Basic Hydrological Concepts

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Basic Hydrological Concepts

• AOM 4643
• Principles and Issues in Environmental Hydrology

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Structure and Properties of Water

• Water is a held together by a covalent bond one
  side has a negative charge and the other a
  positive charge. The positive end of one H2O
  molecule attracts the negative end of another gt
  called hydrogen bond
• hydrogen bond is weaker than a covalent bond but
  very important. Hydrogen bond determines most of
  waters unique properties

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Thermal Properties of Water

• boiling point and freezing point are higher than
  expected for its molecular weight (because of
  intermolecular attraction i.e. hydrogen bonds) ?
  water exists in solid, liquid gas phases on
  earth.
• maximum density _at_ 4oC ? ice floats, caused by
  hydrogen bonds forming tetrahedra at low temp ?
  important in determining earths climate
• high specific heat capacity ? a large input of
  energy raises the temperature a relatively small
  amount energy goes into breaking hydrogen bonds
  rather than raising the temperature

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Structural Properties of Water

• cohesive, sticks to itself ? high surface tension
  ? drops of water are spherical.
• capillarity results of combination of adhesion
  to solid surfaces i.e. glass (water molecules are
  attracted to oxygen atoms in glass) by hydrogen
  bonds and cohesion to itself through surface
  tension important for circulation of blood in
  body and water in soil
• capillary forces are what allow moist sand to
  maintain vertical trench walls, whereas dry sand
  can only maintain a slope of 30o ? tiny menisci
  hold sand grains together through the hydrogen
  bonds

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Water as a Solvent

• universal solvent ? given enough time only a few
  natural substances will not dissolve in water.
• water dissolves substances by
• forming hydrogen bonds with its molecules (polar
  molecules)
• surrounding individual ions of the substance
  (electrolytes)
• water alone cannot carry an electrical current
  due to the hydrogen bonds which do not allow
  hydrogen and oxygen atoms to move around freely
  of one another.
• Electrolytes in water can cause the solution to
  carry a charge. The higher the salt content the
  higher the electrical conductivity.

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Basic Hydrologic Concepts

• 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
• Hydrologic cycle is closed only globally, not on
  a watershed or continental scale.
• Hydrologic phenomena (precipitation, ET,
  infiltration, groundwater, overland, streamflow)
  are extremely complex and although quantifiable
  at lab scale, may never be fully predictable at
  the watershed scale. Thus we represent them in a
  simplified way by means of the systems concept.

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Definition

• A hydrologic system is defined as a structure
  (surface or subsurface) or volume (atmospheric)
  in space, surrounded by a boundary, that accepts
  water and other inputs (such as air or heat
  energy), operates (physical, chemical,
  biological) on them internally and produces them
  as outputs.
• We treat the hydrologic cycle as a system whose
  components are precipitation, evapotranspiration,
  interception, runoff, infiltration, etc.. We
  give up the quest to know the precise
  spatiotemporal water flow patterns within the
  system and settle instead for knowing total water
  storage, and spatially averaged water fluxes in
  and out of the control volume.

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Example
P(t)
ET(t)

• The basic relations of physical hydrology for
  this system are derived from fundamental laws of
  classical physics. Particularly
• Conservation of mass (m mass of water)
• Conservation of energy (internal energy,
  kinetic energy and potential
• 
  energy of the fluid)

overland flow
surface runoff
Q(t)
groundwater discharge
G(t)
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Conservation of Mass

• The most useful principal in hydrologic analysis
  and is required in almost all problems.
• Stated mathematically
• For our watershed problem
• If have a steady flow problem, inflowsoutflows

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Conservation of Energy

• Second fundamental physical law utilized in
  physical hydrology is the conservation of energy.
• Total energy internal energy kinetic energy
  potential energy
• Total Energy E Eu 1/2
  mV2 mgz
• Energy per e eu
  1/2 V2 gz
• unit mass

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Internal Energy

• Internal energy is the sum of sensible heat and
  latent heat.
• Sensible heat is that part of the internal energy
  that is proportional to the substances
  temperature, i.e.
• deu CpdT
• Latent heat - Amount of heat exchange required
  for inducing a phase change per gram of substance
  without a change in temperature. Usually a
  function of temperature.

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Latent Heat Values for Water

• liquid water to vapor
• Le latent heat
  of evaporation
• 597.3 -
  0.57 T cal/g (2.5x106 - 2370T J/kg)
• This is heat absorbed (by vaporized water from
  surroundings) to break H bonds so evaporation can
  take place
• ? evaporation always accompanied by transfer
  of heat out of water body or surroundings to
  vapor ? latent heat transfer
• vapor to liquid water
• Lc latent heat of
  condensation
• -597.3
  0.57 T cal/g (-2.5x106 2370T J/kg)
• This is heat released to surroundings when
  H bonds formed during condensation

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Latent Heat Values for Water

• ice to liquid
• Lm latent heat of
  melting
• 79.7 cal/g ( 0.33 106
  J/kg)
• This is energy required to disrupt tetrahedral
  molecular structure.
• liquid to ice
• Lf latent heat of
  fusion
• -79.7 cal/g ( 0.33
  106 J/kg)
• This is energy released as tetrahedral molecular
  structure is formed.
• ice to vapor
• Ls latent heat of
  sublimation
• 677 - 0.07 T cal/g
• This is energy needed to a) disrupt
  molecular structure then b) break H bonds

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Latent Heat Values for Water

• At typical atmospheric temp. and pressure on
  earth, energy required to sublimate ice to vapor
  generally greater than that required to melt ice
  through evaporation. Therefore, usually water
  goes through liquid phase first.

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Latent Heat Transfer

• Jumps in curve ? latent heat transfer to water
• Slope in curve ? sensible heat transfer to water

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Examples of Use of Latent Heat Properties

• In the SW use the latent heat of evaporation for
  air-conditioning houses ? water and air is run
  into evaporative cooler on roofs of houses -- as
  water evaporates absorbs heat from air. Cooled
  air is returned to house.
• Irrigation of plants to protect from freezing. ?
  when irrigation water freezes it releases heat to
  the environment which increases air temperature
  slightly and protects plant.
• Latent heat transfer is the dominant cause of
  internal energy change for water in most
  hydrologic applications ? temperatures usually
  only change a few degrees C so sensible heat
  transfer is small.