Geology Reference
In-Depth Information
In-flows mainly take place across the southern border of the watershed
(Fig. 4) due to the regional south-north gradient linked to regional
topographical slope. Other in-flows located west and east are due to the
depletion of water table near the boundaries, due to pumping wells. The
balance between horizontal in- and out-flows is finally positive due to the
capture of water from outside. The balance is higher during pre-monsoon
period according to higher water table depletion in such dry period.
Evaporation from Groundwater Table
This component is evaluated using the relation developed by Coudrain et al.
(1998). Evaporation flux is expressed as an inverse power function of the
piezometric depth below the soil surface, independently of the soil
characteristics:
EVAP = 71.9
z
-1,49
(1)
where
z
is water table depth from soil (m).
It was calculated for both pre- and post-monsoon periods. Then, an average
was determined. The value is about 1.7 mm/yr, very less compared to the
other components of the water balance. This is due to high depth (18 m
before the monsoon and 12 m after the monsoon) of depleted water table in
the study area.
Return Flow from Irrigation
Most of the pumped water in the basin is used for irrigation, a large part of
this volume returns back to the aquifer by direct infiltration in the irrigated
lands. This can be written as:
RF
=
C
×
Q
with
C
= return coefficient (0 <
C
< 1),
Q
: pumping flow.
Because return coefficient is different from paddy fields to other crops,
the global return coefficient is weighted by the repartition of crops:
A
×
C
=
A
RICE
×
C
RICE
+
A
OTHER
×
C
OTHER
(2)
where
A
is the total irrigated surface,
A
RICE
= surface of irrigated paddy
fields,
A
OTHER
= surface of other irrigated crops,
C
RICE
= return coefficient
in paddy fields,
C
OTHER
= return coefficient in other crops.
The return coefficient for each crop is quite difficult to assess but some
studies are available and mention that 0.55 <
C
RICE
< 0.80 for paddy fields.
In granite areas encountered in Andhra Pradesh, the coefficient for paddy
fields was evaluated to be 0.60 (APGWD, 1977). For other crops, the proposed
coefficient is
C
OTHER
= 0.20 (CGWB, 1998). With 79% of paddy fields and
21% of other crops, the obtained value is
C
= 0.516.
Percolation and Specific Yield
The satisfying knowledge of groundwater levels in the basin leads us to
estimate the infiltration using the basic relationship between balance over a
