Runoff MCQ Quiz - Objective Question with Answer for Runoff - Download Free PDF

Explanation:

Surface runoff is the water from rainfall, melting snow, or irrigation that flows over the land surface when it cannot soak into the ground. It typically moves toward streams, rivers, lakes, or drainage systems.

Factors affecting surface runoff:

1. Rainfall Intensity and Duration
   Heavy or prolonged rain can exceed soil absorption capacity, leading to increased runoff.

2. Soil Type and Permeability
   Sandy soils allow more infiltration; clayey or compacted soils cause more runoff.

3. Vegetation Cover
   Plants slow rainfall, improve infiltration, and reduce runoff; bare land does the opposite.

4. Land Slope
   Steeper slopes accelerate water flow, reducing infiltration time and increasing runoff.

5. Land Use and Surface Conditions
   Impervious surfaces like roads prevent absorption and increase runoff significantly. 

 Additional InformationKey concepts of surface runoff 

1. Water Accumulation

   Water that doesn't infiltrate the ground accumulates on the surface and flows away as runoff.

2. Runoff Volume

   The amount of surface runoff is influenced by rainfall amount, intensity, and duration.

3. Hydrological Cycle Impact

   Runoff is a key component of the hydrological cycle, transferring water from land to bodies of water.

4. Impact of Impervious Surfaces

   Urbanization and construction increase runoff due to the presence of non-absorptive surfaces like roads and buildings.

5. Natural Barriers

   Features like wetlands, forests, and vegetation help reduce runoff by promoting water infiltration and slowing flow.

Concept:

Index of wetness is used to find the rainfall variation or deviation for a particular year and is given as,

Index of wetness = (Rainfall in a particular year at a place/Avg. Annual Rainfall) × 100

Calculation

 Given, 

Rainfall in a particular year = 90 cm

 Average annual rainfall = 150 cm

Index of wetness = rainfall in a year / avg. annual rainfall

= (90/150) × 100 = 60%

Explanation:

Inglis and Desouza Formula has given  two regional formulae, between annual runoff R in cm and annual rainfall P in cm as follows:

• For Ghat regions of western India
  • ​Highlands
  • ​​R = 0.85P - 30.5​
• ​For Deccan Plateau
  • Plain areas
  • ​R = \(​​\frac{{1\;}}{{254}}\)P(P - 17.8)

Barlow’s Tables:

• Barlow, the first Chief Engineer of the Hydro-Electric Survey of India (1915)
• Barlow studied small catchments (area ~ 130 km²) in Uttar Pradesh and expressed runoff R as:
• R = KbP ,
• Where K_b = Runoff coefficient which depends upon the type of catchment and nature of monsoon and P is the rainfall.

​​Strange’s Tables:

• Strange ​studied the available rainfall and runoff in the border areas of present-day Maharashtra and Karnataka.
• Obtained yield ratios as functions of indicators representing catchment characteristics.​

Explanation:

Runoff is affected by various topographical and environmental factors. A steep slope plays a significant role in increasing runoff due to the following reasons:

• Reduced infiltration: Water flows rapidly down a steep slope, preventing it from soaking into the ground.

• Higher velocity of flow: Water moves quickly over steep terrain, reducing the time available for absorption.

• Minimal retention time: Since water spends less time on the surface, it does not percolate effectively, increasing runoff.

Explanation:

To calculate the total volume of rainfall over the catchment area, we use the formula:

Total Volume of Rainfall= Catchment Area × Total Rainfall Depth

Given Data:

Catchment Area ( A) = 5 square km

Rainfall intensities per hour for a five-hour duration: 10 mm, 15 mm, 20 mm, 22 mm and 5 mm

Convert the catchment area to square meters (since 1 km²= 1,000,000 m²):

5km²=5×1,000,000m²=5,000,000m²

Total Rainfall Depth=10mm+15mm+20mm+22mm+5mm= 72 mm

72mm=72×0.001m=0.072m

Total Volume of Rainfall= Catchment Area×Total Rainfall Depth

Total Volume of Rainfall=5,000,000m²×0.072m

Total Volume of Rainfall=360× 10³ m³

Explanation:

Isochrones:

Isochrones is a line on the map which connects points having an equal time of travel of the surface runoff to the catchment outlet. These are some properties of Isochrones:

• Isochrones vary with time, It's most commonly used to depict travel times, such as drawing a 30-minute travel time perimeter around a start location. The isochrone below joins up all points within a 45-minute drive from the origin. 
• Isochrones depict the variation of the pore water pressure along with the depth of the soil sample.
• Isochrones are mainly used for transport planning, property search, sales territory planning, etc. ​

Important Points

Concept:

ϕ_index = \(\frac{P_e-R}{t_e}\) and W_index = \(\frac{P-R}{t_r}\)

Where,

P_e = effective rainfall causing runoff, R = runoff, 

t_e = duration of effective rainfall, P = total rainfall and t_r = total duration of rainfall

Calculation:

Given,

ϕ_index = 35 mm/hr

∵  We know that,  runoff occurs when the intensity of rainfall (i) >  ϕ_index 

∴ From rainfall intensity data, the intensity of 30 mm/hr < 35 mm/hr, hence ignored in the calculation of ϕ_index. 

