Infiltration MCQ Quiz - Objective Question with Answer for Infiltration - 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:

• 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.

Isohyets: It is a line joining points of equal rainfall for a given interval. Isohyets are contours of equal precipitation analogous to contour lines on a topographic map.

In the isohyetal method, precipitation values are plotted at their respective stations on a suitable base map, and isohyets are drawn to create an isohyet map.

Evapotranspiration Rates

Evapotranspiration is the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants. The rates of evapotranspiration are influenced by several environmental factors, such as temperature, humidity, wind speed, and soil moisture:

• High temperature increases the rate of evaporation.

• Low humidity creates a greater vapor pressure deficit, enhancing evaporation.

• Strong winds remove the saturated air layer around the evaporating surface, increasing evaporation.

• Soil moisture provides the necessary water for evaporation and transpiration.

Analyzing the Given Options

1. "High temperature, high humidity, strong winds, and saturated soil." (Option 1)

   • While high temperature and strong winds promote evapotranspiration, high humidity reduces it by lowering the vapor pressure deficit.

   • Saturated soil ensures water availability but does not overcome the reducing effect of high humidity.

2. "Low temperature, high humidity, strong winds, and dry soil." (Option 2)

   • Low temperature and high humidity both reduce evapotranspiration rates.

   • Dry soil limits the water available for evapotranspiration, further reducing rates.

3. "High temperature, low humidity, calm winds, and saturated soil." (Option 3)

   • High temperature and low humidity significantly enhance the vapor pressure deficit, promoting high evapotranspiration rates.

   • Saturated soil ensures water availability, though calm winds might slightly reduce the overall rate by not removing the saturated air layer.

   • Despite calm winds, the combined effect of high temperature, low humidity, and water availability leads to the highest evapotranspiration rates.

4. "High temperature, low humidity, strong winds, and dry soil." (Option 4)

   • High temperature, low humidity, and strong winds all promote high evapotranspiration rates.

   • However, dry soil limits water availability, which is crucial for high rates of evapotranspiration.

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

Explanation:​.

• Atmometer is a device that is used to measure the rate of water evaporation. It is also known as the evaporimeter.
• Rotameter is used to measure discharge but the current meter is used to measure velocity in open channels.

Important Points

Instruments used in the measurement

Confusion point:

The pitot tube is used to measure the velocity of fluid whereas an anemometer is used to measure gas and air velocity

Concept:

DAD curve: The areal characteristics of a rain storm are represented by a depth-area-duration curve. Once the sufficient rainfall records for the region are collected the basic or raw data can be analysed and processed to produce useful information in the form of curves or statistical values for use in the planning of water resources development projects.

Many hydrologic problems require an analysis of time as well as areal distribution of storm rainfall. Depth-Area-Duration (DAD) analysis of a storm is done to determine the maximum amounts of rainfall within various durations over areas of various sizes.

Hyetograph: is a graph between rainfall intensity and time. Rainfall intensity progressively increases until it reaches a maximum and then gradually decreases.

Mass curve: is graphical representation between accumulated rainfall v/s time. A mass curve of inflow can be prepared from the flow hydrograph of a stream for a large number of consecutive previous years

Double mass curve: It is used to check consistency of a rainfall for a particular area. The current mass curve is plotted and compared with previous data to check variation over a single curve.

Concept:

Direct Runoff: It is the part of runoff that enters the stream immediately after the precipitation. It includes surface runoff, prompt interflow, and precipitation on the channel surface. It is sometimes termed as direct storm runoff or storm runoff.

The volume of runoff = Area of catchment × Depth of rainfall

Precipitation: It is any liquid or frozen water that forms in the atmosphere and falls back to the earth. Precipitation is any product of the condensation of atmospheric water vapour that falls under gravity from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail.

Infiltration Capacity: It is the rate at which water infiltrates into ground is called infiltration capacity. For consistency, in hydrological calculations, a constant value of infiltration rate for the entire storm duration is adopted.

