INFILTRATION is well-known that 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 illustration and plays a very significant role in the runoff process by affecting the timing distribution and magnitude of the surface runoff.
Further, infiltration is the prim step in the natural groundwater recharge.
Infiltration is the flow of water into the go through the soil surface and the process can be easily understood through a simple analogy.
Consider a small container covered with wire gauze a$ in Fig. 3.8. If water is poured over the gauze, a part of it will go into the container and a part overflow.
Further, the container can hold only a fixed quantity and when it is full no more flow into the contain can take place.
This analogy, though a highly simplified one, underscores two important aspects, viz., (1) the maximum rate at which the ground can absorb water, the infiltration capacity and (ii) the volume of water that it can hold, the field 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 the infiltration f and is expressed in units of cm/h.
where i = intensity of rainfall.
The infiltration capacity of a soil is high at the beginning of a storm and has an exponential decay as the time elapses.
The infiltration process is affected by a large number of factors and a few important ones affecting are described below.
Characteristics of Soil
The type of soil, viz, sand, silt or clay, its texture, structure, permeability and under drainage are the important characteristics under this category.
A loose, permeable, sandy soil will have a larger infiltration capacity than a tight, clayey soil.
A soil with good under drainage, i.e. the facility to transmit the infiltered water downward to a groundwater storage would obviously have a higher infiltration capacity.
When the soils occur in layers, the transmission capacity of the layers determine the overall infiltration rate.
Also, a dry soil can absorb more water than one whose pores are already full.
The land use has a significant influence on fc For example, a forest soil rich in organic matter will have a much higher value of ft under identical conditions than the same soil in an urban area where it is subjected to compaction.
Surface of Entry
At the soil surface, the impact of raindrops causes the fines in the soils to be displaced and these in turn can clog the pore spaces in the upper layers. This is an important factor affecting the infiltration capacity. Thus a surface covered by grass and other Vegetation which can reduce this process has a pronounced influence on the value of
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.
The temperature of the water is a factor in the sense that it affects the viscosity of the water which in turn affects the infiltration rate. Contamination of the water by dissolved salts can affect the soil structure and in turn affect the infiltration rate.
MEASUREMENT OF INFILTRATION
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 an infiltrometer.
There are two kinds of infiltrometers:
This is a simple instrument consisting essentially of a metal cylinder, 30cm diameter and 60cm long, open at both ends.
This cylinder is driven into the ground to a depth of 50cm (Fig. 3.10). Water is poured into the top part to a depth of 5cm and a pointer is set to mark the water level.
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 the infiltration capacity vs time is obtained.
The experiments are continued fill a uniform rate of infiltration is obtained and this may take 2-3 h.
The surface of the soil is usually protected by a perforated disk to prevent formation of turbidity and its settling on the soil surface.
A major objection to the simple infiltrometer as above is that the infiltered water spreads at the outlet from the tube (as shown by dotted lines in Fig. 3.10) and as such the tube area is not representative of the area in which infiltration takes place.
To overcome this a ring infiltrometer consisting of a set of two concentric rings (Fig 3.11) is used. In this two rings as inserted into the ground and water is maintained on the soil surface, in both the rings, to a common fixed level.
The outer ring provides a water jacket to the filtering water of the inner ring and hence prevents the spreading out of the f water of the inner tube. The measurements of water volume is done on the inner ring only.
Disadvantages of flooding-type infiltrometers:
1.The raindrop-impact effect is not simulated
2. The driving of the tube or rings disturbs the soil structure
3. The results of the infiltrometer depend to some extent on their size with the larger meters giving less rates than the smaller ones; this is due to the border effect.
In this a small plot of land, of about 2 m x 4 m size, is provided with a series of nozzles on the longer side with arrangements to collect and measure the surface runoff rate.
The specially designed nozzles produce raindrops falling from a height of 2 in and are capable of producing various intensities of rainfall.
Experiments are conducted under controlled conditions with various combinations of intensities and durations and the surface runoff is measured in each case.
Using the water-budget equation involving the volume of rainfall, infiltration and runoff, the infiltration rate and its variation with time are calculated.
If the rainfall intensity is higher than the infiltration rate, infiltration-capacity values are obtained. Rainfall simulator type infiltrometers given lower values than flooding type infiltrometers.
This is due to the effect of the rainfall impact and turbidity of the surface water present in the former.