This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Empirical Evaporation Equations”.

1. Which one of the following is not an empirical equation to estimate the evaporation?

a) Fitzgerald’s equation

b) Meyer’s equation

c) Rohwer’s equation

d) Thornthwaite’s equation

View Answer

Explanation: The rate of evaporation can be estimated using empirical formulae like Fitzgerald’s, Meyer’s, Rohwer’s and Lake Mead’s equations. The Thornthwaite equation is a commonly used empirical formulae used to calculate the potential evapotranspiration.

2. Which of the following represents the general form of an empirical evaporation equation? E = lake evaporation rate, p_{s} = saturation vapour pressure, p_{a} = actual vapour pressure, K = coefficient, f(u) = function including wind speed term.

a) E=K.f(u).(p_{s}-p_{a})

b) E=f(u).(p_{s}-p_{a})

c) E=K.(p_{s}-p_{a})

d) E=f(u).p_{s}-p_{a}

View Answer

Explanation: Most of the empirical formulae that are available for the estimation of lake evaporation are based on Dalton’s equation. The variation is that of the coefficient and the wind speed function.

3. What is the unit of the vapour pressure terms used in the empirical equation for evaporation?

a) Pascal

b) Bar

c) Atm

d) Torr

View Answer

Explanation: Most of the empirical equations developed to find the evaporation rate as based on Dalton’s law. The terms in the equations need to be substituted in the units in which they were derived. The unit for vapour pressure and also atmospheric pressure (Rohwer’s formula) are in mm of mercury or torr.

4. What is the correct wind speed correction function in Meyer’s equation? u_{0}, u_{7} and u_{9} represent the wind speed at the ground level, 7 m above ground level and 9 m above ground level, respectively.

a) \(\left(\frac{1+u_9}{16}\right)\)

b) \(\left(1+\frac{u_9}{16}\right)\)

c) \(\left(1+\frac{u_0}{16}\right)\)

d) \(\left(1+\frac{u_7}{16}\right)\)

View Answer

Explanation: The wind speed correction function in Meyer’s formula uses the monthly mean wind speed at above 9 m above the ground level. Therefore, for Meyer’s equation, f(u)=\(\left(1+\frac{u_9}{16}\right)\).

5. Which of the following are the correct coefficients in Meyer’s equation?

a) 0.36 for shallow water, 0.5 for deep water

b) 0.5 for shallow water, 0.36 for deep water

c) 0.5 for shallow water, 0.72 for deep water

d) 0.72 for shallow water, 0.5 for deep water

View Answer

Explanation: The coefficient in Meyer’s formula accounts for many different parameters and the final values are as 0.36 for large, deep water bodies and 0.5 for small, shallow water bodies.

6. Rohwer’s formula considers an additional correction function for what parameter?

a) Water temperature

b) Radiation storage

c) Air pressure

d) Dissolved salts

View Answer

Explanation: Rohwer’s empirical equation considers a correction function for the effect of atmospheric pressure in addition to the wind speed function. This pressure function is given as (1.465 – 0.000732p

_{a}), where p

_{a}is the mean barometric reading in mm of Hg.

7. The wind speed is usually assumed to follow which power law?

a) 1/3^{rd}

b) 1/5^{th}

c) 1/7^{th}

d) 1/9^{th}

View Answer

Explanation: The empirical equations for evaporation require the wind speed values at a particular height, but this data might not be always available. Thus, a method is required to determine the wind speed at a required height from a known data at other heights. So in the lower part of the atmosphere, the wind velocity is assumed to follow the 1/7

^{th}power law, that is, u=C.\(h^{\frac{1}{7}}\).

8. Rohwer’s equation uses the mean velocity of wind at a height of about 0.6 m above the ground level.

a) True

b) False

View Answer

Explanation: Unlike Meyer’s equation, which uses wind speed values at 9 m above ground level, Rohwer’s formula uses the mean wind velocity in kmph at the ground level, which is measured at a height of approximately 0.6 m from the ground surface.

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