Question

Which of the following statements are correct with respect to Radiation method for estimation of reference crop evapotranspiration.
(A) Radiation method is essentially an adaptation of Hawksley formula.
(B) Radiation method is suggested for areas where available climate data include measured wind and humidity.
(C) In equatorial zone the radiation method may be more reliable than Blanney-Criddle approach.
(D) The amount of radiation received at the top of the atmosphere (Ra) is dependent on latitutde and the time of year only.
Choose the correct answer from the options given below :

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

Hint:

Extraterrestrial radiation (\(R_a\)) does not depend on local atmospheric or weather conditions (like clouds or dust) because it is measured outside the earth's atmosphere.
It is calculated purely using trigonometric functions of latitude and day of the year.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Crop evapotranspiration (\(\text{ET}_o\)) is estimated using various empirical and micrometeorological methods.
The FAO-24 Radiation method is recommended when reliable solar radiation data is available, but wind and relative humidity data are lacking or incomplete.

Step 2: Detailed Explanation:
Let us analyze each statement individually:
- Statement (A): The FAO-24 Radiation method is essentially an adaptation of the Makkink formula (originally developed in the Netherlands), not the Hawksley formula.
Thus, this statement is incorrect.
- Statement (B): The radiation method is suggested for areas where measured wind and relative humidity data are not available or are of poor quality, but solar radiation or sunshine hours are reliably measured.
If wind and humidity data are available, the Penman-Monteith method is preferred.
Thus, this statement is incorrect.
- Statement (C): In the equatorial zone, temperature variations throughout the year are extremely small, making temperature-based approaches like the Blaney-Criddle method unreliable.
Since solar radiation is the dominant driving force of energy in these zones, the radiation method is much more reliable, making this statement correct.
- Statement (D): Extraterrestrial radiation (\(R_a\)), which is the solar radiation received at the top of the atmosphere, is a purely astronomical quantity.
It depends solely on the latitude of the location and the time of the year (Julian day), making this statement correct.
Therefore, statements (C) and (D) are correct.

Step 3: Final Answer:
The correct option is (C), representing (C) and (D) only.