Subsurface Movement of Water

Instructional Objectives
At the end of this lesson, the
student shall be able to learn
1.
The ways by which water moves below ground.
2.
How water is stored underground in soil and fractured bedrock.
3.
What is water table and its relation to saturated and unsaturated ground water.
4.
Pressure measurement in uns
aturated and saturated soil water.
5.
The theory behind movement of
water in the unsaturated zone.
6.
The theory behind movement of water in the saturated zone, also called as
ground water.
7.
What are equipotential and flow lines.
8.
What are aquifers and confining layers.
9.
Qualitatively, the ground water flow
movement in confined and unconfined layers.
10.
The definitions of water storage, and t
he portion of that wh
ich can be withdra3wn
from an aquifer.
2.5.0 Introduction
In the previous lectures of
this module, we talked about th
e hydrologic cycle, which is a
continuous process of transfo
rmation of water in the form of water vapour as it
evaporates from land and ocean, dr
ifts away to clouds and cond
enses to fall as rain. Of
the rain falling over the land surface, a part of it infiltrates into the soil and the balance
flows down as surface runoff. From the point
of view of water
resources engineering,
the surface water forms a direct source wh
ich is utilized for a variety of purposes.
However, most of the water that infiltrates into the soil travels down to recharge the vast
ground water stored at a depth wit
hin the earth. In fact,
the ground water reserve is
actually a huge source of fres
h water and is many times that of surface water. Such
large water reserves remain mostly untapped though locally or regionally, the
withdrawal may be high. Actually, as a result
of excess withdrawal of ground water in
many places of India (and also of the world)
, a number of problems have arisen. Unless
the water resources engineer is
aware of the consequent damag
es, this type of situation
would lead to irreversible change in the
quality and quantity of subsurface water which
likely to affect our future generations.
In this lecture it is proposed to study how t
he water that infiltrates into the soil and the
physics behind the phenomena. We have
deliberately separated the study of
subsurface movement of water from that of
surface flow, as discussed in the earlier
lectures, because of the fact that the scale of mo
vement of these two types of flows can
vary by an order of magnitude
10 to more than 1000! This
would be clear from Figure