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
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
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
Energy Method

• CV contains liquid and vapor phase water
• Continuity - Liquid phase

No flow of liquid water through CS
8
Energy Method

• Continuity - Vapor phase

Steady flow of air over water
9
Energy Method

• Energy Eq.

10
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
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
Aerodynamic Method

• Log-velocity profile
• Momentum flux

Thornthwaite-Holzman Equation
14
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

• 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,

17
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,

18
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,

19
Example

• Use Priestly-Taylor Method to find Evaporation
  rate for a water body

• Net Radiation 200 W/m2,
• Air Temp 25 degC,

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.p
  l
• Evapotranspiration variability in the US
• http//geochange.er.usgs.gov/sw/changes/natural/et
  /