1. EVAPORATION
Definition: Process by which water is changed from the liquid or solid state into the gaseous state through the transfer of heat energy (ASCE, 1949).
It occurs when some water molecules attain sufficient kinetic energy to break through the water surface and escape into the atmosphere (~ 600 cal needed to evaporate 1 gram of water).
Depends on the supply of heat energy and the vapor pressure gradient (which, in turn, depends on water and air temperatures, wind, atmospheric pressure, solar radiation, etc).

2. TRANSPIRATION (T)
Transpiration is the evaporation occurring through plant leaves (stomatal openings).
Transpiration is affected by plant physiology and environmental factors, such as:
- Type of vegetation
- Stage and growth of plants
- Soil conditions (type and moisture)
- Climate and weather

3. EVAPOTRANSPIRATION (ET)
Combined “loss” of water vapor from within the leaves of plants (“transpiration”) and evaporation of liquid water from water surfaces, bare soil and vegetative surfaces.
Globally, about 62% of the precipitation that falls on the continent is evapotranspired (~72,000 km3/yr); 92% of which from land surfaces evapotranspiration and 3% from open water evaporation (source: Dingman, “Physical Hydrology”).
Approximately 70% of the mean annual rainfall in the U.S. is returned to the atmosphere as evaporation or transpiration.

4. EVAPOTRANSPIRATION (ET)
In practice, the terms E and ET are often used to mean the same thing - the evaporation from the land surface.
Therefore, you must use the context to determine what the term evaporation means in a specific case (i.e., is it just from an open water surface or the entire land surface?).

5. POTENTIAL EVAPORATION (PE)
is the climate controlled evaporation from an open water surface with unlimited supply (and no thermal capacity).

6. POTENTIAL EVAPOTRANSPIRATION (PET)
is the ET that would occur from a well vegetated surface when moisture supply is not limiting (often calculated as the PE).
Actual evapotranspiration (AET; ET) drops below its potential level as the soil dries.

7. DESIGN
Evaporation must be considered in the design of large water storage reservoirs, large-scale water resources planning and water supply studies.
For flood flow studies, urban drainage design applications it may be neglected.
Example: during typical storm periods with intensities of 0.5 in/hr, evaporation is on the order of 0.01 in/hr.

8. METHODS FOR ESTIMATING EVAPORATION
Water budget methods
Energy budget methods
Mass transfer techniques (e.g., Meyer, Thornthwaile-Holzman)
Combination of energy budget and mass transfer methods (e.g.,Penman)

9. Energy budget method Total solar radiation - Rt
Net energy advected (net energy content of incoming and
outcoming water - Ee
Reflected solar
radiation - Rr
Net long-wave radiation exchange between the atmospere and the water body- R1
Sensible heat loss from the water body to the atmosphere - Hn
Energy used for evaporation
(latent heat)- Ee
Energy stored - Es
R1 includes long-wave (LW) radiation from the atmosphere, reflected LW radiation, LW radiation emitted by water

10. Energy budget method R1 Rr Rt Ee Hn Ee Es
R1 includes long-wave (LW) radiation from the atmosphere, reflected LW radiation, LW radiation emitted by water

11. Energy budget method
Amount of evaporation - E

12. Energy budget method Characteristics:
most accurate method (evaporation is a function of the energy state of the water system)
difficult to evaluate all terms
energy balance equation has to be simplified
empirical formulas are used (although radiation measurements are preferable)

13. Water budget method Precipitation - P Inflow- Q Evaporation- E
Surface runoff - Qr
Subsurface runoff - Qs
Subsurface seepage losses- Qd
Outflow- Q0

14. Water budget method Units: Depth of evaporation: n – number of days
Ap – area of the pond [ac]

15. Water budget method Characteristics: Simple
Difficult to estimate Qd and Qs
Unreliable, accuracy will increase as Δt increases

16. Example on water balance model

17. Mass transfer methods - definitions
e – actual vapor pressure (difference in the atmospheric pressure with and without the vapor)
es – saturated vapor pressure (partial pressure of water vapor in saturated air)
T [ºC] – air temperature
Rh – relative humidity

