Step 1: Define the Pan Coefficient (\(C_p\)).
The pan coefficient (\(C_p\)) is a correction factor that relates evaporation measured from an evaporation pan (\(E_{pan}\)) to actual evapotranspiration (ET) from a lake or vegetated surface, using the formula \( ET = C_p \times E_{pan} \). Pans that are less similar to natural surfaces will exhibit higher evaporation and thus receive a lower pan coefficient.
Step 2: Analyze factors influencing the pan coefficient for different pan types.
- D. USGS Floating Pan: This pan floats directly in the water body, maintaining a water temperature close to the lake. It is susceptible to splashing. Due to its high similarity to lake conditions, it requires minimal correction, resulting in the highest pan coefficient. (Typical \(C_p \approx 0.9-1.0\)).
- C. Colorado Sunken Pan: Buried in the ground, this pan experiences moderated temperature fluctuations. Its evaporation rate is lower than a surface pan and more representative of a natural surface, yielding a relatively high coefficient. (Typical \(C_p \approx 0.85-0.95\)).
- B. ISI Pan (IS: 5973): Similar to the Class A pan but equipped with a wire mesh cover to prevent bird interference. The mesh reduces evaporation by limiting wind and solar radiation. Consequently, its measured evaporation is lower, necessitating a higher correction factor than an uncovered pan to estimate actual ET. (Typical \(C_p \approx 0.8-0.9\)).
- A. Class A Land Pan: This pan is positioned above ground, exposed to direct solar radiation on its sides and full wind influence. This leads to higher evaporation rates than a natural surface, thus requiring a greater correction (a lower pan coefficient). (Typical \(C_p \approx 0.7\)).
Step 3: Arrange the pan types in increasing order of their \(C_p\) values.
Class A Pan (\(\sim\)0.7) \(\rightarrow\) Colorado Sunken Pan (\(\sim\)0.9) \(\rightarrow\) ISI Pan (mesh-covered, generally higher \(C_p\) than Class A) \(\rightarrow\) USGS Floating Pan (\(\sim\)0.95).
The order of increasing pan coefficients is generally A \textless C \textless B \textless D. This corresponds to option B.