Evaporation and Transpiration Notes
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
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