Types of rainfall:

• Convective rainfall: A packet of air which is warmer than the surrounding air due to localized heating rises upward and consequently undergoes cooling, condensation and precipitation.
• Orographic rainfall: The moist air masses get lifted up to higher altitudes due to the presence of the mountain barriers and consequently undergo cooling, condensation and precipitation.
• Frontal or Cyclone rainfall: When two air masses meet, they do not mix readily due to differences in temperature and density. A front is an imaginary line separting two contrasting air masses.

• The rainfall is collected and measured in a rain gauge, sometimes also referred as pluviometer, ombrometer and hyetometer.
• Rain gauge must be installed in such a way that:
1. The ground must be level and in the open and the instrument must present a horizontal catch surface.
2. The gauge must be set as near the ground as possible to reduce wind effects but it must be sufficiently high to prevent flooding, etc.
3. The instrument must be surrounded by an open fenced area of at least 5.5m X 5.5m.
4. No objects must be nearer to the instrument than 30 m or twice the height of the obstruction.

1. Non-Recording gauges: Symon's Gauge
2. Recording Gauges
• Tipping bucket type
• Weighing bucket type
• Natural Siphon type

• Latitude
• Temperature
• Moisture
• Air Masses
• Frontal Activity
• Differential Heating
• Mountain Barriers
• Distribution of Land and Water

Solved Example: 9083-01

Which one of the following points should be kept in mind while selecting the site for a rain gauge station? (ESE Civil 2019)

A. The site where a rain gauge is set up should be close to a meteorological observatory.

B. The rain gauge should be on the top of a hill.

C. A fence, if erected to protect the rain gauge from cattle etc. should be located within twice the height of the fence.

D. The distance between the rain gauge and the nearest object should be at least twice the height of the object.

Correct Answer: D

Solved Example: 9083-02

Rainfall with an intensity of 6 mm/h is classified as: (DFCCIL Executive Civil Sept 2021 Shift I)

A. Trace rain

B. Moderate rain

C. Heavy rain

D. Light rain

Correct Answer: B

Solved Example: 9083-03

The Rainfall Intensity of Light Rain is: (MPSC AE Civil Mains 2019 Paper II)

A. Upto 2.5 mm/Hr

B. Upto 3.0 mm/Hr

C. Upto 5.00 mm/Hr

D. Upto 7.5 mm/Hr

Correct Answer: A

Solved Example: 9083-04

The maximum average depth due to one day storm over an area of 100 km$^2$ is 100 mm. Depth-Area-Duration (DAD) curves indicate that for the same area of 100 km$^2$ the maximum average depth for a 3 hour storm will be: (MPPSC AE CE 2016 Official Paper II)

A. 100 mm

B. More than 100 mm

C. Less than 100 mm

D. None of these are correct

Correct Answer: C

Solved Example: 9083-05

Which of the following is a non-recording rain gauge? (SSC JE CE Sep 2019 Morning)

A. Weighing type rain gauge

B. Tipping Bucket rain gauge

C. Simon's rain gauge

D. Float type rain gauges

Correct Answer: C

Solved Example: 9083-06

The theissen weights of 4 rain gauges A, B, C and D covering a river basin are 0.15, 0.25, 0.30 and 0.30 respectively. If the average depth of rainfall for the basin is 5 cm and rainfall recorded at B, C and D are 5 cm, 4 cm and 5 cm respectively, what is the rainfall at A? (UPPSC AE Civil Dec 2020)

A. 5 cm

B. 6 cm

C. 7 cm

D. 8 cm

Correct Answer: C

Solved Example: 9083-07

Imaginary lines joining equal ___________ is known as isohyets. (MP Sub Engg Civil Sep 2018 Shift II)

A. Height

B. Humidity

C. Rainfall

D. Pressure

Correct Answer: C

Solved Example: 9083-08

What is the diameter of funnel which collects rain water in tipping bucket rain gauge? (UPPCL JE CE 2020)

A. 100 mm

B. 200 mm

C. 300 mm

D. 400 mm

Correct Answer: C

Solved Example: 9083-09

The method used for estimating missing rainfall is: (GPSC AE CE 2017 Part B Civil)

