Lecture 5
- 1. The Hydrologic Cycle The Blue Planet: Chapter 8
- 2. Outline • Water and the Hydrologic Cycle • Water on the Ground • Water Under the Ground • Water and Society
- 3. Water and the Hydrologic Cycle • Water plays a central role in moderating temperature and controlling climate • The erosional and depositional effects of streams, waves, and glaciers, coupled with tectonic activity have produced the diversity of Earth’s landscapes • The unique chemical properties of water make life possible • The hydrologic cycle maintains a mass balance on a global scale
- 4. Water and the Hydrologic Cycle
- 5. Water and the Hydrologic Cycle • The largest reservoir in the hydrologic cycle is the ocean – Contains more than 97.5% of Earth’s water – Most of the water in the hydrologic cycle is saline, and not usable by humans • The largest reservoir of fresh water is the polar ice sheets – Contain 74% of the Earth’s fresh water • The largest reservoir of unfrozen fresh water is groundwater
- 6. Water and the Hydrologic Cycle
- 7. Water and the Hydrologic Cycle • Movement of water through the hydrologic cycle is powered by the Sun – Evaporation – Condensation – Precipitation – Transpiration – Surface runoff – Infiltration
- 8. Water and the Hydrologic Cycle
- 9. Outline • Water and the Hydrologic Cycle • Water on the Ground • Water Under the Ground • Water and Society
- 10. Water on the Ground • During a heavy rainfall, water moves downhill – Initially as sheet flow (overland flow) – Gets concentrated into stream flow • Consists of storm flow and base flow • Streams with no base flow are ephemeral • Streams with base flow are perennial
- 11. Water on the Ground • Streams are part of a complex natural system that includes – A channel – A drainage basin – A divide • The stream’s load is the total sediment and dissolved matter it is transporting
- 12. Water on the Ground
- 13. Water on the Ground
- 14. Water on the Ground • Streams and drainage systems – Play a fundamental role in both the hydrologic cycle and the rock cycle – Support complex ecosystems – Are constantly evolving in response to changing relief, climate and vegetation
- 15. Water on the Ground • The size and shape of a stream channel are controlled by several factors – Erodibility of rock – Steepness of descent – Volume of water • The vertical distance that a stream channel descends along its course is its gradient, overall this decreases downstream, though not smoothly
- 16. Water on the Ground • Stream behavior is controlled by 5 basic factors 1. Average channel width and depth 2. Channel gradient 3. Average water velocity 4. Discharge 5. Sediment load • All streams experience a continuous interplay among these factors
- 17. Water on the Ground • Following a stream from its source to its mouth, orderly adjustments occur – Width and depth increase – Gradient decreases – Flow velocity and discharge increase – Turbulence decreases
- 18. Water on the Ground
- 19. Water on the Ground • Meandering channels – Straight channels are rare – Low gradient streams typically assume a sinuous shape, each bend is a meander – The shape reflects the way the stream minimizes resistance to flow – Velocity is lowest along inside meanders – Velocity is highest along outside meanders
- 20. Water on the Ground • Meandering channels – Sediment accumulates on the inner side of a meander, forming a point bar – Collapse of the stream bank occurs on the outside of a meander, forming a cut bank – In this way meanders tend to migrate – Sometimes a stream bypasses a channel loop, cutting it off and forming an oxbow lake
- 21. Water on the Ground
- 22. Water on the Ground
- 23. Water on the Ground • Braided channels – A stream that is unable to transport the entire available sediment load tends to deposit the coarsest and densest sediment to form a bar, which locally divides and concentrates the flow – A stream with many interlacing channels and bars is called braided – Tends to have variable discharge and easily erodible banks
- 24. Water on the Ground
- 25. Water on the Ground • The size of clasts a stream can transport is mainly related to velocity • However, the size of clasts decreases downstream from the rocky headwaters • A stream’s load consists of three parts – Bed load – Suspended load – Dissolved load
- 26. Water on the Ground • Bed load – 5-50% of total sediment load – Move by rolling, sliding, or saltation • Suspended load – Particles of silt and clay provide the muddy character of many streams • Dissolved load – Comprised primarily of 7 ions • Bicarbonate, calcium, sulfate, chloride, sodium, magnesium, and potassium
