Use of a water resources model for basin planning in the
Nile Equatorial Lakes region
- April 2013 -
Julien VERDONCK
IWRM expert
BRL Ingénierie (France)
[email protected]
Emmanuel OLET
Water resources expert / Program manager
NBI / NELSAP
[email protected]
Introduction
The Nile Equatorial Lakes (NEL) region is the southern part of
the Nile River basin and comprises the White Nile River basin,
upstream of the outlet of the Sudd. The NEL region therefore
covers parts of 8 countries, namely Burundi, Democratic
Republic of Congo, Kenya, Rwanda, South Sudan, Sudan,
Tanzania and Uganda, as shown on the Fig. 1 [NBI, 2012].
The NEL region has a very complex water system with
different hydrological patterns [Sutcliff et al., 1999]:
• The Lake Victoria basin is closed by Lake Victoria, a large
buffer zone allowing inter-annual storage and regulating its
outflows. These outflows therefore experience a little seasonal
variation. Rainfall and evaporation dominate the Lake Victoria
water balance, whereas its outflows (i.e. the Nile River)
dominate the water balances of the downstream hydrological
sections.
• Lake Kyoga and Lake Albert have the overall effect of
delaying the outflows from Lake Victoria and increasing the
flows in magnitude and seasonal variability.
• The Albert Nile and Bahr el Jebel receive torrential tributaries
with highly seasonal flows.
• The Sudd swamps between Mongalla and Malakal have a
water balance dominated by very high evaporation losses and
delay the Nile River flows.
• The Bahr el Ghazal basin is a large basin, where evaporation
in its downstream swamps makes it almost an endhoreic
system.
Fig. 1. NEL region part of the Nile River basin.
The Nile Basin Initiative (NBI) is a transitional international organization created in 1999 by the Nile Council of
Ministers of Water Resources (Nile-COM). Through its Nile Equatorial Lakes Subsidiary Action Program
(NELSAP), NBI has elaborated in 2012 a NEL region Multi-Sector Investment Opportunity Analysis (NELMSIOA) [NBI/NELSAP, 2012]. One of the objectives of the NEL-MSIOA was to prioritize and sequence
potential investments in water resources management and development at the regional scale. To do so, several
tolls, which include a water resources planning model, named the NEL Basin Planning Model (NEL-BPM), were
prepared and utilised to analyse scenarios which resulting in an optimised trajectory for growth in the region.
This article points out the lessons learnt from the use of the NEL-BPM in multi-sector investment planning for
river basins.
1. Description of the NEL Basin Planning Model
1.1 Key characteristics of the NEL Basin Planning Model
The objective of the NEL-BPM was (i) to be the simplest model, (ii) capable of modelling the complex NEL
region water system and (iii) producing the anticipated outputs listed in section 1.6 of this article, (iv) under an
envisaged list of scenarios close to the final one shown in Tab. 2, and (v) under a given time and resources
constraint.
The NEL-BPM is a process-oriented, surface water distribution model, based on MIKEBASIN® software. It
simulates the distribution of water through the main branches of the NEL region water system, on a monthly
basis, over the period 1951-1990. Because the time of concentration of the water in the various branches is much
less than one month, no river routing algorithm was used. Water propagation only occurs through the virtual
surface water reservoirs which are used to simulate the lakes and significant wetlands. Large lakes incorporated
are Lake Victoria, Lake Kyoga and Lake Albert. Other wetlands incorporated are Lake Ihema and Lake Rushwa
(on the Kagera River), the lower Bahr el Ghazal wetlands and the Sudd. Each of these wetlands is modelled by a
single reservoir with a storage variation determined by the inflows, rainfall, evaporation and outflows, except for
the Sudd which is modelled by a more complex network of reservoirs. Fig. 2 shows the architecture of the NELBPM. Groundwater flows were not considered in the architecture of the NEL-BPM as they are negligible for the
main lakes and Nile River water balances. However, the effects on the surface water of the relations between
groundwater and surface water, upstream of an entry point of the model, are taken into account when using the
surface inflows at this entry point.
The density of components shown in Fig. 2 (runoff inflows, junctions, reaches and reservoirs) gives the
geographical level of detail of the NEL-BPM. It is therefore worth mentioning that the NEL-BPM is appropriate
for analysis at the regional scale for the entire NEL region. However, detailed analysis of local water systems
(e.g. the Mara River sub-basin alone) should be undertaken using other detailed models at that scale. The notion
of “regional” flows used in this article designates the Nile River flows or the contribution of any reach to the
Nile River flows.
