Engineering Hydrology Questions and Answers – Analytical Methods of Evaporation Estimation

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Analytical Methods of Evaporation Estimation”.

1. Which of the following is not an analytical method of estimating evaporation?
a) Blaney-Criddle method
b) Energy balance method
c) Mass transfer method
d) Water budget method
View Answer

Answer: a
Explanation: The analytical methods for estimating lake evaporation can be broadly classifies into three categories, namely energy balance method, mass transfer method and water budget method. The Blaney-Criddle method is an empirical equation to estimate the potential evapotranspiration of a crop.

2. Which analytical method for evaporation estimation is the least reliable?
a) Energy balance method
b) Mass transfer method
c) Water budget method
d) Radiation method
View Answer

Answer: c
Explanation: Out of the three major analytical methods, the water budget equation method is the simplest. There are uncertainties and possibility of errors in some of the terms of the water budget equation, affecting the accuracy and making it less reliable.

3. For the case of decrease in storage, which of the following equations is the correct representation of continuity? I = total inflow, O = total outflow, S = increase in storage.
a) I – O = S
b) O – S = I
c) I + S = O
d) I ± S = O
View Answer

Answer: c
Explanation: For decrease in storage, the total inflow must be lesser than the total outflow. Since, the sign convention for change in storage is given as plus for increase and minus for decrease, the correct representation will be (I – O = -S) or (I + S = O).
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4. Which of the following pairs is not considered as outflow in water budget equation?
a) Precipitation and seepage
b) Seepage and runoff
c) Evaporation and seepage
d) Precipitation and runoff
View Answer

Answer: d
Explanation: In the water budget equation, the quantities considered as inflow are precipitation, surface inflow and groundwater inflow, and the outflow terms are evaporation, transpiration, seepage and water used.

5. Which of the following units cannot be used for the terms of a water budget equation?
a) cm
b) ha-m
c) km2
d) Million m3
View Answer

Answer: c
Explanation: The terms of a water budget equation represent the quantities of water and is to be represented in units of volume (litres, Mm3, ha-m) or if divided by the area under consideration, the units may be of depth (mm, cm, m).

6. If the total inflow into a reservoir is 35 units and total outflow (excluding evaporation) is 33 units, find the evaporation loss from the reservoir, given that the water stored decreased by 1 unit?
a) 0 units
b) 1 unit
c) 2 units
d) 3 units
View Answer

Answer: d
Explanation: Since the total water stored decreased by 1 unit, it implies that the total outflow is more than the total inflow by 1 unit.
⇒ Outflow-Inflow = Decrease in storage ⇒ (33+evaporation)-35=1
∴ Evaporation loss=1+35-33=3 units

7. If the total inflow into a reservoir is 17 units and total outflow (excluding evaporation) is 14 units, find the evaporation loss from the reservoir, given that the water stored increased by 2 units?
a) 0 units
b) 1 unit
c) 2 units
d) 3 units
View Answer

Answer: b
Explanation: Since the total water stored increased by 2 units, it implies that the total inflow is more than the total outflow by 2 units.
⇒Inflow-Outflow=Increase in storage ⇒ 17-(14+evaporation)=2
∴ Evaporation loss=17-14-2=1 unit
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8. Which of the following equations correctly represents the evaporation loss (E) from a reservoir as per the water budget equation? R = rainfall, I = surface inflow, O = surface outflow, G1 = groundwater inflow, G2 = groundwater outflow, S = decrease in reservoir storage.
a) E = R + (I – O) + (G1 – G2) – S
b) E = R + (I – O) + (G1 – G2) + S
c) E = R + (O – I) + (G2 – G1) – S
d) E = R + (O – I) + (G2 – G1) + S
View Answer

Answer: b
Explanation: Since ‘S’ is given as decrease in storage,
Total outflow-Total inflow=S
⇒(O+G2+E)-(R+I+G1)=S
⇒O+G2+E-R-I-G1=S
⇒E=S+R+I+G1-O-G2
∴ E=R+(I-O)+(G1-G2)+S

9. The average inflow and outflow from runoff of a lake (surface area = 35 km2) in a given week is 3.6 m3/s and 4 m3/s. The total rainfall during the week was recorded to be 40 mm and the water level was found to rise by 26 cm at the end of the week. What is the evaporation loss from the reservoir during the week in question?
a) 9.7 mm
b) 0.25 Mm3
c) 198 million litres
d) 17.5 ha-m
View Answer

Answer: b
Explanation: Since there is a rise in water level,
Total inflow-Total outflow=increase in storage
⇒$$\left(\frac{3.6*60*60*24*7}{35*10^6}+0.04 \right)$$m-$$\left(\frac{4*60*60*24*7}{35*10^6}+evaporation \right)$$m=0.026 m
⇒(0.06221 + 0.04) – (0.06912 + evaporation)=0.026
∴ Evaporation=0.06221+0.04-0.06912-0.026 = 0.00709 m=7.09 mm
= 0.00709*35*106 = 248150 m3 = 0.248 Mm3=248.15 Mlitres ≅ 0.25 Mm3

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10. In the water budget method if the time of study is kept short, better accuracy is obtained in the estimation of evaporation.
a) True
b) False
View Answer

Answer: b
Explanation: For larger units of time, like weeks or months, the water budget method gives more reliable values of lake evaporation. This is because the various parameters considered are averaged out to a more consistent value over longer periods.

Sanfoundry Global Education & Learning Series – Engineering Hydrology.

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