COMMISSION INTERNATIONALE
DES GRANDSBARRAGES
------VINGT TROISIEMECONGRES
DES GRANDSBARRAGES
Brasilia, juin 2009
-------
NEED FOR OPERATIONAL CONSIDERATIONS IN WATER RESOURCES
ALLOCATION AND TRADING AGREEMENTS*
B. Mwaka
Director of Water Resource Planning Systems
Department of Water Affairs and Forestry, South Africa
C. Ntuli
Systems Operation Analyst: Water Resource Planning Systems
Department of Water Affairs and Forestry
Abstract: Water storage dams in South Africa are used for a variety of purposes
like irrigation, hydropower generation, domestic and industrial supplies, among
others. Operation of such multi-purpose reservoirs requires numerous decisions
to regulate the water resources in an attempt to reconcile water availability with
the requirements. The regulations are based on operating rules, which are
instructions for managing release or pumping flow rates depending on the
system’s hydrological state and allocation agreements. Due to economical and
socio-political developments rigid operating rules can easily be derailed with
serious consequences. Besides the allocable volume of water, other variables
like risk, flow rates and pressure head count significantly in the resulting system
performance especially for systems involving hydropower generation. The
prevailing situation where Eskom is not able to meet the electricity requirements
fully, is a good case study that will be used to illustrate how once-off water
allocation operating agreement are not good for efficient use of the water
resource. The agreement between Eskom and the Department of Water Affairs
and Forestry (DWAF) for the operation of Gariep and Vanderkloof Dams on the
Orange River system will be used to demonstrate how the legal agreement,
without due consideration to other operating principles, can lead to even higher
risk of failure for both all water users on the system. Because of the increasing
water demand and competition, various concepts are proposed for sharing and
trading the resources. Water banking is one of the concepts that are being
debated with much vigor for DWAF to adopt for water allocation and trading. A
critical review of the water banking philosophy will be made.
Key words: Water allocation, sustainable water supply
1.
INTRODUCTION
Water resource systems operation is the regulation of the systems in order to
reconcile water availability with requirements and mitigate against risks over a
given period of time. The regulation is based on operating rules, which are
instructions for controlling release and/or pumping flow rates depending on the
system’s hydrological condition, water allocation agreements and infrastructural
maintenance schedules, among others.
In earlier phases of water resources management water was not a limitation. It is
the infrastructure that was limiting. The main focus was therefore on the
development of dams and their associated infrastructure, which characterized the
period as a development phase, to increase yield and facilitate storage and
transfer of water. Initially operating rules were once-off statements and because
competition for water was still low, the focus was on supplying water to those with
the ability to pay so that more money could be generated for further
infrastructural developments.
However, over time especially due to economical and socio-political
developments rigid operating rules can easily be derailed with serious
consequences. Water itself is an annual renewable resource and the allocated
water rights licensed to users are based on long-term average yields of systems,
which cannot be guaranteed for all years. Every year the hydrology and level of
development of a given system, yields an amount of water that is seldom the
same for different years. The main function of water resources systems
operation, therefore, is to repeatedly assess the prevailing hydrological
conditions against expected demand patterns in order to balance the available
water against the requirements
But presently water for allocation is the main limitation given the ever-increasing
demands (competition) against the finite water resource. This means that water
management is now in the water allocation phase. Besides, the hydrology is
becoming highly variable, probably due to changing climate and/or land use.
Because of this the once-off rules cannot suffice. Regular operational analyses
are required to make water reconciliations based on envisioned hydrological
conditions, water allocation agreements and infrastructural maintenance
schedules. The main deliverables of systems operation analyses include annual
water allocation, operating rules and related (DSS) for benchmarking and
managing the performance of the systems in terms of efficiency and risk.
Effective water resources systems operation, therefore, requires good reflection
of systems hydrology and hydraulics. Systems water allocation and trading
licensing, however, is largely driven by socio-economical and legal
considerations, and sometimes even emotional historical legacies. Because of
this, systems operation to implement water allocation is increasingly being
compromised.
This paper aims to illustrate the need for operating considerations in water
allocation and trading management. Two examples, one on hydropower
generation and another on water banking allocation approach, will be used to
demonstrate the significance of operating parameters in order to effectively
implement water supply agreements.
