Chapter 5
Groundwater Recharge and Discharge
Brian K. Rawlins
Department of Hydrology, University of Zululand, P Bag X1001,
KwaDlangezwa 3886. South Africa
The following competencies can be achieved on completion of this chapter:
 The learner will have an understanding of recharge and discharge
processes to and from aquifers
 The learner will be aware of various recharge estimation techniques
 The learner will be aware of artificial recharge methods and the use of
artificial recharge in the management of aquifers
 The learner will have an understanding of the interaction between
groundwater and surface water
In the previous sections, the flow equations in confined and unconfined
groundwater systems were derived using both the statement of conservation of
matter (mass) or the continuity equation and Darcy law. This chapter takes the
consideration of flow in aquifers further by looking at the recharge and discharge
mechanisms and processes that take place in aquifers, gives an overview of
recharge estimation techniques, gives an overview of artificial recharge concepts
and methodologies and also outlines various aspects of the interaction between
groundwater and surface water.
5.1
Natural Recharge and Discharge
5.1.1 Recharge and Discharge areas
Under natural conditions, an aquifer is usually in a state of dynamic equilibrium
with recharge and discharge processes acting together over time. It is unusual for
recharge and discharge to and from an aquifer to exactly balance at any given
time, but over the long-term these two processes usually balance with the volume
of water entering an aquifer being equalled by the volume of water that leaves the
aquifer. When recharge exactly equals discharge, the potentiometric surface
(water table) is steady and the amount of water in storage in the aquifer is
constant. Since recharge tends to be intermittent and governed by the variability
of precipitation and discharge is more constant, governed by the principles of
groundwater flow and the hydraulic characteristics of the aquifer, in the short term
the potentiometric surface rises and falls in response to the balance between
recharge and discharge.
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
The aquifer transmits water from recharge zones (which can be widespread) to
discharge areas (which are usually more isolated in space). The rate of the flow is
a function of aquifer characteristics such as the transmissivity and the hydraulic
conductivity and the potentiometric gradient (in unconfined aquifers this is
equivalent to the slope of the water table).
5.1.2 Recharge Processes
Natural recharge to an unconfined aquifer is essentially all derived from meteoric
sources, but this may be direct (from precipitation) or indirect (from precipitation
via surface water bodies or adjacent aquifers). Other contributions known as
“artificial recharge” occur where water is either deliberately or incidentally
recharged to aquifers through human activities. This component of recharge is
dealt with in the next section.
The amount of water that recharges an unconfined aquifer is determined by the
amount of water available for recharge, the vertical hydraulic conductivity of the
zone between the water table and the ground surface, and the transmissivity of
the aquifer and the hydraulic gradient which determines how quickly water moves
away from the recharge area.
a) Direct recharge from precipitation
In general terms, the proportion of precipitation infiltrating to the water table
depends largely on characteristics of the precipitation itself as well as the physical
characteristics of the ground surface (topography, vegetation) and the type and
structure of the soil and the underlying rocks as well as the antecedent soil
moisture conditions.
In theory, recharge from infiltrating precipitation should be greater for long
duration low intensity rainfall (as the likelihood of overland flow is diminished),
shallow water tables (as the depth of material through which infiltrating water must
pass is low), high soil moisture concentrations (as infiltrating water is free to drain
under gravity), and in soils with high hydraulic conductivity (as the rate of
movement is high) and/or low specific moisture capacity (as less water is held in
the soil against the force of gravity).
In reality infiltration to groundwater is isolated in both space and time and it
normally results only when there are favourable conditions of water table depth,
5-2
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
soil type, and antecedent soil moisture conditions together with significant
quantities of rainfall occurring over a significant length of time.
Attempts to estimate recharge simply from a knowledge of soil texture and
saturated hydraulic conductivity are not likely to give correct estimates. There is a
need for detailed information on the functional relationships between suction,
hydraulic conductivity, specific moisture capacity and moisture content since very
small differences in these properties can account for large differences in the
reaction of similar soils to the same infiltration event.
There might also be a process of displacement whereby the water which is added
to the water table during rainfall is not 'new' rainfall but previously stored rainfall
which has been displaced downwards by successive bouts of infiltration. This
helps explain the often rapid response of the water table to precipitation in areas
of low permeability and porosity.
b) Seepage from surface water storage
Recharge occurs when the groundwater body is in direct contact with an open
surface water body such as a river, lake, reservoir, canal or drain. There will
normally be some water movement between the two bodies as a result of a
difference in the potential of the two water bodies (Figure 5.1). Flow will be from
the surface water body (e.g. a stream) to the groundwater, if the elevation of the
surface water surface is above the adjacent water table. This is termed influent
seepage. The reverse situation, effluent seepage, occurs when the elevation of
the water table is higher than the elevation of the surface water body and in this
case, groundwater discharge will occur. The relationship between the two water
bodies is seldom static as both the water table elevation and the surface water
elevation fluctuate over time. Consequently, at a particular point of contact,
recharge and discharge may occur at different times.