- 27. Water on the Ground
- 28. Water on the Ground
- 29. Water on the Ground
- 30. Water on the Ground • Streams form three major depositional landforms – Floodplain: deposition of fine sediment beyond natural levees during a flood – Alluvial fan: a fan-shaped body of alluvium at the base of an upland area – Delta: triangular shaped deposit formed when a stream enters the standing water of a sea or lake
- 31. Water on the Ground
- 32. Water on the Ground
- 33. Water on the Ground • All continents are divided into large regions from which major rivers flow to one of the world’s major oceans – The line separating any two of these is called a continental divide • Continental divides often coincide with crests of mountains, the result of uplift, we know that there is a close relationship between plate tectonics and the locations of stream basins
- 34. Water on the Ground
- 35. Water on the Ground • Water can remain stored in any of several surface water reservoirs – Ice caps are the greatest of these – Lakes: mainly in high latitudes • formed by glaciation, volcanism, tectonism, streamflow, natural dams, cave collapse, ice dam collapse, permafrost thaw, and coastal processes – Wetlands: permanently or intermittently moist • Include swamps, marshes and bogs • Highly biologically productive
- 36. Water on the Ground
- 37. Water on the Ground • Flooding – Occurs when a stream’s discharge becomes so great that it exceeds the capacity of the channel – Major floods occur infrequently, but can be devastating or catastrophic – During a flood, the peak discharge comes well after the rains that produced it • After rainfall, surface runoff moves into stream channels, quickly increasing discharge
- 38. Water on the Ground
- 39. Water on the Ground
- 40. Water on the Ground
- 41. Water on the Ground • Because floods can be so dangerous, prediction has become essential • Frequency of past floods can be plotted, calculating the average time interval between two floods of equal magnitude – This is called the recurrence interval • In addition, real-time monitoring during storms in combination with information about the river basin’s geometry helps
- 42. Water on the Ground
- 43. Water on the Ground • Flood prevention and channelization – River channels are often modified for the purpose of flood control • This is called channelization – Channelization protects our well-being, for a time, and at a price • Interferes with ecosystems • Can aggravate pollution • Does not always protect against flooding, and in fact increases the chances of it • Can lead to subsidence • Renders historic hydrologic data invalid
- 44. Outline • Water and the Hydrologic Cycle • Water on the Ground • Water Under the Ground • Water and Society
- 45. Water Under the Ground • Less than 1% of the liquid water in the hydrosphere lies beneath the ground – Groundwater • It comprises a volume 35 times larger than the volume of all the freshwater lakes and streams, and is nearly a third as large as all the glaciers and sea ice • More than 50% of it is within 750 m depth
- 46. Water Under the Ground • The elements dissolved in groundwater consist of chlorides, sulfates, and bicarbonates of calcium, magnesium, sodium and potassium – Which dissolve from common rock-forming minerals – Consequently the composition of ground- water varies from place to place depending on the surrounding rocks
- 47. Water Under the Ground • From the ground surface to beneath the water table, the regolith – Is filled with air: aerated zone – Is filled with water: saturated zone • The upper surface of this is the water table • The water table represents the upper limit of all readily usable groundwater – It follows the shape of the ground surface, higher under hills, and lower under valleys
- 48. Water Under the Ground
- 49. Water Under the Ground • Groundwater flows between pore spaces by percolation – This flow depends on the porosity and permeability of the rock it moves through – Porosity: the percentage of the total volume of rock that consists of open pore spaces – Permeability: a measure of how easily a solid allows fluids to pass through it
- 50. Water Under the Ground
- 51. Water Under the Ground • Groundwater flows from high water table areas to low water table areas in response to gravity • Replenishment occurs when rainfall and snowmelt enter the ground in areas of recharge • Water moves through the system to areas of discharge, where it meets the surface, streams, lakes, ponds or wetlands
- 52. Water Under the Ground
