1. EVAPORATION

2. INTRODUCTION
A process whereby liquid water is converted to water vapour and removed from the vapour surface.
Evaporation consists of 2 components i.e. The energy component and the aerodynamic component
The energy component is responsible conversion of liquid water to water vapour

3. INTRODUCTION
The aerodynamic component is responsible for vapour removal from the evaporating surface
Energy used for evaporation is the net all wave radiation and to a lesser extend ambient temperature
Evaporation take place from water surfaces, soil surfaces, plant surfaces and animal surfaces.

4. Factors affecting evaporation
Meteorological factors
Radiation, relative humidity, wind speed, air temperature
Radiation and temperature provide the energy required to break water bonds to form vapour
RH and wind speed constitute the aerodynamic component (vapour removal from the surface)

5. Factors affecting evaporation
Evaporation proceeds when there is a vapour pressure gradient between the evaporating surface and the ambient
Wind speed is required for the creation of the vapour pressure gradient
Low ambient RH increases the vapour pressure gradient and vice versa

6. Factors affecting evaporation
The higher the vapour pressure gradient, the faster evaporation will take place
Crop factors
Crop type, variety, development stage, resistance to transpiration (stomatal resistance), crop roughness, ground cover, root charactreristics

7. Factors affecting evaporation
These factors are summarised in the crop coefficient, Kc
Management / Environmental factors
Soil fertility, soil salinity, mulching, hard soil horizons, pests and diseases, plant density, water availability

8. Factors affecting evaporation
These may limit crop development and reduce ET.
Evaporation can be measured in depth or energy terms per unit time
The most common unit is mm/day or MJ/m2/day

9. Conversion factors for evaporation
Depth
Volume per unit area
Energy per unit area
mm/day
mm/ha/day
l/s/ha
MJ/m2/day
1mm/day 1 10
0.116
2.45
1m3/ha/day
0.1
0.012
0.245
1l/s/ha
8.640
86.40
21.17
1MJ/m2/day
0.408
4.082
0.047

10. Conversion factors for evaporation
Example On a summer day , net solar radiation received at a dam was 18 MJ/m2/day. If 80 % of the energy was used to vapourise the water, what is the evaporation rate in mm/day? Answer 0.8*18*0.408 = 5.88 mm/day

11. Terms Reference evapotranspiration, ETo
Is ET rate from a reference surface, not short of water
A reference surface is a hypothetical grass closely representing an extensive surface of 8 – 15 cm tall, green grass cover of uniform height, actively growing & completely shading the ground
Solely a function of meteorological factors i.e. The evaporative demand of the atmosphere

12. Terms Crop evapotranspiration under standard conditions, ETc
is the evaporative demand from crops grown under optimal soil water, excellent management and environmental conditions and achieve full production under the given climatic conditions

13. Terms Crop evapotranspiration under non standard conditions, ETc adj
Is ET from crops grown under management and environmental conditions that differ from standard conditions – calculated by a water stress coefficient, Ks and or by adjusting Kc

14. Measurement of evaporation
Can be measured from soil, plant, water surfaces
Measurement can be direct or in direct
Some of these methods are:
evaporation pan, lysimetry, energy budget, eddy covariance, whole plant evaporation chambers, evaporimeters / atmometers, sap flow gauges, thermal dissipation probe, soil water balance

15. Measurement of evaporation

16. Estimation of ETo
Usually done using empirical relationships from long term studies
Most methods are temperature and radiation based
Correlations were established (and are still being established) between ETo and meteorological parameter(s) using long term data

17. Estimation of ETo
Empirical relationships employ simpler models with less input variables and can be broadly applied
Not limited to data availability
Best suited for humid conditions where the aerodynamic term is relatively small
Empirical equations are however less likely accurate in arid and semi arid conditions (aerodynamic component is significant)

18. Estimation of ETo
Local calibration is required for satisfactory results
Examples of evaporation equations are:
Blaney Criddle Jensen - Haise
Hargreaves Priestly & Taylor
Senami – Hargreaves Thornthwaite
Penman
FAO Makkink

19. Estimation of ETo
Blaney Criddle method p is the ratio of actual daily daytime hours to annual mean daily day time hours Tmean is the mean air temperature (oC)

20. Estimation of ETo
FAO Makkink method c is the FAO-24 adjustment factor for mean RH and day time wind speed (c is usually assumed to be 1) Rs is the solar / short wave radiation (MJ/m2/day) ∆ is the slope of the vapour pressure curve (kPa/oC) γ is the psychrometric constant (kPa/oC)

21. Estimation of ETo
Jensen – Haise method Rs is the incoming short wave radiation (W/m2) Ta is the average air temperature at 2m (oC)

22. Estimation of ETo
Senami – Hargreaves method Tmean is the average daily air temperature (oC) Tmax and Tmin are the daily maximum and minimum temperatures respectively (oC) Ra is the extraterrestrial radiation (mm/day)

23. Estimation of ETo
Thornthwaite method ETo = montly reference evapotranspiration (cm) T is the mean monthly temperature (oC) I is a heat index for a given area which is the sum of 12 monthly index values, i

24. Estimation of ETo
i is derived from mean monthly temperatures using: a is an empirically derived exponent which is a function of I

25. Estimation of ETo The FAO Penman – Monteith method
Is the standard method for measuring ETo
Was developed based on correlation between energy conservation and the aerodynamics of an area (combination method)
The equation is a close, simple representation of the physical and physiological factors governing ETo

26. Estimation of ETo
The method requires all meteorological data required for evaporation to take place
Missing data for a site must be estimated before using the method
The method is mainly used for calibration of other methods

27. Estimation of ETo
Rn is the net radiation at the surface (MJ/m2/day), G is the soil heat flux (MJ/m2/day), T is the average air temperature (oC), U2 is wind speed measured at 2m height (m/s), (ea-ed) is the vapour pressure deficit (Kpa)

28. Estimation of ETo
∆ is the slope of the vapour pressure curve (Kpa/oC), γ is the psychrometric constant (kPa/oC) and 900 is a conversion factor