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46 Chapter Two

Table 2.2 Discharges of ten of the world’s largest karst springs. 2.11 Groundwater flow theory
After Ford and Williams (1989).
In this section, the mathematical derivation of the
3
−1
Spring Discharge (m s ) Basin area
2
(km ) steady-state and transient groundwater flow equa-
Mean Max. Min. tions will be presented followed by a demonstration
of simple analytical solutions to groundwater flow
Matali, Papua New Guinea 90 >240 20 350 problems. Following from this, different scales of
Bussento, Italy – 117 76 –
flow systems are shown to exist in regional aquifer
Dumanli*, Turkey 50 – 25 2800
systems. Before starting, it is necessary to define the
Trebisˇnijca, Bosnia-Herzegovina 50 250 3 –
Chingshui, China 33 390 4 1040 two aquifer properties of transmissivity and storativ-
Vaucluse, France 29 200 4.5 2100 ity for both unconfined and confined aquifers.
Frió, Mexico 28 515 6 >1000?
Silver, USA 23.25 36.5 15.3 1900
Waikoropupu, New Zealand 15 21 5.3 450
2.11.1 Transmissivity and storativity of
Maligne, Canada 13.5 45 1 730
confined aquifers
* Dumanli spring is the largest of a group of springs that collectively
3 −1
yield a mean flow of 125–130 m s at the surface of the Manavgat For a confined aquifer of thickness, b, the transmissiv-
River.
ity, T, is defined as:
where the overlying strata are removed by erosion.
Since the emerging water is usually rapidly equilibrat- T = Kb eq. 2.23
ing to atmospheric pressure, dissolved gases can cre-
ate a ‘boiling’ appearance within the spring pool. and represents the rate at which water of a given
In rivers that flow over an aquifer outcrop, both density and viscosity is transmitted through a unit
influent and effluent conditions can develop depend- width of aquifer or aquitard under a unit hydraulic
2
−1
ing on the position of the water table in relation to the gradient. Transmissivity has the units of L T .
elevation of the river bed. With the seasonal fluctu- The storativity (or storage coefficient), S, of a
ation of the water table, the sections of river that confined aquifer is defined as:
receive groundwater discharge in addition to surface
runoff will also vary. A good example of this type of S = S b eq. 2.24
s
river is the intermittent streams that appear over
areas of Chalk outcrop in southern England. In these where S is the specific storage term, and represents
s
areas, the low specific yield of the Chalk aquifer the volume of water that an aquifer releases from
causes large fluctuations in the position of the water storage per unit surface area of aquifer per unit
table between the summer and winter and, therefore, decline in the component of hydraulic head normal
in the length of the intermittent streams. The inter- to that surface (Fig. 2.22a). Storativity values are
mittent streams, or Chalk bournes or winterbournes dimensionless and range in value from 0.005 to
as they are known, flow for part of the year, usually 0.00005, such that large head changes over extensive
during or after the season of most precipitation. An areas are required to produce significant yields from
example is the River Bourne located on the north-east confined aquifers.
of Salisbury Plain as shown in Fig. 2.21a. In this area
of undulating Chalk downland in central southern
England, the intermittent section of the River Bourne 2.11.2 Release of water from confined aquifers
is 10 km in length until the point of the perennial
stream head is met below which the Chalk water At the beginning of the last century, Meinzer and
table permanently intersects the river bed. In drought Hard (1925) observed in a study of the Dakota sand-
years, the intermittent section may remain dry while stone that more water was pumped from the region
in wet years, as shown in Fig. 2.21b, the upper reaches than could be accounted for (as water was pumped,
sustain a bank full discharge. a cone of depression developed and the rate of