Case A - Normal condition with the water table sloping towards the stream leads
to groundwater discharge (effluent seepage - covered in the next section)
Case B - During periods of high runoff, the water surface level rises above the
level of the adjacent ground water and this leads to recharge (influent seepage)
which will continue as long as the surface water level remains elevated and until
the water table has risen to equal the surface water level.
5-3
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
Case C - In arid or semi-arid areas where evapotranspiration losses far exceed
precipitation gains, the water table is often below the river bed level. Groundwater
recharge (influent seepage) will therefore result from this 'losing stream' during
the periods that it is flowing (generally after high intensity rainfall either in the
locality or further up the catchment). (The lower Nile in Sudan and Egypt, the
Okavango in Botswana, the lower Orange river in South Africa, the Sahel section
of the River Niger and many westerly flowing rivers in Namibia are typical
examples of this situation).
The rate of influent or effluent seepage in all of these cases will depend on
channel characteristics (shape, length of wetted perimeter, permeability of the
river bed) and water characteristics (temperature, quality, depth).
In ephemeral streams where flow occurs as flash floods after heavy rainfall, the
total flow may be completely absorbed by evaporation and influent seepage along
the length of the river. This influent seepage will lead to the development of
groundwater mounds beneath surface channels and depressions. These mounds
will generally have a markedly lower salt content than the main body of
groundwater and therefore will be preferable locations for groundwater extraction
in arid regions.
c) Recharge and discharge via groundwater leakage
As aquitards rarely form an absolute barrier to water movement there is always
some slow drainage of water from and to adjacent aquifers via intervening
5-4
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
aquitards. The magnitude of such flow can be difficult to quantify as it is
dependent upon many factors such as the relative hydraulic conductivity of the
aquifers and aquitards, their configuration, and the hydraulic gradient. Estimates
of this flow can therefore be seriously in error. However, since flow rates are likely
to be low in comparison to other forms of groundwater recharge, the importance
of this is small except in specific conditions.
5.1.3 Discharge Processes
Natural discharge from aquifers parallels recharge to aquifers and can be
classified into three components: evapotranspiration, effluent seepage by means
of spring flow and discharge direct to surface water bodies, and groundwater
leakage through aquicludes to adjacent aquifers (previously covered under
recharge). A further component of discharge, namely artificial abstraction through
boreholes and wells is dealt with elsewhere in this course.
a) Evapotranspiration
The magnitude and variability of discharge through evapotranspiration is
complicated and this can affect groundwater storage both directly and indirectly.
Directly, groundwater is abstracted via evapotranspiration only where the water
table is close enough to the ground surface for water vapour to leave the
groundwater body to the atmosphere or for plant roots to draw water from the
groundwater body or its capillary fringe. Indirectly, the processes of evaporation
and transpiration act to decrease the soil moisture content in the unsaturated
zone above the water table and this reduction in moisture content then has the
effect of reducing the effectiveness of recharge through infiltration.
Evaporation and transpiration are governed by physical factors such as
temperature, humidity, surface roughness and wind speed and it is beyond the
scope of this section to cover these processes in detail. Of particular importance
in the estimation of evapotranspiration losses from groundwater bodies are, in
addition to the meteorological and surface factors, the depth of the water table
below the surface, the type of the vegetation and the rooting depth.
As a result of evapotranspiration, where water tables are close to the surface,
short term direct effects can be seen as a diurnal fluctuation of shallow water
tables. In valley bottom areas and river flood plains the losses due to
evapotranspiration during the hottest part of the day can exceed the rate of
5-5
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
groundwater inflow from surrounding higher areas. This causes the water table to
drop. At night, when evapotranspiration rates drop, groundwater inflow
replenishes the area and the water table recovers. This cycle is maintained during
the hotter months of the year but is interrupted by periods of rainfall and reduced
evapotranspiration. If night time recovery equals daytime drawdown there will be
no long term effects, however if daily drawdown exceeds recovery there will be a
progressive drop of water table. This process will last as long as the groundwater
is capable of being evaporated, as the water table drops it will attain a level below
which capillary rise is unable to satisfy the transpiring demands of the surface
vegetation. Thus the progressive water table drop will follow an exponential curve.
b) Natural discharge via effluent seepage
This major form of groundwater discharge occurs where the upper surface of the
saturated zone intersects the surface. In areas where rainfall exceeds evaporation
there will be net effluent seepage as the water table slopes gently downwards
towards a surface water body (river, stream, lake, reservoir, canal, drain etc).
