Evaporation is the process of change of water into steam in which water molecules leave the surface of water by receiving 600 Calories. This process can be carried out either from the surface of open water or soil or by plants. The amount of water that plants release into the atmosphere is called transpiration.
The amount of evaporation from soil and plant surfaces cannot be separated, so the sum of the two is called evapotranspiration. The significance of evapotranspiration can be witnessed by considering the fact that 57% of the water which falls on lands all over the world is directly evaporated.
The factors affecting evaporation are many, including: solar radiation, moisture content in nature, wind and its speed. The sun's radiation provides the energy needed for evaporation; hence during the day and in the summer there is more energy for evaporation due to more radiation, and evaporation is at its peak at this time.
The same is true about northern latitudes. The humidity or degree of air saturation is also effective in evaporation, as evaporation in dry air is more than humid air; furthermore, wind intensifies evaporation by removing the saturation layer and replacing dry air. Measurement of evapotranspiration in different regions is important in several respects:
First, it controls the humidity of the region through evaporation, and therefore its measurement can be effective in determining the type of the region’s climate.
Secondly, evaporation from the surface of rivers, lakes and various reservoirs causes water losses that need to be calculated.
Thirdly, evapotranspiration from the vegetation within the basin area, which is also considered as water loss, is regarded as an important component of water cycle.
Fourthly, the amount of water used in irrigation schemes for agriculture and trees is also based on evapotranspiration, and determination of dimensions of irrigation networks is directly related to the amount and time of the loss of water entered into atmosphere through evapotranspiration.
Since the province of Hamedan is considered as one of the agricultural regions of the country, it is necessary to study evapotranspiration and its amount in this province in order to estimate the actual needs of plant to water and also further the economic development of the region. Measurement of evapotranspiration is carried out in two ways: the direct method, which is measured by the lysimeter and evaporation pan, etc., and the second method, which is an experimental measurement in which practical formulas and equations are used to estimate the evapotranspiration rate.