Evaporation and transpiration
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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

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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


Mass transfer aerodynamic method1

Mass Transfer (Aerodynamic) Method


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

    • alfalfa0.85

    • beans0.65

    • corn0.75

    • pasture0.75

  • f= monthly consumptive use factor


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