There will be a continuous discharge of groundwater to the surface water body.
The rate of discharge will depend on the head difference between the surface
water body and the adjacent water table (the hydraulic gradient), the hydraulic
conductivity of the aquifer material and the permeability of the river or lake bed
and banks.
Streams and lakes are major discharge points, however broadly distributed
seepages and springs (Figure 5.2) occur over lower valley slopes in many
instances. (Springs can be defined as a concentrated discharge at a point,
whereas seepages indicate a slower and wider spread movement of groundwater
to the surface). Discharge via springs is variable, some are perennial, others are
intermittent. Springs and seepages can move up and down slope in response to
the movement of the interface as groundwater storage increases and decreases
and springs are commonly located where a low permeability bed intersects the
ground surface. Springflow from thick porous aquifers is relatively constant as
volumes of storage change represent a small proportion of the total storage
volume. Thin aquifers can have very variable spring flows, with these often only
occurring immediately after heavy rainfall. It is also relevant that springs and
springflow can be disturbed by human use of them as a water source.
5-6
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
5.2
Recharge estimation methods
Many methods have been developed to estimate the quantity and rate of recharge
to aquifers. The selection of which method or combination of methods is most
appropriate to a particular situation is dependent upon both the availability of data
and on the purpose of the recharge estimation. Recharge estimation techniques
can be broadly categorised into surface methods, unsaturated zone methods,
water table methods and saturated zone methods. The first two methods
generally only provide estimates of potential recharge whereas the latter two
methods can provide estimates of actual recharge. Actual recharge means the
amount of water that actually reaches the saturated zone, whereas potential
recharge means water that has infiltrated that may or may not reach the water
table because of existing conditions within the overlying unsaturated region. For
instance, for a shallow water table, infiltrated water could be lost through
evaporation but would recharge the aquifer if the water table were at a lower
depth. In this section, a broad introduction will be given to all four categories and
it must be stressed that since there are considerable uncertainties inherent in all
recharge estimation methods and consequently when recharge to an aquifer is to
be estimated, it is advisable to use a combination of methods in order to increase
the confidence in the recharge estimation.
5.2.1 Surface methods
Groundwater recharge from surface water bodies generally occurs in arid regions
where surface water systems (streams and lakes) are separated from
groundwater systems by thick unsaturated sections. The surface water bodies in
these settings often form localized recharge sources and consequently the
5-7
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
groundwater recharge can be estimated from surface water data. In humid areas
where surface water systems are fed by groundwater discharge, recharge from
surface water bodies tends to be minimal and isolated in space and time.
Surface water gains or losses can be estimated using a channel water budget
based on river flow gauging data measured at two points in a stream. The
difference between the flow at the upstream site and the flow at the downstream
site will reflect the potential groundwater recharge (or discharge) once
evaporation and any surface inflow to or outflow from the channel has been
accounted for as illustrated in equation 5.1.
R  Qup  Qdown   Qin   Qout  Ea 
S
t
(5.1)
Where
R = recharge rate
Qup = The flow at the upstream site
Qdown = The flow at the downstream site
Qin = The inflow from tributaries or other sources along the reach
Qout = The outflow along the reach
Ea = The evaporation from the surface water or stream bed
S = The change in channel and unsaturated storage over time (t)
The resultant recharge (or transmission loss) reflects the potential recharge to the
underlying aquifer. This could in many cases be an overestimation of the actual
recharge as it does not include any subsequent evapotranspiration and assumes
that all water leaving the stream contributes to recharge to the aquifer (this might
not be the case if there are perched aquifers or if transmissivity is low.
An alternative to the channel water budget method would be to measure seepage
from a surface water body directly using seepage meters which measure the
infiltration rate under water bodies in a similar way to infiltrometers.
Tracer techniques have also been applied to identify and quantify groundwater
recharge from lakes and rivers. In rivers where rivers have headwaters at high
elevations, the river water is often depleted in stable isotopes (of oxygen and
hydrogen) relative to the local precipitation. If the rivers retain the depleted
isotopic signature of the headwaters, the difference between the isotopic
5-8
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
signatures of rivers and precipitation can be used to determine the relative
contribution of the two sources of groundwater recharge.
5.2.2 Unsaturated zone methods
Unsaturated zone techniques are applied mostly in semiarid and arid regions
where the unsaturated zone is generally thick. These techniques provide
estimates of potential recharge based on percolation rates below the root zone.
