The change of state of water from solid and liquid to the vapour and its diffusion into the atmosphere is referred to as evaporation. In agricultural meteorology, evaporation is defined as the maximum possible loss of moisture form a wet, horizontal, flat surface exposed to weather parameters, which exist in the vicinity of plants.
Evaporation is an important process of hydrologic cycle. The evaporation from the soil is an important factor deciding the irrigation water requirements of a crop. In modifying the microclimate of a crop, the evaporation from the soils is an important factor. It is the most important of all the factors in the heat budget, after radiation and in the water economy. Since, a certain amount of evaporation also demands a definite amount of heat, it provides a link between water budget and heat budget.
Factors affecting evaporation – The evaporation from a fully exposed water surface is the function of several environmental factors.
1. Environmental factors
Water temperature – With an increase of temperature, the kinetic energy of water molecules increases and surface tension decreases which increases evaporation.
Wind – The evaporation from fully exposed surface is directly proportional to the velocity of wind and vice-versa, because dry wind replaces the moist air near water. The process of evaporation takes place continuously when there is a supply of energy to provide latent heat of evaporation (540 calories/ gram of water).
Relative humidity – The evaporation is greater at low RH than at high RH.
Pressure – The evaporation is more at low pressure and less at high pressure.
2. Water factors
Composition of water – The dissolved salts and other impurities decrease the rate of evaporation. The evaporation is inversely proportional to the salinity of water.
Area of evaporation – The larger the area of exposure, greater will be the evaporation. Those affecting water supply at the evaporating surface. i.e., soil and plants including soil storage capacity, rainfall and irrigation and those affecting energy supply to the evaporating surface like solar radiation.