2.
STUDY AREA AND ITS ALLOCATIONS
The Orange River rises in the eastern highlands of Lesotho where it is known as
the Senqu River and is the largest and longest river in South Africa. This paper
will focus on the hydropower dams in the Upper reaches of the Orange River,
namely Gariep (total output of 360MW) and Vanderkloof (total output of 240MW).
These dams are the two largest reservoirs in South Africa.
Katse and Mohale dams in Lesotho are not located in the two WMAs, but have a
significant impact on the available water in the Orange River, as the bulk of the
water flowing in the Orange River is generated in Lesotho. Water from Katse
Dam is transferred via the Lesotho Highlands Water Project to Vaal Dam for
Gauteng’s (the country’s economic heartland) water use and Gariep Dam. Also
from Gariep Dam water is transferred via the Orange-Fish tunnel to the Fish to
Tsitsikamma water management area for Port Elizabeth town. The GIS layout of
the Orange River System is given in Figure 1 below.
Figure 1: GIS layout of ORS
3.
ALLOCATION CHALLENGES
Water is released from the two dams to supply downstream users. Eskom
uses this downstream water requirement to generate power as water
released from the two dams passes through the turbines. However, fluctuating
releases from hydropower turbines impact negatively on some users. For
example, downstream consumptive users which is mainly irrigation, require a
high demand during summer months and a low demand in winter, to overcome
this, the mirror image release pattern was developed, whereby storage space is
created in Vanderkloof Dam during the summer months and then allow the higher
releases from Gariep Dam during winter to be captured in Vanderkloof Dam and
at the same time the energy generation from Gariep dam is increased during this
period.
Upstream water transfers also impact negatively on water availability for power
generation. Water is transferred from Gariep dam to Port Elizabeth (PE) through
the Orange-Fish transfer and the current allocation is expected to increase in the
near future given the economic and social developments in PE. This increase in
demand will reduce the amount of water available for power generation.
According to the operational agreement between DWAF and Eskom, Eskom is
allowed to generate electricity at its full load for four hours per day if they are in
crisis/ emergency, unless a drought emergency has been gazetted for the
Orange River system. Considering the prevailing situation, where Eskom is not
meeting the full supply for power demand/requirement in the country, the Orange
River System management may want to employ this emergency Operating rule
agreement.
However, in hydropower generation, besides volumetric allocation, other
variables like water level (H), turbine efficiency as shown in the turbine
characteristic diagram below are equally significant. The characteristic curve
indicates that the turbines perform most efficiently (93% efficiency) when the net
head is between 50m and 65m and the discharge per turbine is between 150
m3/s and 175m3/s. Outside this range, the turbine efficiency decreases. It is also
evident that once Gariep dam storage reaches 40%, power generation becomes
inefficient.
Figure 2: Gariep Turbines characteristic curves
4.
PRESENTATION OF WRPM RESULTS
The behaviour of selected system components (e.g. projected reservoir storages
and simulated flows through water abstraction/supply routes) is presented as
probabilistic distribution plots (box plots). A typical box plot indicating the various
lines that depict specified exceedance probabilities of a probability distribution is
provided in Figure 3 below.
Figure 3: A typical box plot
Annually, systems operation analyses include annual water allocation, operating
rules and related (DSS) for benchmarking and managing the performance of the
systems in terms of efficiency and risk. The projected storage derived from 1000
stochastic sequence as shown in the diagram below is among the examples of
the DSS for monitoring the performance of the system. Releasing water for power
generation at full load for 4 hours as per DWAF/Eskom agreement when the
starting storage in May is 90% as it was the case this year, indicates that 90MW
of power can be generated for only four months but at the sharp storage drop
and the curve plots outside the projected storage as indicated by the pink
trajectory. Gariep dam was drawn down to its Minimum Operating Level (MOL) at
the 99.5% exceedance probability level towards September 2010. This may
result to the implementation of restrictions to other water users in order to protect
the resource.