This may result in inaccuracies since the drainage rates through the unsaturated
zone may not always reflect the recharge rate at the water table as there may be
lateral movement of water within the unsaturated zone.
Lysimeters have been developed to measure the water balance within a section of
soil. The lysimeter consists of a container filled with disturbed or undisturbed soil
with or without a vegetation cover. This container is hydrologically isolated from
the surrounding soil. At the base of the lysimeter, drainage water is collected and
measured and this, in conjunction with precipitation and evaporation data will
allow estimates to be made of the percolation rate of water through the
unsaturated zone. It is difficult to extrapolate lysimeter results to derive recharge
estimates as the establishment of the lysimeter tends to distort actual field
conditions and in addition, an artificial horizon is created at the base of the
lysimeter which may affect the drainage of water. Consequently lysimeters are
more suitable for the measurement of evapotranspiration than recharge.
The zero-flux plane method simplifies the soil water budget by equating recharge
to changes in soil water storage below the zero-flux plane which represents the
plane where the vertical hydraulic gradient is zero (it separates upward
evapotranspirational flow from downward drainage movement). The rate of
change in the storage term between successive measurements is assumed to be
equal to the drainage rate to the water table (the recharge rate). The method
requires that soil matric-potential measurements are made to determine the
location of the zero-flux plane and that soil-water content measurements are
made to estimate storage changes over time. The method cannot be used when
the water flux is downward throughout the entire profile or when water storage is
increasing since the downward movement of a wetting front will mask the zeroflux plane. The technique is relatively expensive in terms of the equipment needed
and the amount of data required.
5-9
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
Darcy’s law can be used to calculate recharge in the unsaturated zone according
to equation 5.2:
R   K ( )
dH
d
  K ( ) ( h  z )
dz
dz
(5.2)
Where:
K() = the hydraulic conductivity at the ambient soil water content, 
H
= the total head
h
= the matric pressure head
z
= the elevation
This application of Darcy’s law requires measurements or estimates of the vertical
total-head gradient and the unsaturated hydraulic conductivity at the ambient soilwater content. For thick unsaturated zones, below the zone of fluctuations related
to climate, in uniform or thickly layered porous media, the matric pressure gradient
is often nearly zero, and water movement is essentially gravity driven. Under
these conditions, little error results by assuming that the total head gradient is
equal to 1 (the unit-gradient assumption). This removes the need to measure the
matric pressure gradient and sets recharge equal to the hydraulic conductivity at
the ambient water content.
Tracer techniques can be applied in the unsaturated zone where chemical or
isotopic tracers are applied as a pulse at the soil surface or at some depth within
the soil profile to estimate recharge. As precipitation infiltrates, the tracer is
transported through the profile. The vertical distribution of tracers is used to
estimate the velocity of water movement and the recharge rate.
Environmental tracers such as chloride which are produced naturally in the
Earth’s atmosphere can be used to estimate recharge rates. The mass of chloride
into the system (from precipitation and dry fallout, P) times the chloride
concentration in P (Cp) is balanced by the mass out of the system (drainage, D)
times the chloride concentration in drainage water in the unsaturated zone (Cuz) if
surface runoff is assumed to be zero (equations 5.3 and 5.4).
PC p  DCuz
(5.3)
5-10
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
D
PC p
Cuz
(5.4)
Chloride concentrations generally increase through the root zone as a result of
evapotranspiration and then remain constant below this depth. Drainage is
inversely related to chloride concentration in the unsaturated zone pore water.
This inverse relationship results in the chloride mass balance (CMB) approach
being much more accurate at low drainage rates because chloride concentrations
change markedly over small changes in drainage.
5.2.3 Water table methods
The water table fluctuation method is based on the premise that rises in
groundwater levels in unconfined aquifers are due to recharge water arriving at
the water table. Recharge is calculated as shown in equation 5.5
R  Sy
dh
h
 Sy
dt
t
(5.5)
Where
Sy = the specific yield
h = the water table height
t = time
This method is best applied over short time periods in regions having shallow
water tables that display sharp rises and declines in water levels. Difficulties in
applying this method are related to determining a representative value for specific
yield and ensuring that groundwater level fluctuations are directly related to
recharge and not as a result of changes in atmospheric pressure, the presence of
entrapped air, pumping or other phenomena.
5.2.4 Saturated zone methods
Using Darcy’s law, the flow through a cross-section of an aquifer can be
estimated. If steady flow conditions exist and there is no water extraction. The
subsurface volumetric flux (q) through a vertical cross section of an aquifer (of
cross sectional area A) is equated to the recharge rate (R) multiplied by the
surface area that contributes to the flow (S) (equation 5.6). This only is applicable
if the water level remains constant. Recharge estimates based on this method are
highly uncertain because of the high variability of hydraulic conductivity.
