Water is constantly evaporated from the earth, and is precipitated back on the earth, mainly in the form of rainfall. One part of this rainfall sinks into the ground, forming groundwater reservoir; second major part flows as runoff in the form of rivers; and the rest is lost in evaporation and transpiration. The part of the rainfall which sinks into the ground is discussed in this chapter.
Infiltration Process
It 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 infiltration 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 primary step in the natural groundwater recharge. Infiltration is the flow of water into the ground through the soil surface and the process can be easily understood through a simple analogy. Consider a small container covered with wire gauze, if water is poured over the gauze, a part of it will go to container and a part overflows. Further, the container can hold only a fixed quantity and when it is full no more flow into the container can take place. This analogy, though a highly simplified one, underscores two important aspects, viz., the maximum rate at which the ground can absorb water, the infiltration capacity and the volume of water that it can hold, the field capacity.
Factors Affecting Infiltration Rate
The major factors affecting the infiltration of water into the soil are,
1. Initial moisture content
2. Condition of the soil surface
3. Hydraulic conductivity of the soil profile
4. Texture
5. Porosity
6. Degree of swelling of soil colloids
7. Organic matter
8. Vegetative cover
9. Duration of irrigation or rainfall
10. Viscosity of water
The antecedent soil moisture content has considerable influence on the initial rate and total amount of infiltration, but decreasing as the soil moisture content rises. The infiltration rate of any soil is limited by any restraint to the flow of water into and through the soil profile. The soil layer with the lowest permeability, either at the surface or below it, usually determines the infiltration rate. Infiltration rates are also affected by the porosity of the soil which is changed by cultivation or compaction. Cultivation influences the infiltration rate by increasing the porosity of the surface soil and breaking up the surface seals. The effect of tillage on infiltration usually lasts only until the soil settles back to its former condition of bulk density because of subsequent irrigations. Infiltration rates are generally lower in soils of heavy texture than in soil of light texture. It has been established that in surface irrigation, increased depth increases initial infiltration slightly but the depth of application has negligible effect after prolonged irrigation. Infiltration rates are also influenced by the vegetal cover. Infiltration rates on grassland are subsequently higher than bare uncultivated land. Addition of organic matter increases infiltration rate substantially. The hydraulic conductivity of soil profile often change during infiltration, not only because of increasing moisture content, but also because of the puddling of the surface caused by reorientation of surface particles and washing of finer materials into the soil. Viscosity of water influences infiltration. The high rates of infiltration in the tropics under otherwise comparable soil conditions are due to the low viscosity of warm water.
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:
1. Flooding-type infiltrometer
2. Rainfall simulator
Flooding-Type lnfiltrometer
This is a simple instrument consisting essentially of a metal cylinder, 30 cm diameter and 60 cm long, open at both ends. This cylinder is driven into the ground to a depth of 50 cm (Fig.14.1). Water is poured into the top part to a depth of 5 cm 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 lime is obtained. The experiments are continued till 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 capacity vs time is obtained. The experiments are continued till a uniform rate of infiltration is obtained and this may take 2-3 h.
Fig. 14.1.Simple Infiltrometer.
The surface of the soil is usually protected by a perforated disk to preventformation of turbidity and it’s settling on the soil surface.A major objection to the simple infiltrometer as above is that the infiltered waterspreads at the outlet from the tube (as shown by dotted lines in Fig. 14.1) and assuch the tube area is not representative of the area in which infiltration takesplace.
Fig. 14.2.Ring Infiltrometer.
To overcome this ring infiltrometer consisting of a set of two concentric rings (Fig. 14.2) is used. In this two rings are 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 infiltering water of the inner ring and hence prevents the spreading out of the infiltering water of the inner tube. The measurement of water volume is done on the inner ring only.
The main disadvantages of flooding-type infiltrometer are:
(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 thelarger meters giving fewer rates than the smaller ones; this is due to the border effect.
Rainfall Simulator
In this a small plot of land, of about 2 m X 4 m size, is provided with a size 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 m 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 is calculated. If the rainfall intensity is higher than the infiltration rate, infiltration-capacity values are obtained.
Rainfall simulator type infiltrometers give 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.
Infiltration indices
Hydrological calculations involving floods it is found convenient to use a constant value of infiltration rate for the duration of the storm. The average infiltration rate is called infiltration index and two types of indices are in common use.
Φ-index
The Φ index is the average rainfall above which the rainfall volume is equal tothe runoff volume. The Φ index is derived from the rainfall hyetograph with theknowledge of the resulting runoff volume. The initial loss is also considered as infiltration. The Φ value is found by treating it as a constant infiltration capacity. If the rainfall intensity is less than 0, then the infiltration rate is equal to the rainfall intensity; however, if the rainfall infiltration. TheΦ value is found by treating it intensity is larger than Φ the difference between rainfall and infiltration in an interval of time represents the runoff volume (Fig. 14.3).
Fig. 14.3.Φ-index.
The amount of rainfall in excess of the Φ index is called rainfall excess. The Φ -index thus accounts for the total abstraction and enables runoff magnitudes to be estimated for a givenrainfall hyetograph.
W- Index
In an attempt to refine the Φ-index the initial losses are separated from the total abstraction and an average value of infiltration rate called the W index is defined as
(14.1)
Where, P is total precipitation (cm), R is total storm runoff (cm), Ia is initial losses (cm), te is the duration of the rainfall excess, i.e. the total time in which the rainfall intensity is greater than W (in hours) and W is the average rate of infiltration (cm/h). Since Ia values are difficult to obtain, the accurate estimation of the W index is ratger difficult. The minimum value of the W index obtained under very wet soil conditions, representing the constant minimum rate of infiltration of the catchment, is known as Wmin. Both theW-index and Φ index vary from storm to storm.
