Evaporation and Transpiration

1. 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.

2. 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.

3. 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)

4. 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)

5. 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.

6. Energy Balance Method • If neglect sensible heat transfer to atmosphere (Hs) and ground (G ) • Substitute equation for Q into energy balance • Recall

7. 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

8. 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

9. Mass Transfer (Aerodynamic) Method

10. 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

11. 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.

12. 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.

13. 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

14. 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

15. 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