⇒ P_e = (36 + 40 + 120 + 85 + 45 + 45) × 0.5

⇒ Pe = 185.5 mm and t_e = 3 hr

∵ We know that, ϕindex = \(\frac{P_e-R}{t_e}\)

⇒ 35 = \(\frac{185.5-R}{3}\) 

⇒ R = 80.5 mm

Now, P = (36 + 40 + 120 + 85 + 45 + 45 + 30) × (30/60)

⇒ P = 200.5 mm and t_r = 3.5 hr

∵ We know that, Windex = \(\frac{P-R}{t_r}\)

⇒ Windex = \(\frac{200.5-80.5}{3.5}\)

⇒ W_index = 34.28 mm/hr

Concept:

The volume of water evaporated (V) = E × L × B

Where, E = Evaporation measured per day, L = Stretch of evaporation  & B = Average surface width

Calculation:

Given, L = 80 km = 8 × 10⁴ m 

B = 15 m

Evaporation = 0.5 cm / day

Pan co-efficient = 0.7

Total volume of evaporation (V) = 8 × 10⁴ × 15 × 0.5 x 10^-2 × 0.7 × 30 m³

Explanation

∵ We know that,

Runoff coefficient = Runoff/Rainfall

Given,

Runoff = 150 Mm3

Rainfall depth = 750 mm

Area = 500 km2

Rainfall depth in terms of volume = 0.75 × 500 = 375 Mm3 (1 million-m = 106 m)

Runoff coefficient = 150/375

= 0.4

Concept:

ϕ index of a catchment is defined as the constant infiltration capacity that would yield the actual total runoff for a given rainfall amount. 

Mathematically, ϕ_index =  \(\frac{P_e -R}{t_e} \)

Where,

P_e = effective rainfall causing runoff, R = runoff, and t_e = effective rainfall period

As we know that, for runoff to occur, Rainfall intensity (i) > ϕ_index

Calculation:

Given,

P = 15 cm, R = 8.7 cm, duration of rainfall = 8 hrs

Trial 1: Considering all rainfall values in a period of 8 hrs causes runoff.

∵ We know that, ϕ_index = \(\frac{P_e -R}{t_e} \)

⇒ ϕ_index = \(\frac{15 - 8.7}{8}\)

∴ ϕ_index = 0.7875 cm/hr

\(\because \) We know that, for runoff to occur, i > ϕ_index,

\(\Rightarrow\) rainfall intensities 0.6 and 0.75 cm/hr are less than ϕ_index = 0.7875 cm/hr

\(\therefore \) for trial 2, rainfall intensities 0.6 and 0.75 cm/hr are ignored.

Trial 2:

Total rainfall, P = 1.35 + 2.25 + 3.45 + 2.7 + 2.4 + 1.5 = 13.65 cm, R = 8.7 cm

Duration of rainfall, t = 6 hrs

\(\Rightarrow\) ϕ_index = \(\frac{13.65 - 8.7}{6}\)

 \(\therefore\) ϕ_index = 0.825 cm/hr

Concept:

According to Khosla's method monthly runoff is given as -

\(R_{m}=P_{m}-L_{m}\)

Where \(R_{m}=Monthly\, Runoff\)

\(P_{m}=\) Monthly rainfall in cm

\(L_{m}=\) monthly losses in cm

 \(L_m=0.48\times Mean\, Temperature\,(T_m),\, T_m>4.5^{\circ}C\)

Calculation:

Given data:

Mean monthly temperature in June (T_m) = 28^∘ C

Average (mean) rainfall in June (P_m) = 16 cm

June runoff (R) =?

\(R_m=16-0.48\times 28^{\circ}C=16-13.44\)

\(R_m=2.56\, cm\approx2.6\, cm\)

The runoff for the month of June is 2.6 cm

Concept:

Run-off : It is defined as the part of water cycle that flows over land as surface water instead of being absorbed into the ground water or evaporated. Run-off is that part of precipitation that appears in uncontrolled surface streams, rivers drains or sewers.

Calculation:

Concept:

Hydrograph is used to measure runoff discharge.The area under the discharge and time curve gives the volume of runoff.

Calculation:

Catchment area = 777.6 km² = 777.6 × 10⁶ m²

Total volume of water = Area of Hydrograph

Total volume \(= \frac{1}{2} \times 72 \times 60 \times 3600\)

= 7776000 m³

Runoff storm \(= \frac{{Volume}}{{Area}} = \frac{{7776000\;{m^3}}}{{777.6 \times {{10}^6}{m^2}}}\)

Runoff storm = 10^-2 m

∴ Runoff storm = 1 cm

Explanation:

Runoff

• Runoff means the draining or flowing off of precipitation from a catchment area through a surface channel.
• Runoff represents the response of a catchment to precipitation. It reflects the integrated effects of a wide range of catchment, climate and rainfall characteristics such as magnitude, intensity, distribution according to time and space, and variability.
• High-stream discharges occur during monsoon month, and low flow during the rest of the year.
• Physical characteristics of the catchment, such as area, shape, slope and drainage channel pattern in the catchment, are some of the major static characteristics that affect the volume of the surface runoff and shape of the runoff hydrograph from a catchment due to a storm.

The factors affecting the runoff can be summarised in below tabulated form