Question

The principal meteorological factor influencing evapotranspiration is:

• A. air temperature
• B. solar radiation
• C. humidity
• D. wind speed

Hint:

Remember that in physics and thermodynamics, a change of state (from liquid to gas) requires energy. Solar radiation is the only primary source of energy in the agricultural ecosystem, making it the dominant driver of ET.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Evapotranspiration (ET) is the combined process of water loss from the soil surface (evaporation) and water loss through plant tissues (transpiration).
Both processes require a continuous input of energy to convert liquid water into water vapor.
While several meteorological factors influence this rate, one primary energy source drives the entire process.

Step 2: Detailed Explanation:
Let us evaluate the role of each meteorological factor in evapotranspiration:
1. Solar Radiation (B):
Solar radiation is the primary source of energy on Earth.
It provides the latent heat of vaporization ($2.45\text{ MJ/kg}$ at $20^{\circ}\text{C}$) required to break the hydrogen bonds between liquid water molecules, converting them into water vapor.
Without this solar energy input, evaporation and transpiration would slow down significantly, regardless of air temperature or wind speed.
Therefore, solar radiation is the principal meteorological driver of ET.
2. Air Temperature (A):
Air temperature reflects the sensible heat in the atmosphere.
While it influences the rate of ET, it is itself a secondary effect driven by solar radiation heating the Earth's surface.
3. Relative Humidity (C):
Humidity determines the vapor pressure gradient between the evaporating surface and the surrounding air.
Lower humidity increases this gradient, which increases ET, but it is not the energy source that drives the phase change.
4. Wind Speed (D):
Wind removes the saturated boundary layer of air next to the leaf or soil surface, maintaining a favorable vapor pressure gradient, but it is a secondary transport mechanism rather than the primary driver.

Step 3: Final Answer:
The principal meteorological factor that provides the energy required for evapotranspiration is solar radiation.
Hence, the correct option is (B).