This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Evapotranspiration Equations – Set 3”.

1. In what terms is the amount of sunshine received by the crop substituted as in the Blaney-Criddle equation?

a) Hourly percentage of daily sunshine hours

b) Daily percentage of weekly sunshine hours

c) Weekly percentage of monthly sunshine hours

d) Monthly percentage of annual sunshine hours

View Answer

Explanation: The PET as per Blaney-Criddle formula is estimated in inch for a given month. Hence, the sunshine received is substituted in terms of per month percentage of the total sunshine for that year. This percentage depends on the latitude of the region.

2. Which of the following derivatives of Blaney-Criddle formula gives the value of potential evapotranspiration in m/year? K = crop coefficient, P = monthly percentage of sunshine hours in a year, T = mean monthly temperature in °F.

a) \(\frac{KPT}{100} \)

b) \(\frac{KPT}{328} \)

c) \(\frac{KPT}{1181} \)

d) \(\frac{KPT}{3937} \)

View Answer

Explanation: The standard Blaney-Criddle equation which gives the PET in inch/month is,

PET=\(\frac{KPT}{100} \)

Now to convert inch to m, multiply by 0.0254 and to convert per month to per year, multiply by 12.

⇒PET=\(\frac{KPT}{100} \)*0.0254*12=0.3048*\(\frac{KPT}{100} = \frac{KPT}{\frac{100}{0.3048}}≅ \frac{KPT}{328}\) m/year

3. Blaney-Morin equation is a modification of the Blaney-Criddle equation with an additional correction for which of the following?

a) Wind speed

b) Humidity

c) Wind speed and humidity

d) Effective rainfall

View Answer

Explanation: Blaney-Morin equation is an empirical equation to determine PET. It is a modified version of the Blaney-Criddle equation with an additional factor considering the effect of relative humidity on the PET.

4. The percent of annual sunshine received by a place during December is 7.66% and mean temperature is 21.4°C. If the crop coefficient during December is 0.85, find the minimum daily consumptive use of the crop during the month.

a) 1.14 mm

b) 1.39 inch

c) 3.76 mm

d) 11.66 cm

View Answer

Explanation: K = 0.85, P = 7.66, T = 21.4°C = 21.4*1.8 + 32 = 70.52°F

From Blaney-Criddle formula,

Consumptive use=\(\frac{KPT}{100} = \frac{0.85*7.66*70.52}{100}\) = 4.59 inch/month = \(\frac{4.59}{31}\) = 0.148 inch/day

= 116.63 mm/month = \(\frac{116.63}{31}\) = 3.76 mm/day

= 11.66 cm/month = \(\frac{11.66}{31}\) = 0.376 cm/day

Therefore, the daily minimum consumptive use (or PET) is 3.76 mm.

5. Find the total PET (in cm) during March for a crop at 50% growth (K = 0.65) exposed to a mean temperature of 29.9°C. Assume the annual sunshine time to be distributed equally for every day of the year.

a) 4.19

b) 4.74

c) 11.80

d) 12.03

View Answer

Explanation: Percentage of annual sunshine for one day = \(\frac{100}{365}\) = 0.274%

So percentage of sunshine for March, P=0.274*31=8.49%

Temperature,T=29.9℃=29.9*1.8+32=85.82℉

∴ PET = \(\frac{KPT}{100} = \frac{0.65*8.49*85.82}{100}\)*2.54=12.03 cm/month

6. Find the daily PET (in mm) during September for a crop at 80% growth (K = 0.77) exposed to a mean temperature of 23°C. Assume the annual sunshine time to be distributed equally for each month.

a) 1.25

b) 3.75

c) 4

d) 5.5

View Answer

Explanation: Percentage of annual sunshine for September, P = \(\frac{100}{12}\) = 8.33%

Temperature,T=23℃=23*1.8+32=73.4℉

∴ PET = \(\frac{KPT}{100} = \frac{0.77*8.33*73.4}{100}*\frac{25.4}{30}\) = 3.986 mm/day≅4 mm/day

7. The only climatological data required for using Thornthwaite equation is the mean monthly air temperature.

a) True

b) False

View Answer

Explanation: The Thornthwaite equation was developed for the regions of eastern USA and requires only the mean air temperature of the month for which PET is required. Other than this, it involves an adjustment factor and heat index.

8. Calculate the heat index term to be used in the Thornthwaite equation for estimating the PET during March, in a year where all the months are assumed to have the same mean temperature of 28°C.

a) 13.6

b) 163

c) 835

d) 5936

View Answer

Explanation: The heat index term (It) is defined as the sum of the heat index of all the months of the given year.

I

_{t}= \(\displaystyle\sum\limits_{January}^{December} i = \displaystyle\sum\limits_{n=1}^{n=12} \left(\frac{T_n}{5}\right)^{1.514}\) = 12*\(\left(\frac{28}{5}\right)^{1.514}\) = 12*5.6

^{1.514}= 162.9063≅163

9. Thornthwaite equation considers an adjustment factor for which of the following?

a) Wind speed

b) Latitude

c) Day lengths

d) Humidity

View Answer

Explanation: The Thornthwaite equation has a different adjustment factor (L

_{a}) for each month depending upon the number of daylight hours and also the latitude of the place. The value of L

_{a}lies in the range of 1.01-1.27 from March to September and 0.81-1.04 from October to February.

10. What is the unit of PET as obtained from Thornthwaite equation?

a) inch/month

b) mm/day

c) m/sec

d) cm/month

View Answer

Explanation: The Thornthwaite equation is given as,

PET = 1.6*L

_{a}*\(\left(\frac{10*T}{I_t}\right)^a \)

Where, the PET is given for one month in cm for temperature substituted in °C.

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