Geoscience Reference
In-Depth Information
Figure 6.2
Dams and barrages of Pakistan.
exacting a heavy toll on the environment. In
particular, there is concern that the upstream
abstraction of large volumes of water from rivers
has, in many cases, left insufficient flow to meet
the needs of downstream ecosystems. Coastal and
marine regions, because they lie at the end of
rivers, have been affected most heavily by this
upstream abstraction (IUCN, 2007).
The increase in use of Indus River water for
irrigation has changed the hydrological balance
of the basin significantly. The annual water flow
to the Arabian Sea diminished from 170 million
acre feet (MAF) (210 billion m
3
) to approximately
10 MAF (12 billion m
3
) (Gonzalez
et al
., 2005)
and in 2008-2009 it was only 5.8 MAF (7 billion
m
3
) (Government of Pakistan, 2009). Sediment
transport has also changed. The flow of alluvium
- the fine-grained nutrient-rich soil brought by the
rivers during its course through the fertile plains -
has declined from 400 million to 100 million tons
(363 to 91
×
10
6
t) per year (Amjad
et al
., 2007).
The overall impacts of man-made changes
in the Indus River system are best observed
downstream of Kotri Barrage. In the pre-Kotri
period (1956-1961), there was not a single
day with a zero flow downstream from Kotri
Barrage. The occurrence of zero flow days in the
winter season increased progressively following
the commissioning of the Kotri and Guddu
barrages and the Mangla dam (Figure 6.3). During
2001-2003 the National Institute of Oceanography
(Karachi) observed zero flow during most part of
the year between Sajawal, which is equidistant
from Kotri Barrage (
90 km from upstream) and
the river mouth at Khobar creek. Fresh water
reached the deltaic area infrequently during the
summer, i.e. July-September (Inam
et al
., 2004).
As a result of upstream water abstraction, mainly
∼
