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Instream Flow Requirements: Theory and Practice
The analysis of environmental flow requirements is based on the premise that river flow is the primary determinant of a number
of physical and biological parameters in aquatic and riparian ecosystems. A crucial relationship exists between river flow or
discharge (analyzed as a number of components, notably base flows and floods), hydraulics and geomorphology. The volume
of flow determines the depth and size of the river and a number of ecological processes are linked to this.
An IFR is a description of a modified flow regime, invariably due to the presence of a control structure in the river, such as a
dam or weir, that is linked to a description of the condition or health of the river that this flow might be expected to produce.
Furthermore, because the characteristics of a river change as one moves downstream, flow-ecology relationships change too.
One must, therefore, establish different relationships for each affected river reach. Thus, IFR studies focus their analyses on
carefully selected IFR Sites, representative of particular river reaches (the LHWP IFR Phase 1 Sites are shown in the map on
Page 1).
In Southern Africa, the Building Block Methodology (BBM) has been developed and used for the assessment of IFRs. The BBM
requires that a desired future state of the river be identified first; then the scientists
construct the seasonal flow regime, comprising base flows, small and large floods,
required to maintain that desired state. In the case of the LHWP, the in-stream
flow assessment (IFA) was needed to assist in determining a desired future state,
not the other way round, so a different approach was developed. This Downstream
Response to Imposed Flow Transformations (DRIFT) methodology allows the
biophysical and socio-economic consequences of flow changes to be determined
for a number of potential future flow regimes. That is, it is a scenario-based approach.
A Caveat. It is important to bear this in mind when reading the findings of these
studies, that the findings are based on a specific set of conditions defined as
the scenario and not on actual conditions. The findings are, thus, all predictions
and are only as good as the primary information or databases that support them.
For instance, the assessment of changes in riparian vegetation patterns requires
a detailed knowledge of the auto-ecology of individual species, but such data
are not available for the Highlands environment and species specifically, although
experience in other systems will be applicable to some extent. The results of
these studies must, therefore, be interpreted with caution; they must not be viewed
as fact, but rather as the best predictions currently available. Careful monitoring
of the system, after implementation of the IFR Policy, will be essential to establish
the actual system responses, to establish actual losses to resource users and to
permit adaptation as necessary.
The LHWP IFR analysis proceeded on the basis of eight IFR sites in the four rivers
of interest (see Map). Detailed, holistic field characterizations of each site field
were undertaken over a 12-month period to establish a baseline, although a preproject
baseline for the Malibamats’o River could not be established, due to the
presence of Katse Dam. The DRIFT methodology was then used to predict the
effects on different biophysical system components. Four modified flow regimes
were hypothesized, based on design and operational constraints and the nature of
the rivers. Finally, the consequences of the predicted biophysical changes on the
human Population at Risk were analyzed and the costs of potential resource losses
were calculated.
Bringing all the
elements together
The final phase of policy development
will be to assess the trade-offs between
river health, communities and the
implications of different release options
in terms of lost benefits to the two
countries – all within the context of the
Treaty requirement that the project
should “pay due regard to the
maintenance of the welfare of persons
and communities”, and “take all
reasonable measures to ensure…the
protection of the existing quality of the
environment”. If a purely financial
approach were to be taken, the project
would maximize revenue at the expense
of the downstream environment and
communities; if a purely environmental
approach were to be taken, the project
would release as much water as was
necessary to protect the environment
and communities at a high cost in terms
of lost revenue. The policy development
process seeks an intermediate position
that will represent a fair and reasonable
balance between the two extreme
positions.
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