1. Chapter-4 Infiltration
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

2. Chapter-4: Infiltration
Course Outcome
On completion of this chapter you will be able to understand and / or identify and / or quantify
What is infiltration?
The process of infiltration
Infiltration capacity
Factors influencing infiltration
Infiltration measurement
Infiltration estimation
Horton’s Infiltration Equation
Green Ampt Infiltration Equation
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

3. Chapter-4: Infiltration
When water is applied to the surface of a soil, a part of it seeps into the soil. This movement of water through the soil surface is known as infiltration
Infiltration plays a very significant role in the runoff processes by affecting the timing, distribution and magnitude of the runoff
Further infiltration is the primary step in the natural ground water recharge
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

4. Process of infiltration
The process of infiltration can be easily understood through a simple analogy
Consider a small container covered with wire mesh as shown in the Figure. If water is poured over the mesh, a part of it will go into the container and a part overflows
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

5. Process of infiltration
Further, the container can hold only a fixed quantity of water and when it is full no more flow into the container can take place
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

6. Process of infiltration
This analogy, though a highly simplified one, underscores two important aspect;
The maximum rate at which the ground can absorb water, the infiltration capacity
The volume of water that it can hold, the field capacity
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

7. Process of infiltration
Since the infiltrated water may contribute to groundwater discharge in addition to increasing the soil moisture, the process can be schematically modeled as in the Figure.
The figure considers two situation
Low-intensity rainfall
High-intensity rainfall and is self explanatory
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

8. Infiltration Capacity
The maximum rate at which a given soil at a given time can absorb water is defined as the infiltration capacity.
It is designated as fc and is expressed in units of cm/h or mm/h.
The actual rate of infiltration f can be expresses as;
where i is the rainfall intensity
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

9. Factors influencing infiltration
The infiltration process is affected by a large number of factors and a few important ones are;
Characteristics of soil
Surface condition
Fluid characteristics
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

10. Characteristics of soil
The type of soil, e.g. sand, silt or clay, its texture, structure, permeability and its under drainage are the important characteristics that influence infiltration.
A loose, permeable, sandy soil will have large infiltration capacity than a tight, clayey soil
A soil with good under drainage, i.e. facility to transmit the infiltrated water downward to a groundwater storage would obviously have a higher infiltration capacity
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

11. Characteristics of soil
When the soils occur in layers the transmission capacity of layers determine the overall infiltration rate
Also a dry soil can absorb more water than one whose pores are already full
Land use has a significant effect on fc. For example, a forest soil rich in organic matter will have a much higher vale of fc under identical conditions than the same soil in an urban area where it is subjected to compaction
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

12. Soil surface condition
At the soil surface, the impact of raindrops causes the fines in the soils to displaced and these in turn can clog the pore space in the upper layer. This is an important factor affecting infiltration capacity.
Thus a surface covered with grass and other vegetation which can reduce this processes has a pronounced influence on the value of fc
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

13. Fluid characteristics
Water infiltrating into the soil will have many impurities, both in solution and in suspension.
The turbidity of the water, especially the clay and colloid content is an important factor as such suspended particles block the fine pores in the soil and reduce its infiltration capacity
Contamination of water by dissolved salts can affect the soil structure and in turn affect infiltration rate.
The temperature of water is also a factor in the sense that it affects the viscosity of the water which in turn affects infiltration rate
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

14. Infiltration Measurement
Information about the infiltration characteristics of the soil at a given location can be obtained by conducting controlled experiments on small areas
The experimental set-up is called infiltrometer test and is shown in the Figure
(Figure: A double-ring infiltrometer and quick draw tensiometer assembly from Rahardjo et al., 2002)
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VAB2063 Hydrology
Chapter-4: Infiltration

15. Infiltration Measurement
The infiltrometer consists of two concentric rings
This two rings are inserted (about 10 cm) into the ground and water is maintained in both the rings to a common fixed level
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VAB2063 Hydrology
Chapter-4: Infiltration

16. Infiltration Measurement
The outer ring provides a water jacket to the infiltrating water of the inner ring and hence prevents the spreading out of the infiltrating water of the inner ring
Water is poured into the rings (both) to a depth of 5 cm and a pointer is set to mark the water level
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VAB2063 Hydrology
Chapter-4: Infiltration

