# Engineering Hydrology Questions and Answers – Reservoir Evaporation and Methods for its Reduction

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Reservoir Evaporation and Methods for its Reduction”.

1. Which of the following is true regarding the methods of evaporation estimation?
a) Analytical methods give unreliable results and is difficult to obtain
b) Analytical methods give reliable results and is expensive to conduct
c) Empirical equations give reliable results and is economical
d) Empirical equations give reliable results and is expensive

Explanation: The analytical methods give more realistic results compared to empirical methods, but the parameters involved are difficult to determine and it is also more expensive. Empirical methods can at best give the results within the expected order of magnitude.

2. Which of the following most closely represents the amount of annual evaporation across India?
a) 2 mm
b) 20 mm
c) 20 cm
d) 2 m

Explanation: The amount of evaporation depends on the time of the year. The expected annual evaporation as per IMD is about 150 cm to 250 cm over a major part of the subcontinent. A large portion of the annual evaporation takes place during the summer months of April and May.

3. Which of the following is true regarding reservoir evaporation?
a) It is continuous and depends on the reservoir area
b) It is intermittent and depends on the reservoir area
c) It is continuous and does not depend on the reservoir area
d) It is intermittent and does not depend on the reservoir area

Explanation: Evaporation from a reservoir is a continuous process and takes place at the surface of the water. It also depends heavily on the surface area of natural or man-made reservoir in a directly proportional relationship.

4. The average surface area of an Indian reservoir during April to July is 378 km2. If the evaporation loss measured from a nearby pan (Cp=0.8) is 6.2 mm/day, estimate the volume of water lost by evaporation from the reservoir in the month of June? (Answer in x106 m3).
a) 5.6
b) 5.8
c) 56.25
d) 58.12

Explanation: Pan evaporation loss in June is given as,
Ep=6.2 mm/day*30 days=186 mm=0.186 m
Therefore, volume of water lost in evaporation in June,
VE=A*Ep*Cp=378*106*0.186*0.8=56246400 m3=56.25*106m3

5. During a month, 0.1 km3 of water is lost from a reservoir due to evaporation. The amount of water in the reservoir at the start of the month was observed to be 11,780 Mm3. A nearby pan (coefficient = 0.8) indicated an evaporation of 171 mm during the month. What is the average depth of water left at the end of the month?
a) 13.7 m
b) 14.4 m
c) 16.0 m
d) 16.6 m

Explanation: First let us find the surface area of the reservoir using pan data.
Depth of evaporation during the month = 0.8 * 171 = 136.8 mm = 0.137 m
⇒ Surface area of the reservoir=$$\frac{Volume \, of \, evaporation}{Depth \, of \, evaporation}=\frac{0.1 km^3}{0.137 m}$$
= $$\frac{0.1 km^3}{0.137*10^{-3}km}$$=730 km2
Now, volume of water left at the end of the month (11780*106)-(0.1*109)=11680 Mm3
∴ Depth of water left=$$\frac{Volume \, left}{Surface \, area} = \frac{11680*10^6}{730*10^6}$$=16 m

6. A water reservoir is proposed with the aim of reducing evaporation losses. The proposed minimum capacity is 2500 Mm3. Which of the following models is the most suitable?
a) Model A: surface area = 115 km2; depth = 22 m
b) Model B: surface area = 140 km2; depth = 18 m
c) Model C: surface area = 90 km2; depth = 28 m
d) Model D: surface area = 80 km2; depth = 30 m

Explanation: The model D does not satisfy the minimum capacity as it is 2400 Mm3. The models A, B and C satisfy the capacity requirement. Model C has the least surface area and the largest depth which implies it will undergo the least evaporation as the volume of water loss is directly proportional to area.

7. Which of the following compounds is used as an evaporation inhibitor?
c) Dodecanol

Explanation: Chemical compounds like hexadecanol and octadecanol are able to form thin monomolecular films on the surface of water. This acts as a boundary which prevents the water molecules to escape to the atmosphere.

8. Which of the following is not a disadvantage of using chemical films as evaporation reducers?
a) Loss due to oxidation
b) Penetration of sunlight
c) Sweeping action of wind
d) Damage caused by birds

Explanation: The chemical films need to be regularly monitored and replenished. This is because it may deteriorate due to oxidation, or by the action of strong wind taking it to the shore, or it may be removed by birds and insects.

9. Better reservoir planning and operation help in reduction of evaporation.
a) True
b) False

Explanation: The process of reservoir planning involves the decision on the location, altitude, area, depth and other features of the reservoir. Proper steps taken keeping in mind the aim of reducing evaporation will help a long way during its operation.

10. Which of the following is not generally use as mechanical cover for evaporation reduction?
a) Palm fronds
b) Buoys
c) Floating rafts
d) Aquatic plants

Explanation: Mechanical covers work on the principle that it blocks most of the incoming solar radiation, transmitting minimum heat energy to the water surface, resulting in reduced evaporation.

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