Evaporation Slides prepared by Daene C Mc Kinney

Evaporation • Slides prepared by Daene C. Mc. Kinney and Venkatesh Merwade • Reading: Applied Hydrology Sections 4. 1 and 4. 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

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

Transport Processes • Sun is the major source of energy in hydrologic cycle • Energy transport takes place through: – Conduction – Convection – Radiation Sun Energy & Mass Ocean Energy & Mass Land

Flux • The rate of flow of extensive property per unit area of surface through which it passes is called the flux. Volumetric flux Mass flux Momentum flux Energy flux Mass flux = density x volumetric flux Momentum flux = mass flux x velocity

Conduction • Heat is transferred as molecules with higher temperature collide with lower temperature molecules • Results from random molecular motion in substances • Eg. Heating of earth’s land surface

Conduction of mass, momentum and energy • Flux is proportional to the gradient of a potential Momentum flux (laminar flow) Newton’s law of viscosity Mass flux Fick’s law of diffusion Energy flux Fourier’s law of heat conduction m is dynamic viscosity, D is diffusion coefficient, and k is heat conductivity. Dynamic viscosity (m) is related to kinematic viscosity (n) as m = r n The direction of transport of extensive properties is transverse to the direction of flow.

Convection • Energy transfer through the action of turbulent eddies or mass movement of fluids with different velocities. • Turbulence – mechanism causing greater rate of exchange of mass, energy, and momentum than molecular exchanges • Unlike conduction, convection requires flowing fluid • Eg. Convection causes vertical air circulation in which warm air rises and cool air sinks, resulting in vertical transport and mixing of atmospheric properties

Convection of mass, momentum and energy Momentum flux (turbulent flow) Km is momentum diffusivity or eddy viscosity Mass flux Kw is mass diffusivity Energy flux Kh is heat diffusivity • Km is 4 -6 orders of magnitude greater than n. • tturb is the dominant momentum transfer in surface water flow and air flow. The direction of transport of extensive properties is transverse to the direction of flow.

Velocity Profile • Determining momentum transfer requires knowing velocity profile • Flow of air over land or water – log velocity profile Von Karman constant Roughness height Shear velocity Wall shear stress Velocity gradient

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/