Figure 2: Gariep storage with starting storage at 90% Full Supply Level
Assuming a scenario where the storage starts at 100% full supply storage in
May, which is taken as the decision month as this is the month when most
summer rains have been captured into storage and the dams are mostly full and
if Eskom generates at full load for four hours. It was found that 90MW of power
per machine can be generated for the whole year as the storage manages to
recover during the summer rains. Gariep dam was drawn down to its Minimum
Operating Level (MOL) at the 99.5% exceedance probability level towards the
end of November 2010. The short term analysis results for Gariep Dam storage
for this scenario are presented in Figure 6 and indicate that the full starting
storage operating rule protects the resource adequately
Figure 6: Short-term Projected Storage Trajectories for Gariep Dam (100% full scenario)
For the scenario where the dam is assumed to start at 60% live storage and
Eskom generates as per emergency agreement, it was found that the dam level
drops drastically and does not even reach the summer period for the storage to
recover. Projected storage trajectories of Gariep Dam show that this Scenario
operating rule does not protect the resource to such an extent that the worst
sequence projection (99.5% exceedance) is drawn down to the MOL as early as
December 2009, Figure 7. This is due to the fact that the dam staring storage
was low and the resource was fully utilized.
Figure7: Short-term Projected Storage Trajectories for Gariep Dam (100% full scenario)
Emergency Release Drawdown Curve
57
56
Water level (head)
m
55
54
53
100%start
90%start
80%start
52
51
50
49
48
47
46
May
June
July
Aug
Sept
Oct
Nov
Dec
Jan
Time (months)
Figure 83: Emergency release drawdown curve
Feb
Mar
April
The figure above shows the different starting storages of the dam and the impact
of generating at full load for four hours on power generated. When the dam starts
at 100% in decision month, May 90MW power per machine can be generated at
full load for the whole year, however when the dam starts at 90% power can be
generated efficiently i.e. at 90MW on 4 machines until September, however, in
October power generation will be reduced to 80MW per machine. It is however
very rare to have the 100% full storage in the dams which seems to be the only
curve running throughout the year when the power is generated at Eskom’s full
load for 4hours per day as indicated in the figure below.
Time (Months)
80%Full
90%Full
70%Full
Ap
ril
ar
ch
M
t
Se
pt
em
be
r
O
ct
ob
er
No
ve
m
be
r
De
ce
m
be
r
Ja
nu
ar
y
Fe
br
ua
ry
Au
gu
s
Ju
ly
Ju
ne
M
ay
%Exceedance
% Exceedance of Storage
100
90
80
70
60
50
40
30
20
10
0
60%Full
Figure 9: Storage % of exceedance
As indicated previously, when Gariep dam level reaches 40% which is equivalent
to 42m power generation is stopped. The power generation at this level is 60MW
at 91% turbine efficiency, Figure 10 below.
100%Full
Power generation at different water levels
58
56
54
Water Level (m)
52
90MW
80MW
70MW
60MW
50
48
46
44
42
40
150
155
160
165
170
175
180
185
190
195
200
Flow (m3/s)
Figure 40: Gariep Power generation levels at different water levels
5.
WATER BANKING
Water banking also known as capacity sharing, refers to the apportionment
system of issuing water use entitlements against shares of the storage capacity
of large government dams, storage shares can be available for rental or
purchase by individual stakeholders. (Pott et al). The system enables water users
to bank their water in large government dams. Where there is storage works, the
total potential storage capacity is divided and portions of the total available
storage made available for rental or purchase by individual stakeholders, hence
the term capacity sharing (Dudley, 1992). Fractional water allocation refers to
entitlements issued against run-of-river flows (where there is no storage).
Fractional Water Allocation and Capacity Sharing (FWACS) system allows water
users to get entitlements on both the portions of dam storage and inflow into the
dam. The proportions in the dam are allocated according to the price the user
pays for water or the priority classification of the user. The National Water Act
(1998) No. 36 (the Act) classifies the water use into levels of priority with the
reserve (basic human and ecological needs) having the highest priority, followed
by international obligations, strategic use, future use and all other uses. The
same classification is used in water pricing.
