Geoscience Reference
In-Depth Information
Thus, although records vary about the minimum Ganga discharge at Farakka,
it is clear that in 1913, the minimum was 50,000 cusecs (1,416 cumecs), which
came down to 40,000 cusecs in 1952 and 1953. Those were the driest years when
the Ganga flow was exceptionally low. As continuous prototype observation data at
Farakka were then not available, it is difficult to establish, whether the discharge
in lean season at the point was gradually reducing, or not. Development activi-
ties in the upper reaches from olden days before and after India's Independence
call for a closer look to ascertain the volume of lean-season flow, utilized in such
activities.
Daily discharges in a river vary from year to year. Since the commissioning of
the barrage at Farakka in 1975, the highest discharge up to the 1990s, was 73,923
cumecs, or 2,610,960 cusecs, on 19th September 1987 and the minimum at 991
cumecs (35,000 cusecs) occurred on 13th April 1993. In several other years, the
discharge at Farakka fell to 1,132 cumecs (40,000 cusecs), or even less, e.g., in
1980, 1983 and 1992. In 1984-1985, 1987-1989 and 1994, the minimum discharge
hovered between 1,132 and 1,416 cumecs (40,000 and 50,000 cusecs). In 1980
and 1996, discharge soared to 70,793 cumecs (2,500,000 cusecs) and in 11 years,
it remained above 56,635 cumecs (2,000,000 cusecs); the years were 1976 (for
13 days), 1978 (28 days), 1980 (30 days), 1982 (12 days), 1983 (4 days), 1984
(7 days), 1987 (7 days), 1988 (20 days), 1991 (13 days), 1994 (5 days) and 1996
(10 days).
An analysis of the flow data, as given in Tables 4.6, 8.2, 8.3, and 8.4, can be made
to find out, whether the flow has been reducing, or maintaining uniformity since the
commissioning of the barrage. Graphs have been plotted to show year-wise variation
of flow (years horizontal and flows vertical), as shown in Figs. 8.5 and 8.6 for annual
as well as lean season. The line drawn shows a diminishing trend of the flow. As the
points are scattered and a definite trend in these years being elusive, three-yearly and
five-yearly moving averages have been worked out for the annual and lean-season
flows at Farakka from 1975 to 1995 and shown in Table 8.5 below.
Figures 8.7 and 8.8 show the graphs of three-yearly and five-yearly moving aver-
ages for different years. It is seen that annual and the lean-season flows in the Ganga
are diminishing since 1975, i.e., after the commissioning of the barrage, which could
be due to
a) Melting of less snow in the Himalayas;
b) Blockage of the natural flow and its storage at the mouths of the Ganga and its
tributaries;
c) Increased evaporation loss of the Ganga water;
d) Increased under-ground storage of water;
e) Less precipitation of rains and less run-off; and
f) More withdrawal of the Ganga water in the upper reaches for irrigation, hydro-
electric and thermal power generation, drinking and other domestic use and for
industries.
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