Evaporation Slides prepared by Daene C. McKinney 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 Evaporation Air Flow u

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

Evaporation from a Pan National Weather Service Class A type Area, A CS Sensible heat to air Net radiation Vapor flow rate Heat conducted to ground 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

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 h Steady flow of air over water

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

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

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 Mass flux = density x volumetric flux Momentum flux Momentum flux = mass flux x velocity Energy flux

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 Evaporation Air Flow

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

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

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 kPa, Wind speed = 3 m/s, Net Radiation = 200 W/m2, Air Temp = 25 degC, Rel. Humidity = 40%,

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

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

Example Use Priestly-Taylor Method to find Evaporation rate for a water body Net Radiation = 200 W/m2, Air Temp = 25 degC, 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/climaps.pl Evapotranspiration variability in the US http://geochange.er.usgs.gov/sw/changes/natural/et/