Geology Reference
In-Depth Information
the watershed does not coincide with the hydrogeological limit of the aquifer
comprised in the surface watershed. Pumping flow (
Q
) is the major negative
component in such exploited basin. Water is mainly used for irrigation and
domestic purposes. Evaporation (EVAP) from the groundwater table must
be considered regarding the hot temperatures observed in this semi-arid area.
Except when specially mentioned, the flows are expressed as rates in
mm/yr.
The watershed is divided in a grid of 1324 cells of 200 × 200 metres size.
The flows identified above are evaluated in each cell, except for HIN and
HOUT, which are determined only along the boundary of the basin.
Pumping Flow
A database of the bore-wells present in the watershed has been constituted.
In all 929 wells were located using GPS portable and their discharge rate
measured. Information about daily duration of pumping and annual number
of pumping days was gathered in order to assess annual abstracted volume.
The discharge rates are comprised between 5 l/min and 700 l/min. A map
of annual abstracted volume is presented (Fig. 3). Pumping is concentrated
in low zones, on flat areas allowing agriculture.
Horizontal Flow Across the Boundaries of the Watershed
Because weathering cover of the geological profile is dry and permeability
of the fresh basement is very low, horizontal flows in these layers of the
aquifer can be neglected. They are only located in the fissured layer of
the aquifer.
These flows are dependent on horizontal permeability, thickness of saturated
zone and local hydraulic gradient. They were computed using a finite
differences model in order to obtain a spatial distribution on a grid of squared
cells. Modflow model was used with a grid corresponding to the one selected
for the balance. A kriged grid of the altitude of the summit of the fresh
basement, constituting the bottom of the aquifer, was imported along with
the grid of water table. Hydraulic potentials were imposed on the four limits
of the modelled domain. Flows are computed during a few days with an
average horizontal permeability
K
= 0.87 × 10
-5
m/s determined by upscaling
(from slug-test scale to 200 × 200 metres cells) using a DFN (Discrete
Fractures Network) model, on the basis of 25 slug-tests. The results are
presented in Table 1.
T
able 1:
HIN—Horizontal In-Flow, HOUT—Horizontal Out-Flow
Flow
Pre-monsoon
Post-monsoon
HIN (m
3
/d)
1997
1637
HOUT (m
3
/d)
176
686
HIN-HOUT (m
3
/d)
1821
951