- 53. Water Under the Ground • An aquifer is a body of rock or regolith sufficiently porous and permeable to store and conduct significant quantities of groundwater – If it has a water table, it is unconfined – If the rate of withdrawal exceeds the rate of local groundwater flow, a cone of depression may form – A confined aquifer is bounded above and below by impermeable rock (aquiclude) – If it has high hydrostatic pressure, it is an artesian aquifer, freely flowing
- 54. Water Under the Ground
- 55. Water Under the Ground • A spring is a flow of groundwater emerging naturally at the ground surface
- 56. Water Under the Ground • Slow moving groundwater has the capacity to dissolve a lot of material – Limestone and marble are very susceptible • A cave will form when circulating groundwater dissolves an underground void with only no opening to the surface • The passage is enlarged in the most favorable flow route by the water that fills the opening
- 57. Water Under the Ground • Spectacular cave formations are deposited by precipitation of materials from the groundwater
- 58. Water Under the Ground • In contrast to a cave, a sinkhole is a large dissolution cavity open to the sky • In regions of exceptionally soluble rock, sinkholes and caves are so numerous that they combine to form a distinct topography of smalls basins, ridges, and pinnacles called karst – This is best developed in moist, tropical regions underlain by limestone
- 59. Water Under the Ground
- 60. Outline • Water and the Hydrologic Cycle • Water on the Ground • Water Under the Ground • Water and Society
- 61. Water and Society • A reliable water supply is critical – For human survival and health – For industry and agriculture – For environmental services • Water is under threat almost everywhere in the world in terms of quality and quantity • Laws and policies are confusing and complicated, and groundwater is difficult to monitor
- 62. Water and Society • Crop irrigation demands 75% of water • Industry demands 20% • Domestic use demands 5% • However, proportions vary greatly from one region to another • Population growth is partly responsible for increasing demand, as are improvements in living standards
- 63. Water and Society • 29 countries worldwide suffer from water shortages (450 million people) • Interbasin transfer of water from one drainage basin to another to meet high water demands raises political issues, and can have environmental impacts • Excessive groundwater withdrawal can lead to lowering of the water table, drying of springs, compaction and subsidence
- 65. Water and Society • About 1.2 billion people, mainly In developing countries, do not have access to clean drinking water • In North America, water is drawn from relatively clean sources, but is still monitored and treated with chlorination to kill microorganisms
- 66. Water and Society • The accessibility of surface water makes them useful resources, but highly susceptible to contamination • Contaminants come from – Urban, suburban and agricultural runoff – Industrial and landfill effluents – Mining, logging and petroleum discharge – Airborne contaminants – Thermal pollution
- 67. Water and Society • A common form of surface water contamination results from excess plant nutrients from fertilizers and detergents • Triggers algae growth, and aquatic weeds get out of control: algal bloom • When they die, their breakdown causes oxygen depletion, killing other organisms in the water: eutrophication • If accelerated by the addition of anthropogenic pollutants, it is called cultural eutrophication
- 69. Water and Society • Groundwater contamination is caused by many of the same pollutants that affect surface water • However, it can be much more difficult to detect, control, and clean up – Passive remediation involves relying on natural environmental processes to clean up the site – Active remediation involves intervention by injecting oxygen or other chemicals to speed the breakdown of contaminants
- 70. Extras
- 71. Extras
- 72. Extras
- 73. The Cryosphere The Blue Planet: Chapter 9
- 74. Outline • Earth’s Cover of Snow and Ice • Glaciers • Glaciation • Sea Ice • Ice in the Earth System
- 75. Earth’s Cover of Snow and Ice • The part of Earth’s surface that remains perennially frozen is the cryosphere – Sea ice – Glaciers: 10% of Earth’s land surface – Frozen ground: 20% of Earth’s land
- 76. Earth’s Cover of Snow and Ice • In the Northern hemisphere almost 1/4 of the land is covered by snow and frozen ground during the winter • The accumulation of snow fall that is greater than seasonal melt is the main contributor to glaciers and ice caps • This highly reflective surface bounces sunlight back into space, reducing surface air temperature
