The hydrologic cycle

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The hydrologic cycle
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Soil Vegetation Column
Water Balance Equations
Energy Balance Equation
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Sunshine Ball measures hours of bright sunshine
Sunshine card
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Sunshine Card
The glass ball focuses the light and burns the
card as the sun track across the sky. The length
of the burn indicates hours of bright sunshine.
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5. Mass Transfer / Aerodynamic Method

• It estimates evaporation from modeling mass
  momentum transport of water vapor from
  evaporating surface by convection.
• Ea f(U)(es - ea)
• Where f(U) Mass transfer coefficient
• U windspeed (m/day or m/s) usually measured at
  a height of 2 m above the ground
• es Saturation vapor pressure of air at Ta,
• ea Actual atmospheric vapor pressure of air
  (air is often not saturated with vapor).

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• Zo Roughness height of vegetation (0.005 m to a
  few meters)
• Za Height where measurements are taken
• U Wind velocity in m/sec
• k Von Karmans constant (0.4)
• Patmospheric pressure
• ?a and ?w Density of air and water

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6. Penman (1948) Combination Method

• By assuming a thin free-water surface, Penman
  derived a equation that combines both features of
  the aerodynamic and the energy methods, known as
  the combination method. The most physically-based
  evaporation model derived from several basic
  equations.

ea, Ta
Rn
Air
Wind (U)
Water
eo, To
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0

• Net radiation Rn lvEn G H (1)
• Bowens ratio ? H/lvE (2)
• Saturation vapor pressure gradient D des/dT
• Mass transfer Ea f(U)(es - ea) (3)
• D saturation vapor pressure gradient,
• ? (psychrometric constant) ? 66.6 Pa/oC

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ER D calibrated from ET simulated by 2-Source
model for Paddle River Basin that of Granger
Grey (1989)
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2-Source versus Penman-Monteith
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2-Source versus Modified Penman