Question

Evapotranspiration is directly measured by

• A. Pan evaporimeter
• B. Penman-Monteith equation
• C. Hygrometer
• D. Lysimeter

Hint:

Lysimeter = Actual ET (Direct).
Open Pan = Potential Evaporation.
Penman-Monteith = Calculated/Estimated ET.
Think of a Lysimeter as a "Giant Weighing Scale" for a whole crop patch.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify the specific physical instrument used for the direct measurement of evapotranspiration (ET) from a soil-plant system.
Detailed Explanation:

• The Lysimeter:
A lysimeter is a large container buried in the ground and filled with soil and the target crop.
It works on the principle of the water balance equation: \( \text{Inflow} - \text{Outflow} = \Delta \text{Storage} \).
By accurately weighing the entire container (Weighing Lysimeter) or measuring the drainage water (Drainage Lysimeter), the amount of water lost through the combined process of soil evaporation and plant transpiration can be measured directly.

• Why other options are not direct:
Pan Evaporimeter: Measures "Evaporation" from a free water surface, not ET from a crop. It provides a reference (PET), not the actual ET.
Penman-Monteith Equation: This is a theoretical model or mathematical formula based on energy balance and mass transfer. It is an estimation method, not a direct measurement.
Hygrometer: This measures the relative humidity of the air, which is only one factor influencing ET.

• Types of Lysimeters:
Weighing type: Most accurate; measures weight changes in the soil-water-plant system.
Non-weighing/drainage type: Measures ET by subtracting drainage and runoff from the applied irrigation/rainfall.

Step 2: Final Answer:
Among the given choices, the Lysimeter is the only instrument that provides a direct physical measurement of evapotranspiration.