5-11
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
qA  RS
(5.6)
Groundwater recharge may be estimated by means of dating the water in the
saturated zone. Historical tracers or event markers such as bomb-pulse tritium
(3H) can be used to estimate recharge in both the saturated and the unsaturated
zones. The use of 3H to date groundwater is generally being replaced by the use
of tracers such as chloroflurocarbons (CFCs) and tritium/helium-3 (3H/3He). These
gas tracers can only be used in the saturated zone where they can no longer
exchange with the atmosphere. The first appearance of tracers such as CFCs or
3H/3He can be used to estimate recharge rates where flow is primarily vertical.
Recharge rates can also be determined by estimating ages of groundwater where
the age is the time since the water entered the saturated zone. Groundwater ages
are estimated by comparing CFC concentrations in groundwater with those in
precipitation.
In unconfined porous media aquifers, groundwater ages increase with depth, the
rate of which depends on aquifer geometry, porosity and recharge rate. The
vertical groundwater velocity decreases with depth to zero at the lower boundary
of the aquifer. The age increases linearly with depth near the water table and
nonlinearly at greater depths. Near the water table, the influence of aquifer
geometry is greatly reduced. The recharge rate can be determined by dating
water at several points in a vertical profile, calculating the groundwater velocity by
inverting the age gradient and extrapolating this velocity to the water table and
multiplying the velocity by the porosity for the depth interval.
5.3 Artificial Recharge
Artificial recharge, which can be conducted in a variety of ways and may be
deliberate or incidental, is becoming increasingly important in groundwater
management and in conjunctive use of surface water and groundwater resources.
Artificial recharge is used to reduce, stop or reverse declines of groundwater
levels and it enables the storing of surplus surface water (from rivers and
reservoirs as well as from storm runoff in urban areas) below the ground either for
a variety of reasons. These reasons include storing the water for future use,
counteracting the development of unwanted groundwater conditions such as
saline intrusion, and also as part of a treatment process for wastewater.
5.3.1 Methods of artificial recharge
There are two major forms of artificial recharge, spreading and the use of pits or
wells. Artificial recharge by the spreading method consists of increasing the
5-12
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
surface area of infiltration by releasing water through flooding or irrigation from the
source to the surface of a basin, pond, stream channel, or a ditch and furrow
system. This is certainly the most efficient and most cost-effective method for
aquifer recharge. However, these methods require large surface areas to
accommodate the recharge scheme, allowing water to evaporate if percolation in
the ground is slow. Pit and well methods of artificial recharge involve the
construction of wells or boreholes specifically for the purpose of encouraging
recharge into aquifers either using only the force of gravity or injecting water
under pressure to achieve the end.
a)
Basin method
The basin method operates by releasing surplus surface water into artificially
created shallow basins (either formed by digging or by constructing walls around
the basin). Since the permeability of the bed of the basin is of paramount
importance in determining the rate of infiltration from the basin, it is important that
the water is silt free (as this silt will settle out and form a lining to the basin). Even
so, the basins do need maintenance which might involve breaking up the bed to
increase the permeability of the bed and therefore enhance infiltration capacity.
Basins are often found within urban areas where storm runoff is directed through
a storm drainage system. Multiple linked basins are also used where water is fed
into a series of basins from which it may infiltrate (Figure 5.3).
5-13
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
The advantage of multiple basin systems is that at any one time, one or more
basins may be out of use for cleaning and maintenance while the other basins in
the system continue to provide infiltration potential. Recharge rates may reach up
to 2m per day from well designed basins situated on ideal soil conditions.
b)
Stream channel method
In arid areas where streams are naturally “losing streams” which recharge the
aquifers beneath them, recharge may be enhanced by constructing weirs or check
dams using gabions across the width of the rivers. These structures are often “L”
shaped (Figure 5.4) and these have the effect of retaining water over the whole
width of the river at a reasonable depth. As with the basin method, proper
maintenance of the river bed will assist in maintaining the infiltration capacity. If
reservoirs exist upstream of such a scheme then water may be released into a
system such as this at a rate that does not exceed the absorptive capacity of the
channel.
c)
Ditch and furrow method
In this method, water is released into a network of flat bottomed closely spaced
ditches or furrows. In this way the water flows along the ditches infiltrating as it
goes. The ditches should have a downward gradient that is sufficient to keep fine
grained material from settling out which would reduce the permeability of the ditch
beds. The networks of ditches can be such that they follow contours, alternatively
they could be dendritic (tree-shaped), or lateral with smaller ditches leading off a
main stem in a purpendicular fashion (Figure 5.5).
