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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.
evaporation
Evaporation
  • 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

Hs

Rn

Qe

heat stored in system

G

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.

z

z

u

T

qv

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
evapotranspiration
Evapotranspiration
  • 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
evapotranspiration1
Evapotranspiration
  • 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
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