Introduction

• Evaporation is the processes in which a liquid change to the gaseous state at the free surface, below the boiling point through the transfer of heat energy.
• Evaporation is expressed as an evaporation rate in mm/day, cm/day, in/day.
• The process of evaporation is dependent on the driving force and resistance force.
• Evaporation will occur only if the actual vapor pressure of air above the evaporating body is less than that of the evaporating surface.

Factors influencing evaporation and transpiration rates

I. Area of water available: The amount of evaporation is directly proportional to the area of evaporation. If the exposed area is large, the evaporation will be more and vice versa.

ii. Amount of water available: Rate of the evaporation increases with increase in the amount of available water. Thus, evaporation is greater over oceans than over continents. In soils, the rate of evaporation at field capacity is higher than that at the lower moisture level.

iii. Solar radiation and air temperature: High radiation or more temperature means more energy available to the water molecules for escaping from liquid to gaseous stage. If temperature is more, water vapor capacity of the air mass also increased leading more evaporation. Thus, evaporation in summer season is higher than that of the winter season.

iv. Wind velocity: If the winds are light, a thin layer of the air above the surface gets saturated and vapor pressure deficit between the surface and the atmosphere becomes small, which reduces the rate of evaporation. When wind velocity is high, moisture evaporated from the ground is mixed with upward atmosphere and the vapor pressure deficit remains high, thereby increasing the evaporation rate.

v. Humidity: Vapor pressure deficit is reduced at higher level of atmospheric humidity and thus evaporation is lowered.

vi. Atmosphere pressure: If the atmospheric pressure is more, naturally will be lesser evaporation (Dallon’s law). At the higher altitude, the atmospheric pressure is less; hence the evaporation should be normally higher. This is not true exactly so, because of the decreased of temperature at higher altitude (means decreased atmospheric pressure) which reduce the evaporation.

vii. Soil characteristics: When moisture content of the soil is at field capacity, soil characteristics does not affect the rate of evaporation critically. When the moisture content of the soil decreased and the soil dried out, the rate of evaporation depends primarily on the soil texture, configuration and soil profile, uniformity etc.

viii. Quality of water: Presence of any dissolved salts in water reduces the saturated vapor pressure of the water which consequently reduces the rate of evaporation. So, the evaporation from fresh water is greater than that from saline water.