5-14
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
d)
Flooding method
If the topography is relatively flat, water may be diverted from a source to spread
evenly over a wide area such as a flood plain. The water should flow at a
minimum velocity and infiltration rates are greatest when the natural vegetation
and soils are left undisturbed
e)
Irrigation method
Artificial recharge through irrigation often occurs incidentally in any area where
irrigation takes place. If more water is applied to irrigated lands than the irrigated
crops can use then the excess water will infiltrate through to the underlying
aquifer. In addition, artificial recharge from irrigation may be deliberate and be
conducted during periods when plant activity is dormant such as during winter. In
all cases, however, care should be taken to ensure that this “over irrigation” does
not result in the salts in the upper layers of the soils being leached out which
would possibly reduce crop yields.
f)
Pit method
A pit excavated into a permeable formation can serve as an ideal facility for
artificial recharge (Figure 5.6). The advantages of a pit over a shallow basin are
that the pit may penetrate through a layer of low permeability close to the ground
surface which would render basins inoperable, additionally with a pit, infiltration
into the aquifer will occur both through the walls of the pit as well as the base.
This means that even if the base of the pit becomes clogged through the
deposition of fine material, recharge will continue through the side walls. The
depth of these pits may range from 2 to 3 m up to 30 to 40 m. Pits may also be
5-15
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
excavated for the specific purpose of artificial recharge or existing abandoned
excavations such as gravel pits or quarries may be used. Aquifer recharge simply
consists of diverting water from the main channel to the pit. Even with a deep pit,
it may be advisable to have a smaller settling pit between the main channel and
the larger recharge pit. Both recharge and settling pits should be fenced and have
a suitable inlet so that the inflowing water does not erode the walls of the pits.
g)
Recharge well method
A recharge well is essentially the reverse of a pumping well and these may be
used to recharge both confined and unconfined aquifers. These wells do not
require a large surface area as do most of the foregoing methods so they are
ideal for establishment in an urban area where land area is at a premium.
As water is pumped into a recharge well, a cone of recharge develops around the
hole in a similar fashion (but the reflection of) the cone of depression that occurs
around a pumping well. Recharge rates rarely match potential pumping rates and
care should be taken to ensure that the recharge water is free of silt or any other
contaminant as this may clog the zone surrounding the well or lead to pollution of
the aquifer.
h)
Incidental recharge
Incidental recharge from a variety of human activities often occurs. In many cases
this incidental recharge is unwanted as it leads to the contamination of aquifers.
Water loss from broken sewers, septic tanks, latrines, soakaways, landfills and
5-16
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
waste disposal facilities are prime examples of this and care should be taken to
minimise this recharge. In other cases, incidental recharge may be beneficial (if
expensive) where water loss from broken water mains or canals can recharge
aquifers.
5.3.2 Artificial Recharge as a groundwater management tool
Artificial recharge has several roles to play in the management of groundwater.
These include its role in the conjunctive use and management of surface water
and groundwater resources, the role it plays in preventing or controlling saline
water intrusion, and the role played in the treatment of wastewater for future use.
These aspects are covered more comprehensively in the groundwater
management module so here these roles are just briefly described.
a) Artificial recharge as a tool in conjunctive use
In basins approaching full development of water resources, optimal beneficial use
can be obtained by conjunctively using both surface water and groundwater
resources. This involves a coordinated and planned operation of both resources in
such a manner that water requirements are met and water is conserved. The
separate firm yields of both resources are replaced by a larger and more
economic joint yield of the two resources. It is beyond the scope of this module to
fully investigate the methodologies and process of conjunctive use as these are
covered in the groundwater management module, however artificial recharge is
an integral component of such schemes as surplus or excess surface water is
recharged to aquifers through the methods described above. During periods of
above average precipitation or when surface water reservoirs have more water in
storage than is required immediately, aquifers are recharged with this water to
augment groundwater storage and raise groundwater levels. During drought
periods or when surface water resources are insufficient to meet demands, the
groundwater resource is exploited to meet the demand. In this way it is possible to
both reduce water losses (through evaporation from surface storage facilities) and
to operate the groundwater resource in a manner that would not be sustainable if
artificial recharge did not take place.
b) Artificial recharge as used to manage saline water intrusion
In coastal regions where groundwater extraction takes place, the possibility of
disturbing the balance between fresh groundwater and saline groundwater exists.
