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Evaporation and Transpiration. Evaporation- change of water from liquid to vapor phase Potential Evaporation - climatically controlled evaporation from a surface when the supply water to the surface is unlimited

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evaporation and transpiration
Evaporation and Transpiration
  • Evaporation- change of water from liquid to vapor phase
  • Potential Evaporation - climatically controlled evaporation from a surface when the supply water to the surface is unlimited
  • Transpiration - evaporation occurring from plant’s leaves through stomatal openings. Function of stomata is to provide a place where CO2 can dissolve into water and enter plant tissue. Evaporation unavoidable in this process - driven by same process as evaporation.
  • Potential Transpiration - Transpiration which would occur if water supply to plant roots and through vascular system to stomata was unlimited. Controlled by climate and plant physiology.
  • Two main forces influencing evaporation rate are:
    • Supply of solar energy to provide the latent heat of evaporation.
    • Ability to transport evaporated water away from surface  affected by wind velocity and vapor gradient.
  • Transpiration affected by above plus ability of plant to extract and transmit water from soil to stomata.
methods of estimating evaporation
Methods of Estimating Evaporation
  • energy balances methods
  • mass transfer or aerodynamic methods
  • combination of energy and mass transfer (Penman equation)
  • pan evaporation data

All these methods were developed to estimate evaporation from free water surfaces (or completely saturated soil)

energy balance method
Energy Balance Method
  • Assumes energy supply the limiting factor.
  • Consider energy balance on a small lake with no water inputs (or evaporation pan)

sensible heat transfer to air

net radiation

energy used in evaporation




heat stored in system


heat conducted to ground (typically neglected)

energy balance method1
Energy Balance Method
  • Steady state conservation of energy equation. (assume water temperature does not change, no flow into or out of lake)

energy inflows = energy outflows

  • Hs - sensible heat flux to atmosphere (by convection)
  • G - heat conducted to ground are typically small and difficult to measure.
energy balance method2
Energy Balance Method
  • If neglect sensible heat transfer to atmosphere (Hs) and ground (G )
  • Substitute equation for Q into energy balance
  • Recall
energy balance method assumes
Energy Balance Method assumes
  • no water inflow/outflow to lake
  • no change in water temperature of lake
  • neglects sensible heat transfer to ground and atmosphere
  • neglects heat energy lost with water which leaves system as vapor
  • calculates evaporation on a daily time interval
mass transfer aerodynamic method
Mass Transfer (Aerodynamic) Method
  • based on the concept that rate of turbulent mass transfer of water vapor from evaporating surface to atmosphere is limiting factor
  • Mass transfer is controlled by (1) vapor gradient and (2) wind velocity which determines rate at which vapor is carried away.






combination method penman
Combination Method (Penman)
  • Evaporation can be computed by aerodynamic method when energy supply not limiting and energy method when vapor transport not limiting  Typically both factors limiting so use combination of above methods
  • Weighting factors sum to 1. Deviation of weighting factors is based on physical processes,
  •  = vapor pressure deficit
  • g = psychrometric constant
combination method penman1
Combination Method (Penman)
  • Combination method is most accurate and most commonly used method if meteorological information is available. Particularly good for small, well-monitored areas.
  • Need: net radiation, air temperature, humidity, wind speed
  • If all this information is not available can use Priestly-Taylor approximation:
  • Based on observations that second term in Penman equation typically  30% of first. This is better for large areas.
  • Based on observations that second term in Penman equation typically  30% of first.
  • This is better for large areas.
  • All equations suitable for daily time intervals or longer.
evaporation pan
Evaporation Pan
  • Since expensive to maintain weather stations required to use Penman equation, evaporation pans are often used to directly measure evaporation.
  • Standard (Class A) Evaporative Pans are built of unpainted galvanized iron. 4 ft. diameter, 10 inches deep, set on a platform 12 inches above ground.
  • Water level in pan recorded daily with high precision micrometer. Evaporation determined by mass balance.
evaporation pan1
Evaporation Pan
  • Mass balance equation
  • Pans measure more evaporation than natural water bodies because:
    • 1) less heat storage capacity (because smaller volume water)
    • 2) heat transfer through pan sides
    • 3) wind effects caused by pan itself
  • Typically estimate
  • Pan factor varies with season and location. Should be calibrated at each site. Set up complete weather station  calculate Penman E and Ep
  • Same factors which govern water evaporation from water surfaces govern evapotranspiration because essentially transpiration is mainly due to evaporation from stomata.
  • In addition plant physiology (plants can control size of stomata and resistance to flow through roots and vascular systems) and soil moisture conditions (resistance of flow to roots) play a role.
  • Estimate Evaportranspiration using
  • Alternative empirical equation- Blaney-Criddle equation
  • K= monthly crop coefficient
    • alfalfa 0.85
    • beans 0.65
    • corn 0.75
    • pasture 0.75
  • f= monthly consumptive use factor