Evaporation and Transpiration Notes

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  • Topic: Water, Vapor pressure, Water vapor
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Evaporation and Transpiration
Evaporation and transpiration (evapotranspiration) are components of hydrologic cycle, which at principle, can be calculated from meteorological observations such as temperature, humidity, radiation, wind speed, etc. The process, however, is quite complex and a substantial literature has been produced on it. In this chapter our focus is mainly on the concept and simple estimations of evaporation and transpiration. Mechanism of evaporation from water surfaces

Here we firs deal with evaporation from water surfaces. At the surface of a water body water molecules are exchanged between the liquid and the atmosphere. The escape of water from the surface of the liquid lowers its temperature. Similarly, condensation of water causes its temperature to rise. The energy exchange due to evaporation or condensation is large, 540 Cal per gram. Note that 1 Cal equals 4.184 j (Joule). At the event that the molecules are prevented from escaping, the vapor pressure in the liquid rises. This vapor pressure is directly proportional to concentration of water molecules as vapor in the air above the liquid. The higher the vapor pressure, the more water molecules will re-enter the liquid. Eventually, equilibrium is reached when the vapor pressure becomes equal to saturated vapor pressure. Because above the earth we have the atmosphere, it is always possible for water molecules to diffuse into the air and therefore, under normal conditions, we do not have saturation. Now, let us consider the situation of the air above the water surface with partial vapor pressure less than saturated vapor pressure. We can assume that immediately above the surface of water, a very thin film of vapor-water molecules forms with a temperature the same as the water temperature. Since the vapor pressure of this thin film is equal to the saturated vapor pressure, it is more than the partial vapor pressure of the surrounding air. Therefore, the water molecules escape from the saturated film and diffuse into the air. The rate of evaporation depends on the difference between the vapor pressure of air and saturated vapor pressure. Pan evaporation

The pan evaporation is a measurement that combines the effects of climate elements and reflects the net evaporation from the surface of the water. Pan evaporation is the maximum in hot, dry, windy days. For actual field measurement, an evaporation pan is used for measuring the pan evaporation. There are many sizes and shapes of evaporation pans. The most common evaporation pan is called “U.S. Class A evaporation pan”, cylinder with the diameter of 47.5 in. (120.7 cm) and the depth of 10 in. (25 cm). The pan is placed leveled and is filled up to 2 inches (5 cm) from the pan top. After 24 hours the amount of water that is needed to refill the pan to the same level is reported as the evaporation.

Figure 4.1 Class A pan evaporimeter
Aerodynamic method for determining evaporation
Vapor pressure
For any gas in a mixture in a container, the partial pressure is defined as the pressure of that that specific gas would have if it was alone in the container. The partial pressure of water vapor is called vapor pressure. The vapor pressure is defined as: e=ρWRT0.622

e = vapor pressure in (mb)
ρw = vapor density (g/cm3)
R = dry air constant = 2.87 × 103 mb cm3/g°K
T = absolute temperature (°K)
Note:
°K=273.15+°C
°F = [(°K-273.15)-32] × 1.8
1 mb = 100 Pa = 100 N/m2 (close to earth)
Saturated vapor pressure:
The air can contain certain amount of water vapor. There is a point that if you add more water vapor to water, there will be some vapor that will go out of the air in the form of condensed vapor (water). This situation is called saturation. The vapor pressure that is essentially the maximum possible water vapor pressure under that condition is called saturated vapor pressure. It is usually shown by es Relative humidity

The relative humidity is the ratio of actual vapor pressure to the vapor...
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