The Science of Surface and Ground Water
Subsurface Movement of Water
2.5.0 Introduction: hydrologic cycle, which is a continuous process of transformation
of water in the form of water vapour as it evaporates from land and ocean, drifts away
to clouds and condenses 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 which 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 within the earth. In fact, the ground water reserve is
actually a huge source of fresh water and is many times that of surface water. water is
present in the pores of soils and fissures of rock up to a depth beyond
which there is solid rock with no gaps which can store water. Although water is present
in the pores of the soil and permeable rocks, there is difference between that stored
above the water table and below it. The soil above the water table has only part of the
voids filled up with water mol
Some ofthe definitions related to subsurface water are as follows:
o Soil water: The water stored in the upper layers of the soil from the ground
surface up to the extent of roots of plants
o Vadose water: That stored below in the region between soil water zone and the
capillary fringe. It is a link between water infiltrating from the ground surface and
moving down to the saturated layer of ground water
o Capillary water: That which has risen from the saturated ground water region
due to capillary action. Naturally, the pressure here would be less than
atmospheric.
o Ground water: This is the water in the fully saturated zone. Pressure of water
here would be more than atmospheric.
o Water table: An imaginary surface within ground below which all the voids ofthe
soil or permeable rock are completely filled with water. Below this imaginary
surface, the pore water pressure is atmospheric. As one moves downwards from
the water table, the pressure increases according to the hydrostatic law. Above
the water table, the voids of soil/porous rock are only partially saturated with
water clinging to the surface of the solids by surface tension. Hence, the
pressure here is sub-atmosphere.
2.5.2 Water pressure in unsaturated zone
the term ‘ground water fiow’ is used generally to describe the flow of water
in the saturated portion of soil or fractured bedrock. No doubt it is important from
the
point of extraction of water from the zone using wells, etc. But the unsaturated zone,
too, is important because of the following reasons:
o The water in the unsaturated zone is the source of moisture for vegetation (the
soil water)
o This zone is the link between the surface and subsurface hydrologic processes
as rain water infiltrates through this zone to recharge the ground water.
o Water evaporated or lost by transpiration from the unsaturated zone (mainly from
the soil water zone) recharges the atmospheric moisture.
Further, the process of infiltration, quite important in hydrologic modeling catchment,
is
actually a phenomenon occurring in the unsaturated zone.

At the water table, the pressure head is zero (that is
atmospheric), that in the unsaturated zone is positive.
Movement of water in unsaturated zone
The negative pressure head in the unsaturated zone of the soil can be metaphorically
expressed as the soil being “thirsty”. All the pores of the soil here are not filled
up.
Hence, as soon as water is applied to the soil surface, it is “Iapped up” by the soil
matrix. Only if the water is applied in excess of the amount that it can “drink”, would
water fiow over the land surface as surface runoff. This capacity of the soil in the
unsaturated portion to absorb water actually depends on the volumetric water content, 0
expressed as:
9 Zn
V
Where Vw is the volume of water and V is the unit volume of soil or rock.
What happens to the water that got absorbed (that is infiltrated) at the surface of the
unsaturated soil during application of water from above? It moves downward due to
gravity through inter connected pores that are filled with water. Vlfith increasing water
content, more pores fill, and the rate of downward movement of water increases.
A measure of the average rate of movement of water within soil (or permeable bed rock)
is the hydraulic conductivity, indicated as ‘K’, and has the unit of velocity. Though
it is
more or less constant for a particular type of soil in the saturated zone, it is
actually a
function of the moisture content in the unsaturated portion of the soil. As 0
increases,
so does K, and to be precise, it should correctly be written as K(0), indicating K to
be a
function of 0. Figure 5 shows such a typical relation for an unsaturated soil.
Version 2 CE HT, Kharagpur
oo
-5
2><’l U
U 0.1 0.2 0.3
Water Content (8)
FIGURE 5. Variation of Hydraulic Cionductivity (K)
with Water Content (3)
Actually, the moisture potential (IIJ) is also a function ofe, as shown in Figure 6.
FIGURE 6. Variation of Moisture Potential and Water Content
Version 2 CE HT, Kharagpur
The relationship of unsaturated hydraulic conductivity and volumetric water content is
determined experimentally. A sample of soil placed in a container. The water content is
kept constant and the rate at which water moves through the soil is measured. This is
repeated for different values of 0 (that is different saturation levels). It must be
recommended that both K and IIJ very with 0 and by its very nature, unsaturated fiow
involves many changes in volumetric moisture content as waves of infiltration pass.
The movement of a continuous stream of water infiltrating from the ground into the
unsaturated soil may be typically seem to be as shown in F
FIGURE 7. Variation of soil moisture for water infiltrating from surface

