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

Explanation:

• The Blaney–Criddle equation is widely used in India for estimating consumptive use (evapotranspiration) of crops due to its simplicity and reliance on readily available climate data like temperature and crop coefficients.

• Data Required: Only temperature and daylight hours, which are easily available.

• Advantages: Simple, practical, cost-effective.

• Limitations: Less accurate in extreme climates or non-standard crop conditions.

• Suitability: Preferred in Indian irrigation projects and rural planning.

Additional InformationPenman's equation

• Type: Physically based method (semi-theoretical).

• Formula Basis: Combines energy balance and aerodynamic terms.

• Parameters Used:

  • Temperature

  • Solar radiation

  • Humidity

  • Wind speed

• Advantages:

  • More accurate and scientific

  • Useful in precision agriculture or academic research

• Limitations:

  • Requires multiple climate data inputs

  • Complex calculations

• Suitability: Used in research, high-accuracy modeling, and where detailed meteorological data is available.

Tanks and Lysimeter

a) Lysimeter:

• Description: A device used to measure the actual evapotranspiration by isolating a soil volume and monitoring water inputs/outputs.

• Advantages:

  • Direct measurement

  • Highly accurate

• Limitations:

  • Expensive

  • Requires controlled setup and long-term monitoring

• Suitability: Used in research stations for calibration of empirical formulas.

b) Tanks (Class A Evaporation Pan, etc.):

• Description: Evaporation pans are used to measure evaporation losses; crop coefficient is applied to estimate consumptive use.

• Advantages:

  • Simple equipment

  • Easy to operate

• Limitations:

  • Indirect method for consumptive use

  • Influenced by pan location and wind

• Suitability: Supplementary tool for estimating evaporation, not commonly used alone for consumptive use.

Explanation:

Thornthwaite equation

• Used to estimate potential evapotranspiration (PET) based on temperature and day length.

• It is a climatological method useful in water balance studies and irrigation planning.

• Suitable for regions with limited climatic data.

Hazen-Williams equation

• Used to estimate head loss due to friction in pressurized water flow through pipes.

• It is empirical and applicable mainly for water supply systems.

• Not related to evapotranspiration or atmospheric processes.

Chezy's equation

• Used in open channel flow to calculate flow velocity based on slope and hydraulic radius.

• It is a fundamental hydraulic equation, not related to PET.

• Mainly applied in river engineering and channel design.

Manning's equation

• Also for open channel flow, used to determine the velocity of water.

• Considers channel roughness, slope, and shape.

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.

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.

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

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 

R 

R 

∴ 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,

∴ Infiltrated depth is 9 cm.

Concept:

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

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,

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.

ii) If f p then infiltration rate will be will be f and no 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

Concept:

(i) Infiltration is the movement of water through soil. It is the process by which water enters into surface strata of soil to meet initial soil moisture deficiency. Infiltration Index is a method used for measurement of Infiltration. It represents average rate of infiltration. There are two types of infiltration index:-

(ii) φ- Index – It represents average infiltration rate during the period of rainfall excess. In the calculation of φ- Index initial losses is also considered as infiltration quantity.

(iii) W-Index- This is the average infiltration rate during the entire/whole period of storm. In the calculation of W-Index initial loss is separated from the total infiltration.

φ- Index is greater than or equal to W-index.

If in a certain duration T, the rainfall is P, Runoff is Q, initial losses is l, then W-index is given by

W 

φ- Index = , here T=Effective time duration in which rainfall intensity is greater than φ- Index

When intensity of rainfall is less than or equal to φ- Index then there will be no runoff.

Calculation:

A rain gauge recorded hourly rainfall as 5 cm, 2 cm, 4 cm and 3 cm for a four hour storm respectively.

φ- index =3 cm/hour

Intensity of rainfall for 2^nd hour and 3^rd hour is 2 cm/hour and 3 cm/hour which is less than and equal to φ- index respectively, so it is not required to consider those amount of rainfall during φ- index calculation.

So Total Precipitation = 5 + 4 = 9 cm

T_e= Effective time duration in which rainfall intensity is greater than φ- Index=2 hour

So 3 

R = 9 – 6 = 3 cm