soil moisture utilization, the plants must exert greater effort to extract the same amount of moisture. Whether naturally occurring or artificially applied, water infiltrates soil and replenishes available water content, a process known as soil moisture recharge.

Example (Fig. 9.10): In the case of silt-loam soil from, roughly what is the available water capacity? How is this value derived?

The lower line on the graph plots the wilting point; the upper line plots field capacity. The space between the two lines represents the amount of water available to plants given varying soil textures. Different plant types growing in various types of soil send roots to different depths and therefore are exposed to varying amounts of soil moisture. For example, shallow-rooted crops such as spinach, beans, and carrots send roots down 65 cm (25 in.) in a silt loam, whereas deep-rooted crops such as alfalfa and shrubs exceed a depth of 125 cm (50 in.) in such a soil. A soil blend that maximizes available water is best for supplying plant water needs.

Example: Water balance and water management scheme in Snowy Mountain, Southeastern Australia.

In the Snowy Mountains, part of the Great Dividing Range in extreme southeastern Australia, precipitation ranges from 100 to 200 cm (40 to 80 in.) a year, whereas interior Australia receives under 50 cm (20 in.), and drops to less than 25 cm (10 in.) further inland. POTET (potential evapotranspiration) values are high throughout the Australian outback and lower in the higher elevations of the Snowy Mountains.

The plan was designed to take surplus water that flowed down the Snowy River eastward to the Tasman Sea and reverse the flow to support newly irrigated farmland in the interior of New South Wales and Victoria. The westward flow of the Murray, Tumut, and Murrumbidgee rivers is augmented, and as a result, new acreage is now in production in what was dry outback, formerly served only by wells drawing on meager groundwater supplies.

In the 1990s the Scheme passed its 50th anniversary of operation, a major milestone.

Are groundwater resources independent of surface supplies,

or are the two interrelated? (Movie at the end of lecture)

Groundwater is the part of the hydrologic cycle that lies beneath the ground and is therefore tied to surface supplies. Groundwater is the largest potential source of freshwater in the hydrologic cycle–larger than all surface reservoirs, lakes, rivers, and streams combined. Between Earth's surface and a depth of 3 km (10,000 ft) worldwide, some 8,340,000 km3 (2,000,000 mi3) of water resides. (See next slide).

Fig. 9.15: Groundwater resource potential for the United States and Canada. Highlighted areas of the United states are underlain by productive

aquifers capable of yielding freshwater to wells at 0.2 m3/per minute or more- for Canada the figure is 0.4 liters per second.

At what point does groundwater utilization become groundwater mining?

Aquifers frequently are pumped beyond their flow and recharge capacities; groundwater mining refers to this overutilization of groundwater resources. Large tracts of the Midwest, West, lower Mississippi Valley, and Florida experience chronic groundwater over drafts. In many places the water table or artesian water level has declined more than 12 m (40 ft). Groundwater mining is of special concern today in the High Plains aquifer.

What is the nature of groundwater pollution? Can contaminated groundwater be cleaned up easily?

When surface water is polluted, groundwater also becomes contaminated because it is fed and recharged from surface water supplies. Groundwater migrates very slowly compared with surface water. Surface water flows rapidly and flushes pollution downstream, but sluggish groundwater, once contaminated, remains polluted virtually forever. Pollution can enter groundwater from industrial injection wells, septic tank outflows, seepage from hazardous-waste disposal sites, industrial toxic-waste dumps, residues of agricultural pesticides, herbicides, fertilizers, and residential and urban wastes in landfills. Thus, pollution can come either from a point source or from a large general area (a non-point source), and it can spread over a great distance. Because surface water flows rapidly, it can flush pollution downstream. Yet, if groundwater is polluted, because it is slow moving, once its contaminated, it will remain polluted virtually forever.

Water Scarcity

Water Sustainability Issues

World Water Day, March 22, 2003

Water Consumption1.3 gallons/day needed to survive, on average

13 gallons/day needed for drinking, cooking, bathing, and sanitation

U.S.: 65-78 gallons/day for drinking, cooking, bathing and lawn watering

Somalia: 2.3 gallons/day

Product Unit cubic metersCattle head 4,000Sheep and goats head 500Beef kilogram 15Sheep kilogram 10Poultry kilogram 6Cereals kilogram 1.5Citrus kilogram 1Roots & tubers kilogram 1

http://water.usgs.gov/watuse/graphics/wuto.fact.3d.gif

Summary: Why the interest?~1.2 billion people lack clean drinking water~250 million cases of water-related disease/year, with 5 – 10 million deathsOver last 100 years, nearly ½ of all wetlands lostWater pollution growing problem (50% people in developing countries have polluted water sources)

End of Chapter 9

Geosystems 5eAn Introduction to Physical Geography

Robert W. ChristophersonCharlie Thomsen

MovieGroundwater Approximately three-quarters of Earth’s surface is covered by water. But most fresh water comes from underground. Topics of this program include aquifers, rock porosity and permeability, artesian wells, the water table, cave formation, sinkholes, and how groundwater may become contaminated.