Hydrological modelling applications for water resources management in the Mkomazi Catchment.
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Date
2001
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Abstract
Predictions that water shortages will constrain economic growth in South Africa by
2025 have led to increased concerns among water resource managers that there is a need
for comprehensive water management strategies. To this extent the new South African
Water Act requires that water resource allocation be approached in a more equitable and
conservative way than in the past in order to sustain water resources for catchment
development. This includes protection of the water resource base by the setting aside
of a health Reserve for basic human needs and for the ecological functioning of rivers.
At a time when water resource management is shifting from the practice of large dam
construction to reconciling water demand with water supply in more holistic strategies,
the Mkomazi Catchment in KwaZulu-Natal provides an opportunity to investigate some
of the major issues that dominate contemporary water resource management. Presently
(2001), there are no impoundments on the Mkomazi River and the catchment is
generally underdeveloped. These factors have provided the impetus for the Department
of Water Affairs and Forestry's proposed inter-basin transfer scheme to use the surplus
flow in the Mkomazi Catchment to augment the water resources of the neighbouring
Mgeni system.
Impact-of-Iand-use and development scenario studies, using the ACRU agrohydrological
modelling system, were performed to simulate the impacts of (a) baseline land cover,
(b) present land use, (c) the first phase of the Mkomazi-Mgeni Transfer Scheme and (d)
potential climate change on the hydrological dynamics of the Mkomazi Catchment.
The results indicate that the change from baseline land cover conditions to present land
use conditions has little impact on the annual water resources of the Mkomazi River.
This is especially so in the upper catchment where there is little anthropogenic
development and from where the planned inter-basin transfer will be made from the
proposed Smithfield Dam. Although the impacts of commercial forestry and irrigation
in the middle and lower catchment impose local stress on streamflow generation, they
do not detract substantially from the main downstream flows. Evaluation of the impacts
of the proposed Smithfield Dam on annual streamflow generation revealed that there is
more than sufficient water in the upper Mkomazi Catchment to sustain the inter-basin
transfer under present climatic conditions. However, under potential climate change the
median annual Mkomazi streamflows at the estuary could be reduced by 46% if the
dam was constructed, compared with a 22% reduction under present climatic conditions.
The impacts of catchment development on the seasonal low flows within the Mkomazi
Catchment indicated that those areas which are already heavily utilised by afforestation
and, particularly, by irrigated land use are unlikely to be able to support any further
large scale commercial agricultural development, even under present climatic
conditions.
Water management strategies for the Mgeni system will impact on potential water
allocation within the Mkomazi Catchment. The results of the impacts studies were used
to assess the water demand of the major water-use sectors and the availability of
streamflows for further allocation was assessed. Present total annual water demands of
Mkomazi streamflows is minimal. Even allowing for the environmental demand in the
Mkomazi Catchment, as identified by the Building Block Methodology during an
instream flow requirements workshop, as well as the first phase of the inter-basin
transfer, there would be surpluses of 66%, 43%, 42% and 45% of streamflows,
respectively, at the four instream flow requirement sites on the Mkomazi River.
The results of the Mkomazi instream flow requirements workshop were revisited to
assess the achievability of the recommended flows within the ACRU generated daily
time series of streamflows for each of the scenarios simulated, at the each of the four
instream flow requirement sites on the Mkomazi River. The results confirmed the need
to ascertain the Mkomazi River's natural flow variability, and to assess how much
alteration is likely under development of the Mkomazi Catchment. The Indicators of
Hydrologic Alteration and Range of Variability Approach methodologies were used to
determine which components of the streamflow regime would be most impacted by the
inter-basin transfer. Hypothetical, yet realistic, upper and lower management target
thresholds were applied to determine the range of variation experienced by the
streamflow regime of the Mkomazi, under both pre- and post-dam construction
conditions, and to evaluate a preliminary assessment of the characteristics of the
streamflow regime required to meet environmental sustainability.
The issues raised by potentially conflicting water uses within catchments in South
Africa have indicated that any approach to address the increasing complexity of water
resource problems, and the management thereof, requires effective hydrological
modelling.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2001.
Keywords
Hydrologic models., Water-supply--Management., Hydrology--KwaZulu-Natal--Mkomazi Catchment., Mkomazi Catchment (KwaZulu-Natal), Theses--Bioresources Engineering and Environmental Hydrology.