Question

Write the sequence of the following type of Pan according to the value of pan coefficient in increasing order.
A. Class A Land Pan
B. ISI Pan
C. Colorado Sunken Pan
D. USGS Floating Pan

• A. A, B, C, D
• B. A, C, B, D
• C. B, A, D, C
• D. C, B, A, D

Hint:

The more a pan's environment mimics a large natural water body, the higher its pan coefficient will be. A floating pan is most similar (highest \(C_p\)), while an exposed land pan is least similar (lowest \(C_p\)).

Answer 1

The Correct Option is B

Step 1: Define Pan Coefficient (\(C_p\)).
The pan coefficient is a correction factor used to relate the evaporation measured from an evaporation pan to the actual evapotranspiration (ET) from a lake or vegetated surface. \( ET = C_p \times E_{pan} \). A pan that loses more water (less similar to a natural surface) will have a lower pan coefficient.
Step 2: Analyze the factors affecting the pan coefficient for each pan type.
- D. USGS Floating Pan: It floats in the water body itself. Its water temperature is very close to the lake temperature, and it is subject to splashing. Evaporation is most similar to the lake, so it requires the least correction, meaning it has the highest pan coefficient. (Typical \(C_p \approx 0.9-1.0\)).
- C. Colorado Sunken Pan: It is buried in the ground, which moderates temperature fluctuations, making its evaporation rate lower than a surface pan and more representative of a natural surface. It has a relatively high coefficient. (Typical \(C_p \approx 0.85-0.95\)).
- B. ISI Pan (IS: 5973): This pan is similar to the Class A pan but has a wire mesh cover to prevent birds from drinking. This mesh reduces evaporation due to reduced wind and solar radiation, meaning its measured evaporation is lower. To get to the actual ET, it needs a higher correction factor than an uncovered pan. (Typical \(C_p \approx 0.8-0.9\)).
- A. Class A Land Pan: It is exposed above ground, so it absorbs radiation on its sides and is fully exposed to wind. This makes it evaporate more water than a natural surface, thus requiring a larger correction (i.e., a lower pan coefficient). (Typical \(C_p \approx 0.7\)).
Step 3: Arrange in increasing order of \(C_p\). Class A Pan (\(\sim\)0.7) \(\rightarrow\) Colorado Sunken Pan (\(\sim\)0.9) \(\rightarrow\) ISI Pan (mesh makes it complex, but generally higher C\(_p\) than Class A) \(\rightarrow\) USGS Floating Pan (\(\sim\)0.95).
The order from lowest to highest coefficient is generally: A \textless C \textless B \textless D. This matches option B.