A DECISION SUPPORT SYSTEM FOR BENEFICIAL USE OF
NATIONAL DAM WATER RESOURCES IN SWAZILAND
By Prof. J.I. Matondoa and Dr. K. M. Msibib
ABSTRACT
Globally water is unevenly distributed in time and space. In Swaziland the highveld region receives high
rainfall ranging from 1200 to 1500mm annually while the lowveld region receives the lowest amount of
rainfall ranging from 300 to 500mm annually. The Planning, development and management of water
storage and transmission facilities is an attempt to change the spatial and temporal availability of water, to
make it available where and when needed.
The Government of Swaziland has undertaken the development of water storage and distribution facilities
as a process towards overcoming the unevenly distribution of the water resource. However, the current
water storage facilities with a total capacity of 568.65x10 6 m3 can not meet the water demand which
stands at 1795x106 m3 annually. The overall objective of this project was to develop an optimum and
sustainable utilization decision support tool for water allocation and rationing options for national water
reservoirs.
A Decision Support System (DSS) is defined as a user-oriented computer system which supports
decision-makers in addressing unstructured problems. The rationing mode in the developed DSS was
used to distribute the available stored water at the beginning of April in order to arrive at a priori
predetermined reservoir storage level at the end of October. The DSS has three major components and
that is the model input, modeling options and outputs screens. The model input component comprises of
technical data (inflow, reservoir level, intended % volume at end of October and allowed lowest rationing
%), and policy data (uniform rationing or differential rationing). The modeling options comprise of
commands to determine optimum rationing %, provide % rationing and observed dam response and
imposed rationing %. The output of the DSS comprises of the optimal rationing (%), monthly reservoir
volume from April to end of October as well as a graphical representation of the reservoir response and
the final recommendation. The DSS also provides the total crop hectarage and thus the net benefits for the
different initial and end of season (October) for the corresponding optimal irrigation water rationing (%).
1
,The results of the DSS with a starting reservoir level at 15m at the beginning of April and a 10% reservoir
level at the end of October, is that there is no need for rationing (that is the water demand for all uses are
met) and the reservoir level stands at around 29% at the end of October. The results of the DSS with a
starting reservoir at 6m at the beginning of April is that the rationing is 46% for all uses in order to arrive
at a 10% reservoir storage level at the end of October is not recommended. This is because, with 54%
deficit, there is a danger of crops to wilt beyond recovery. The final DSS recommendation starting with a
reservoir level at 6m at the beginning of April is irrigation rationing at 60% (i.e. 40% water deficit)
combined with a reduction of 14% of the hectares of irrigated fields.
The Nyetane dam was used as a pilot reservoir in the development of the DSS. The developed DSS with
all the relevant input data can be used to allocate water efficiently in other reservoirs in the country.
Keywords: Beneficial, Decision support system, Optimum, Rationing,
Sustainable use
BACKGROUND
The Kingdom of Swaziland, is situated in South Eastern Africa between latitude 25 o and 28o South and
between longitudes 31o and 32o East. It lies some 48 to 225 kilometers inland of the Indian Ocean littoral
and hence physically land locked. The country has a total surface area of 17,360 km 2 and therefore, is the
smallest country in the southern hemisphere.
It is bounded by the Republic of South Africa in the north, west and south, and by Mozambique on the
east. Although small in size, Swaziland is characterized by a great variation in landscape, geology and
climate. It also lies within the Maputoland Centre, an area reported to have the greatest biodiversity in
Southern Africa.
There are four distinct physiographic regions within the country namely: highveld, middleveld, lowveld
and lubombo, which are clearly distinguished by elevation and relief (Murdoch, 1970). Swaziland enjoys
a climate which is generally subtropical, with hot and wet summers and cold and dry winters. Further
variations in climatic conditions occur within the different physiographic regions giving rise to three
clearly distinguishable climate types.
2
, The highveld and upper middleveld are characterized by a Cwb climate. The lower middleveld and
lubombo range have a Cwa climate whilst the western and eastern lowveld have a Bsh climate (Murdoch,
1970). Mean annual rainfall ranges from about 1500 millimetres in the highveld to less than 500
millimetres in the southern lowveld.
