Evaporation What is evaporation How is evaporation measured

Evaporation • What is evaporation? • How is evaporation measured? • How is evaporation estimated? • Reading: Applied Hydrology Sections 4. 1 and 4. 2 on Infiltration and soil water movement

Evaporation – process by which liquid water becomes water vapor – Transpiration – process by which liquid water passes from liquid to vapor through plant metabolism – Evapotranspiration – evaporation through plants and trees, and directly from the soil and land surface – Potential Evaporation – evaporation from an open water surface or from a well-watered grass surface

Factors Influencing Evaporation • Energy supply for vaporization (latent heat) – Solar radiation • Transport of vapor away from evaporative surface – Wind velocity over surface – Specific humidity gradient above surface • Vegetated surfaces – Supply of moisture to the surface – Evapotranspiration (ET) • Potential Evapotranspiration (PET) – moisture supply is not limited Net radiation u Air Flow Evaporation

Evapotranspiration (ET) Over land surfaces, we cannot distinguish between water vapor that evaporated from the soil and water vapor that was transpired through plants

Evaporation from an Open Water Surface • Simplest form of evapotranspiration – This is the amount of water lost from lakes and reservoirs – Often estimating by measuring the loss from a National Weather Service Class A pan • This is referred to as Potential Evapotranspiration (ETp) because it is the maximum potential rate of ET under the given meteorological conditions

Lysimeters Measurement of evapotranspiration

Flux Towers (Marcy Litvak)

Flux tower instruments

Flux tower instruments Pyrronometer Net radiometer 3 -D Sonic anemometer Quantum sensor IRGA

Air Temperature at 1 m and 10 m Freeman Ranc Flux Tower (Marcy Litvak) 40 35 30 25 20 15 t_hmp_10 m 10 t_hmp_1 m 5 0 8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

Vapor Pressure and Saturated Vapor Pressure (k. Pa) 6 5 4 3 2 1 e_Avg 0 8/5/04 8/6/04 8/7/04 e_sat_Avg 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

Relative Humidity at 1 m and 10 m 1 0. 8 Average = 0. 71 0. 6 Average = 0. 61 0. 4 rh_hmp_10 m 0. 2 0 8/5/04 rh_hmp_1 m 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

Wind Speed (m/s) 25 20 15 10 5 0 8/5/04 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

Net Radiation (W/m 2) 800 700 600 500 400 300 200 100 0 8/5/04 -100 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

ET -Eddy covariance method • Measurement of vertical transfer of water vapor driven by convective motion • Directly measure flux by sensing properties of eddies as they pass through a measurement level on an instantaneous basis • Statistical tool

Basic Theory Instantaneous signal Time averaged property Instantaneous Perturbation from The mean All atmospheric entities show short-period fluctuations about their long term mean value

Turbulent mixing Propterties carried by eddies: Mass, density ρ Vertical velocity w Volumetric content qv Temperature T

Sensible Heat Flux (W/m 2) 500 400 300 200 100 0 8/5/04 -100 -200 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

Latent Heat Flux (W/m 2) 500 400 300 200 100 0 8/5/04 -100 -200 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

Evaporation (mm/day) 20 16 12 8 Average = 3. 15 mm/day 4 0 8/5/04 -4 8/6/04 8/7/04 8/8/04 8/9/04 8/10/04 8/11/04 8/12/04 8/13/04 8/14/04 8/15/04

Energy Balance Method Can directly measure these variables How do you partition H and E? ?

Energy Balance Method