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Evaporation and Transpiration

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- 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.

- 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)

- 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)

- 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.

- If neglect sensible heat transfer to atmosphere (Hs) and ground (G )
- Substitute equation for Q into energy balance
- Recall

- 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

- 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

- 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 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.

- 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.

- 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
- alfalfa0.85
- beans0.65
- corn0.75
- pasture0.75

- f= monthly consumptive use factor