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The Hydrologic Cycle

The Hydrologic Cycle

  • Water is distributed very unevenly around Earth.
  • Less than 1 percent of Earth’s total moisture is involved in the hydrologic cycle.
  • Hydrologic cycle—a series of storage areas interconnected by various transfer processes, in which there is a ceaseless interchange of moisture in terms of its geographic location and its physical state.
    • Surface-to-Air Water Movement
      • Evaporation is responsible for most of the moisture that enters the atmosphere from Earth’s surface.
        • Of the moisture evaporated, more than 84 percent comes from ocean surfaces.
        • The water evaporated becomes water vapor, and though it stays in the atmosphere only briefly (hours to days), it can travel a considerable distance, either vertically or horizontally.
    • Air-to-Surface Water Movement
      • Water vapor either condenses to liquid water or sublimates to ice to form cloud particles.
      • Clouds drop precipitation (rain, snow, sleet, hail).
      • Precipitation and evaporation/transpiration balance in time.
        • They do not balance in place.
          • Evaporation exceeds precipitation over oceans.
          • Precipitation exceeds evaporation over lands.
    • Movement on and Beneath Earth’s Surface
      • Runoff—flow of water from land to oceans by overland flow, streamflow, and groundwater flow.
        • Runoff is why the oceans do not dry up and continents become flooded despite the imbalance of evaporation and precipitation through space (oceans and continents).
        • Runoff water amounts to 8 percent of all moisture circulating in the global hydrologic cycle.
    • Residence Times
      • At any given movement, the atmosphere contains only a few days’ potential precipitation.
      • Residence time of a molecule of water can be hundreds of thousands of years to only a few minutes.
    • Energy Transfer in the Hydrologic Cycle
      • Hydrologic cycle is powered by the Sun.
      • It represents a vast reservoir of moisture and energy.
      • Latent heat is released during condensation.
      • Fuels storms and hurricanes and transfers energy from the tropics to the poles.

The Oceans

  • Knowledge of the seas has been very limited until recently.
  • Only in about the past four decades have we developed technology that allows us to catalog and measure details of the ocean environment.
  • The world ocean has a surface area of 360 million square kilometers and contains 1.32 billion cubic kilometers of salt water.
  • Just one ocean, which is divided into five principal parts:
    1. Pacific
    2. Atlantic
    3. Indian
    4. Arctic
    5. Southern
  • Most smaller bodies of water are considered portions of the ocean.
  • A few are so narrowly connected that they warrant separate consideration.
  • Examples are the Black Sea, Mediterranean Sea, and Hudson Bay.

Characteristics of Ocean Waters

  • Significant difference from place to place.
  • Almost all known minerals are found to some extent in seawater, but sodium and chloride are the most important.
    • Salinity—a measure of the concentration of dissolved salts.
      • Geographic distribution of surface salinity varies because of:
      • Varying evaporation rates.
      • Varying fresh water discharge rates.
    • Increased Acidity
      • The oceans take in carbon dioxide and form carbonic acid.
      • As a result of increased carbon emissions from industrialization, the oceans are estimated to be more acidic than they were during the preindustrial era.
      • Currently, oceans possess a pH of 8.1, and it is estimated that the pH of the oceans could drop to 7.7 by the end of the century.
      • The possible consequences of a slightly more acidic ocean are:
      • The limiting of the growth of organisms such as coral polyps and foraminifera.
      • Creatures such as these will have a difficult time building their shells because there are fewer calcium ions in acidic seawater.
      • This could lead to the decline of coral reefs that provide habitats for many organisms.
      • Foraminifera are at the bottom of the food web, so their decline could possibly affect other organisms higher up the food web.
    • Temperature—decreases with increasing latitude.
      • Western sides of oceans are nearly always warmer than eastern margins (movement of major ocean currents).
    • Density—varies with temperature, degree of salinity, and depth.

Movement of Ocean Waters

  • Most motion occurs in waves, currents, and tides.
  • Movement affects the surface more than deeper water.
  • Disturbances in Earth’s crust under the ocean can trigger motion.

Tides  cause the greatest vertical movements of ocean waters and can cause horizontal movement.