Total infiltrated volume = Total rainfall – Runoff

Calculation:

Given that:

Rainfall intensity = 20 mm/hr = 0.02 m/hr

Watershed area = 1 km²

Total duration = 6 hour

Direct runoff = 30000 m³

Total rainfall = Rainfall intensity × Total duration area

Total rainfall = (0.02 × 6) × 1 × 10⁶ = 1,20,000 m³

Precipitation not available for runoff means precipitation that is infiltrated.

Total infiltrated volume = Total rainfall – Runoff

Total infiltrated volume = 1,20,000 – 30,000 = 90,000 m³

For 1 km² watershed area,

\(The\;infiltrated\;depth = \frac{{90,000}}{{1 \times {{10}^6}}} = 0.09\;m\)

∴ Infiltrated depth is 9 cm.

Concept:

\(\emptyset - {\rm{index}} = \frac{{{\rm{P}} - {\rm{R}}}}{{\rm{t}}}\)

Where,

R (Runoff) (m): 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. Runoff is that part of precipitation that appears in uncontrolled surface streams, rivers drains or sewers.

P (Precipitation) (m): It is any liquid or frozen water that forms in the atmosphere and falls back to the earth. Precipitation is any product of the condensation of atmospheric water vapour that falls under gravity from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail.

Infiltration Capacity: It is the rate at which water infiltrates into ground is called infiltration capacity. For consistency, in hydrological calculations, a constant value of infiltration rate for the entire storm duration is adopted.

The average infiltration rate is called the infiltration index and two types of infiltration indices commonly used:

1. 𝛗 – Index: It is defined as the rate of infiltration above which volume equals the runoff volume.
2. w – index: It is the average infiltration rate during the time when the rainfall intensity exceeds the infiltration rate.

Explanation:

The ϕ-index represents an infiltration rate where the infiltration rate surpasses the threshold at which the volume of runoff equals the volume of rainfall.

Calculation:

If,        Rainfall < ϕ-index; Runoff = 0

and     Rainfall ≥ ϕ-index; Runoff = Rainfall - ϕ-index

The total direct runoff from the catchment = 0+(6-3) + (9-3) + (5-3) + (3-3) =11 cm

Explanation:

Infiltration is the process in which water enters into soil and the rate at which water entering into soil is known as infiltration rate. Under the ponding condition, infiltration rate decreases exponentially, and it can be represented by Horton equation.

According Horton’s equation,

\({\rm{f}} = {{\rm{f}}_{\rm{c}}} + \left( {{{\rm{f}}_0} - {{\rm{f}}_{\rm{c}}}} \right) \times {{\rm{e}}^{ - {\rm{kt}}}}\)

where,

fp = infiltration capacity at any time ‘t’ from start of rainfall.

fo = initial infiltration capacity at t = o.

fc = ultimate infiltration capacity occurring at t = tc

kn = Horton’s decay coefficient

Note: The equation is applicable only when rainfall intensity is greater than or equal to fp.

Concept:

Infiltration: It is the flow of water into the ground through the soil surfaces.

Infiltration Capacity: The maximum rate at which a given soil at a given time can absorb water is defined as infiltration capacity. It is denoted as fp and its unit is cm/hr.

Hence, if the intensity of rainfall is more than the infiltration capacity then, infiltration rate will be equal to infiltration capacity (fp) and rest of the amount of rainfall will be in the form of runoff.

Important points related to f_p:

The intensity of rainfall is “i”. Then, three conditions arise from here:

i) If the actual rate of infiltration is f such that f = f_p and i > f_p then certain amount of runoff will be observed.

Concept:

Pan coefficient (CP) = Pond evaporation (E) / Pan evaporation (EP)

Calculation:

Given,

Pan coefficient (C_P) = 0.70

Pan evaporation (E_P) = 45 mm

Pond evaporation (E) = C_P x E_P

E = 0.70 x 45 = 31.5 mm

Groundwater exploration is the investigation of underground formations to understand the hydrologic cycle, know the groundwater quality, and identify the nature, number and type of aquifers.

The main objective is to study and understand the hydrological cycle of the region, to have an overall concept about the type, nature & number of aquifers, the quality of ground water.

The top surface of the zone of saturation or groundwater is known as phreatic surface. This phreatic surface is also known as water table.