Question

Thornthwaite's formula is used for the calculation of

• A. Evaporation (E)
• B. Transpiration (T)
• C. Evapotranspiration (ET)
• D. Potential evapotranspiration (PET)

Hint:

Thornthwaite's equation is a temperature-based method used to estimate "PET." If you encounter an exam question asking for a PET estimation method that requires only temperature data, Thornthwaite is the classic answer.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Potential Evapotranspiration (PET) is the maximum amount of water that can be lost through evaporation and transpiration from a healthy, fully covered, well-watered grass crop under specific climatic conditions.
Several empirical formulas have been developed to estimate PET using available meteorological data.
Key Formula or Approach:
The Thornthwaite method (1948) estimates monthly PET based on the mean monthly air temperature and a heat index.
The basic monthly PET formula is given by:
\[ \text{PET} = 1.6 \left( \frac{10 T}{I} \right)^a \]
where \( T \) is the mean monthly temperature in \( ^\circ\text{C} \), \( I \) is the annual heat index (the sum of 12 monthly heat indices), and \( a \) is an empirical cubic function of \( I \).

Step 2: Detailed Explanation:
Thornthwaite's method is widely used because it only requires mean monthly temperature and day-length data (determined by latitude).
While other formulas like Penman-Monteith are more comprehensive (incorporating wind, humidity, and radiation), Thornthwaite's empirical formula remains a standard, practical tool for calculating Potential Evapotranspiration (PET) in climatology and hydrology, especially where meteorological data are limited.
It does not measure simple evaporation (E) or transpiration (T) alone, nor does it measure actual evapotranspiration (AET), which depends heavily on soil moisture availability.
Thus, it is specifically used to calculate PET.

Step 3: Final Answer:
Thornthwaite's formula is used to calculate Potential Evapotranspiration (PET), which corresponds to option (D).