  • Rhythmic oscillations about every 6 hours from the gravitational attraction of nearby heavenly bodies.
  • Causes of Tides
    • Although both the Sun and Moon have an influence on Earth’s tides, because of its considerably greater distance, the Sun produces a smaller percentage of Earth’s tides than does the Moon.
    • As Earth rotates, tidal progression appears to move westward.
  • There are two tidal cycles a day.
    • Two high tides and two low tides every 25 hours.
    • Tidal magnitude varies greatly in time and place.
    • Water flows toward the coast in a period of 6 hours and 13 minutes in what is known as a “flood tide.”
    • After reaching high tide, the water then begins to recede over a period of 6 hours and 13 minutes in what is known as an “ebb tide.”
    • Once the water has reached its lowest level, the cycle begins again.
  • Tidal range—refers to the vertical distance in elevation between the high and low tide.
    • Changes in the positions of Earth, Sun, and Moon have influences on periodic variations in tidal ranges.
    • When all are aligned, Earth experiences spring tides.
    • When out of alignment, Earth experiences neap tides.
    • Tidal range is also affected by the distance between Earth and the Moon.
    • During the Moon’s perigee (its closest distance to Earth), tidal ranges are greater than when it is at its apogee (its farthest distance from Earth).
  • Global Variations in Tidal Range
    • Coastline configuration and shape also have an influence on tidal ranges.
    • Greatest tidal range found in the Bay of Fundy in eastern Canada.
    • A tidal bore (a wall of sea water) several centimeters to more than a meter high rushes up the Petitcodiac River in New Brunswick.
    • Inland bodies of water experience the smallest tidal ranges.

Ocean Currents  –  Currents shift water both horizontally and vertically.

  • Primarily caused by wind flow, but also by contrasts in temperature and salinity.
  • Influenced by the size and shape of a particular ocean basin, configuration and depth of the sea bottom, and the Coriolis effect.
  • Deep Ocean Circulation
    • Deep ocean circulation occurs because of differences in water density that arise from differences in salinity and temperature.
    • This circulation is also known as the thermohaline circulation.
    • Sinking happens predominantly at higher latitudes because more fresh water is locked in glacial ice, which causes ocean water to be more saline and denser in these regions.
    • Global conveyor belt circulation—circulation pattern formed from deep ocean water movement through thermohaline circulation combined with surface currents
    • Waves
    • Waves tend to be just shapes, with very little forward progress.

 Permanent Ice—The Cryosphere

  • Second largest storage reservoir for moisture (still minuscule in comparison to the ocean).
  • Land portion of ice is larger than oceanic ice.

Ocean ice  has several names:

    • Ice pack—an extensive and cohesive mass of floating ice.
    • Ice shelf—a massive portion of a continental ice sheet that projects out over the sea.
    • Ice floe—a large, flattish mass of ice that breaks off from larger ice bodies and floats independently.
    • Iceberg—a chunk of floating ice that breaks off from an ice shelf or glacier.
      • Because of its lower density than water, only about 14 percent of the mass of an iceberg is exposed above water.
    • Oceanic ice is made up of fresh water because the ice crystals do not take in the minerals of seawater.
    • Several large, once-stable ice shelves have recently broken off of Antarctica.

Permafrost—permanent ground ice of permanently frozen subsoil; makes up most of the ice beneath the land surface.

  • Active layer—the upper 30 to 100 centimeters of the soil that thaws during the summer.
  • Beneath the active layer, the soil is frozen to a depth of approximately 50 meters.
  • Higher average temperatures have led to ground temperatures high enough to melt the permafrost.
  • Problems are associated with melting of permafrost:

Wet thermokarst conditions—where the ground surface subsides and it becomes saturated with water.

  • This in turn leads to the subsidence of structures such as roads and pipelines.
  • This also makes many roads impassible.
  • May also lead to an increase in the activity of microorganisms in the soil, which in turn will decompose organic material.
  • This will then release carbon dioxide and methane that will contribute to further warming.

 Surface Waters

Make up only 0.25 percent of world’s total moisture supply.

Lake—a body of water surrounded by land.

