Question

The drawbacks of Thornthwaite formula as enumerated by Chang (1968) are: (A). Air humidity alone is a good indication for the energy available for ET., (B). Air temperature at a place lags behind radiation., (C). According to formula ET will cease when mean temperature is below 0 degree C which by no means is true., (D).The formula does not take into account the wind effect which might be an important factor in some areas., (E). It considers the effect of warm and cool air on the temperature of a place. Choose the correct answer from the options given below:

• A. (B), (C) and (D) only
• B. (A), (B) and (C) only
• C. (B), (C) and (E) only
• D. (C), (D) and (E) only

Hint:

Thornthwaite is a temperature-only empirical method.
Its main drawbacks are the temperature-radiation phase lag, assuming zero ET below \(0^{\circ}\text{C}\), and neglecting wind speed and relative humidity.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Thornthwaite formula (1948) is an empirical method used to estimate potential evapotranspiration (\(\text{PET}\)) based on mean monthly air temperature and daylight hours.
Although simple to use, the formula has several physical limitations because it does not account for key aerodynamic and radiation parameters.

Step 2: Detailed Explanation:
Let us analyze the limitations of the Thornthwaite formula as discussed by Chang (1968):
- (B) Air temperature lags behind radiation: Solar radiation is the primary energy source driving evapotranspiration, but air temperature exhibits a seasonal phase lag relative to radiation.
This lag causes the Thornthwaite formula to underestimate evapotranspiration in the spring and overestimate it in the autumn.
- (C) Cessation of ET below \(0^{\circ}\text{C}\): The formula assumes that potential evapotranspiration is zero whenever the mean monthly temperature falls below \(0^{\circ}\text{C}\).
In reality, sublimation and evaporation can still occur below freezing due to direct solar radiation and wind, making this assumption incorrect.
- (D) Neglecting wind and advection: The formula does not include wind speed or relative humidity.
Wind is a major driver of evapotranspiration, especially in arid or semi-arid regions where warm, dry winds (advective energy) significantly increase water loss.
- Statement (A) is incorrect because humidity alone does not represent available energy, and Statement (E) is not a drawback of the formula.
Therefore, statements (B), (C), and (D) are the primary drawbacks of the Thornthwaite formula.

Step 3: Final Answer:
The drawbacks of the Thornthwaite formula are (B), (C) and (D) only.