GATE 2025 CE (CE1) – Question 61
A symmetrical trapezoidal canal is 100 km long. The bottom width is 10 m and the side slope is 1 Horizontal : 1 Vertical. The average flow depth in the canal is 2.5 m throughout the month of April. The measurement from a Class-A evaporimeter in the vicinity of the canal indicated an average evaporation rate of 0.5 cm/day in April.
The volume of water evaporated from the canal (in m$^3$) in the month of April is close to __________________ $\times 10^3$ (*rounded off to 1 decimal place*).
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Correct answer: 156.71 to 158.29
Explanation
The water surface width is $10 + 2 \times 1 \times 2.5 = 15$ m, so the water surface area is $15 \times 100\,000 = 1.5 \times 10^6$ m$^2$. A Class A pan overestimates the evaporation from a large water surface, so it is multiplied by the pan coefficient of 0.7. The evaporation depth in April is $0.7 \times 0.005 \times 30 = 0.105$ m. The volume is $1.5 \times 10^6 \times 0.105 = 157\,500$ m$^3$, which is $157.5 \times 10^3$.