Question

According to Doorenbos and Pruitt, the Blaney-Criddle formula is not suitable for use in: (A) Equatorial regions where temperatures remain fairly constant, but other weather parameters change, (B) Large islands where air temperature is affected by the surrounding sea temperature showing little response to seasonal change in radiation, (C) High altitude where daytime radiation is practically independent of night temperature, (D) Climates with a high variability in sunshine hours during transition months, (E) Blaney-Criddle method should normally be applied for periods not greater than one month period. Choose the correct answer from the options given below:

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

Hint:

Temperature-based formulas like Blaney-Criddle assume temperature is a reliable proxy for solar radiation.
In equatorial, maritime, or high-altitude regions where this correlation is weak, more comprehensive methods like Penman-Monteith must be used.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Blaney-Criddle method is a temperature-based empirical formula used to estimate reference crop evapotranspiration (\(\text{ET}_o\)).
Because it relies primarily on air temperature and daylength factors, its accuracy can be limited in climates where temperature does not correlate well with solar radiation or advective energy.

Step 2: Detailed Explanation:
According to FAO-24 guidelines by Doorenbos and Pruitt, the Blaney-Criddle formula has limitations under specific climatic conditions:
- (A) Equatorial regions: Near the equator, temperatures remain relatively constant year-round.
However, evapotranspiration can vary significantly due to changes in wind speed, relative humidity, and cloud cover, which are not captured by a temperature-only formula.
- (B) Large islands: The surrounding ocean moderates island air temperatures, resulting in minimal seasonal temperature fluctuations.
Yet, solar radiation levels change seasonally, causing actual evapotranspiration to deviate from temperature-based estimates.
- (C) High altitudes: At high elevations, high daytime solar radiation is often followed by cold nights, creating a large diurnal temperature range.
This decouples the average daily temperature from the actual solar energy available for evapotranspiration during the day.
- Thus, conditions (A), (B), and (C) represent the primary regions where the Blaney-Criddle method is not suitable.

Step 3: Final Answer:
The Blaney-Criddle formula is not suitable for use in (A), (B) and (C) only.