This concept is being debated with much vigor for DWAF to adopt for water
allocation and trading. Pott (2008) argues that defining water use entitlements in
terms of water banking encourages the development of a water market thus
improving water use efficiency while accounting for undesirable externalities
associated with water quality, the environment and equity considerations in South
Africa.
FWACS was developed in Australia as a possible solution to efficient allocation
of water (Dudley and Musgrave, 1988). It was developed to enable individual
irrigators to manage their own risks and avoid being adversely affected by the
decisions of others (Dudley and Musgrave, 1988). They could allocate their own
water through time so as to satisfy individual supply sufficiency requirements.
In Australia with the generally arid climate, water rights were mostly defined in
terms of stored water. Thus capacity sharing was developed as a best
management practice for water systems in which the operation of water in
storage was paramount (Dudley, 1992). In the context of Australia water resource
availability is highly uncertain and integration of supply and demand management
is essential in order to promote economically efficient resource allocation. The
aim was to have a system of water entitlements in which individual irrigators
could decide on the levels of risk they were prepared to shoulder. In this system
the water right holder decides how much water to carry over between irrigation
periods. The amount carried over is available to the irrigator when required plus
any other allocation due. The water user not the water authority manages this
transaction. This enables the right holder to manage variability in risk regarding
availability of water. FWACS was devised to provide a basis for private rights to
water. Australia users were industry, domestic, irrigation and the environment a
case similar to South Africa.
This concept has also been widely used in Zimbabwe’s Mazowe catchment as far
back as 1984 (DWAF 2006). In Zimbabwe, a combined irrigation scheme
consisted of eleven commercial farmers, each subscribing to a defined
percentage of cost of construction of a dam and its related infrastructure. This
percentage was the basis on which their percentage of the potential annual
storage capacity of the dam was calculated. The Mazowe catchment comprised
of irrigation users only.
6.
OPERATIONAL CHALLENGES
Establishing and implementing daily extraction management requires additional
infrastructure and management effort including:
(a) actual or simulated stream flow data,
(b) knowledge of requirements including environmental requirements
(c) operational gauges providing daily flow information,
(d) announcements to all users of the daily flow class,
(e) daily pumping/metering information,
(f) improved data storage and management,
(g) audit and compliance.
In addition, to meet the requirements of constant reviewing of the water sharing
plans, reserve allocations, reduction of storage capacity over time, growing basic
needs in catchment, etc, constant monitoring is required.
The water banking concept might look simple, however its application can be
more complicated in South Africa a country with the history of inequalities (the
issue of trust). It is still not clear how the issues of risk and water pricing,
infrastructure capacity, water quality, seepages as well as the evaporation losses
would be addressed once the FWACS concept is applied.
6.1
Risk
Currently the assurance of supply is given to water users in terms of the priority
levels as classified in the Act. This classification ensures that the resource is
protected, managed and used effectively, efficiently and sustainably. During
drought periods, users within the lowest priority level and pay less amount for
their allocation are restricted first allowing basic human needs and ecological
water requirements to be met as well as water for strategic use to be supplied. In
the capacity sharing concept, the water use entitlements are homogenous
resulting in different allocation system as addressed by the Act.
If assurance of supply is incorporated in the banking option analysis, the results
show that banking results in negative impacts on water supply, assurance of
supply as well as on storage levels particularly during periods of high storage
levels in the dams (Fig11) where Scenario A, use a starting storage
representative of a relative wet year. The actual storage as applicable to the May
2008 operating analysis was used for Scenario A as the Gariep and Vanderkloof
dams were 89% and 83% full respectively. Scenario 1 represents the introduction
of the banking option so that in year one the irrigation demand is reduced to 50%
of the full requirement. This is then banked to be used the next year, resulting in
a 150% of the irrigation demand for the following year. Scenario 2 represents the
inverse of the banking option meaning that in year one 150% of the irrigation
requirement is used with the following year only 50% of the irrigation demand.
From year 3 onwards all scenarios including the base scenarios uses a maximum
of 100% of the irrigation demand and curtailments will be imposed on the system
for the rest of the analysis period when required. It is also evident from the
analysis that water banking results in higher evaporation and spillages from the
dams particularly when the starting storages are high
4500
Supply (million m3 /a)
4000
3500
3000
2500
2000
2007
2008
2009
2010
2011
2012
2013
2014
2015
Years
Base A - 98%
Base A Curt - 98%
A1- 98%
A2 - 98%
Figure 11 Supply for different scenarios within main scenario A at 98% exceedance probability
level.
