Chapter-4 Infiltration

1. Chapter-4Infiltration VAB2063 Hydrology Chapter-4: Infiltration

2. 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 VAB2063 Hydrology Chapter-4: Infiltration

3. 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 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 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 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 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 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 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 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 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 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 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 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) 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 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 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) 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 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) 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 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 VAB2063 Hydrology Chapter-4: Infiltration

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

25. 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 VAB2063 Hydrology Chapter-4: Infiltration

26. Solution-1: Let the equation for the Infiltration Capacity curve be VAB2063 Hydrology Chapter-4: Infiltration

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

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

29. 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 Green Ampt Equation …(5.19) …(5.18) …(5.20) VAB2063 Hydrology Chapter-4: Infiltration

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

31. 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 VAB2063 Hydrology Chapter-4: Infiltration