The highveld’s temperate climate is characterized by wet summers and dry winters, and annual rainfall
averaging 1500 millimetres. Temperatures vary between a maximum of about 33 oC in mid-summer and
0oC at night in mid-winter. On the other extreme end is the lowveld which experiences a sub-tropical
climate. This region receives the lowest annual rainfall of about 450 mm. There is also a large diurnal
temperature range experienced here with maximum temperatures reaching the upper 30 oC are not
uncommon. Semi-arid pockets of areas are found in this region, which is also liable to desertification.
The frequency of heavy downpours is more uniform across Swaziland than the total rainfall. Between
75% and 83% of precipitation (summed mean monthly amounts) comes in summer months (October –
March).
The water sources in Swaziland are mainly surface waters (rivers, reservoirs), ground water and atmospheric
moisture. Figure 1 shows the major drainage basins of Swaziland namely: Komati, Usutu, Mbuluzi,
Ngwavuma, Lomati, Pongola and Lubombo. The Komati and Usutu rivers both originate in South Africa
while the rest of the basins originate within Swaziland. It should also be noted that all the rivers in Swaziland
are international rivers and therefore, the development and management of the surface water resources must
be undertaken in collaboration with the other riparian states namely: South Africa and Mozambique.
Water is unevenly distributed in time and space all over the world and Swaziland is no exception. The
major objective of water resources planning and development is to alter the temporal and spatial
availability of water. To make the water available where and when needed and this is possible through the
construction of storage and distribution facilities.
There are ten big dams in Swaziland which are greater than 5 million cubic metres (MCM). The dams are
located in the Komati, Usuthu and Mbuluzi catchments. Three of these reservoirs are government owned
located in each of the three catchments whilst the remaining six reservoirs are owned privately.
The existing infrastructure is not sufficient to carter for the whole country and currently the lowveld
region continues to suffer from drought related problems. The challenge to the government remains that
of constructing water storage and distribution facilities and transfers it to areas of acute water shortage.
Water resources management is posing a complex challenge due to the increasing competing and
3
NATIONAL DAM WATER RESOURCES IN SWAZILAND
By Prof. J.I. Matondoa and Dr. K. M. Msibib
ABSTRACT
Globally water is unevenly distributed in time and space. In Swaziland the highveld region receives high
rainfall ranging from 1200 to 1500mm annually while the lowveld region receives the lowest amount of
rainfall ranging from 300 to 500mm annually. The Planning, development and management of water
storage and transmission facilities is an attempt to change the spatial and temporal availability of water, to
make it available where and when needed.
The Government of Swaziland has undertaken the development of water storage and distribution facilities
as a process towards overcoming the unevenly distribution of the water resource. However, the current
water storage facilities with a total capacity of 568.65x10 6 m3 can not meet the water demand which
stands at 1795x106 m3 annually. The overall objective of this project was to develop an optimum and
sustainable utilization decision support tool for water allocation and rationing options for national water
reservoirs.
A Decision Support System (DSS) is defined as a user-oriented computer system which supports
decision-makers in addressing unstructured problems. The rationing mode in the developed DSS was
used to distribute the available stored water at the beginning of April in order to arrive at a priori
predetermined reservoir storage level at the end of October. The DSS has three major components and
that is the model input, modeling options and outputs screens. The model input component comprises of
technical data (inflow, reservoir level, intended % volume at end of October and allowed lowest rationing
%), and policy data (uniform rationing or differential rationing). The modeling options comprise of
commands to determine optimum rationing %, provide % rationing and observed dam response and
imposed rationing %. The output of the DSS comprises of the optimal rationing (%), monthly reservoir
volume from April to end of October as well as a graphical representation of the reservoir response and
the final recommendation. The DSS also provides the total crop hectarage and thus the net benefits for the
different initial and end of season (October) for the corresponding optimal irrigation water rationing (%).