(a) lntial condition, water is yet to be applied
(b) Soon after water has been applied by ponding
(c) Some time after ponding
Ifthe source of the water is now out off, then the distribution of water content with
depth
may look like as shown in Fig
SATURATED
SOIL
(Bl (bl (Cl
FIGURE 8. Variation in soil water content after the ponded water is exhausted
(a) Soon after
(b) Some time after
(c) A long time after
2.5.4 Movement of water in saturated zone
The water that infiltrates through the unsaturated soil layers and move vertically
ultimately reaches the saturated zone and raises the water table. Since it increases
the
quantity of in the saturated zone, it is also termed as ‘recharge’ ofthe ground water.

FIGURE 9. Variation of ground water table: (a) Before infiltration;
(b) During lsoon alter infiltration
Version 2 CE HT, Kharagpur
It may be observed from Figure 9 that both before any infiltration took place, there
existed a gradient of the water table which showed a small gradient towards the river.
However, the rise of the water table after the recharge due to infiltrating water is not
uniform and thus the gradient of the water table after recharge is more than that
before
recharge. This has a direct bearing on the amount of ground water flow, which is
proportional to the gradient. Based on actual observation or on mathematical analyses,
we may draw lines of equal hydraulic head (the equipotentials) within the saturated
zone, as shown in Figure 10. We may also draw the fiow lines, which would be
perpendicular to the equipotential lines. The flow lines, indicating the general
direction
of flow within the saturated soil zone is also drawn in the figure.

FLOWLINES
& EQUIPOTENTIAL
LINES
FIGURE 10. Equipotential lines and Flow lines of ground water movement:
(a) Before infiltration, with a low water table
(b) During I’ soon after infiltration, with a raised water table
The rate of movement of the ground water, of course, varies with the material through
which it is flowing since actually the flow is taking place through the voids which is
different for different materials. The term hydraulic conductivity of a porous medium
is used to indicate the ease with which water can flow through it. It is defined as the
discharge taking place through a flow tube (which may be thought of as a short pipe

along a fiow line) per unit area of the tube under the influence of unit hydraulic
gradient
(which is the difference of potential heads in unit distance along the fiow line).
Hydraulic
conductivity is generally denoted by ‘K’ and if the porous material is homogeneous,
then
K is also likely to be the same in any direction. However, in nature, the soil layers
are
often formed in layers resulting in the hydraulic conductivity varying between
different
directions. Even porous bed rock, which is usually fractured rock, may not be fractured
to the same extent in all directions. As a result, in many natural fiows the fiow is more
in
some preferential direction. This type of conducting media is referred to as being
heterogeneous and the corresponding hydraulic conductivity is said to be anisotropic.
d any such body. Here we
qualitatively discuss the fiow of ground water through a few examples which show the
relative interaction between the fiow and the geological properties of the porous
medium. Here, the two — dimensional plane is assumed to be vertical
2.5.6 Water table contours and regional flow
For a region, like a watershed, if we plot (in a horizontal plane) contours of equal
hydraulic head ofthe ground water, then we can analyse the movement ofground water
in a regional scale.
GROUND WATER CONTOURS
FLOW LINES
:TRACE OF RIVERS
2.5.7 Aquifer properties and ground water flow
Porosity
Ground water is stored only within the pore spaces of soils or in the joints and
fractures
of rock which act as a aquifers. The porosity of an earth material is the percentage of
the rock or soil that is void of material. It is defined mathematically by the equation
100v,
n = T
v
(2)
Where n is the porosity, expressed as percentage; v., is the volume of void space in a
unit volume of earth material; and vis the unit volume of earth material, including
both
voids and solid.
Specific Yield
While porosity is a measure ofthe water bearing capacity of the formation, all this
water
cannot be drained by gravity or by pumping from wells, as a portion ofthe water is held
in the void spaces by molecular and surface tension forces. Ifgravity exerts a stress
on
a film of water surrounding a mineral grain (forming the soil), some of the film will
pull
away and drip downward. The remaining film will be thinner, with a greater surface
tension so that, eventually, the stress ofgravity will be exactly balanced by the

