Watershed Hydrology, a Hawaiian Prospective: Evapotranspiration

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Watershed Hydrology, a Hawaiian
ProspectiveEvapotranspiration

• Ali Fares, PhD
• Evaluation of Natural Resource Management, NREM
  600
• UHM-CTAHR-NREM

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Objectives of this chapter

• Explain and differentiate among the processes of
  evaporation from a water body, evaporation from
  soil, and transpiration from a plant
• Understand and be able to solve for
  evapotranspiration (ET) using a water budget
  energy budget method
• Explain potential ET and actual ET relationships
  in the field.

• Under what conditions are they similar?
• Under what conditions are they different?
• Understand and explain how changes in vegetative
  cover affect ET.
• Describe methods used in estimating potential and
  actual ET

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Conservation of Energy

• The conservation equation as applied to energy,
  or conservation of energy, is known as the energy
  balance.
• How precipitation is partitioned into
  infiltration, runoff, evapo-transpiration, etc.,
  similarly, we can look at how incoming radiation
  from the sun and from the atmosphere is
  partitioned into different energy fluxes (where
  the term flux denotes a rate of transfer (e.g. of
  mass, energy or momentum) per unit area).

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Water Energy relationship

• There is strong link between the water and energy
  balance
• Partitioning of incoming radiation into the
  various fluxes of energy ( energy for ET, energy
  to heat the atmosphere and energy to heat the
  ground) depends on the water balance and how much
  water is present in soils and available for
  evapotranspiration.
• the partitioning of precipitation into the
  various water fluxes (e.g. runoff and
  infiltration) depends on how much energy is
  available for ET.
• Just as changes in water balance were reflected
  in changes in storage in water amounts (soil
  moisture in a root zone level of a lake) changes
  in energy balance are reflected in temperature
  changes.
• Just as we wrote water balances for a number of
  different control volumes, we could write energy
  balances for the same control volumes.

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Evapotranspiration
ET P Q ?S - ?D

• ?S watershed storage variation (mm)
  SendSbeginning
• P Precipitation (mm)
• Q Stream flow (mm)
• ?D Seepage out seepage in (mm)
• ET evaporation and transpiration (mm)

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Energy Budget for an ideal surface

• Energy budget is
• Rn H LE G
• where Rn is net radiation at the surface
• H is sensible heat exchanged with the atmosphere
  
• LE is latent heat exchanged with the atmosphere
  and
• G is heat exchanged with the ground.

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Net Solar Energy Flux

• The net flux of solar energy entering the land
  surface is therefore given as
• K Kin - Kout Kin (1-a)
• where
• K in is the incident solar energy on the surface,
  and it includes direct solar radiation (i.e. that
  which makes it through the atmosphere unscathed)
  and diffuse (due to scattering by aerosols and
  gases)
• Kout is the reflected flux
• a is the albedo
• Solar radiation is measured in specialized
  meteorological stations with radiometers.

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Evapotranspiration

• For humid climates, vegetative cover affects the
  magnitude of ET and thus, Q (stream flow).
• In Dry climate, effect of vegetative cover on ET
  is limited.
• ET affects water yield by affecting antecedent
  water status of a watershed ? high ET result in
  large storage to store part of precipitation

• More than 95 of 300mm in Arizona
• gt 70 annual precipitation in the US
• In General ET/P is
• 1 for dry conditions
• ET/P lt 1 for humid climates ET is governed by
  available energy rather than availability of water

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Evapotranspiration

• evapotranspiration summarizes all processes that
  return liquid water back into water vapor
• - evaporation (E) direct transfer of water from
  open water bodies or soil surfaces
• - transpiration (T) indirect transfer of water
  from root-stomatal system
• of the water taken up by plants, 95 is
  returned to the atmosphere through their stomata
  (only 5 is turned into biomass!)
• Before E and T can occur there must be
• A flow of energy to the evaporating or
  transpiring surfaces
• A flow of liquid water to these surfaces, and
• A flow of vapor away from these surfaces.
• Total ET is change as a result of any changes
• That happens to any of these 3.

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• Three main factors affect E or T from evaporating
  transpiring surfaces
• Supply of energy to provide the latent heat of
  evaporation
• Ability to transport the vapor away from the
  evaporative surface
• Supply of water at the evaporative surface

• Source of energy? Is solar radiation
• What take vapors away from evaporating surface?
  Wind and humidity gradient
• Evaporation includes
• Soil -- vegetation surface transpiration
• gt Evapotranspiration, ET

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The linkage between water and energy budgets

• Is direct
• the net energy available at the earths surface
  is apportioned largely in response to the
  presence or absence of water.
• Reasons for studying it are
• To develop a better understanding of Hydrological
  cycle
• Be able to quantify or estimate E and ET (soil,
  water or snowmelt)

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Energy Budget

• L is latent heat of vaporization, E evaporation,
  H energy flux that heats the air or sensible
  heat, G is heat of conduction to ground and Ps is
  energy of photosynthesis.
• LE represents energy available for evaporating
  water
• Rn is the primary source for ET snow melt.

• Net radiation
• Rn(Wsws)(1- a)Ia-Ig
• Rn is determined by measuring incoming outgoing
  short- long-wave rad. over a surface.
• Rn can or
• If Rn gt 0 then can be allocated at a surface as
  follows
• Rn (L)(E) H G Ps

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• An island of tall forest vegetation presents more
  surface area than an low-growing vegetation does
  (e.g. grass).
• The total latent heat flux is determined by
• LE Rn H
• Advection is movement of warm air to cooler
  plant-soil-water surfaces.
• Convection is the vertical component of
  sensible-heat transfer.

• In a watershed Rn, (LE) latent heat and sensible
  heat (H) are of interest.
• Sensible heat can be substantial in a watershed,
  Oasis effect were a well-watered plant community
  can receive large amounts of sensible heat from
  the surrounding dry, hot desert.
• See Table 3.2 comparison
• See box 3.1 illustrates the energy budget
  calculations for an oasis condition.

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Water movement in plants

• Illustration of the energy differentials which
  drive the water movement from the soil, into the
  roots, up the stalk, into the leaves and out into
  the atmosphere. The water moves from a less
  negative soil moisture tension to a more negative
  tension in the atmosphere.

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Yw -1.3 MPa
Yw -1.0 MPa
Yw -0.8 MPa
Yw -0.75 MPa
Yw -0.15 MPa
Ys -0.025 MPa
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Soil Water Mass Balance

• There are different ways to estimate drainage.
• The direct method is the use of lysimeters.

• Lysimeters have a weighing device and a drainage
  system, which permit continuous measurement of
  excess water and draining below the root zone and
  plant water use, evapotranspiration.

Lysimeters have high cost and may not provide a
reliable measurement of the field water balance.
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Water Mass balance Equation
?S (I R U) - (D RO ET)

• ET Evapotranspiration
• R, I Rain Irrigation
• D Drainage Below Rootzone
• RO Runoff
• ?S Soil Water Storage variation
• U upward capillary flow

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Rain
Transpiration
Evapo-transpiration
Irrigation
Evaporation
Runoff
Root Zone
Water Storage
Below Root Zone
Drainage
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Effects of Vegetative Cover
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ET / Potential ET
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Available Soil Water
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ET Available Soil Water
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