A. Normal ratio method

B. Station year method

C. Plotting position method

D. Rational method

Correct Answer: A

Solved Example: 9083-10

When rain falls as water droplets of size less than 0.5 mm, so light in weight to appear as floating in air, is termed as: (UPPCL JE CE Paper II 2014)

A. Drizzle

B. Snow

C. Dew

D. Rain

Correct Answer: A

Solved Example: 9901-01

The rainfall figures for successive 30-minute intervals are 35, 40, 120, 85, 45, 45 and 30 mm/hr. If the $\phi$-index is 35 mm/hour, determine W-index. (NWDA JE CE 2019)

A. 24.3 mm/hour

B. 44.3 mm/hour

C. 34.3 mm/hour

D. 54.3 mm/hour

Correct Answer: C

Solved Example: 9901-02

The observed annual runoff from a basin of area 500 $km^2$ is 150 $Mm^3$ and the corresponding annual rainfall over the basin during the same year is 750 mm. What is the runoff coefficient? (UPPSC AE Civil Paper II Dec 2020)

A. 0.2

B. 0.67

C. 0.4

D. 0.5

Correct Answer: C

Solved Example: 9901-03

A storm with 150 mm precipitation produces a direct runoff of 8.7 cm, with incremental hourly rainfall values being 0.6, 1.35, 2.25, 3.45, 2.7, 2.4 1.5 and 0.75 cm/hr. Estimate the $\phi$-index of the storm. (NWDA JE CE 2019)

A. 0.925 cm/hr

B. 0.825 cm/hr

C. 0.625 cm/hr

D. 0.725 cm/hr

Correct Answer: B

Solved Example: 9901-04

A catchment consists of 40% area with run-off coefficient 0.30 with the remaining 60% area with run-off coefficient 0.50. The equivalent run-off coefficient will be: (RSMSSB JE Civil Degree Sep 2016)

A. 0.38

B. 0.42

C. 0.48

D. 0.52

Correct Answer: B

Solved Example: 9901-05

Which of the following statements is/are correct?

1. Isochrones are curves of equal pore water pressure
2. Isochrones depict the variation of the pore water pressure along with the depth of the soil sample
3. Isochrones vary with time

A. 1 only

B. 2 and 3 only

C. 1 and 2 only

D. 1, 2 and 3

Correct Answer: B

Solved Example: 9901-06

A canal is 80 km long and has an average surface width of 15 m. If the evaporation measured in a class A pan is 0.5 cm/day, the volume of water evaporated in a month of 30 days is (in m$^3$) (SSC JE CE Sep 2019 Morning)

A. 12600

B. 18000

C. 180000

D. 126000

Correct Answer: D

Solved Example: 9901-07

The run off increases with: (RSMSSB JE Civil Diploma Oct 2016)

A. Increase in intensity of rain

B. Increase in infiltration capacity

C. Increase in permeability of soil

D. All of the above

Correct Answer: A

Solved Example: 9901-08

A direct runoff hydrograph due to a storm idealized into a triangular shape has a peak flow rate of 60 m$^3$/s occurring at 25 hours from its start. If the base width of this hydrograph is 72 hours, and the catchment area is 777.6 km$^2$, the runoff from the storm is: (ESE Civil 2014 Paper II

A. 1 cm

B. 2 cm

C. 5 cm

D. 10 cm

Correct Answer: A

Solved Example: 9901-09

The streams that do not receive any base flow contribution are termed as: (MP Sub Engg Civil Jul 2017 Shift I)

A. Intermittent streams

B. Ephemeral streams

C. Perennial streams

D. None of the above

Correct Answer: B

Solved Example: 9901-10

The term base flow denotes: (MPSC AE Civil Mains Paper II 2018)

A. Delayed groundwater flow reaching a stream

B. Delayed groundwater and snowmelt reaching a stream

C. Delayed groundwater and interflow

D. The annual minimum flow in a stream

Correct Answer: A

Solved Example: 9084-01

Watershed is a:

A. Drainage basin contributing the runoff to a single outlet

B. Hydrological entity

C. Unit of area extent in which hydrologic cycle completes

D. All are correct

Correct Answer: D