surface
tension (Hygroscopic water is the
surface tension). Considering the
ofthe volume of water that drains
of
gravity to the total volume ofthe

moisture clinging to the soil particles because of
above phenomena, the Specific Yield (8,) is the ratio
from a saturated soil or rock owing to the attraction
aquifer.

Iftwo samples are equivalent with regard to porosity, but the average grain size of one
is much smaller than the other, the surface area ofthe finer sample will be larger. As a
result, more water can be held as hygroscopic moisture by the finer grains.
The volume of water retained by molecular and surface tension forces, against the force
of gravity, expressed as a percentage of the volume of the saturated sample of the
aquifer, is called Specific Retention 8,, and corresponds to what is called the Field
Capacity.
Hence, the following relation holds good:
n=Sy +5, (3)
Specific storage (ss)
Specific storage (ss), also sometimes called the Elastic
amount of water per unit volume of a saturated formation
from
storage owing to compressibility of the mineral skeleton
change in potentiometric head. Specific Storage is given

Storage Coefficient, is the
that is stored or expelled
and the pore water per unit
by the expression

S. = Mac + nfl) (4)
where y is the unit weight of water, or is the compressibility ofthe aquifer skeleton;
n is
the porosity; [3 is the compressibility of water.
Specific storage has the dimensions of length"
The storativity (8) of a confined aquifer is the product ofthe specific storage (85)
and
the aquifer thickness (b).
s = bs, (5)
All ofthe water released is accounted for by the compressibility ofthe mineral skeleton
and pore water. The water comes from the entire thickness ofthe aquifer.
In an unconfined aquifer, the level of saturation rises or falls with changes in the
amount
of water in storage. As water level falls, water drains out from the pore spaces. This
storage or release due to the specific yield (Sy) of the aquifer. For an unconfined
aquifer, therefore, the storativity is found by the formula.
S :5, +hS, (6)
Where h is the thickness of the saturated zone.
Since the value of 8,, is several orders of magnitude greaterthan hs, for an unconfined
aquifer, the storativity is usually taken to be equal to the specific yield.
2.5.8 Aquifers and confining layers
It is natural to find the natural geologic formation of a region with varying degrees of
hydraulic conductivities. The permeable materials have resulted usually due to
weathering, fracturing and solution effects from the parent bed rock. Hence, the

physical size of the soil grains or the pre sizes of fractured rock affect the movement
of
ground water flow to a great degree. Based on these, certain terms that have been
used frequently in studying hydrogeology, are discussed here.
o Aquifer: This is a geologic unit that can store and transmit water at rates fast
enough to supply reasonable amount to wells.
0 Confining layers: This is a geologic unit having very little hydraulic conductivity.
Confining layers are further subdivided as follows:
0 Aquifuge: an absolutely impermeable Iayerthat will not transmit any water.
0 Aquitard: A layer of low permeability that can store ground water and also
transmit slowly from one aquifer to another. Also termed as “Ieaky
aquifer’.
o Aquiclude: A unit of low permeability, but is located so that it forms an
upper or lower boundary to a ground water flow system.
Aquifers which occur below land surface extending up to a depth are known as
unconfined. Some aquifers are located much below the land surface, overlain by a
confining layer. Such aquifers are called confined or artesian aquifers. In these
aquifers, the water is under pressure and there is no free water surface like the water
table of unconfined aquifer.