Geoscience Reference
In-Depth Information
45.00
Mean Flow
1 in 5 Drought
40.00
35.00
30.00
Figure 4.4
Great Ruaha River
monthly flows for an average year,
and a one in five year drought, in the
middle reaches in the Ruaha National
Park (modified from SMUWC, 2000).
Present day flows are not substantially
affected in the wet season, but now
cease completely in the dry season,
from September/October through to
December, due to upstream
abstraction.
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10.00
5.00
0.00
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hydropower for Dar es Salaam. The study reach
for the environmental flow assessment organized
by the WWF was from the outlet of the wetlands
to the inflow at the Mtera Dam. The river flow is
highly seasonal (Figure 4.4) with wet season flows
replenishing the impoundment. The dry season
flows, reduced naturally by evapotranspiration
from the wetlands, have disappeared progressively
from the study reach since the early 1990s, for
increasing periods, owing to the development of
extensive irrigation for rice, and also probably
because of progressive vegetation removal in the
upper catchment. In recent years, the river within
the national park (the extent of the study) has been
reduced to an average of one pool per kilometre by
the end of the dry season.
The result, apart from a serious reduction in
fish and invertebrates, has been the concentration
of hippos and crocodiles in the remnant pools,
resulting in poor water quality and increased
aggressive interactions, as well as a concentration of
large mammals round the pools, with overgrazing
resulting in erosion caused by floods. The entire
national park is threatened by this reduction in low
flows in what is its only perennial water source.
As with the Mara case study, the Ruaha is a river
flowing into a national park, in which the flows
are being modified by upstream abstraction and
landscape alteration. The Ruaha, however, is in a
much more serious crisis than the Mara, for which
historical records show that flows have never
ceased. In the case of the Ruaha, therefore, the
decision was taken to carry out a relatively short-
term environmental flow assessment, which would
concentrate on the short-term requirement to
restore at least some dry-season flow in the reaches
of the river flowing through the Ruaha National
Park. With limited resources and an urgent time
constraint, this assessment was combined with a
more detailed study of the options for restoring
at least some dry-season flow into the study area
as soon as possible, and a preliminary study of
the effects of reducing dry-season inflows to the
Usangu wetlands. In the longer term, the flow
requirement could be refined, and options for more
sustainable flow restoration could be examined.
The various options for dry-season flow restoration,
which are not mutually exclusive, were determined
as follows (WWF-TCO, 2010) (Figure 4.5):
To build storage in the upper river, which
could be used for dry-season irrigation, reducing
abstraction from the river, which would therefore
allow more natural flows into the wetlands, and
through to the study area. The problem with this
option is that the storage might encourage further
development of rice farming, in the expectation
that the stored water could provide increased
irrigation water in addition to (rather than instead
of) the abstraction from the river.
To build storage in the Ndembera River, a
tributary which flows into the Ihefu wetland. The
stored water could be transferred into an adjacent