17. Infiltration Measurement
As infiltration proceeds, the volume is made up by adding water from a burette to keep the water level at the tip of the pointer
Knowing the volume of water added at different time intervals, the plot of infiltration capacity versus time is obtained
(Figure: Plan and sectional view of layout of a double-ring infiltrometer and quick draw tensiometers from Rahardjo et al., 2002)
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VAB2063 Hydrology
Chapter-4: Infiltration

18. Infiltration Measurement
The experiment are continued till a uniform rate of infiltration is obtained and this may take 2-3 hours
The surface of the soil is usually protected by a perforated disk to prevent formation of turbidity and its settling on the soil surface
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

19. Infiltration Measurement
A graph of cumulative infiltration rate versus time, prepared from known volume of water added at different time intervals, during an infiltration experiment is shown in the Figure
Cumulative infiltration rates from a double ring infiltration test on a flat grass-covered surface (from Rahardjo et al, 2002)
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

20. Infiltration Estimation
Horton Infiltration Equation:
The infiltration capacity curve is the graphical representation of the variation of infiltration capacity with time, during and a little after rainfall
Since the curve is reaching a constant value and therefore it is a curve of the exhaustion type
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

21. Infiltration Estimation
Horton has suggested a mathematical form by which this curve is generally represented, as given below
Where,
f = infiltration rate at any time t
t = time from the beginning from rainfall
fc = the value of infiltration rate after it reaches a constant value
fo = infiltration rate at the start
K = a constant
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

22. Infiltration Estimation
Taking log on both sides;
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VAB2063 Hydrology
Chapter-4: Infiltration

23. Infiltration Estimation
Hence the above equation represents a straight line having a slope= -1/Klog10e
The –ve sign shows that as t increases, f decreases, and therefore (f - fc) decreases and hence log10 (f - fc) decreases
The straight line graph is shown in the adjacent figure
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

24. Infiltration Estimation
If two values of f at two known times are known and fc is also known, then a straight line can be drawn through these two points and the slope of the line can be obtained
Thus equating the slope to -1/Klog10e, K can be determined and hence an equation for infiltration capacity can be written
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

25. Chapter-4: Infiltration
Exercise-1:
The infiltration capacities of an area at different intervals of time are indicated in table below.
Find an equation for the infiltration capacity in the exponential form
Time t (hr)
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
Infiltration capacity f (cm/h)
10.4
5.6
3.2
2.1
1.5
1.2
1.1 1
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

26. Chapter-4: Infiltration
Solution-1:
Let the equation for the Infiltration Capacity curve be t
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00 f
10.4
5.6
3.2
2.1
1.5
1.2
1.1 1
f-fc
9.40
4.60
2.20
1.10
0.58
0.20
0.10
0.00
log10 (f - fc)
0.973
0.663
0.324
0.042
-0.301
-0.699 -1
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

27. Chapter-4: Infiltration
Solution-1:
From the graph: slope of the straight line
The infiltration Eqn. is;
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

28. Chapter-4: Infiltration
Green Ampt Equation (1911)
The Green Ampt method of infiltration estimation is based on Darcy’s law;
…(5.17)
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

29. Chapter-4: Infiltration
Green Ampt Equation
The volume of water in the soil is a product of the difference in the initial soil moisture content and the final moisture content times the depth of the percolating water
Substituting
In Equation (5.18) results
Setting Eqn. (5.17) equal to Eqn. (5.19) gives
…(5.19)
…(5.18)
…(5.20)
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

30. Chapter-4: Infiltration
Green Ampt Equation
When integrated with initial conditions H = 0, at t=0 and combined with Eqn. (5.18) yields
…(5.21)
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration

31. Chapter-4: Infiltration
Green Ampt Equation (1911)
Assumptions:
The soil surface is covered by a pool of water whose depth can be neglected
There is a distinctly defineable wetting front in the soil which can be viewed
Once the soil is wetted the water content in the wetted zone does not change as infiltration continues (i.e. hydraulic conductivity is constant)
There is a negative constant pressure just above the wetting front
UNIVERSITI TEKNOLOGI PETRONAS
VAB2063 Hydrology
Chapter-4: Infiltration