- 77. Earth’s Cover of Snow and Ice
- 78. Earth’s Cover of Snow and Ice • Snow melt is a major source of water for rivers and moisture for soils • During a typical Northern hemisphere year: – Sept-Oct: snow appears in Alaska/Siberia – Nov: expands south and thickens – Dec: snow reaches Russia/Europe/USA – Dec-Mar: snowpack thickens, southern edge retreats – Late Spring: snowpack recedes rapidly – June: confined to high mountains and Arctic
- 79. Earth’s Cover of Snow and Ice • The lower limit of perennial snow is the snowline – Its shape is controlled by variations in thickness of winter snowpack and local topography – Altitude typically changes from year to year • Dependent on winter snow accumulation and summer melting • Above this, part of winter’s snow has survived summer • In polar regions annual snowfall is very low because the air is too cold to hold moisture – Polar deserts
- 80. Earth’s Cover of Snow and Ice
- 81. Outline • Earth’s Cover of Snow and Ice • Glaciers • Glaciation • Sea Ice • Ice in the Earth System
- 82. Glaciers • When snow and ice become so thick that the pull of gravity causes the frozen mass to move, this is a glacier – Cirque glacier: the smallest – Valley glacier: extends down from a cirque – Ice cap: cover mountain highlands or low lying land at high latitudes, flow radially out – Fjord glacier: glacier in a fjord – Piedmont glacier: spreads out from a valley glacier
- 83. Glaciers
- 84. Glaciers • Earth’s high mountain ranges contain glacier systems tens of kilometers long • Continent-sized ice sheets overwhelm nearly all the land within their margins – Greenland and Antarctica include 95% of Earth’s glaciers and reach 3000 m thick • Ice shelves hundreds of meters thick occupy Antarctica’s embayments
- 85. Glaciers
- 86. Glaciers • Glaciers form from snow that has accumulated and been compacted until it is so dense that it is impenetrable to air – Glacier ice is considered a rock • Ice grains recrystallize at depth within the glacier, reaching 1 cm near the base
- 87. Glaciers
- 88. Glaciers • Glaciers form wherever snow and ice can accumulate – High latitudes – High mountains at low latitudes • Ice temperatures vary among glaciers – Warm (temperate) glaciers: at pressure melting point, can coexist with water – Cold (polar) glaciers: below pressure melting point
- 89. Glaciers
- 90. Glaciers • A glacier’s advance or retreat is the balance of the amount of snow and ice added (accumulation) and lost (ablation) – Upper zone is the accumulation area – Below this is the ablation area – Between these is the equilibrium line – The front of the glacier is called the terminus
- 91. Glaciers
- 92. Glaciers
- 93. Glaciers • The terminus of a glacier responds to changes in mass balance • The movement of glacier ice occurs by 1. Internal flow • Ice at critical thickness deforms and is pulled downslope by gravity • Occurs within individual ice crystals under stress • The surface is brittle, cracks under tension, forms crevasses, <50 m deep 1. Basal sliding • Meltwater at the base acts as lubricant
- 94. Glaciers
- 95. Glaciers • The uppermost ice in the central part of a glacier flows faster than the sides and the base - similar to a river • Flow velocities vary from cm to m per day • Even as a terminus is retreating, down- glacier flow of ice continues • The response lag is the time for effects of change in accumulation to be transferred through the glacier to the terminus
- 96. Glaciers
- 97. Glaciers • Coastal glacier retreat is characterized by frontal calving – Where the terminus is in deep water – Front breaks off to form icebergs • Fjord glacier termini may remain grounded on a shoal, preventing calving
- 98. Glaciers
- 99. Glaciers
- 100. Glaciers • Most glaciers move slowly, but some experience episodes of unusual rapid behavior called surges – Ice in the accumulation area moves rapidly down-glacier – Produces a chaos of crevasses and broken pinnacles in the ablation area – May advance several km during a surge – The cause is not fully understood
- 101. Glaciers
- 102. Outline • Earth’s Cover of Snow and Ice • Glaciers • Glaciation • Sea Ice • Ice in the Earth System
- 103. Glaciation • The Earth fluctuates between periods of extended cooler and warmer temperatures – Glaciations: glaciers expand, and new ones form – Interglacials: ice sheets retreat, sea level rises • We are in an interglacial period