1
,The results of the DSS with a starting reservoir level at 15m at the beginning of April and a 10% reservoir
level at the end of October, is that there is no need for rationing (that is the water demand for all uses are
met) and the reservoir level stands at around 29% at the end of October. The results of the DSS with a
starting reservoir at 6m at the beginning of April is that the rationing is 46% for all uses in order to arrive
at a 10% reservoir storage level at the end of October is not recommended. This is because, with 54%
deficit, there is a danger of crops to wilt beyond recovery. The final DSS recommendation starting with a
reservoir level at 6m at the beginning of April is irrigation rationing at 60% (i.e. 40% water deficit)
combined with a reduction of 14% of the hectares of irrigated fields.
The Nyetane dam was used as a pilot reservoir in the development of the DSS. The developed DSS with
all the relevant input data can be used to allocate water efficiently in other reservoirs in the country.
Keywords: Beneficial, Decision support system, Optimum, Rationing,
Sustainable use
BACKGROUND
The Kingdom of Swaziland, is situated in South Eastern Africa between latitude 25 o and 28o South and
between longitudes 31o and 32o East. It lies some 48 to 225 kilometers inland of the Indian Ocean littoral
and hence physically land locked. The country has a total surface area of 17,360 km 2 and therefore, is the
smallest country in the southern hemisphere.
It is bounded by the Republic of South Africa in the north, west and south, and by Mozambique on the
east. Although small in size, Swaziland is characterized by a great variation in landscape, geology and
climate. It also lies within the Maputoland Centre, an area reported to have the greatest biodiversity in
Southern Africa.
There are four distinct physiographic regions within the country namely: highveld, middleveld, lowveld
and lubombo, which are clearly distinguished by elevation and relief (Murdoch, 1970). Swaziland enjoys
a climate which is generally subtropical, with hot and wet summers and cold and dry winters. Further
variations in climatic conditions occur within the different physiographic regions giving rise to three
clearly distinguishable climate types.
2
, The highveld and upper middleveld are characterized by a Cwb climate. The lower middleveld and
lubombo range have a Cwa climate whilst the western and eastern lowveld have a Bsh climate (Murdoch,
1970). Mean annual rainfall ranges from about 1500 millimetres in the highveld to less than 500
millimetres in the southern lowveld.
The highveld’s temperate climate is characterized by wet summers and dry winters, and annual rainfall
averaging 1500 millimetres. Temperatures vary between a maximum of about 33 oC in mid-summer and
0oC at night in mid-winter. On the other extreme end is the lowveld which experiences a sub-tropical
climate. This region receives the lowest annual rainfall of about 450 mm. There is also a large diurnal
temperature range experienced here with maximum temperatures reaching the upper 30 oC are not
uncommon. Semi-arid pockets of areas are found in this region, which is also liable to desertification.
The frequency of heavy downpours is more uniform across Swaziland than the total rainfall. Between
75% and 83% of precipitation (summed mean monthly amounts) comes in summer months (October –
March).
The water sources in Swaziland are mainly surface waters (rivers, reservoirs), ground water and atmospheric
moisture. Figure 1 shows the major drainage basins of Swaziland namely: Komati, Usutu, Mbuluzi,
Ngwavuma, Lomati, Pongola and Lubombo. The Komati and Usutu rivers both originate in South Africa
while the rest of the basins originate within Swaziland. It should also be noted that all the rivers in Swaziland
are international rivers and therefore, the development and management of the surface water resources must
be undertaken in collaboration with the other riparian states namely: South Africa and Mozambique.
Water is unevenly distributed in time and space all over the world and Swaziland is no exception. The
major objective of water resources planning and development is to alter the temporal and spatial
availability of water. To make the water available where and when needed and this is possible through the
construction of storage and distribution facilities.
There are ten big dams in Swaziland which are greater than 5 million cubic metres (MCM). The dams are
located in the Komati, Usuthu and Mbuluzi catchments. Three of these reservoirs are government owned
located in each of the three catchments whilst the remaining six reservoirs are owned privately.
The existing infrastructure is not sufficient to carter for the whole country and currently the lowveld
region continues to suffer from drought related problems. The challenge to the government remains that
of constructing water storage and distribution facilities and transfers it to areas of acute water shortage.
Water resources management is posing a complex challenge due to the increasing competing and
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