Hydraulic conductivity is a measure of the ability of a fluid to flow through a porous
medium and is determined by the size and shape of the pore spaces in the medium and
their degree of interconnection and also by the viscosity of the fluid. Hydraulic
conductivity can be expressed as the volume of fluid that will move in unit time under
a unit hydraulic gradient through a unit area measured at right angles to the direction
Stream flow and groundwater flow
If the unsaturated zone of the soil is uniformly permeable, most of the infiltrated
water percolates vertically. Infiltrated water that reaches the ground water reserve
raises the water table. This creates a difference in potential and the inclination of the
water table defines the variation of the piezometric head in horizontal direction. This
difference in energy drives the ground water from the higher to the lower head and
some of it ultimately reaches the stream flowing through the valley. This contribution
of the stream flow is known as Base flow, which usually is the source of dry-weather
flow in perennial streams.
During a storm event, the overland flow contributes most of the immediate flow of the
stream. The total flow of the stream, however, is the sum ofoverland flow, interflow
and base flow. It must be remembered that the rates at which these three components
of runoff move varies widely. Stream flow moves fastest, followed by interflow and
then ground water flow, which may take months and sometimes even years to reach
Note that for some streams, the water table lies quite some distance below the bottom
of the stream. For these streams, there is a loss of water from the river bed percolating
into the ground ultimately reaching the water table. The reason for a low water table
could possibly be due to natural geographic conditions, or a dry climate, or due to
heavy pumping of water in a nearby area.
The hydrograph and hyetograph
As the name implies, Hydrograph is the plot of the stream flow at a particular location
as a function of time. Although the flow comprises of the contributions from overland
flow, interflow and groundwater flow, it is useful to separate only the groundwater
flow (the base flow) for hydrograph analysis, which is discussed in Lesson 2.3.
In Lesson 2.1, precipitation was discussed. The hyetograph is the graphical plot of the
rainfall plotted against time. Traditionally, the hyetograph is plotted upside down as
shown in Figure 3, which also shows a typical hydrograph and its components.
Splitting up of a complete stream flow hydrograph into its components requires the
knowledge of the geology of the area and of the factors like surface slope, etc.
Nevertheless, some of the simpler methods to separate base flow are described