Question

Lake evaporation measured by

• A. USWB Class A pan
• B. Lysimeter
• C. Field plot
• D. Tensiometer

Hint:

Remember the classic relationship: $\text{Lake Evaporation} = C_p \times \text{Pan Evaporation}$.
For the standard USWB Class A pan, the average value of the pan coefficient ($C_p$) is $0.70$.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Evaporation from open water bodies like lakes represents a major component of the hydrological cycle.
Because direct measurement of evaporation from a large lake is physically difficult, meteorological instruments are used to estimate it.
The most common method involves measuring evaporation from a standard water pan and applying a correction factor.

Step 2: Detailed Explanation:
Let us analyze each instrument and its specific application:
1. USWB Class A pan (A):
This is a standard pan evaporimeter recommended by the United States Weather Bureau.
It is a cylindrical pan made of unpainted galvanized iron, measuring $1.22\text{ m}$ in diameter and $255\text{ mm}$ in depth.
The pan is filled with water, and the daily drop in water level is recorded to measure open-pan evaporation.
Because a pan has a much smaller volume of water than a lake, it warms up faster, resulting in higher evaporation rates.
To estimate the actual lake evaporation, the pan evaporation rate is multiplied by a pan coefficient ($C_p$), which typically ranges from $0.6$ to $0.8$ (with an average value of $0.7$):
\[ \text{Lake Evaporation} = C_p \times \text{Pan Evaporation} \] Therefore, the USWB Class A pan is the standard tool used to estimate lake evaporation.
2. Lysimeter (B):
A lysimeter is a device used to measure the actual evapotranspiration of crops under field conditions by monitoring the water balance of a soil column.
3. Field plot (C):
Field plots are used to study runoff, soil erosion, or crop yields under specific experimental treatments.
4. Tensiometer (D):
A tensiometer measures soil moisture tension (suction), which indicates how tightly water is held by soil particles.

Step 3: Final Answer:
Lake evaporation is standardly measured and estimated using the USWB Class A pan.
Hence, the correct option is (A).