Evaporation is defined as the process by which water moves out of the water surface or soil surface in the form of water vapour to atmosphere due to pressure gradient. Evaporation from natural surface such as open water, bare soil or vegetative cover is a diffusive process by which water in the form of vapour is transferred from the underlying surface to the atmosphere.
The essential requirement for evaporation process are:
• Source of heat energy to vaporize the irrigated water.
• The presence of a concentration gradient of water vapour between the evaporating surface and surrounding air of atmosphere.
Evaporation can occur only when vapour concentration of evaporating surface exceeds that of the surrounding air. The sources of heat energy are solar energy and wind energy. The energy required for evaporation is 590 calories per gram of water to evaporate at 20°C. The fundamental principle of evaporation from a free surface has indicated evaporation as the function of difference in the vapour pressure of water surface and the vapour pressure of air.
Measurement of evaporation –
This can be made by the following methods.
• Pan evaporimeter
• Tin can evaporimeter
• Pitche evaporimeter
1. Pan Evaporimeter- Evaporimeter is an instrument which integrates the effect of all the different climatic elements and furnishes their combined effect. It is relatively simple, cheap and more useful in irrigation practices.
2. “U.S.W.B. Class A pan” Evaporimeter – It is the standard type used globally. It is made of GI pan having a diameter of 120 cm with a depth of 25 cm. It is painted with white colour to reduce heat absorption and mounted on a wooden platform at a height of 15 cm from ground level to reduce the effect of soil temperature. The water level is measured by the Hook gauge or a fixed scale attached to a stilling well. The pan is covered with a mesh to prevent animals’ and bird’s disturbances to the water. Evaporation is recorded at a fixed time in the still well by adding water in the evaporimeter to compensate the daily loss of water by evaporation. Evaporimeter is to be cleaned periodically and tested for it’s leakage. Development of algae etc., should be avoided.
3. “Tin can” evaporimeter – A small tin is fitted with a scale and water is filled in the tin and kept in the cropped field at different locations. The daily loss will be taken as evaporation.
4. “Pitche” evaporimeter – It consists of a graduated tube of 30 cm with one open end and covered by a drier paper and is attached in a metallic stand. The tube is filled with water and turned upside down. The water slowly wets the paper and evaporates and the water loss in the tube is considered as a measurement of evaporation.
Evaporation, transpiration and consumptive use are the important factors in estimating irrigation requirement and planning irrigation system.
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