1.2 Inputs data used for the calibration of the NEL Basin Planning Model
The period 1951-1990 used for the calibration of the NEL-BPM is considered as natural (N): water uses between
1951 and 1990 are not taken into account as (i) the water abstractions were negligible during this period
compared to the regional flows, (ii) operation of reservoirs was also negligible given the fact that the Lake
Victoria water release curve followed the natural release curve for the greater part of that period.
Most of the hydrological data used for the calibration of the NEL-BPM comes from the Nile Basin Decision
Support System (Nile-DSS) discharge datasets. The Nile-DSS is a larger-scale tool recently prepared by NBI.
Some complementary datasets come from the hydrological databases of the institutions in charge of water
management in the NEL region’s countries, including the Lake Victoria Basin Commission (LVBC) which
already participated to a modelling exercise of the Lake Victoria basin [LVBC, 2002]. The Semliki River
(upstream Lake Albert) inflows and the Lol, Jur and Tonj rivers inflows (in the Bahr el Ghazal sub-basin) have
been modelled thanks to the use of GR2M, a rainfall-runoff optimization model [Mouehli et al., 2005] used for
filling the gaps of the observed discharge data. The torrential tributaries between Lake Albert and Mongalla are
not systematically gauged and a comparison between the observed Albert Nile and Bahr el Jebel flows is used to
estimate the torrents’ contribution. When no Albert Nile observed flow was available, the modelled flows have
been used. The modelling of the hydrological processes taking place in Lake Victoria, Lake Kyoga, Lake Albert,
the Kagera River and the Sudd has been inspired by recent works from NBI for the testing of the Nile-DSS [NBI,
2011]. The meteorological data, rainfall and evaporation, used for the reservoirs water balances and for the
rainfall-runoff model come from the same sources listed above.
1.3 Reliability of the NEL Basin Planning Model
The precision with which the NEL-BPM can reproduce observed conditions has been analysed through 3
criteria: (i) visual analysis, (ii) conservation of mean annual runoff and (iii) Nash coefficient. The visual analysis
is very good (see for instance Fig. 3), except for the years 1961 to 1965 when a sudden rise of Lake Victoria was
caused by changed rainfall over the lake and its basin. If one excludes these 5 years from the period 1951-1990,
the conservation of mean annual runoff is higher than 95% at key nodes and the Nash coefficient higher than
83% for the same nodes and for the main lakes water levels (e.g. Lake Victoria outflows, Lake Kyoga outflows,
Lake Albert outflows, Sudd outflows). Therefore, the NEL-BPM can be considered as reliable for broad scale
water resources management studies, especially because the main uncertainties have been documented.
Fig. 2. Architecture of the NEL Basin Planning Model.
Fig. 3. Comparison of measured and modelled values for Lake Victoria outflows and Lake Albert levels.
1.4 Scenario elaboration
The various water resources management and development scenarios (simply referred to as “scenarios” in this
article) tested by the NEL-BPM are combinations of selected options for key drivers, as listed in Tab. 1. The
scenarios are listed in Tab. 2.
Four of the drivers represent water uses: potable water supply, environment, hydropower and irrigation. This list
is non-exhaustive but is representative of the key types of water uses: consumptive and non-consumptive uses,
river flow requirements and water storage requirements.
Another driver concerns the possibility of introduction of water storage in order to regulate river flows for
downstream water requirements.
The final driver is related to climate change and gives the possibility of testing a single increase in temperature
of 6°C in the NEL region. This increase in temperature directly affects evaporation from the reservoirs and water
requirements for irrigation in the NEL-BPM. This scenario is quite pessimistic as the current climate change
projections indicate an increase in temperature of 2 to 6°C by the year 2100; the same projections are less
converging for precipitation with a range of variation going from negative to significantly positive, and a
generally increasing trend in the annual precipitation [NBI, 2012 and Booij. et al., 2011].
The rationale for the selection of the drivers’ levels and the 11 scenarios was a compromise between (i) trying to
find the limits of the system, (ii) trying to build scenarios representing plausible futures for the management and
development of the NEL region water resources and (iii) trying to build scenarios which provide equitable
development among the different countries. The irrigation driver is a good example of this approach as level I3
represents an equitable and plausible irrigation development, whereas level I4 represents the maximum land
areas available for irrigation [NBI/NELSAP, 2011], to be tested in order to determine the maximum areas it
would be possible to irrigate with the water resources available.
