Evaporation & transpiration

Meteorological factors

In the above interactive animation, the greater the number, the greater the value of the designated factor.

Evaporation is the process to converse water into water vapour below the boiling point. In water resource studies, it affects the yield of river basins, yield of underground supplies and reservoir capacity. The rate of evaporation depends on the color and reflection properties of the surface, vapour pressure, air and water temperatures, wind speed, atmospheric pressure, quality of water and size of the water body. The more important meteorological factors are discussed below.

Solar radiation

As evaporation take place below the boiling point, an energy input should be participated to change the state of the molecules of water from liquid to gas. The energy input becomes strong when the water is under direct radiation of the sun. So, As clouds prevent the full spectrum of the sun's radiation reaching the earth's surface, the energy input would be reduced and slow down the rate of evaporation.

Wind

Wind aids in removing the evaporated water vapour from the zone of evaporation and consequently creates greater scope for evaporation. When the water vaporises into the atmosphere, the boundary layer (between earth and air or sea and air) becomes saturated. This layer continually replaced by drier air because of evaporation and the movement of water is a function of wind speed. For large water bodies, high-speed turbulent wands are needed to cause maximum rate of evaporation.

Relative humidity

If the air has high humidity, it's ability to absorb water molecules is decreased and hence rate of evaporation becomes slow.

Temperature

As mentioned in Solar radiation, energy input is the key object to proceed evaporation. If temperatures of the air and ground are high, rate of evaporation will become faster as heat energy act as an input. Since the capacity of air to absorb water vapour increases with temperature rises, air temperature can say to be have a double effect on evaporation rate.

Transpiration

(picture from http://www.fao.org/)

As common knowledge, growing vegetations need water to sustain life. However, only a small fraction of water is retained in the plant for needs, most of the water is transpired into atmosphere through the leafy part of the plant.
It is practically impossible to differentiate with evaporation in covered vegetation. So evaporation and transpiration are commonly linked together as "Evapotranspiration". In order to distinguish potential evapotranspiration and what actually takes place, a potential figure under abundant water supply is given.

Methods of calculating Evaporation

Water budget

where      E = evapotranspiration
                P = total precipitation
                I = surface inflow
                U = underground outflow
                O = surface outflow
                S = change in storage

Energy budget method
Energy budget method is similar to water budget approach. It involves an equation that lists all the sources and the thermal energy, the only unknown is the evaporation. This method is not available if many data are missed, so it is a specialist approach.

Empirical formulae

where      Ea = evaporation of lake (mm/day)
                ew = saturation vapour pressure (mmHg)
                e = actual vapour pressure (mmHg)
                u = wind velocity (m/s)

Thornthwaite's formula

where



                j is the monthly 'heat index'
                J is the yearly 'heat index'
                PEx is the theoretical potential evapotranspiration

As PEx is a theoretical value based on 30 days and 12 hours of sunshine per day, the actual PE should adjusted as:

where
                D is the number of days in the month
                T = average number of hours between sunrise and sunset in the month.

Modification by Serra:
The method has been tested by Serra, she suggested that equations of j and a can be rewritten as

Penman's Theory
In 1948, Penman presented a theory and formula for the evaporation from weather data. In order to make continuous evaporation occurs, two requirements should be met:
1. Energy supply to provide latent heat of vaporization must exists.
2. Mechanism for removing vapour must be participated.

Evaporation measurement by Pan

(picture from http://www.fao.org/)

This is a direct measurement with a evaporation pan with standard size 1.83m square and 610 mm deep filled to a depth of 550 mm in Britain. The pan is set in the ground with the rim of pan projects 76 mm above the ground. Evaporation pans are placed near waterbody and to set water level daily after measurement. Measurements are usually by hook-gauge with allowance made for input rainfall. Compared to lake and river volumes, pans have relatively small capacities and shallow depths which allow proportionately greater amounts of advected heat from the atmosphere to be absorbed by the water in the pan. Therefore, pan evaporation usually too high and need to apply a pan coefficient (0.65 ~ 1).