  • Lakes make up more than 90 percent of the surface water of the continents.
  • Lake genesis and continued existence occurs through two conditions:
  • Some sort of natural basin having a restricted outlet.
  • Sufficient inflow of water to keep the basin at least partly filled.
  • More than 40 percent of lake water is salt water.
  • Lakes are distributed unevenly throughout world.
  • Most common where glaciers had been.
  • Human Alteration of Natural Lakes
    • Diversion of streams by humans has had a large influence on reducing the volume of some lakes.
    • Mono Lake, CA, has been reduced by 50 percent of its previous volume.
    • The destiny of most lakes is to disappear.
      • Aral Sea
      • 1960s Soviet irrigation projects diverted vast quantities of water from the two rivers that flow into the Aral Sea.
      • Today the Aral Sea is 10 percent of its former size.
      • This destroyed the fishing industry and has generated choking, wind-blown dust and salt from the dry lake bottom.
      • Recent reengineering of the Syr Darya River will allow the northern remnant of the Aral Sea to remain near its current size.
      • The southern portion of the sea will most likely disappear within a couple of decades.
      • Lake Chad
      • Ongoing drought has reduced the lake to about 5 percent of its former size.
      • Some water diversion projects have contributed to the problem, but the greatest cause is climate change in the region.


  • Creation of artificial lakes has had immense ecological and economic consequences, not always beneficial.
  • In the southwestern United States, population growth has led to increased water consumption.
  • One visible consequence is “bathtub” rings around the reservoirs.


  • Land areas where saturation with water is the overriding factor that influences soil development and plant and animal communities.
  • Play important roles as local ecosystems and act as filters for surface runoff.
  • Swamps and Marshes
    • Swamp—water body with water-tolerant plants, predominantly trees.
    • Marsh—water body with water-tolerant plants, primarily grasses and sedges.
    • Both are flattish surface areas that are submerged in water at least part of the time but shallow enough to permit the growth of water-tolerant plants.

Rivers and Streams

    • Physical geographers call any flowing water a stream, no matter its size.
    • Drainage basin is all the land area drained by a river and its tributaries.


  • The total amount of water underground is more than 2.5 times that in lakes and streams.
  • Underground water is more widely distributed than surface water.
  • Almost ubiquitous
  • Quantity is sometimes limited.
  • Quality is sometimes poor;
  • Sometimes found at great depths.

Movement and Storage of Underground Water

  • All underground water originally comes from above.
  • Two factors affect underground water flow.
  1. Porosity—a measure of the capacity of rock or soil to hold water and air; the percentage of total volume of a material that consists of voids.
  2. Permeability—capacity of soil or rock to transmit water; determined by the size of pores and by the degree of interconnectedness.
  • Interstices—pore spaces; a labyrinth of interconnecting passageways among the soil particles that makes up nearly half the volume of an average soil.
  • Aquifers—where underground water is stored; a permeable subsurface rock layer that can store, transmit, and supply water.
  • Aquiclude—an impermeable rock layer that hinders or prevents water movement. Excludes water because of high density or, as in the case of clay, because interstices are many but too small to transmit water.
  • Hydrologic zones—underground layers involved in the general distribution of underground water.
  • Zone of aeration  – the topmost hydrologic zone within the ground, which contains a fluctuating amount of moisture (soil water) in the pore spaces of the soil (or soil and rock). A mixture of solids, water, and air; of variable depth.
  • Zone of saturation  the second hydrologic zone below the surface of the ground, where uppermost boundary is the water table. The pore spaces and cracks in the bedrock and the regolith of this zone are fully saturated.
  • Zone of confined water  – the third hydrologic zone below the surface of the ground, separated from the zone of saturation by impermeable rock. Occurs in many, but not most, parts of world. It contains one or more permeable rock layers (aquifers) into which water can infiltrate. If drilled into, confining pressure will force water to rise in the well.
  • Waterless zone  – the lowermost hydrologic zone that generally begins several kilometers or miles beneath the land surface and is characterized by the lack of water in pore spaces as a result of the great pressure and density of the rock.
  • Groundwater—water found in the zone of saturation.
  • Water table—the top of the zone of saturation within the ground.
  • Where the water table intersects Earth’s surface, water flows out.
  • A lake, swamp, marsh, or permanent stream is almost always an indication that the water table reaches the surface there.
  • Perched water table—occurs when a localized zone of saturation develops above an aquiclude.
  • Cone of Depression—occurs when water is removed from a well faster than underground water can replace it; this lowers the water table, which becomes the approximate shape of an inverted cone in the immediate vicinity of the well.
  • Artesian Wells—the free flows that result when a well is drilled from the surface down into a zone of confined water and the confining pressure is sufficient to force the water to the surface without artificial pumping.
  • Subartesian well—the free flow that results when a well is drilled from the surface down into a confined aquifer but requires artificial pumping to raise the water to the surface because the confining pressure forces the water only part way up the well shaft.