2016
Gariep & Vanderkloof Dam Storage
Base A
9000
Combined Storage (million m 3 )
Full Supply Volume
8500
8000
7500
7000
Curtailments imposed when required
6500
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
Years
Base a 50%
Base A Curt 50%
A1 - 50%
A2 - 50%
FSV
Figure 12: Combined storage for different scenarios within main scenario A
The Water Resources Planning Model (WRPM) is an extremely powerful tool
used by the DWAF for operational and planning analyses of water resource
systems. The WRPM uses the yield characteristics in combination with the user
priority classification to supply the system demands at the required level of
assurance. Irrigation or domestic users will, for example, be supplied at a lower
assurance than domestic users. In the analysis, the users’ demands are
therefore sub-divided into different user categories according to their level of
priority, which represent different assurance or reliability levels. It is also
important to note that water requirement within the lower assurance category
(irrigation) is priced less than that in the higher assurance (domestic). The
example of the user priority classification definition for the system with two user
categories is presented in Table 1 below.
Table 1
User category and priority classifications
USER CATEGORY
LOW
(95% ASSURANCE)
(1:20 YEAR)
HIGH
(99,5%
ASSURANCE)
(1:200 YEAR
Portion of water requirement supplied (Mm3)
Domestic
0
1 000
Irrigation
2 200
0
Curtailment level
Accordingly, the two water allocation categories are distributed using the shortterm yield curves as indicated in figure13 below. It can be noted that in case of
shortages such that the target draft is to be restricted, it is the lower assurance
water (irrigation in blue block) that is cut-off first. The high assurance water
(domestic, in red) will be the last to be curtailed after the low assurance.
Figure 13: Short-term yield curve showing water availability against assurance level.
However if the water banking concept is introduced and the irrigators decide to
draw all of their annual allocation in the beginning of the year, this then implies
that their supply risk is changed to the higher assurance category, consequently
pushing the domestic water to the lower priority as indicated in figure 14 below.
Figure 14: Short-term yield curve showing water availability against assurance level
6.2
Infrastructure
As previously mentioned, water banking allows water users to accumulate the
part of their share not taken out in a particular year if storage is available. It is
however not clear how the issue of dam safety in case of floods control will be
addressed. For example, if water has been banked and the big flood comes, this
might put the dam as well as the people and infrastructure downstream at risk.
This might result to the emergency release of the banked water and in some
cases the infrastructure i.e sluice gates and valves might not be sufficient to
release that large amount of water. Dam breaks as a result of NOC level being
exceeded.
To ensure that users abstract their allocated volume only strong voluntary
collaboration between users is required and communication of information should
be good. The water accounting and ordering system must be developed to
enforce transparency and ensure users’ faith in the accounting system. The cost
of implementing the system thus become extremely high as computer models will
be required to determine water availability and monitor weather patterns on real
time basis. Access to share of storage space based on legislation must be highly
regulated.
6.3
Inequalities in South Africa
A large amount of water use licenses is already in the hands of white south
Africans, the government in its initiatives to redress the imbalances of the past is
faced with challenges of over allocated catchments and the emerging farmers
can not be allocated with water for the their irrigable land. It is said that new
users will be accommodated by reduction in allocation fractions of existing users
if system is fully allocated. However, capacity sharing concept allows users to
trade their share; this trade involves financial transactions between users. The
existing users might prefer to trade their allocation before it is reduced. Thus
permanent trade needs to be highly regulated by water resource managers due
to third party effects that may result (Pott et al, 2008). This may result to the price
of water being extremely high than the irrigable land further exacerbating the
inequalities problems.
In addition, if due to emergency situations during floods water is released and the
users are required to abstract their water, some farmers especially emerging
farmers might not have the capacity in terms of pumps to abstract the large
volumes of their banked water and also the storage capacity to store such
volumes. This then implies that they will forfeit their allocation.