This may cause the interface between the two to migrate landwards and this could
lead to the deterioration of the aquifer as a water resource. It is common for a
5-17
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
problem to develop before the situation is fully understood and so often artificial
recharge schemes are implemented to reverse or halt this migration of saline
water. Through artificial recharge either using spreading or injection techniques a
mound of fresh water can be maintained along the coast that will restrict saline
water intrusion. This obviously requires that a supplemental water source be
located and that this source has high quality water.
c) Treatment of waste water through artificial recharge
Artificial recharge as a means of treating waste water and also augmenting
groundwater resources is practised in phreatic aquifers under certain conditions.
While it may seem unwise to deliberately allow polluted waste water to enter the
groundwater domain, if conditions are suitable then the waste water will be
naturally treated as it passes through the vadose zone. This will be possible if the
vadose zone is of sufficient depth to allow natural processes to remove or reduce
organic matter. If the waste water contains significant concentrations of inorganic
or non biodegradable constituents then this option should be avoided.
5.4
Groundwater-surface water interactions
Traditionally, the hydrological sciences have been subdivided into two supposedly
distinct and separate disciplines, namely groundwater hydrology and surface
water hydrology. This subdivision has led to the training of specialists in either of
the two subdivisions. Groundwater hydrologists have primarily come from
geological backgrounds whereas surface water hydrologists tend to have
engineering, agriculture or geography backgrounds. This somewhat arbitrary
division of the hydrological sciences fails to recognise in any substantial way that
both surface water and groundwater are storage components within the
hydrological cycle that are influenced by and have influences on the other one. It
has recently become very evident that this strict subdivision of hydrology is
impractical and that it is vital for these two subdivisions to become more
integrated in order for the full comprehension of the processes and interactions to
be attained. It is ill advised for instance to develop groundwater models without
including traditionally surface water aspects such as evaporation, precipitation
and river flow. Similarly, the assessment of riverflow depends to a great extent on
the full comprehension of the roles that groundwater discharge and recharge play
on the generation of runoff. This section offers a brief overview of several aspects
of the interaction between groundwater and surface water in order to highlight
certain important points.
5-18
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
5.4.1 Vadose zone processes
The vadose or unsaturated zone forms the transition zone between groundwater
and surface water except where there is a direct connection between the two at
locations such as rivers and lakes. In the context of the interaction between
groundwater and surface water the vadose zone is important in that it is through
this layer that recharge to the groundwater (saturated) zone takes place and also
it is from this zone that the saturated zone may be depleted through the capillary
rise of water and its subsequent evaporation or transpiration.
When the front of infiltrating water passes through the unsaturated zone and
reaches the capillary fringe, it displaces air in the pore spaces and causes the
water table to rise. The top of the capillary fringe will also rise and the most
recently arrived water will be found at the top of the capillary fringe. The time of
movement of infiltrating water is a function of the thickness of the unsaturated
zone and the vertical unsaturated hydraulic conductivity. The presence of layers
of low permeability (silts and clays) will retard the rate of recharge even if the
layers are thin.
If the capillary fringe reaches the land surface, direct evapotranspiration of
groundwater is possible. Evapotranspiration occurs primarily when the sun is
shining, hence it is at a maximum on warm sunny days. Groundwater levels of
shallow aquifers show diurnal variation in the summer months when plants are
actively growing and direct evapotranspiration is occurring during the daylight
hours. During winter months when plants are relatively dormant, there is little
evapotranspiration and no diurnal water level variation.
5.4.2 Transmission Losses
Transmission losses refer to the water lost by a surface water stream to the
groundwater domain. This will generally occur in arid or semi-arid regions where
there is no direct connection between the surface water body and the
groundwater body. As the river or stream travels downstream, it provides water for
infiltration beneath the stream bed. Of primary importance in determining the
magnitude of the transmission loss in any given situation are the infiltration
characteristics of the river bed material, the temperature, the sediment
5-19
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
concentration of the river water, the antecedent flow in the river and the degree of
turbulence of the water in the river.
In situations where the river bed material is composed of low permeability material
and the sediment concentration is high and the river flow is not turbulent,
transmission losses are likely to be minimal even if there is no direct connection
between the river bed and the groundwater zone. However, if the bed material is
coarse and the flow is turbulent, transmission losses can be considerable.
In arid areas, where ephemeral flows occur, it is common for a layer of silt or clay
to be deposited in the river bed. When flow occurs, the river will flow over the top
of this low permeability layer and transmission losses are likely to be low.
However, with an increase in flow as is experienced with “flash floods” the flow
may become turbulent and this turbulence is likely to break up the low
permeability layer and then induce transmission losses. The estimation of
transmission losses is therefore a complex procedure that involves taking into
account all of the factors that control the rate of loss and additionally takes into
account the fact that these factors may change during the course of a flood or
over the course of a wet season.