Tab. 1. Drivers and their levels used for the elaboration of scenarios.
Drivers
Levels
P0 No potable water supply
Potable water
P1 Current situation (supply of the NEL region population in 2012)
supply
P2 Future situation (supply of the NEL region population in 2030)
E0 Current situation without application of any environmental flows rules
Environment
E1 Future situation with application of one environmental flows rule
H0 No hydropower
H1 Current situation of hydropower schemes in the NEL region (installed capacity of 700 M W)
Hydropower
H2 Future situation including the near future identified hydropower projects (installed capacity of 1,400 M W)
H3 Future situation including all the identified hydropower projects (installed capacity of 6,000 M W)
I0
No irrigation
I1
Current situation of irrigation schemes in the NEL region (objective to irrigate 50,000 ha)
Irrigation
I2
Future situation including the near future identified irrigation projects (objective to irrigate 400,000 ha)
I3
Future situation with a normal rate for irrigation development up to 2035 (objective to irrigate 610,000 ha)
I4
Future situation including all the identified potential irrigation areas (objective to irrigation 24,000,000 ha)
S0
No storage can be added for the satisfaction of the water uses
Water storage
S1
Storage can be added for the satisfaction of the water uses
Climate
C0 Hydrological and meteorological data of the years 1951-1990
change
C1 Same as C1 with an increase in ETP corresponding to an increase in temperature of 6°C
Tab. 2. Scenarios tested by the NEL Basin Planning Model.
S cenarios \ drivers and levels
N
0
1
2
3
4
5
6
7
8
9
Natural
Current situation
Near future
Scenario 1 + storage
Scenario 2 + environmental flows
Scenario 3 + large development of hydropower
Scenario 3 + try to develop large irrigation area
Scenario 5 + try to develop potential irrigation area
Scenario 4 + scenario 5
Scenario 4 + scenario 6
Scenario 3 + increase in temperature of 6°C
Potable
Environment
water supply
P0
X
P1 P2
X
X
X
X
X
X
X
X
X
X
E0
X
X
X
X
E1
X
X
X
X
X
X
X
Hydropower
H0 H1 H2 H3
X
X
X
X
X
X
X
X
X
X
X
Water Climate
storage change
Irrigation
I0
X
I1
I2
I3
I4
X
X
X
X
X
X
X
X
X
X
S0
X
X
X
S1
X
X
X
X
X
X
X
X
C0
X
X
X
X
X
X
X
X
X
X
C1
X
1.5 Supplementary modelling components required for scenario analysis
On each reach represented in Fig. 2, it was possible to add some water uses modelling components representing
(i) potable water demand, (ii) environmental flow requirements, (iii) hydropower plants characteristics, (iv)
irrigation water demand, (v) water storage required for the satisfaction of the downstream water demand and (vi)
reservoir characteristics and operation rules. These components were added to the NEL-BPM for the simulation
of various scenarios, combinations of the levels listed in Tab. 1. Most of these modelling components were the
results of necessary estimations and some of them are summarized below:
• The potable water demand has been estimated based on the population census and projections per sub-basin
(each reach of the model corresponding to one distinct sub-basin). The unit water demands were taken as 40, 80,
15 and 40 l/day/capita, respectively for the present urban demand, the 2030 urban demand, the present rural
demand and the 2030 rural demand. The total potable water supply demand is therefore estimated at 18 m3/s
today and 98 m3/s in 2030 in the entire NEL region. For simplification, the total potable water demand is
considered to be satisfied by surface waters in the NEL-BPM.
• Two environmental flow constraints have been taken into account. (i) Downstream of all the tributaries, as well
as at various points of the Nile River, a minimum flow has to be respected throughout the year, 4 years out of 5.
This minimum flow is equal to the low annual minimum monthly flow of scenario N (natural) with a return
period of 5 years. For the Nile River itself, this minimum flow has been reduced to 80% of the low annual
minimum monthly flow of scenario N with a return period of 5 years, because this value was already quite high
due to the natural regulation of Lake Victoria, Lake Kyoga and Lake Albert. (ii) In a given sub-basin, the manmade water storage reservoir volume cannot exceed 40% of the low annual flow of scenario N with a return
period of 5 years. This environmental constraint makes it possible to model the preservation of a certain amount
of flood water.
