Groundwater Balance at the Watershed Scale in a Hard Rock Aquifer Using GIS - Groundwater Dynamics in Hard Rock Aquifers - page 139

given period and resulting water level fluctuations, better known as Water-
Table Fluctuation Method (Healy and Cook, 2002):
BAL
S

= h (3)

where S = specific yield, BAL = balance result of the addition of all the
components, h = water level fluctuation (same sign than BAL). This
necessitates the accurate assessment of S at the catchment scale. This is done
through the same way using (3) for a different period.
During the post-monsoon dry period (from November 2001 to June 2002),
the percolation term can be neglected and the balance BAL becomes according
to the budget defined by Schicht and Walton (1961), neglecting baseflow
(groundwater discharge to streams or springs):

BAL RF Q EVAP HIN HOUT (4)

DRY DRY DRY DRY DRY DRY

Including (3) in (4), it is obtained:

RF Q EVAP HIN HOUT

DRY DRY DRY DRY DRY

S = (5)

h

DRY

where all the terms are known (Table 2).

Table 2: Groundwater balance during dry period (all terms
are expressed as rates in mm/yr)

RF DRY Q DRY EVAP DRY HIN DRY - HOUT DRY h DRY (m) S(-)

69 135 1.0 4.9 -6.88 0.0089

The specific yield obtained is consistent with other approaches applied on
this basin such as pumping tests (Maréchal et al., 2004) and global modelling
(Engerrand, 2002).
Consequently, the same approach is applied to determine the percolation.
During the wet season (from June 2001 to November 2001), the BAL
becomes:

(6)

BAL INF RF Q EVAP HIN HOUT

WET WET WET WET WET WET WET

Including (3) in (6), we obtain:

INF . h . S - RF Q EVAP - HIN HOUT

WET WET WET WET WET WET WET

Application of this equation is illustrated in Table 3.

Table 3: Groundwater balance during wet period (flow terms are in mm/yr)

RF WET h WET (m) INF WET

49.6 96.1 0.7 3.5 +5.70 94.5
The percolation is quite coherent for such a hydrogeological context.

Q WET EVAP WET HIN WET - HOUT WET