Question

For upland rice, the crop coefficient generally given for initial stages of rice crop need to be reduced by :

• A. 5-10%
• B. 10-15%
• C. 15-20%
• D. 20-25%

Hint:

- Flooded Rice initial \(K_c \approx 1.10\) (high due to open water evaporation).
- Upland Rice initial \(K_c \approx 0.90\) (reduced by 15-20% due to dry soil surface).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The crop coefficient (\(K_c\)) is the ratio of crop evapotranspiration (\(\text{ET}_c\)) to reference crop evapotranspiration (\(\text{ET}_o\)).
For rice, the coefficient depends on the cultivation system, as flooded (lowland) rice has very different soil wetting characteristics compared to non-flooded (upland) rice.

Step 2: Detailed Explanation:
In standard FAO-56 guidelines, the crop coefficient values for rice are primarily developed for traditional flooded (lowland) paddy systems.
In a flooded system, the soil surface is continuously wet, resulting in a very high rate of evaporation from the standing water, which leads to a high initial crop coefficient (\(K_{c,\text{ ini}}\) around 1.05 to 1.15).
In contrast, upland rice is grown in non-flooded, well-aerated soils, similar to other cereal crops.
Because the soil surface is not continuously flooded, the soil surface evaporation is significantly lower during the initial stages of crop growth when the canopy cover is minimal.
According to FAO hydrology guidelines, to account for this reduced evaporation, the initial crop coefficient (\(K_c\)) value specified for flooded rice must be reduced by approximately 15% to 20% for upland rice systems.

Step 3: Final Answer:
The crop coefficient for the initial stages of upland rice needs to be reduced by 15-20% (Option C).