18. Evaporation is a diffusive process (moves from where its concentration is larger to where its concentration is smaller at a rate that is proportional to the gradient of concentration):
E = b0 (es0 – ea)
es0 – vapore pressure of the evaporating surface; saturation vapor pressure at the water surface temperature Ts
ea – vapor pressure of overlying air at the same height
b0 – empirical coefficient that has to be calibrated

19. b0 = function (air turbulence)=fn(v) E = b1 fn(v)(es – ea)
E = b0 (es0 – ea)
Studies showed that
b0 = function (air turbulence)=fn(v)
E = b1 fn(v)(es – ea)
Meyer’s formula:
E = 0.5 ( v30)(es – ea)
v30 - wind speed [mi/h] at 30 ft height;
es; ea [in Hg]
E [in/day]

20. k – Von Karman constant (0.41)
b0 = f(v, es, ea, Ta, Tw)
Thornthwaite-Holzman equation (no calibration)
b0 = f(v,T,k);
k – Von Karman constant (0.41)

21. Example

22. Combination approach – Penman equation
Combine mass-transfer and energy-balance equations to derive an evaporation equation that does not require water surface temperature data.

23. Penman equation:
Δ [mm Hg/ºC] – slope of the saturation vapor pressure curve at mean temperature
T0 [ºC] – temperature of the water surface
T [ºC] – temperature of the air
e0 [mm Hg] - vapor pressure of the water surface
ea* [mm Hg] - saturated vapor pressure at temperature T

24. Penman equation: En [mm/day] – net radiation
Start with energy equation:
Rn – net radiation
RI – amount of energy absorbed (shortwave)
RB – net outward flow of longwave radiation

25. Penman equation:
RI [g-cal/cm2-day] – amount of energy absorbed (shortwave)
RA [g-cal/cm2-day] – total possible radiation for the period of estimation; it is function of latitude and season; Table 14-3.
r – reflection coef. ( )
a,b – empirical coef. (a=0.2; b=0.5)
n/D – fraction of possible sunshine (from climatic atlas)

26. Rn[g-cal/cm2-day] – net radiation
RI [g-cal/cm2-day] – amount of energy absorbed (shortwave)
RB [g-cal/cm2-day] – net outward flow of longwave radiation
e [mm Hg] – actual vapor pressure
T [ºC] – air temperature
n/D – fraction of possible sunshine (from climatic atlas)

27. Penman equation:

28. Penman equation: En – net radiation Rn – net radiation
Hv –latent heat of vaporization

29. Penman equation:

30. Penman equation:

31. Penman equation:

32. example

33. Measuring evaporation

34. PE = Rainfall + Irrigation - Percolation
Irrigated lysimeter
PE = Rainfall + Irrigation - Percolation

35. Atmometer

36. U.S. Weather Bureau Class A Pan
4 ft
Wooden support
10 in
6 in
Galvanized steel

37. Evaporation pan Subsurface runoff - Qs Inflow- Q Outflow- Q
Evaporation - E
Subsurface seepage losses- Qd
Precipitation - P
Surface runoff - Qr

38. Evaporation Pan
Historical records of daily pan evaporation are available from the National Climatic Data Center (NCDC) for U.S. Weather Buruau Class A Land pans.

39. Evaporation Pan
We are not really interested in what evaporates from a pan; instead we want to know the regional evaporation from land surface or the evaporation from a nearby lake. Unfortunately, pan evaporation is often a poor indicator of these variables (due in part to pan boundary effects and limited heat storage).

40. Evaporation Pan
Evaporation from an open water surface (E) is usually estimated from the pan evaporation (Ep) as:
E = K Ep where K is the pan coefficient (regional coef, usually around ~0.7). Similar expressions are also used in practice to estimate potential evapotranspiration from pan data.

41. Pan coefficient
FIGURE 2. Source: Farnsworth, Richard K., Edwin S. Thompson, and Eugene L. Peck. After Map 4: Pan Coefficients.
In NOAA Technical Report NWS 33, Evaporation Atlas for the Contiguous 48 United States, NWS, NOAA, 1982.

42. evapotranspiration from satellite data
When a surface evaporates, it looses energy and cools itself. It is that cooling that can be observed from space. Satellites can map the infrared heat radiated from Earth, thus enabling to distinguish the cool surfaces from the warm surfaces.
summer
winter