6.4
Water quality
Users can decide to either bank or take up all their allocation once and bank the
water in their own storages. If most users decide to take up their total annual
allocation for the particular year, accumulation of nutrients due to low storage
levels will increase resulting to the bad water quality in the dam. Water banking
concept does not reflect how this issue will be dealt with in such cases and may
need to be modified.
In addition, major sources of pollution of surface waters in South Africa are
agricultural drainage and wash-off (irrigation return flows, fertilisers, pesticides
and runoff from feedlots), urban wash-off and effluent return flows (bacteriological
contamination, salts and nutrients), industries (chemical substances), mining
(acids and salts) and areas with insufficient sanitation services (microbial
contamination). Pollution of groundwater results from mining activities, leachate
from landfills, human settlements and intrusion of sea water. High pollution loads
may render water unsuitable for uptake by some users. Non-point source
pollution may be difficult to monitor on a continuous basis.
Concentration of such pollutants may be higher in drought situations and may
also vary with depth in a reservoir. This situation favour users who have the
infrastructure to draw off water rapidly when its quality is suitable. Water quality
monitoring is a critical component of FWACS. However it is important to modify
FWACS so that the fraction of shared has both usable and unusable water. Other
complications may arise if we consider that users at different locations may have
different requirements on water quality and that there may be dilution along the
river channel making water suitable for downstream users. Thus FWACS can be
an important driver for water quality monitoring.
6.5
Seepage losses
A dam or a conveyance system could experience high seepage losses. Such
losses can be accounted for through monitoring and modelling. Again it is evident
that the total amount of water available to each user is reduced proportionally.
Thus those users who have a larger fraction may have more interest in reducing
losses. However, seepage losses may become alluvial water, which could
become available to some users and not to others because the extent of alluvial
storage could be site specific. This could be a potential cause for conflict but
could be addressed through considering alluvial storage as part of the water
released. Sustainable use of alluvial aquifers can result in optimal use of the
water resource.
7.
CONCLUSIONS
From the above, it can therefore be concluded that the Eskom/DWAF agreement
cannot be effectively operationalized when all these operational considerations
are taken into account. Once-off dam operations agreement alone can lead to
inefficient or in-optimal water use of the resource. Before these agreements
can be applied, operational considerations need to be taken into account. This
will ensure the sustainable use of water both for economic and social
development.
Banking of water to be used in the following year in general has negative impacts
on water supply to the users as storage levels in the dams over time reduces. It
also results to an increase in the level of curtailments required to be imposed on
the system and should be avoided particularly in times of high starting storages.
The FWACS does not consider the resource availability throughout the critical
period and in the long-term resulting to failure in protecting the water resource as
required by the NWA. The modification to the current applications of FWACS in
Australia and Zimbabwe is required before this system can be applied in South
Africa.
This principle requires extensive amount of monitoring since near-real time data
on incremental and cumulative flow and losses must be captured. Releases from
storage also need to be quantified at a point of abstraction. Implementation of all
these measurements depends on the existence of infrastructure (metering) to
obtain the data for monitoring and high regulation of policies by the department.
Cost of installing measuring devices to measure the volume of water should also
be thoroughly quantified. A high level of trust and accountability among users
cannot be overemphasized unless FWACS can generate chaos. It is therefore
important to build the capacity of new users to participate in the FWACS system.
7.
ACKNOWLEDGEMENTS
We would like to thank the following people for their contributions
Jason, Andrew, Manie, Lerato
REFERENCES
[1] DEPARTMENT OF WATER AFFAIRS AND FORESTRY Orange River
System Annual (2008) Operating Analysis. Report compiled by WRP (PTY) Ltd
for the Department of Water Affairs and Forestry
[2] DWAF, 2008. Orange River system: 2008 additional analysis banking
options. Report compiled by WRP (Pty) Ltd for the Department of Water Affairs
and Forestry
[3] DWAF, 2006. A brief overview of water licensing approaches and the water
allocation practice in South Africa.
