# Engineering Hydrology Questions and Answers – Evapotranspiration Equations – Set 2

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Evapotranspiration Equations – Set 2”.

1. Which of the following is not a characteristic of the reference surface given by FAO to evaluate the reference crop evapotranspiration?
a) Crop height = 12 cm
b) Surface resistance = 70 sm-1
c) Reflection coefficient = 0.23
d) Sunshine time = 10 hours

Explanation: PET is regularly used in irrigation calculations and for its standardization, FAO recommends a reference crop evapotranspiration (ETo). The standard crop is assumed as grass with a height of 12 cm, fixed surface resistance of 70 s/m, and a reflection coefficient of 0.23.

2. Which of the following is not a property of the reference crop surface for standard ET as per FAO?
a) Well-watered green grass
b) Actively growing crop
c) Wet soil surface
d) Grass of uniform height

Explanation: The reference crop surface should be similar to a considerable area of well-watered, uniform, green grass with active growth and should completely shade the area. The soil surface should be moderately dry with a weekly irrigation frequency.

3. The Penman-Monteith method requires which of the following sets of data to estimate reference evapotranspiration?
a) Air temperature, humidity, wind speed
b) Air temperature, humidity, soil moisture
c) Air temperature, humidity, soil moisture, radiation
d) Air temperature, humidity, radiation, wind speed

Explanation: The Penman-Monteith method is recommended by FAO to estimate the reference crop evapotranspiration. This is a modified version of the Penman equation and requires data regarding the air temperature, air humidity, radiation and wind speed.

4. The crop coefficient for a given crop depends on which of the following?
a) Number of crops in the area
b) Stage of growth
c) Latitude
d) Reason for crop growth

Explanation: The reference crop evapotranspiration can be used to calculate the expected potential evapotranspiration of any crop with the help of suitable coefficients which vary from crop to crop. The value of this coefficient changes with the stage of the crop.

5. Which of the following correctly represents the crop coefficient? Eo = evapotranspiration of reference crop, E = evapotranspiration of given crop.
a) E.Eo
b) $$\frac{E_o}{E}$$
c) $$\frac{E}{E_o}$$
d) $$\frac{1}{E.E_o}$$

Explanation: The crop coefficient is a property of the crop which is used to predict the potential evapotranspiration. It is defined as the ratio of ET of crop under study to the ET of reference crop under similar conditions.

6. The evapotranspiration from reference crop was found to be 95% of the evapotranspiration of a given crop A for the same conditions. What is the crop coefficient of crop A?
a) 0.95
b) 1.05
c) 1.11
d) 1.95

Explanation: Given ETo = 95% of ETA = 0.95*ETA
∴ Crop coefficient, KA = $$\frac{ET_A}{ET_O} = \frac{ET_A}{0.95*ET_A} = \frac{1}{0.95}$$=1.05263≅1.05.

7. The reference crop evapotranspiration and evapotranspiration for crop ‘K’ over a given period was found to be 3.5 cm and 4.1 cm, respectively. Which of the following is potentially crop ‘K’?
a) Rice
b) Wheat
c) Maize
d) Cotton

Explanation: Crop coefficient = 4.1/3.5 = 1.1714
The crop coefficient values for wheat, maize and cotton lie in the range of approximately 0.5-0.8, with an average value of 0.65. On the other hand, the average coefficient for rice is 1.1 with values ranging from 0.85-1.30.

8. The Blaney-Criddle method is most suitable to estimate the potential evapotranspiration in a region of eastern USA.
a) True
b) False

Explanation: Blaney-Criddle equation is an empirical equation derived based on climatological data of a particular region, namely arid western USA. It may not be applicable for other areas and should be used with caution to predict PET from climatologically differing regions.

9. What is the unit of the temperature term in Blaney-Criddle formula?
a) Celsius
b) Fahrenheit
c) Kelvin
d) Rankine

Explanation: Blaney-Criddle is an empirical equation with all terms to be substituted in the units they were derived in. Since it was developed from the US, the temperature term is also to be substituted in Fahrenheit units.

10. For a particular region, the pan evaporation during May was recorded as 11.2 cm. For maize crop grown in this region, the crop coefficient is 0.55 corresponding to 25% growth. Estimate the daily consumptive use of maize at 25% growth for the month of May.
a) 1.99 mm
b) 2.05 mm
c) 6.16 mm
d) 19.87 mm

Explanation: Evaporation in May, E = 11.2 cm and K = 0.55.
Consumptive use is given as, Cu = K*E = 0.55 * 11.2 = 6.16 cm/month
= $$\frac{6.16}{31}$$ = 0.1987 cm/day
= 1.987 mm/day≅1.99 mm/day

Sanfoundry Global Education & Learning Series – Engineering Hydrology.

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