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

Explanation:

1. When the rate of precipitation exceeds the rate of infiltration, the excess water cannot be absorbed by the soil. As a result, the water flows over the surface of the land as surface runoff.

2. Surface runoff occurs when the ground becomes saturated or when the rate of precipitation is too high for the soil to absorb it. This excess water can flow into rivers, lakes, or reservoirs, contributing to water levels rising.

3. Infiltration is the process by which water enters the soil. If precipitation is faster than infiltration, it can lead to flooding, erosion, and other hydrological issues.

 Additional InformationSurface Runoff:

1. Occurs When Precipitation Exceeds Infiltration Rate: Surface runoff happens when the rate of rainfall or other precipitation is greater than the soil's ability to absorb it.

2. Leads to Erosion: Excess water flowing over the ground can erode soil, carry away nutrients, and cause land degradation.

3. Contributes to Flooding: If surface runoff is significant, it can lead to localized or widespread flooding, especially in urban areas where natural infiltration is reduced due to impermeable surfaces like roads and buildings.

4. Affects Water Quality: Runoff can pick up pollutants such as oil, chemicals, and debris, which can be carried into water bodies, affecting their quality.

5. Impacted by Land Use and Vegetation: Urbanization, deforestation, and the removal of vegetation increase surface runoff by reducing the amount of water that can infiltrate the soil.

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.

Explanation:

• When rainfall intensity exceeds the infiltration capacity of the soil (as shown by the infiltration capacity curve), the soil cannot absorb all the incoming water.
• The excess water then accumulates on the surface and starts flowing overland — this is known as surface runoff.

Additional InformationGroundwater Recharge

• Occurs when water infiltrates through the soil and percolates down to the water table.
• If infiltration capacity is already exceeded, no additional water goes to groundwater — it becomes runoff instead.

Evaporation

• A slow process where water converts from liquid to vapor.
• Does not account for immediate excess water due to intense rainfall.

Evapotranspiration

• Combination of evaporation and plant transpiration.
• Like evaporation, it’s a gradual loss of water, not an immediate response to excess rainfall.

Explanation:

• The infiltration capacity refers to the maximum rate at which soil can absorb water. When rainfall intensity exceeds this capacity, the soil can no longer absorb all the water, and the excess water starts flowing over the surface of the ground.

• This excess water that cannot infiltrate into the soil is what leads to surface runoff, which is the movement of water across the land surface, often resulting in streams, rivers, or flooding.

Additional Information Groundwater recharge:

• This happens when water infiltrates the soil and percolates down to replenish groundwater supplies, but this occurs when rainfall intensity is less than or equal to the infiltration capacity.

Evapotranspiration:

• This is the process of water being transferred to the atmosphere from both soil evaporation and plant transpiration, which is unrelated to excess rainfall.

Evaporation:

• This refers to water turning into vapor from the surface of the land or water bodies, but it is not directly related to the excess rainfall.

Explanation:

A Symons rain gauge is a non-recording type of rain gauge. It collects rain in a cylindrical container, and the collected water is then measured using a measuring glass or cylinder with a graduated scale to determine the depth of rainfall manually, usually in millimeters.

Additional Information A digital display –

• Digital displays are used in modern automated rain gauges, not in the traditional Symons rain gauge.
• Symons rain gauge is a manual instrument and does not involve electronics or digital readouts.

A rotating drum –

• This is typically found in self-recording rain gauges (like the tipping bucket type with chart recorders), not in the Symons type.
• The Symons rain gauge does not use any mechanical recording device.

A pressure sensor –

• Pressure sensors are used in advanced weather instruments but are not part of the Symons rain gauge design.

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:

Hydrograph:

• A hydrograph is a plot between discharge and time at any given section of a river, channel, etc. 
• It is a response of a given catchment to the rainfall input.
• The shape of the rising limb of a hydrograph depends on both catchment characteristics and rainfall characteristics.
• The shape of the falling or recession limb of a hydrograph depends only on catchment characteristics.
• The inflation point on the falling limb is often assumed to be the point where direct runoff ends.
• Time of Concentration is the time required by the entire drainage area to contribute to the runoff is called the time of concentration or time required by the most extreme point in the drainage to reach the point of interest.
   

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Concept

While selecting the site for rain gauge stations the following points should be considered:

• The site should be on level ground and on open space. It should never be on sloping ground.
• The site should be such that the distance between the gauge station and the objects (like a tree, building, etc) should be at least twice the height of the objects.
• In the hilly area, where level ground is not available, the site should be so selected that the station may be well shielded from high wind.
• The site should be easily accessible to the observer.
• The site should be well protected from cattle by wire fencing.

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:

Precipitation- It is the fall of water in various forms on the earth from the clouds.

The usual forms of precipitation are as follows:

Note:

Explanation

Concept:

The ϕ – index is a rate of infiltration in which, the rate of infiltration exceeds the value at which volume of runoff become equals to the volume of rainfall.

\( {ϕ_{\left( {index} \right)}} = \frac{{{Total\; Infiltration}}}{{Total\;time\;of\;the\;storm}}\)

\( {ϕ_{\left( {index} \right)}} = \frac{{{P_{total}} - Q}}{{Total\;time}}\)

where

PTotal = Total precipitation

Q = Runoff

Calculation:

ϕ - index = 7.5 cm/hr

ϕ - index is the average rate of rainfall such that the volume of rainfall in excess of that rate is equal to the volume of surface runoff

So runoff (R) in cm is,

R \(=(12.5-7.5) \dfrac{15}{60}+ (17.5 - 7.5) \dfrac{15}{60}+ (22.5 - 7.5) \dfrac{15}{60}\)

R \(=\dfrac{5\times 15}{60}+\dfrac{10 \times 15}{60}+ \dfrac{15 \times 15}{60}\)

\(R=\dfrac{15}{60}(5+10+15)\)

R \(=\dfrac{15}{60}\times 30\)

∴ R = 7.5 cm

Concept:

Form factor: This is defined as the ratio of average width of the basin to the axial length

Form factor \(\rm = \dfrac{B_{avg}}{L_{axial}}\)

Form factor \(\rm=\dfrac{Area}{L× L}\)

Calculation:

Given:

Area of catchment = 62.5 mm2

axial length of catchment = 10 mm

Form factor \(=\dfrac{62.5}{10\times 10}\)

∴ Form factor = 0.625