Geology Reference
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in order to reconstruct the rough balance of water released by the
surrounding rock;
for each outlet, the evaluation of the coeffi cient of variation between
the fl ood discharge and the dry season discharge. This coeffi cient
describes the average permeability of the reservoir and its subterranean
circulation conditions. It can vary from 5 in isotropic, poorly permeable
environments, to 100 in karst environments;
the tracking over time of each outlet, in order to estimate interseasonal
and interannual variations in discharge, and to allow correlations with
rainfall data and the recharge area of the aquifer.
When deadlines and budget constraints do not allow a suffi cient degree
of investigation, inquiries with locals familiar with the natural environment
can provide a rough sketch of the outlets' principal characteristics, particularly
in terms of their fl ow volume and their coeffi cient of variation.
3 EXCHANGES BETWEEN AQUIFERS
It is very common for aquifers to have no specifi c outlets, even when they
outcrop over a wide area and have a high infi ltration rate.
It is also sometimes observed that the discharge from a reservoir is
lower than the theoretical infl ows from infi ltration (budget defi cit), or, on the
contrary, that the discharge is greater than these infl ows (budget excess).
These situations are a result of exchanges between different aquifers in
the same region, allowing partial or total leakage of the infi ltrated infl ow
over one recharge area, and contributing more or less signifi cantly to the
alimentation of another aquifer.
Only a detailed examination of the mutual relationships that are
possible between different reservoirs and their surroundings (lakes, rivers,
sea) can allow the defi nition of an aquifer's infl ows and the attempt to
quantify its various inputs (infi ltration, losing streams, exchanges with
other aquifers).
This definition and quantification can be obtained through the
measurement, monitoring, and calculation of the budget's principal
parameters over a representative time period (rainfall data, infi ltrated volume,
stream fl ow, spring discharge, piezometry, actual evapotranspiration), but
it is a long and costly process, which, in addition, often requires some
hypothesizing for certain data points (see chap. A4.1).
The analysis of the relationships between rainfall, the surface area of the
recharge area, and spring discharge can sometimes provide adequate data
at lower cost, based on the specifi c module of infi ltration. It can verify the
coherence between parameters examined in a particular natural context, or,
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