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.

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 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 h Area, A CS Sensible heat to air Net radiation Vapor flow rate 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 No flow of liquid water through CS h

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 h Recall: Neglecting sensible and ground heat fluxes

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

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

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

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

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

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

Resources on the web Evaporation maps from NWS climate prediction center – Climate maps from NCDC – Evapotranspiration variability in the US –