[4] DWAF, 2006. A conceptual analysis of fractional water allocation and
capacity sharing. Report compiled by Makgaleng Projects for the Department of
Water Affairs and Forestry
[5] DWAF, 2006. Fractional water allocation and capacity sharing concepts,
principles, assumptions and lessons from selected case studies. Report compiled
by WRP (Pty) Ltd for the Department of Water Affairs and Forestry
[6] DWAF, A guide to the national water act
[7] DWAF, 2005. Orange River System Annual (2005) Operating Analysis.
Report compiled by BKS for the Department of Water Affairs and Forestry
[8] DWAF, 1997. Water resource yield model user guide release 4.1.1
[9] Pott, A.J., Hallowes, J.S., Mtshali S.S., The challenge of water conservation
and demand management particularly for irrigated agriculture in South Africa
Water Research Commission, 2008
SUMMARY
Water resource systems operation is the regulation of the systems in order to
reconcile water availability with requirements and mitigate against flooding and
drought risks. In the past the focus was on development of water resource
infrastructure and operating rules were once-off statements. Competition for
water was still low, the focus was on supplying water to those with the ability to
pay so that more money could be generated for further infrastructural
developments. However, due to the hydrology being highly variable, probably
due to changing climate and/or land use, once-off rules cannot suffice. Regular
operational analyses are required to make water reconciliations based on
envisioned hydrological conditions, water allocation agreements and
infrastructural maintenance schedules. Two examples, one on hydropower
generation and another on water banking allocation approach, will be used to
demonstrate the significance of operating parameters in order to effectively
implement water supply agreements.
Two dams in South Africa on the Orange are used for hydropower generation,
namely Gariep with total output of 360MW and Vanderkloof dam with total output
of 240MW. Fluctuating releases from hydropower turbines of these dams impact
negatively on some users. According to the operational agreement between
DWAF and Eskom, Eskom is allowed to generate electricity at its full load for four
hours per day if they are in crisis/ emergency, unless a drought emergency has
been gazetted for the Orange River system.
Releasing water for power generation at full load for 4 hours as per
DWAF/Eskom agreement when the starting storage in May is 90% as it was the
case this year, indicates that 90MW of power can be generated for only four
months but at the sharp storage drop and the curve plots outside the projected
storage At 100% starting storage, it was found that 90MW of power per machine
can be generated for the whole year as the storage manages to recover during
the summer rains. For the scenario where the dam is assumed to start at 60%
live storage and Eskom generates as per emergency agreement, it was found
that the dam level drops drastically and does not even reach the summer period
for the storage to recover. It is however very rare to have the 100% full storage in
the dams which seems to be the only curve running throughout the year when
the power is generated at Eskom’s full load for 4hours per day
Because of the increasing water demand and competition, various concepts are
proposed for sharing and trading the resources. Water banking is one of the
concepts that are being debated with much vigor for DWAF to adopt for water
allocation and trading. Water banking also known as capacity sharing, refers to
the apportionment system of issuing water use entitlements against shares of the
storage capacity of large government dams, storage shares can be available for
rental or purchase by individual stakeholders The water banking concept might
look simple, however its application can be more complicated in South Africa a
country with the history of inequalities (the issue of trust). It is still not clear how
the issues of risk and water pricing, infrastructure capacity, water quality,
seepages as well as the evaporation losses would be addressed once the
FWACS concept is applied.
If assurance of supply is incorporated in the banking option analysis, the results
show that banking results in negative impacts on water supply, assurance of
supply as well as on storage levels particularly during periods of high storage
levels. Current operation of the dams is such that the lower assurance water (e.g.
irrigation) is cut-off first. The high assurance water (domestic) will be the last to
be curtailed after the low assurance. However if the water banking concept is
introduced and the irrigators decide to draw all of their annual allocation in the
beginning of the year, this then implies that their supply risk is changed to the
higher assurance category, consequently pushing the domestic water to the
lower priority.
In conclusion, once-off dam operations agreement alone can lead to
inefficient or in-optimal water use of the resource. Before these agreements
can be applied, operational considerations need to be taken into account. This
will ensure the sustainable use of water both for economic and social
development. Banking of water to be used in the following year in general has
negative impacts on water supply to the users as storage levels in the dams over
time reduces. It also results to an increase in the level of curtailments required to
be imposed on the system and should be avoided particularly in times of high
starting storages.