Evaporation Slides prepared by Daene C Mc Kinney
Evaporation • Slides prepared by Daene C. Mc. Kinney • 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.
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
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
Evaporation from a Water Surface • Simplest form of evaporation – From free liquid of permanently saturated surface
Evaporation from a Pan Sensible heat to air • • 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 Net radiation Vapor flow rate CS h Area, A Heat conducted to ground
Methods of Estimating Evaporation • Energy Balance Method • Aerodynamic method • Combined method
Energy Method • CV contains liquid and vapor phase water • Continuity - Liquid phase h No flow of liquid water through CS
Energy Method • Continuity - Vapor phase Steady flow of air over water h
Energy Method • Energy Eq. 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 h
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
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 Net radiation Air Flow Evaporation
Aerodynamic Method Net radiation Air Flow • Log-velocity profile Evaporation Z • Momentum flux u Thornthwaite-Holzman Equation
Aerodynamic Method Net radiation Air Flow • Often only available at 1 elevation • Simplifying Evaporation
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
Example • Use Combo Method to find Evaporation – – – Elev = 2 m, Press = 101. 3 k. Pa, Wind speed = 3 m/s, Net Radiation = 200 W/m 2, Air Temp = 25 deg. C, Rel. Humidity = 40%,
Example (Cont. ) • Use Combo Method to find Evaporation – – – Elev = 2 m, Press = 101. 3 k. Pa, Wind speed = 3 m/s, Net Radiation = 200 W/m 2, Air Temp = 25 deg. C, Rel. Humidity = 40%,
Example (Cont. ) • Use Combo Method to find Evaporation – – – Elev = 2 m, Press = 101. 3 k. Pa, Wind speed = 3 m/s, Net Radiation = 200 W/m 2, Air Temp = 25 deg. C, Rel. Humidity = 40%,
Example • Use Priestly-Taylor Method to find Evaporation rate for a water body – Net Radiation = 200 W/m 2, – Air Temp = 25 deg. C, Priestly & Taylor
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
Potential Evapotranspiration • Multiply reference crop ET by a Crop Coefficient and a Soil Coefficient http: //www. ext. colostate. edu/pubs/crops/04707. html
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. pl • Evapotranspiration variability in the US – http: //geochange. er. usgs. gov/sw/changes/natural/et/
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