Ji-Yeon, Yoo

1
Effect of soil properties on evaporation rate
(including empirical and model-using prediction)
2004. 06
Ji-Yeon, Yoo EN629 Dept. of Environmental Science
Engineering, Ewha Womans University, Seoul,
Korea
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Contents

1. Introduction
2. Estimation of evaporation at large scales
3. Analytical estimate of evaporative fraction
4. Equations for soil hydraulic properties
5. Results and Conclusion

3
1. Introduction
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion
4
2. Estimation evaporation at large scales
2.1 Overview
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• Infiltration, evaporation and runoff processes
• Results show that during periods of low
  atmospheric demands (early morning/late
  afternoon, Winter periods, etc.) the use of
  equivalent parameters will over estimate
  evaporation and during periods of high demand
  (midday Summer periods) under estimate
  evaporation
• The sensitivity of the soil-vegetation system to
  low soil moisture

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2.2 Derived distribution of soil moisture and
evaporation
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• Water table depths, runoff, soil moisture, and
  evapotranspiration
• Understanding heterogeneity within natural
  watersheds
• The probability distribution of water table
  depths is mapped into a probability distribution
  for actual evaporation
• By ignoring the spatial variability in soil
  moisture the aggregated model significantly
  underestimates the catchment-scale
  evapotranspiration

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3. Analytical estimate of evaporative fraction
3.1 Analytical model for soil variability and
evaporation
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• Equation 1)
• Equation 2)

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3.2 Derivation of the evaporative fraction
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• Equation 3)
• where ?r the residual soil moisture
• Fs the fraction of the area that is
  under
• soil control and (1 - Fs) the
  fraction
• under atmospheric control

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3.2 Derivation of the evaporative fraction
(continue)
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• Equation 4)

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3.2 Derivation of the evaporative fraction
(continue)
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• Equation 5)
• Eq. (6) to Eq. (4) is Evaporation scaling ratio
• This ratio will be investigated for a range of
  conditions reflective of field conditions.

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3.3 Sensitivity of land characteristics on the
evaporation scaling ratio
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• The sensitivity of the evaporation scaling ratio
  depends on three parameters
• ? The ratio of the equilibrium soil moisture
  
• ß The soil-vegetation sensitivity factor to
  soil moisture stress
• Cv The variability of soil moisture within
  the region measured by its coefficient of
  variation

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4. Equation for soil hydraulic properies
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• The power curve representing the moisture
  characteristic
• Equation 6)
• soil wetness W ?/?s,
• ?s the saturated water content

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4. Equation for soil hydraulic properies
(continue)
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• Equation 8)
• This inflection point is assigned the
  coordinates(Wl, ?l), and the interval Wl W l
  can be described by the parabola

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4. Equation for soil hydraulic properies
(continue)
1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion

• The moisture characteristic using (6) and (8)
  for the hyperbolic and parabolic suction,
  respectively. The broken line segments are
  disregarded.

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5. Result and Conclusion

• The effect of subgrid variability in soil
  moisture on evaporation
• If soil moisture fields can be shown to scale
  statistically then its small scale statistical
  properties can be estimated and combined with
  large scale mean soil moisture estimated from
  either remote sensing or aggregated parameter
  macroscale models.
• The scaling theory would determine the small
  scale statistical behavior of soil moisture that
  would then be used to estimate the surface
  evaporative fluxes through a statistically
  distributed land surface model

1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion
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5. Result and Conclusion (continue)

• Representative values for Hydraulic
  Parameters(SD in Parentheses)

1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion
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5. Result and Conclusion (continue)

• Equations (6) to (8) for the moisture
  characteristic plus Campbells
• Equation (8) for hydraulic conductivity
• ?s presented here provides for parameter
  estimates for infiltration problems using only
  limited soil data
• The exponent b is statistically related to soil
  texture that there is substantial variability in
  ?s within and among textural classes and that
  gradual air entry represented by the inflection
  point on the W versus ? curve may have
  considerable effect on the calculated wetting
  from suction

1. Introduction 2. Estimation evaporation at
large scales 3. Analytical estimate of
evaporative fraction 4. Equation for soil
hydraulic properties 5. Result and Conclusion