5.4.3 Baseflow and river hydrographs
A stream hydrograph shows the discharge of a river at a single location as a
function of time. The total streamflow as shown on a hydrograph gives no
indication of the origin of the water but it is possible, through the application of
hydrograph seperation techniques to break down the hydrograph into its
components (baseflow, interflow, overland flow and direct precipitation). The
interflow component essentially relates to the loss of water from the unsaturated
zone to the stream whereas the baseflow component relates to the loss of water
from the saturated zone to the stream. As the interflow component is usually
short-lived and occurs only during or immediately after rainfall, this discussion will
only concern itself with the baseflow component of groundwater-surface water
interactions.
The hydrograph of a stream during a period with no excess precipitation will
decay over time following an exponential curve. The discharge in a river during
this period is composed entirely of groundwater contributions. As the stream
drains water from the groundwater reservoir, the water table will fall, the hydraulic
gradient will decrease and there will be less groundwater available to continue to
5-20
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
feed the stream and it will flow at a slower rate. If there was no replenishment of
the groundwater reservoir, the baseflow to the stream would eventually become
zero and the water table would become horizontal. With replenishment through
precipitation, infiltration and percolation, the groundwater reservoir gets
recharged, the water table rises and the baseflow contribution increases. The
baseflow therefore generally fluctuates over the course of a season in response to
precipitation events but there is a time lag between the events and the baseflow
responses.
Baseflow in a stream is relatively constant when compared to the other
components of a hydrograph which are more variable and have a more rapid
response to precipitation. Interflow can be highly variable depending upon the
geology of the drainage basin. A deep sandy soil might not induce any interflow
whereas other situations such as those with layered soil horizons may have
significant quantities of interflow. In the task of hydrograph analysis and
hydrograph seperation, the distinction between overland flow including interflow
and baseflow is made. Many techniques are available for this analysis but
essentially they can be classified into graphical methods (where the hydrograph
shape is used to split the hydrograph) and mathematical techniques (where the
relative contribution of the various components are assessed independently).
5.4.4 Land-use, Vegetation and urbanisation
The influence of land-use on the interaction between surface process and
underground processes is complex and highly variable. Land use can be broadly
separated into vegetational influences and urbanisation influences.
Vegetation can be further subdivided into what may be termed “natural” or
“undisturbed” conditions and “modified” conditions where human activities have
changed the vegetation from what it was before to something else. Since most of
the planet has been affected in some way by human activities such as agriculture
or industrial activity it is rare to find a situation where truly natural vegetation exist
outside of long established conservation areas or areas where population
densities are very low. Consequently, in any given situation the vegetation will
usually reflect the activities of man. The vegetational effects on the groundwater
zone are primarily associated with the influence that vegetation has on
precipitation (mainly interception), infiltration capacity (as a result of the root
structure) and evaporation (transpiration by the plants). Different types of
vegetation influence these to a greater or lesser extent. Forsets for instance tend
5-21
Waternet M.Sc in Integrated Water Resources Management: Introduction To Hydrogeology, Chapter 5
to have high transpiration and interception rates, they have deep rooting depths
but the roots themselves create pathways through the soil through which
infiltrating water can travel. Grasslands on the other hand, have far lower
transpiration and interception rates and consequently their effect on groundwater
recharge is lower than forested areas.
Urbanisation has an effect on groundwater in two ways. Firstly, the creation of
impermeable surfaces such as roads and buildings, together with the creation of
stormwater drains tends to reduce the recharge potential as water is not free to
infiltrate and it is channelled away from the area to adjacent surface water bodies.
Secondly, in the urban environment there is a high potential for contaminants to
be picked up by precipitated water. If this water subsequently enters the ground
through the process of infiltration then the likelihood of contaminated groundwater
beneath urban areas increases.
5.5
Bibliography
The following standard groundwater textbooks have been consulted in the
creation of this chapter. They all cover in varying degrees of detail the material
covered in this chapter. Students are recommended to consult these for further
information and clarification of the course content. In addition, students are
encouraged to make use of the Internet where a wide variety of resources can be
found.
Bowen, R (1986). Groundwater. Elsevier Applied Science Publishers, London
Bower, H (1978). Groundwater Hydrology. McGraw-Hill, Tokyo
Driscoll, F G (1986). Groundwater and Wells. Johnson Screens, St Paul.
Fetter, C W (1994). Applied Hydrogeology. Prentice Hall, Englewood Cliffs
Todd, D K (1980). Groundwater Hydrology 2ed. John Wiley, New York
5-22