Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 121

2.3 Discharge and fl ow regime of springs
It is very important to have a thorough knowledge of the fl ow regimes and
discharge variations of an aquifer's outlets.
Some emergences, termed perennial springs, present a permanent outfl ow
and are generally located at the lowest points of the reservoir. They are often
associated with temporary springs enabling the evacuation of overfl ow due
to rainy periods, at a higher elevation, through generally unused passages.
In some cases, the natural outfl ow mechanisms are even complemented by
an exceptional spring, which functions only rarely, following extreme and
prolonged rainfall. Such arrangements are characteristic of karst aquifers,
where access points to the reservoir allow the sudden emptying of important
volumes of water at an elevation signifi cantly above the base level; in France
these events are termed “crevaisons” (burst).
The measurement of discharge over time often requires the creation of
specifi c equipment, which can be costly and constraining: the reconcentration
of different rills into one channel, the raising of little fusible dams, the
construction of a permanent weir, the installation of gaging equipment.
When discharge is low, the timed fi lling of a graduated receptacle can
allow rapid and accurate data collection. For larger values, one can also
proceed by measuring the fl ow velocity with the help of a micro-turbine
fl ow meter, or by dilution of a chemical (dye visible to the naked eye or
NaCl tracked with a conductivity meter), as long as the length and cross-
section of the rivulet generated by the spring allow it.
In extreme cases (high discharge), and in order to follow the discharge
over a certain length of time, it becomes essential to measure the height of
the water above a weir on a staff gage or with the help of a limnigraph, and
to correlate this height to the discharge, by establishing a calibration curve,
which requires numerous measurements under varying conditions.
These measurements are clearly impossible in the case of springs located
underwater, in lakes or oceans, where more complex and less reliable
methods can nevertheless provide orders of magnitude. Outfl ows under
rivers can; however, be quantifi ed, based on differential measurements
upstream and downstream of the emergence. Certain precautions are
necessary to guarantee the reliability of the results (measurement of eventual
infl ows and outfl ows along the length under consideration), which will also
greatly depend on the relative values of the discharge being calculated and
the discharge of the river.
In practice, three distinct and complementary actions can be considered
in order to estimate the behavior of an aquifer:
￿ the instantaneous measure of the total discharge from all outlets during
extreme hydrologic events (fl oods and dry periods, for example),