Engineering Hydrology Questions and Answers – Measurement of Evapotranspiration

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Measurement of Evapotranspiration”.

1. Which of the following methods cannot be used to estimate evapotranspiration?
a) Lysimeters
b) Field plots
c) Horton’s equation
d) Drum culture technique
View Answer

Answer: c
Explanation: The evapotranspiration from a given vegetation can be determined by using lysimeters, field plots or drum culture technique. Each method has its advantages and drawbacks. ET can also be determined by using pan evaporimeters with the help of suitable crop coefficients. Horton’s equation is associated with infiltration.

2. Which of the following is not a type of lysimeter?
a) Siphon lysimeter
b) Floating lysimeter
c) Volumetric lysimeter
d) Gravimetric lysimeter
View Answer

Answer: a
Explanation: Lysimeters are used to measure actual evapotranspiration and are of different types based on the application. Depending on whether the measurement is based on volume or weight it may be a volumetric or gravimetric lysimeter, respectively. Floating lysimeter is used in the case of aquatic plants.

3. Which part of a weighing lysimeter helps to prevent its overheating?
a) Lysimeter tank
b) Retaining tank
c) Dummy tank
d) Perforated plate
View Answer

Answer: c
Explanation: A dummy tank is a smaller tank compared to the lysimeter and retaining tank. It is placed in the gap next to the headwork of the weighing balance and serves the main purpose of preventing the over heating of the main tank.

4. What is the function of the perforated plate in a gravimetric lysimeter?
a) Serves as a platform for weighing machine
b) To allow free drainage of excess water
c) To remove the percolated water
d) To isolate the lysimeter tank from main field
View Answer

Answer: b
Explanation: The lysimeter tank consists of a perforated metallic plate at its bottom. This serves to form a hollow chamber below it, where the drained excess water collects. A tube is inserted into this chamber to remove the percolated water.

5. The retaining tank protects the lysimeter from seepage from the field.
a) True
b) False
View Answer

Answer: a
Explanation: The retaining tank has dimensions larger than the main tank as it houses the whole apparatus to isolate it from the main soil. It also acts as a wall support for the pit in which the apparatus is installed.

6. The main tank weight (in kg) of a gravimetric lysimeter measured at 24-hour intervals over a week is as follows – 1950, 1945.3, 1938.1, 1934.2, 1930.7, 1926.3 and 1920. If there was no rainfall recorded during the week, what is the average per day evapotranspiration during the week?
a) 2.6 mm
b) 3.0 mm
c) 4.3 mm
d) 5.0 mm
View Answer

Answer: b
Explanation: During the course of the week, the total decreases in main tank weight is
=1950-1920=30 kg
For a gravimetric lysimeter, ET=Decrease in weight(kg)*0.6+rainfall
=30*0.6+0=18 mm over 6 days
Therefore, per day ET = 18/6 = 3 mm/day.

7. The weight of lysimeter tank increased by 5820 g from the previous day weight after a total rainfall of 7.5 mm during the 24-hour time period between the two readings. What is the evapotranspiration amount (in mm) in that time?
a) 1 mm
b) 4 mm
c) 10 mm
d) 11 mm
View Answer

Answer: b
Explanation: ET=Decrease in weight (in kg)*0.6+rainfall
⇒ET=(-5.82*0.6)+7.5=4.008 mm≅4mm

8. For a volumetric lysimeter, the conversion factor for volume of water to depth of water is given as 1 cc = ‘k’ x 10-5 mm. What is the value of ‘k’?
a) 0.694
b) 6.94
c) 69.4
d) 694
View Answer

Answer: c
Explanation: The ET in a volumetric lysimeter is measured in terms of volume of water lost or gained. The volume measured in cc is then converted to depth using the conversion,
1 cc=0.000694 mm

9. The volumetric lysimeter is most suitable for which type of crop?
a) Aquatic plants
b) Standing water crops
c) Rabi crops
d) Zaid crops
View Answer

Answer: b
Explanation: Unlike a gravimetric lysimeter, a volumetric one is used to measure evapotranspiration in terms volume. This requires the availability of standing water in the main tank and hence is suitable for standing water crops, like rice.

10. During a given study period, the inflow, outflow and rainfall recorded for a volumetric lysimeter is 4.1 litres, 2.3 litres and 8.5 mm, respectively. Estimate the actual evapotranspiration during this study period.
a) 9.75 mm
b) 9.06 mm
c) 9.13 mm
d) 9.67 mm
View Answer

Answer: a
Explanation: The evapotranspiration from a volumetric lysimeter is given as,
ET=(Inflow-Outflow)*conversion factor+Rainfall
⇒ET=[(4100 cc-2300 cc)*0.000694] mm+8.5 mm
⇒ET=1800*0.000694+8.5=1.2492+8.5=9.7492 mm ≅9.75 mm

Sanfoundry Global Education & Learning Series – Engineering Hydrology.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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