Objectives: To understand infiltration processTo know how to measure infiltrationTo estimate infiltration using Horton’s equation and infiltration index

INFILTRATION

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.

Fig. 1 An analogy for infiltration

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 gauze as in Fig. 1. If water is poured over the gauze, a part of it will go into the 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, through a highly simplified one, underscores two important aspects, viz., (i) the maximum rate at which the ground can absorb the water, the infiltration capacity and (ii) the volume of water that it can hold, the field capacity. Since the infiltrated water may contribute to groundwater discharge in addition to increasing the soil moisture, the process can be schematically modeled as in Fig. 2. (a) and (b). This figure considers two situations, viz., low intensity rainfall and high intensity rainfall, and is self-explanatory.

Fig. 2 An infiltration model

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. The actual rate of infiltration f can be expressed as

f = fc when i fc

and f = i when i < fc

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 fc are listed below.

1. Characteristic of soil2. Surface of entry

3. Fluid characteristicsMeasurement of infiltration

The experimental set up is called an infiltrometer. There are two kinds of infiltrometers:

1. Flooding-type infiltrometer (Fig. 3)2. Rainfall simulator.

Fig. 3 Simple infiltrometer

Infiltration capacity values

The typical variation of the infiltration capacity for two soils and for two initial conditions is shown in Fig. 4. It is clear from the figure that the infiltration capacity for a given soil decreases with time from the start of rainfall; it decreases with the degree of saturation and depends upon the type of soil. Horton (1930) expressed the decay of the infiltration capacity with time as

fct = infiltration capacity at any time t from start of the rainfall fco = initial infiltration capacity at t = 0fcf = final steady state value td = duration of rainfall andKh = constant depending upon the soil characteristics and vegetation cover.

Fig. 4 Variation of infiltration capacity

Infiltration indices

The average infiltration rate is called infiltration index.

Index

The index is the average rainfall above which the rainfall volume is equal to the runoff volume. The index is derived from the rainfall hyetograph with the edge of the resulting runoff volume. The initial loss is also considered as infiltration. The value is found by treating it is a constant infiltration capacity. If the rainfall intensity is less than , then the infiltration rate is equal to the rainfall intensity; however, if the rainfall intensity is larger than the difference between rainfall and infiltration in an interval of time represents the runoff volume (fig. 5). The amount of rainfall in excess of the index is called rainfall excess. The index thus accounts for the total abstraction and enables runoff magnitudes to be estimated for a given rainfall hyetograph.

Fig. 5 Index