Groundwater Mining

  • Accumulation of groundwater is tediously slow, but humans can use it up rapidly.
  • High rates of groundwater use can be likened to mining because a finite resource is being removed with no hope of replenishment.
  • Largest U.S. aquifer, Ogallala, underlies 585,000 square kilometers (225,000 square miles) of eight states.
  • Water accumulated here for some 30,000 years.
  • Farmers began to tap into it in the early 1930s.
  • Water table is sinking.
  • Used to take 50-foot wells, now need ~150 to 250 feet (45 to 75 meters) to access water.
  • Less careful neighbors can harm those farmers who are trying to be very conservative in their water use.
  • Regional variations in saturated thickness.
  • Nebraska Sandhills are in the best shape, with great thickness, small usage, and a rapid recharge rate.
  • The 13 counties of southwestern Kansas have withdrawal rates that far exceed the recharge rate.
  • Continued extraction of groundwater can lead to the compaction of aquifer sediments.
  • Especially a problem if the rate of groundwater extraction exceeds the rate of recharge.
  • Several U.S. regions have been affected by this.
  • In California’s Central Valley, groundwater pumping resulted in 8.5 meters of subsidence.
  • In Las Vegas, NV, the land has subsided as much as 2 meters since the 1950s.
  • Fissures have developed on the surface, and well casings have been damaged.

People and the Environment: The Great Pacific Garbage Patch

  • Because of their durability, plastics have become a problem in marine environments.
  • Of the 90 billion kilograms of plastic produced annually, 10 percent end up in the oceans.
  • These plastics tend to accumulate in ocean regions that possess weak winds and currents (i.e., gyres).
  • Most of this trash floats in the upper 10 meters of the ocean.
  • Estimates of the size of this accumulation of ocean trash range from the size of Texas to twice the size of Texas.
  • Scripts Institution of Oceanography estimates that the trash accumulating in the patch has increased by 100 times in the past 40 years.
  • There are two major regions (or patches) of garbage.
    • The eastern garbage patch is located between Hawaii and California (located in the area of the subtropical high).
    • The western garbage patch extends east of Japan to the western archipelago of the Hawaiian Islands (within the North Pacific Subtropical Convergence Zone).
  • Since humans began ejecting waste into the sea, these patches have always been here.
  • The major difference now is the amount of nonbiodegradable material they contain.
  • Because these materials are so durable, and because of the significant increase in the use of plastics, the size of these garbage patches has increased significantly.
  • Hazards from the garbage patch include:
  • Ingestion of plastics by marine life.
  • Large pieces and those that have photodegraded into smaller pieces.
  • These smaller pieces may have toxic qualities.
  • These toxins then move up the food chain and biomagnify.
  • Entanglement of marine life in garbage.
  • Pollution of the ocean is arguably the biggest threat to marine life around the world.
  • Debris from the 2011 Japanese Tsunami
  • Materials are beginning to show up on the opposite side of the ocean.
  • A 20-meter dock was found on an Oregon beach in 2012.

Global Environmental Change: Monitoring Groundwater Resources From Space

  • UNESCO estimates that 2.5 billion people around the world obtain all of their drinking water from groundwater.
  • UNESCO also estimates that about 40 percent of all irrigation water comes from groundwater.
  • In many regions, groundwater extraction rates currently far exceed the natural recharge rates.
  • Monitoring changes in groundwater is difficult in many parts of the world.
  • New technology is allowing scientists to monitor changes in the status of groundwater from space.
  • GRACE Satellites—Gravity Recovery and Climate Experiment (GRACE)
  • A pair of polar orbiting satellites launched by NASA.
  • They circle the planet about 220 kilometers (137 miles) apart.
  • The satellites measure tiny differences in the distance between them that are caused by slight variations in Earth’s gravity field.
  • These local differences in gravity are caused by differences in the mass of Earth below.
  • GRACE data revealed in 2015 that 13 of the 37 largest aquifers on Earth are being depleted.
  • GRACE data are also used on an experimental basis to measure short-term differences in soil moisture and groundwater levels caused by weather variations such as droughts.
  • The GRACE system is so sensitive that it can detect differences in the mass of water and ice on and within Earth.
  • Can allow scientists to track changes in the exchange of water between ice sheets and the ocean, and between groundwater and the surface. The depletion of groundwater and the lowering of the water table of aquifers can result from overpumping as well as from declines.

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Geog 001

1.  Introduction to Earth