Infiltration
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Infiltration
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Infiltration
• Process by which precipitation water is abstracted by seeping into the soil
below the land surface.
• Water lost due to absorption in the ground.
• Infiltrated water may move as interflow or percolated flow.
• Measured in terms of depth of water per unit time.
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Infiltration
• At any instant ‘d’ depth of infiltrated water divided by time ‘t’ gives us
Instantaneous Infiltration Rate.
• If total infiltrated water over a time ‘T’ is divided by total time, Average
Infiltration Rate is determined.
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Factors affecting Infiltration
• Precipitation
• Soil type
• Soil water content
• Vegetation cover
• Ground slope.
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Measurement
Double Ring Infiltrometer
• The two rings (22.5 to 90 cm diameter) are driven into the ground by a
driving plate and hammer, to penetrate into the soil uniformly without tilt or
undue disturbance of the soil surface to a depth of 15 cm.
• After driving is over, any disturbed soil adjacent to the sides tamped with a
metal tamper.
• Point gauges are fixed in the centre of the rings and in the annular space
between the two rings.
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Double Ring Infiltrometer
• Water is poured into the rings to maintain the desired depth (2.5 to 15 cm
with a minimum of 5 mm) and the water added to maintain the original
constant depth at regular time intervals (after the commencement of the
experiment) of 5, 10, 15, 20, 30, 40, 60 min, etc. up to a period of atleast 6
hours is noted and the results are plotted as infiltration rate in cm/hr versus
time in minutes.
• The purpose of the outer tube is to eliminate to some extent the edge effect
of the surrounding drier soil and to prevent the water within the inner space
from spreading over a larger area after penetrating below the bottom of the
ring.
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Estimation
Infiltration capacity for given soil decreases with time from the start of rainfall,
i.e. it decreases with degree of saturation and depends on type of soil.
Horton (1930) expressed the decay of infiltration capacity with time as
• f = infiltration capacity at any time t from start of rainfall
• fo = initial infiltration capacity at t = 0
• fc = final steady state value
• t = time from beginning of storm
• k = constant depending on soil characteristics and vegetation cover (units
are in h‐1)
• Integration gives total infiltration
F = fc t+ [(fo – fc)/k][1- e-kt ]
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Infiltration Indices
• The average infiltration rate is called infiltration index. There are two types
of indices which are common
Index
W Index
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Infiltration Indices
• Φ Index is average infiltration above which rainfall volume is equal to
runoff volume. Φ index is derived from rainfall hyetograph with edge of
resulting runoff volume. Φ value is found by treating g. It as a constant
infiltration capacity.
• If rainfall intensity is less than Φ then infiltration rate is equal to rainfall
intensity. However, if rainfall intensity is larger than Φ, the difference
between rainfall and infiltration is an interval of time represents the runoff
volume.
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Infiltration Indices
• W‐index gives the average rate of infiltration. It can be found by
subtracting the sum of total direct runoff, interception and depression
storage from the total rainfall.
F = Total Infiltration
T = Duration of Storm
P = Depth of Precipitation
R = Runoff Depth
I = Interception and depression storage, then
F=P‐R‐I
And ‘W’ index = F / T = ( P – R – I ) / T
For I = 0, ‘W’ Index = Φ Index
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