Question Bank • QB_EXP_IDE_ETP_010 NAT • 1 Mark

Section 5: Irrigation and Drainage Engineering

Evapotranspiration, Consumptive Use & Crop Water Needs

Subtopic: Crop evapotranspiration from pan evaporation

Question Statement:

A Class-A evaporation pan installed in an irrigated field measures an average daily pan evaporation rate of $E_{pan} = 6.0\text{ mm/day}$. Taking the pan coefficient $K_p = 0.75$ and crop coefficient during peak flowering stage $K_c = 1.15$, the crop evapotranspiration $ET_c$ is ________ $\text{mm/day}$ (round off to two decimal places).

Answer Key:

5.17

Step-by-Step Solution & Explanation

Reference evapotranspiration $ET_0 = K_p \times E_{pan} = 0.75 \times 6.0 = 4.50\text{ mm/day}$. Crop evapotranspiration is: $ET_c = K_c \times ET_0 = K_c \times K_p \times E_{pan} = 1.15 \times 0.75 \times 6.0 = 5.17\text{ mm/day}$
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