# Irrigation Engineering Questions and Answers – Water Requirements of Crops – Estimation of Consumptive Use – 2

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This set of Irrigation Engineering Interview Questions and Answers focuses on “Water Requirements of Crops – Estimation of Consumptive Use – 2”.

1. Which method involves the use of crop factor?
a) Penman’s equation
b) Hargreaves method
c) Blaney-Criddle Formula
d) Tanks and Lysimeter

Explanation: Blaney-Criddle Formula is expressed as Cu = k.f
Where, f = p/40 [1.8t + 32], Cu = seasonal consumptive use and k = crop factor. Crop factor is determined by experiments under environmental conditions of the particular area for each crop.

2. What are the dimensions of the standard class-A pan?
a) Diameter-1.2 m, Depth – 25 cm
b) Diameter-2 m, Depth – 25 cm
c) Diameter-2 m, Depth – 30 cm
d) Diameter-1.2 m, Depth – 30 cm

Explanation: Class-A pan is used for experimental determination of pan evaporation. This pan has a diameter of 1.2 m, 25 cm deep, and the bottom is raised 15 cm above the ground surface.

3. Penman’s equation for the estimation of PET has been derived by using ___________
a) energy balance approach
b) mass transfer approach
c) combination of energy balance approach and mass transfer approach
d) combination of energy balance approach and energy transfer approach

Explanation: The formula was developed by Penman in 1948 for determining the consumptive use of different areas of the basin. This formula has sound theoretical reasoning and is derived by combining energy balance and mass transfer approaches for the computation of transpiration and evaporation respectively.

4. Albedo or reflection coefficient factor is used in which method?
a) Blaney- Criddle equation
b) Christiansen equation
c) Penman equation
d) Tank and Lysimeter method

Explanation: Albedo is a factor used in Penman’s equation which is used for a different type of areas for calculation of consumptive use. The value of albedo varies for water surface, bare lands and snow.

5. Penman’s equation can also be used to compute evaporation from a water surface like a lake if _________
a) reflection coefficient = 0.05
b) reflection coefficient > 0.05
c) reflection coefficient < 0.05
d) reflection coefficient = 0.45- 0.90

Explanation: The value of reflection factor or albedo varies for water surface, bare lands and snow. When the value is equal to 0.05, it can be used to compute evaporation from a water surface.

6. The pan evaporation can also be determined by Christiansen formula.
a) True
b) False

Explanation: Christiansen formula for pan evaporation is Ep = 0.459 R.Ct.Cw.Ch.Cs.Ce where, R = Extra-terrestrial radiation in cm
Ct = Coefficient of temperature
Cw = Coefficient for wind velocity
Ch = Coefficient for relative humidity
Cs = Coefficient for percent of possible sunshine
Ce = Coefficient of elevation.

7. What is the consumptive use for a crop in the month of April having a consumptive use coefficient equal to 0.80 and pan evaporation is 35 cm?
a) 28 cm
b) 43.75 cm
c) 35.80 cm
d) 40.80 cm

Explanation: The formula is Et = k. Ep
Et = 0.8 x 35 = 28 cm
Hence, the required value of consumptive use = 28 cm.

8. Compute the total consumptive use for a drainage basin by Penman’s formula. The slope of saturation vapour pressure V/s chart at 40°C is 2.95 mm of Hg/°C and the net incoming solar radiation is 5.705 mm of evaporative water/day. Assume, Ea = 18.07 mm/day and Psychrometric constant = 0.49 mm of Hg/°C.
a) 17.01 mm/day
b) 18 mm/day
c) 11.07 mm/day
d) 20.45 mm/day

Explanation: Penman’s formula for Cu is equal to [A.Hn + Ea.Y] / (A + Y)
Cu = [2.95 x 5.705 + 18.07 x 0.49] / (2.95 + 0.49)
Cu = 17.01 mm/day.

9. What is the correct expression of parameter Ea in Penman’s equation?
a) Ea = 0.35 (1 + V2/160) (es – ea)
b) Ea = 0.35 (1 + V2/160) (es + ea)
c) Ea = 0.35 (1 – V2/160) (es – ea)
d) Ea = 0.35 (1 – V2/160) (es + ea)

Explanation: The parameter Ea is estimated by as: Ea = 0.35 (1 + V2/160) (es – ea).
Where, V2 is the wind mean speed at 2m above the ground in km/day, es is Saturation vapour pressure at mean air temperature in mm of Hg, and ea is actual mean vapour pressure of air in mm of Hg.

Sanfoundry Global Education & Learning Series – Irrigation Engineering. 