Green Economy Journal Issue 61

WATER
WATER
WATER MODELLING
is key to managing SA’s scarce water resources
Different user groups
are supplied at different
assurances of supply.
The potential impacts of
climate change increase the
uncertainty of designing,
planning and operating dams.
As a water-scarce country, South Africa will need to rely increasingly on the detailed modelling
of water resources to help manage the needs of its population, industries and ecosystem –
as well as to navigate the uncertainty of climate change.
BY SRK CONSULTING
According to SRK Consulting principal hydrologist Kerry
Grimmer, this modelling is vital to the rigorous planning
and careful management of the country’s water resources.
“Long-term water supply is assessed through hydrological
and yield modelling of water resources to secure and maximise
water availability and supply,” says Grimmer. “The yield of a dam,
is defined as the maximum amount of water which it can supply;
yield analyses are essential in developing operating rules for water
supply systems, whether these are independent dams or interconnected
water systems.”
Growing demands from expanding communities and developing
industry needs to be considered when sizing and designing a potential
dam, and complex hydrological systems must be modelled to allow
careful planning and operation. Adding to the complexity of this
process over the past few decades are other crucial aspects to consider
in water resources modelling – to reflect current and future conditions
more accurately, and to protect the environment.
“Reservoirs and water supply systems are generally sized and
operated based on studies which used long-term historical
hydrological data, most notably streamflow and rainfall data,” she
explains. “From these studies, operating rules are developed, which
help dam operators to manage their dams by maximising water
resources, avoiding dam failure and protecting the environment.”
This helps ensure the security of water supply for users, including
domestic and rural consumption, agricultural food production,
energy production and industry to sustain and develop the economy
of the region.
“Different user groups are supplied at different assurances
of supply, and this is often determined in cooperation with the
stakeholders within the area,” continues Grimmer. “For example,
domestic use is supplied at a higher assurance level than agricultural
because it is critical that people have access to water for basic
human consumption.”
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Other high priority users include strategic users such as energy
generation facilities, international obligations such as transboundary
watercourses and ecological water requirements.
“To allow these users to obtain their water at a higher priority than
other users, operating rules are developed through water resources
modelling,” she says. “This modelling aids in prioritising supply to
different user groups.”
These rules are especially important during times of drought when
water restrictions need to be implemented. During low rainfall or
Kerry Grimmer, Principle Hydrologist, SRK Consulting.
drought periods, dam operators can refer to the existing operating
rules and determine which of their users need to be curtailed and to
what extent – depending on the current level of the dam.
“This means that we can identify specific users of a system who
will be the first to have water restrictions imposed upon them, and to
what extent,” she says. In these analyses, the most important drivers
are long-term historic measurements of rainfall and streamflow.
In hydrological and yield analyses, scientists heavily rely on this
historic data.
Grimmer points out, however, that this data features the natural
spatial and temporal variability for the preceding time period and
does not yet fully incorporate the changes in climate.
“Although climate change has been studied for decades by
researchers, South Africans are beginning to see the effects of climate
change in their own lives,” she says. “We only need to look back over
the past 10 years to see examples of the devastating extremes of
both droughts and floods in South Africa.”
Historic rainfall and streamflow patterns may therefore no longer
accurately reflect the current climate, which causes a greater uncertainty
in predicting future hydrology. Stochastic models are used to analyse
rainfall and streamflow, but they are limited to the base historic data.
“Changes in the hydrological regime include variations in annual
rainfall, rainfall patterns, intensity and distribution, such as seasonal
shifts,” she says. “The cumulative impacts of these changes will
significantly impact streamflow, and subsequently the yield of a
dam or system.”
It is therefore imperative that changes in climate be incorporated
into water resources modelling by assessing various projected
climate scenarios and determining the effect and consequences of
these changes.
“The potential impacts of climate change increase the uncertainty of
designing, planning and operating dams,” explains Grimmer. “A certain
amount of flexibility in operating rules is fundamental in addressing
fluctuating and unpredictable hydrological conditions and reducing
the associated negative implications.”
She also highlights that designs of existing systems may no longer
be optimal in the light of impending hydrological conditions. To
address the expected changes in climate, studies need to be updated
to ensure that these changes are incorporated into system analyses.
“From this, we can project more accurate assessments of river flows
and dam yields,” she adds. “We can also develop strategic responses
and adaptation measures to the potential negative impacts of climate
change, essentially with the goal of ’climate proofing’ our water resources.”
As part of its extensive portfolio of water management work, SRK’s
contribution to water supply projects includes implementing the
results of potential climate change into the system modelling – to
determine changes and possible consequences. In a current study
in Zambia, SRK’s climate change specialists have estimated that the
rainfall in the project area is projected to decrease by approximately
4% in the medium term and up to 22% in the long term, while oneday
flood events will increase. In addition, the relationship between
rainfall and streamflow is not linear.
“This means that if the rainfall decreases by 4%, the corresponding
change in streamflow will be much greater than 4%, as the effect is
amplified,” she adds. “It is crucial to incorporate this new data, which
reflects the changing climate, into our models.”
She notes that another important aspect of water resources
modelling is the development of an approach which will aid in
protecting natural water resources. South Africa needs to meet not
only the demands of people, agriculture and industry, but also the
needs of the ecosystem.
“If we utilise our current water resources to the point that rivers and
dams run dry, that will have catastrophic effects on the biodiversity
of the environment,” says Grimmer. “For this reason, SRK includes
ecological water requirements in our projects to ensure that we
address and protect the environmental integrity of the river system
under analysis.”
Essentially, these are flows which need to remain in the river to
sustain the natural ecosystem of that river, including vegetation,
insects, amphibians and fish. Ecological water requirements are very
specific to each system and must be determined individually for each
river reach or site in a system. It is important to note that these flows
are meant to mimic natural conditions, which includes droughts and
floods, as these are natural events.
“Although ’flooding’ often has negative connotations, some species
of fish will spawn only during flood events, for example,” she explains.
“Flooding also recharges groundwater and wetlands, so it is important
to capture the natural variation within the climate when determining
ecological water requirements.”
“When determining the yield of a dam, the ecological water
requirements represent the water that cannot be abstracted from
the river; these flows must always remain in the river to sustain its
biodiversity and ecological integrity,” she concludes. “This is a delicate
balance between development and economic growth and the protection
of our natural resources, and water resource modelling is a vital tool
in achieving this result.”
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