1. Lecture 8 Evapotranspiration (1)
Evaporation Processes
General Comments
Physical Characteristics
Free Water Surface (the simplest case)
Approaches to Evaporation Estimation

2. Evaporation: General Comments
Change of state of water from liquid to gas and thus released to the atmosphere as water vapour
Represents both a loss of mass and energy from a surface
Globally, evaporation is the main control on precipitation and the hydrologic cycle since it releases moisture from the ocean reservoir
Important locally; accounts for up to 75% of water losses from catchments (drainage basins)

3. Evaporation (Physical Characteristics)
1. Vapour pressure relationship
Vapor pressure at and above evaporating surface
Vapor pressure depends on relative humidity and temperature
Saturation vapor pressure (es)
Actual vapor pressure (ea)
2. Latent heat (E)
Transfer of latent heat from the evaporating body into the air
Reduces the surface temperature
Latent heat of vaporization:
 = 2.47 MJ kg-1 at 10ºC

4. Evaporation (Physical Characteristics)
3. Sensible heat (H)
Heat energy that we can feel (sense)
Upward rate of sensible-heat exchange by turbulent transfer
Depends on the vertical gradients of wind speed and temperature
4. Bowen ratio
A ratio of sensible-heat exchange to latent-heat exchange

5. Free Water Surfaces (Simplest Case)
Evaporation represents the net flux of water molecules from a surface to the atmosphere.
It requires 3 basic conditions
1. Energy Input
2. Vapour Pressure Gradient
3. Exchange Mechanism (Wind)

6. Required conditions for evaporation
1. Energy Input
Increases temperature of the water
Controls the number of molecules with sufficient energy to break free of the liquid mass (i.e., latent heat of vaporization) because:
(1) Water is a polar molecule
(2) Molecules align along the surface
(3) Excess kinetic energy required to overcome barrier

7. Required conditions for evaporation
2. Vapour Pressure Gradient
If: es > ea then evaporation
ea > es then condensation
es = ea then equilibrium
es = Saturation Vapour Pressure
ea = Actual Vapour Pressure (overlying air)

8. Required conditions for evaporation
3. Exchange Mechanism (Wind)
Once the air above water becomes saturated, evaporation will cease, unless water molecules in the air is moved away from the surface layer. Therefore, properties of the receiving air determine how many water molecules remain airborne or are carried away.
With all other factors constant, evaporation is proportional to the wind-speed and the vapour pressure deficit, i.e., the difference between saturation vapour pressure at the temperature of the water surface and the actual vapour pressure of the overlying air.
Dalton’s Equation: 
E = ku (es - ea), kg·(m2·s)-1
E = Rate of evapouration
k = Surface roughness factor
u = Wind speed (m/s)
es= Saturation vapour pressure
ea= Vapour pressure of the air

9. Approaches to Evaporation Studies
Water Balance Approach
Mass-Transfer Approach
Energy Balance Approach
Combined Approach

10. Approaches to Evaporation Studies
Water Balance Approach
Estimating water loss from the water body of interest over a time period t using a water-balance equation:
E = Evaporation
Swin = Surface water in
SWout = Surface water out
GWin = Ground water in
GWout = Ground water out
V = Change in water storage
Approach is simple, application is difficult
High uncertainty in measured values
Accuracy increases as t increases

11. Approaches to Evaporation Studies
2. Mass-Transfer Approach
Evaporation equals the transfer of water vapour (mass) which can be estimated as
E = Evaporation
bo = Empirical constant
b1 = Empirical constant
es = Saturation vapor pressure
ea = Actual vapor pressure
a = Wind speed

12. Approaches to Evaporation Studies
3. Energy Balance Approach
Latent heat of vaporization balances energy inputs and outputs for a given period (Electromagnetic radiation)
Q* = K - K + L - L
= K (1-) + L*
Q* = Net radiation
Qh = Sensible heat flux (surface – atmosphere)
Qg = Sensible heat flux (surface – subsurface)
Qe = Latent heat flux (surface – atmosphere)

13. Approaches to Evaporation Studies
3. Energy Balance Approach
E = Evaporation
K = Net short-wave radiation input
L = Net long-wave radiation input
G = Net output via conduction to ground
H = Net output of sensible heat to atmosphere
Aw = Net input associated with in/outflows of water
Q = Change in heat storage per unit area
t = Change in time
w = Density of water
w = Latent heat of vaporization

14. Points to note for the energy balance approach
Empirical constants depend on height of wind speed and vapor pressure measurements
Difficult to obtain representative measurements of wind speed and vapor pressure from shore-based observation stations
Influenced by atmospheric instability – free convection
Turbulent transport of water vapor (and heat) away from the surface even in the absence of wind
Upward diffusion of water vapour from the evaporating surface
“Drying power” of overlying air (humidity)
Evaporation is highest when: warm, dry conditions (large saturation deficit), i.e., es = high, ea = low
Diffusion controlled by concentration gradient

15. Approaches to Evaporation Studies
4. Combined Approach
Attempts to consider all factors influencing evaporation. To be studied in next lecture