# Irrigation Engineering Questions and Answers – Water Requirements of Crops – Effective Rainfall

This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Water Requirements of Crops – Effective Rainfall”.

1. Which of the following statement is not correct about Effective rainfall?
a) It doesn’t take into consideration precipitation lost through deep percolation
b) It satisfies evapotranspiration needs of the crop
c) It includes surface runoff loss
d) It doesn’t include surface runoff water loss

Explanation: Effective rainfall is that portion of precipitation that remains in the soil and is available for consumptive use. It is available to meet the evapotranspiration needs of the crop and does not include percolation losses and surface runoff losses.

2. What is the field irrigation requirement (FIR)?
a) The water required to meet the evaporation needs of a crop
b) Amount of water required to delivered at the field to meet evapotranspiration and leaching needs
c) Amount of water required to meet the net irrigation requirements plus water lost due to surface runoff and percolation
d) Amount of water required to meet the field irrigation requirements plus water lost in conveyance through the canal system

Explanation: FIR can be defined as the water required meeting the net irrigation requirements plus the amount of water lost as surface runoff and through deep percolation.
Mathematically, FIR = NIR / Na where Na = application efficiency and NIR = Net irrigation requirement.

3. What is the correct representation for the calculation of NIR?
a) Cu – Re
b) CIR + leaching losses
c) FIR + conveyance losses
d) FIR + leaching losses + conveyance losses

Explanation: Net irrigation requirement (NIR) can be defined as the amount of irrigation water required to be delivered at the field to meet the evapotranspiration and leaching losses.
NIR = (Cu – Re) + amount of water required for leaching = CIR + leaching losses requirement.

4. Determine the Gross irrigation requirement for an irrigation system if consumptive use of crop is 200 cm in terms of depth and the rainfall excess is 30 cm. assume water application efficiency = 50% and conveyance efficiency = 90%.
a) 370 cm
b) 380.79 cm
c) 355.56 cm
d) 377.77 cm

Explanation: Given, Cu = 200 cm, Re = 30 cm, Na = 50%, Nc = 90%
NIR = CIR + losses = (200-300) + 0 = 170 cm
FIR = NIR / Na = 170/0.5 = 340 cm
GIR = FIR / Nc = 340/0.9 = 377.77 cm.

5. An outlet irrigates an area of 40 ha. The discharge required at this outlet to meet the evapotranspiration requirement of 40 mm occurring uniformly in 40 days neglecting other field losses is _________
a) 4.52 litres / sec
b) 2.31 litres / sec
c) 4.01 litres / sec
d) 1.52 litres / sec

Explanation: Volume of water required for evapo-transpiration = 40 x 104 x 40 x 10-3 x 103 = 16 x 106 litres
Hence, the discharge required at outlet
= (16 x 106) / (40 x 86400)
= 4.52 litres / sec.
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6. The moisture holding capacity of the soil in a 100-hectare farm is 20 cm/m. The root zone depth of the crop is 1.0 m and peak rate to moisture use is 5 mm/day. The field is to be irrigated when 40% of the available moisture in the root zone is depleted. Calculate the frequency of irrigation.
a) 16 days
b) 18 days
c) 20 days
d) 25 days

Explanation: The moisture holding capacity is 20 cm/m and the root zone depth = 1 m
Moisture holding capacity of soil for crop = 20 x 1 = 20 cm
Allowable depletion of moisture = 40% of 20 cm = 8 cm
Consumptive use of water= 5 mm/day = 0.5 cm/day
The frequency of irrigation = 8 / 0.5 = 16 days.

7. What is the correct formula for Gross irrigation requirement?
a) Cu – Re
b) CIR + leaching losses
c) NIR/Na
d) FIR/Nc

Explanation: It is defined as the amount of water required to meet the field irrigation requirements plus the amount of irrigation water lost in conveyance through the canal system by evaporation and seepage.
Mathematically, GIR = FIR/Nc.

8. The following data gives the value of consumptive uses and effective rainfalls for the periods shown against them for a Jowar crop. Determine NIR of this crop neglecting other losses.

Dates Cu (in mm) Re (in mm)
16-31 October 37 30.8
1-30 November 84.2 20.4
1-31 December 154.9 6.7
1-31 January 188.1 2.4
1-10 February 13.3 1.0

a) 41.60 cm
b) 41.60 mm
c) 44 cm
d) 44 mm

Explanation: NIR = consumptive use – effective rainfall.

Dates Cu (in mm) Re (in mm) NIR = Cu – Re
16-31 October 37 30.8 6.2
1-30 November 84.2 20.4 63.8
1-31 December 154.9 6.7 148.2
1-31 January 188.1 2.4 185.7
1-10 February 13.3 1.0 12.3

Summation of all values of NIR = 416.2 mm = 41.62 cm.

Sanfoundry Global Education & Learning Series – Irrigation Engineering.

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