- 104. Glaciation • As glaciers flow, they sculpt the land – As plows, scraping up weathered rock – As files, rasping firm rock – As sleds, carrying away sediment • A glacier can carry very large boulders and very fine sediment, but due to its viscosity, there is no segregation by size or density – Glacial deposits are neither sorted nor stratified
- 105. Glaciation • Glaciers pluck rock fragments from leeward outcrops • These rocks embedded in the base of the ice scrape underlying bedrock producing long parallel scratches called striations, or deeper glacial grooves • Fine fragments of silt called rock flour polish the bedrock until it has a smooth reflective surface
- 106. Glaciation
- 107. Glaciation • Landforms sculpted by glacial erosion include – Cirques – Aretes – U-shaped valleys – Fjords – Drumlins
- 108. Glaciation
- 109. Glaciation • The debris carried by the glacier eventually gets deposited – Till: unsorted glacial debris deposited by glacial ice – Outwash: debris reworked, transported and deposited by meltwater – Moraines: glacially bulldozed ridges of sediment – Esker: curved ridge of sand and gravel – Kettle: closed basin
- 110. Glaciation
- 111. Glaciation
- 112. Glaciation • Land beyond the limit of glaciers is called periglacial and is mainly found in circumpolar regions – Characterized by permafrost • The active layer of this thaws in summer, becoming unstable, and refreezes in winter • When it melts, ground collapses – Patterned ground: ice-wedge polygons – Pingoes: frost-heaved hills – Gelifluction: mass wasting
- 113. Glaciation
- 114. Glaciation
- 115. Glaciation
- 116. Glaciation • Trapped in the snow that accumulates each year on a glacier is evidence of local and global environmental conditions • The oldest ice in most cirque and valley glaciers is several hundred to several thousand years old • Large ice sheets contain ice that dates far back into the ice ages • This can be examined through ice cores
- 117. Glaciation
- 118. Outline • Earth’s Cover of Snow and Ice • Glaciers • Glaciation • Sea Ice • Ice in the Earth System
- 119. Sea Ice • Forms by the solidification of fresh water at the ocean surface, not by precipitation, salt crystals are excluded – In glacial periods, the ocean becomes saltier, during interglacials, sea ice melts and it becomes less salty • 2/3 of Earth’s persistent ice cover floats as a thin veneer on polar oceans • But it only comprises 1/1000 of Earth’s total volume of ice
- 120. Sea Ice
- 121. Sea Ice • Once the ocean surface cools to the freezing point of sea water, slight additional cooling leads to ice formation – First small platelets or needles called frazil – Then a soupy mixture at the surface – Without waves, crystals freeze together to form a 1-10 cm thick blanket of ice – With waves, crystals form 3 cm diameter pancake-like ice masses
- 122. Sea Ice
- 123. Sea Ice • Sea ice is distributed differently between the two hemispheres due to contrasting geography – Antarctica is covered by a vast, thick ice sheet, and sea ice forms a ring around it – The North Pole is within the deep Arctic Ocean, mostly covered by sea ice • Sea ice is in constant motion and is constantly changing
- 124. Sea Ice
- 125. Sea Ice
- 126. Sea Ice
- 127. Outline • Earth’s Cover of Snow and Ice • Glaciers • Glaciation • Sea Ice • Ice in the Earth System
- 128. Ice in the Earth System • Influence on ocean salinity and circulation – Interactions among ice, water and atmosphere influence ocean structure, salinity and circulation – Sea ice is very sensitive to temperature change, and the exclusion of salt as it forms leads to the production of dense, cold saline water on the continental shelves – This produces deep bottom ocean water
- 129. Ice in the Earth System • Influence on atmospheric circulation and climate – Floating ice isolates the ocean surface from the atmosphere, cutting off heat exchange – Ice has high albedo, reflecting incoming solar radiation rather than absorbing it – This results in a steep temperature gradient between the equator and poles, driving atmospheric circulation
- 130. Ice in the Earth System • Ice cover and environmental change – If the climate became colder and ice over expanded, the result would be a positive feedback due to raised albedo – If the climate warms and ice cover shrinks or disappears, a similar but opposite effect of positive feedback warming would occur as the Earth’s overall albedo decreases • While melting of sea ice would contribute little to ocean levels, however melting of land ice would contribute significantly to water volume • Both would drastically affect ocean salinity
- 131. Extras
- 132. Extras
- 133. Extras
- 134. Extras