AOM 4643

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AOM 4643

• Principles and Issues in Environmental Hydrology

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Definitions

• Hydrologic Science
• Focuses on the global hydrologic cycle and the
  processes involved in the land phase of that
  cycle.
• Predicts spatial and temporal distribution of
  water in the terrestrial, oceanic and atmospheric
  compartments of the global water system
• Predicts movement of water on and under earths
  land surfaces and the physical/ chemical/
  biological processes that accompany, conduct or
  affect movement
• Applied or Engineering hydrology
• uses this understanding to design and operate
  flood control, water supply, irrigation and
  drainage, pollution abatement, wildlife
  protection systems. i.e., for planning and
  management of water resources.

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The Hydrologic Cycle

• Describes the continuous circulation of water
  from land and sea to the atmosphere and back
  again.
• Concept is based on mass balance and is simply
  that water changes state and is transported in a
  closed system which extends approximately 1 km
  down into the earths crust and about 15 km up
  into the atmosphere.
• The cycle is only closed earth-wide, not on a
  watershed or continental scale. Thus practicing
  hydrologists are typically faced with an open
  system.
• The energy required to keep the hydrologic cycle
  going is provided by the sun.

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Definitions

• precipitation - movement of water from the
  atmosphere to the earth as rain or snow
• evapotranspiration - combined consumptive
  evaporative process by which water is released to
  the atmosphere through vegetation and soil
• throughfall - water not intercepted by vegetation
• interflow - water at shallow depths within soil
  structure
• overland flow - precipitated water which moves
  over the land surface ultimately infiltrating
  into the ground or discharging into streams as
  surface runoff.

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Definitions

• infiltration - precipitated water absorbed by
  soil surface
• percolation - water movement into deep aquifers
  (recharge)
• exfiltration - rising of soil moisture due to
  tension and capillary forces
• sublimation - release of water from snowpack and
  icecaps directly to the atmosphere as vapor

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Global Hydrology

• Ocean activity dominates the global hydrologic
  cycle -- receiving 79 of the earths rainfall
  and contributing 88 of the evapotranspiration.
• More rainfall falls on the oceans than land
  (because of larger surface area).
• Land receives more water than it evaporates while
  oceans evaporate more water than receive as
  precipitation.
• Excess water on land returns to ocean as surface
  and groundwater outflow to balance the system.

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Distribution of water throughout the earth

• oceans and saline groundwater 97.5
• fresh water 2.5
• ice caps 1.7
• groundwater 0.7
• lakes, rivers, streams, etc. 0.02
• atmosphere 0.001
• soil moisture 0.001
• biological water 0.0001

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Global Stores and Fluxes of Water
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Residence Times

• The residence time of a water molecule in a
  component of the system equals average volume of
  water stored a component of the system divided by
  the volumetric flow rate through the system
• Tr Storage 12,900 km3
  8.2 days
• Flow Rate 577,000 km3/yr
• Thus very short residence time indicates that
  atmospheric moisture is replaced approximately 40
  times per year. ? part of what makes weather
  prediction so difficult.
• Atmospheric portion of the hydrologic cycle is
  very active and is driving force for surface
  hydrology

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Residence Times
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History of Hydrology
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Challenges in Hydrology

• Understanding the basic physics of individual
  hydrologic processes which occur instantaneously
  at a point does not always extrapolate easily to
  an understanding of hydrologic processes over
  larger space and time scales, due to
• non-linearity of many hydrologic processes
• high degree of spatiotemporal variability in
  natural systems
• difficulty and expense in obtaining data to
  characterize variability.

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Goals for this Class

• We will cover basic physical principals which
  govern the major hydrologic processes
  Precipitation, Evapotranspiration, Infiltration,
  Groundwater Flow, Overland Flow, Streamflow.
• We will focus on relatively simple quantitative
  representations of these processes in order to
  develop a sound intuitive sense of the way water
  moves through the land based portion of the water
  cycle.
• These physical principals are powerful tools
  which constitute the foundation of hydrologic
  science. However, the degree of knowledge that
  can be obtained with the tools we will discuss is
  limited primarily by the availability and quality
  of field data and sometimes because they are
  conceptually inappropriate.