Evaporation

1. Evaporation • Slides prepared by Daene C. McKinney • Reading: Applied Hydrology Sections 4.1 and 4.2 • Quotation for today (from Socrates) "There is only one good, that is knowledge; there is only one evil, that is ignorance."Contributed by Matt Harold.

2. Evaporation • Terminology • Evaporation – process by which liquid water passes directly to the vapor phase • Transpiration - process by which liquid water passes from liquid to vapor through plant metabolism • Sublimation - process by which water passes directly from the solid phase to the vapor phase

3. Net radiation u Air Flow Evaporation 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

4. Evaporation from a Water Surface • Simplest form of evaporation • From free liquid of permanently saturated surface

5. Sensible heat to air Net radiation Vapor flow rate CS h Area, A Heat conducted to ground Evaporation from a Pan • National Weather Service Class A type • Installed on a wooden platform in a grassy location • Filled with water to within 2.5 inches of the top • Evaporation rate is measured by manual readings or with an analog output evaporation gauge

6. Methods of Estimating Evaporation • Energy Balance Method • Aerodynamic method • Combined method

7. h Energy Method • CV contains liquid and vapor phase water • Continuity - Liquid phase No flow of liquid water through CS

8. h Energy Method • Continuity - Vapor phase Steady flow of air over water

9. h Energy Method • Energy Eq.

10. h Energy Method • Energy Eq. for Water in CV Assume: 1. Constant temp of water in CV 2. Change of heat is change in internal energy of water evaporated Recall: Neglecting sensible and ground heat fluxes

11. Wind as a Factor in Evaporation • Wind has a major effect on evaporation, E • Wind removes vapor-laden air by convection • This Keeps boundary layer thin • Maintains a high rate of water transfer from liquid to vapor phase • Wind is also turbulent • Convective diffusion is several orders of magnitude larger than molecular diffusion

12. Net radiation Air Flow Evaporation Aerodynamic Method • Include transport of vapor away from water surface as function of: • Humidity gradient above surface • Wind speed across surface • Upward vapor flux • Upward momentum flux

13. Net radiation Air Flow Evaporation Z u Aerodynamic Method • Log-velocity profile • Momentum flux Thornthwaite-Holzman Equation

14. Net radiation Air Flow Evaporation Aerodynamic Method • Often only available at 1 elevation • Simplifying

15. Combined Method • Evaporation is calculated by • Aerodynamic method • Energy supply is not limiting • Energy method • Vapor transport is not limiting • Normally, both are limiting, so use a combination method Priestly & Taylor

16. Example • Elev = 2 m, • Press = 101.3 kPa, • Wind speed = 3 m/s, • Net Radiation = 200 W/m2, • Air Temp = 25 degC, • Rel. Humidity = 40%, • Use Combo Method to find Evaporation

17. Example (Cont.) • Elev = 2 m, • Press = 101.3 kPa, • Wind speed = 3 m/s, • Net Radiation = 200 W/m2, • Air Temp = 25 degC, • Rel. Humidity = 40%, • Use Combo Method to find Evaporation

18. Example (Cont.) • Elev = 2 m, • Press = 101.3 kPa, • Wind speed = 3 m/s, • Net Radiation = 200 W/m2, • Air Temp = 25 degC, • Rel. Humidity = 40%, • Use Combo Method to find Evaporation

19. Example • Net Radiation = 200 W/m2, • Air Temp = 25 degC, • Use Priestly-Taylor Method to find Evaporation rate for a water body Priestly & Taylor

20. Evapotranspiration • Evapotranspiration • Combination of evaporation from soil surface and transpiration from vegetation • Governing factors • Energy supply and vapor transport • Supply of moisture at evaporative surfaces • Reference crop • 8-15 cm of healthy growing green grass with abundant water • Combo Method works well if B is calibrated to local conditions

21. Potential Evapotranspiration • Multiply reference crop ET by a Crop Coefficient and a Soil Coefficient http://www.ext.colostate.edu/pubs/crops/04707.html

22. Resources on the web • Evaporation maps from NWS climate prediction center • http://www.cpc.ncep.noaa.gov/soilmst/e.shtml • Climate maps from NCDC • http://www.nndc.noaa.gov/cgi-bin/climaps/climaps.pl • Evapotranspiration variability in the US • http://geochange.er.usgs.gov/sw/changes/natural/et/