• The hydropower plants’ characteristics (installed capacity, head and reservoir elevation-volume curves), come
from existing studies. When details were not available, estimations were made with comparisons to similar
projects. Most of the hydropower plants were considered as run-of-river plants. The plants are assumed to be
producing to the maximum possible capacity at all times, with the water available. This simplified strategy does
not optimize the hydropower potential for reservoir-plants, especially in the case of cascades of reservoirs.
• The irrigation water requirements have been calculated with the Penman-Monteith equation. The efficient
rainfall is considered to be 80% of the real rainfall. The crop patterns were estimated based on existing national
statistic surveys. The efficiency of the irrigation schemes is considered to be 25%, but two thirds of the 75%
losses return to the river system. It means that the return flow equals 50% of the abstractions. In a large scale
model, a return flow makes sense if large irrigation schemes are considered. With different efficiency criteria,
the irrigation areas could have doubled or been divided by two.
Once again, given the regional-scale of the model, an irrigation water abstraction component is not associated to
a specific irrigation project, but represents a sum of irrigation projects over a given reach’s sub-basin.
For instance, for the 510,000 ha irrigated in scenario 7, the total average annual gross irrigation water abstraction
is 150 m3/s (or 0.3 l/s/ha), with a peak of 350 m3/s (or 0.7 l/s/ha) during the month of June. Half of it, or 75 m3/s,
is the net abstraction, whereas the other half returns to the NEL region water system.
• When required, supplementary storage reservoirs have been added in the model for the satisfaction of the
downstream water demand. These reservoirs are conceptual in nature since no investigations have been made
with respect to their exact location nor with respect to which other sectors could make use of their construction
to support development. For simplification, their efficiency is considered to be 100% (no losses).
1.6 Outputs from the NEL Basin Planning Model
Some of the key outputs from the NEL-BPM are listed below and represent key drivers for the water resources
management and development in the NEL-region:
• energy production for all the hydropower plants,
• irrigated area,
• water storage capacity required for the satisfaction of the potable water, irrigation water and environmental
water requirements,
• surface and water balance of the main reservoirs (including the modelled lakes and wetlands),
• water discharge at key locations of the NEL region (downstream of the various sub-basins, at the outlet of Lake
Victoria, Lake Kyoga and Lake Albert, and at the outlet of the NEL region).
2. Use of the NEL Basin Planning Model
2.1 Key results from the scenario analysis
Tab. 3 presents some simple statistics of some of the key outputs from the NEL-BPM for the 11 scenarios. The
NEL region outflows are the flows of the White Nile River at the outlet of the Sudd, or upstream the confluence
with the Sobat River. The following key preliminary conclusions can be drawn from Tab. 3:
• Potable water supply is not shown in Tab. 3 because potable water supply requirements are satisfied with the
highest priority in all the scenarios.
• Comparison between scenarios N (natural) and 0 (current situation) shows that the water resources quantities
are hardly affected at all by the current relatively low levels of water resources management and development.
• Comparison between scenarios 0 (natural) and 1 (near future) shows that the water resources quantities will be
scarcely affected by the near future levels of water resources management and development (consisting mainly
of an increase in potable water supply requirements), except for the Sudd area in dry years. Uganda represents a
large part of the hydropower production thanks to the Nile River potential.
• Comparison between scenarios 1 and 2 (scenario 1 + storage) shows that the creation of supplementary storage
reservoirs allows for more irrigation, especially in Kenya, South Sudan (Bahr el Ghazal), Tanzania and Uganda
(small tributaries of Lake Kyoga) where the low flow levels in the dry season limit irrigation. However, this
supplementary irrigation in the NEL region has once again a significant impact on the Sudd areas in dry years (20%).
• Comparison between scenarios 2 and 3 (scenario 2 + environmental flows) shows that the environmental flow
requirements call for more storage (+80%) if the irrigation levels remain the same, mainly in the smaller
tributaries where irrigation was using all the low flows in the dry season in scenario 2.
• Comparison between scenarios 3 and 4 (scenario 3 + large development of hydropower) shows that the planned
hydropower development in the NEL region has no impact on the regional water quantities. However, one can
see that it slightly improves the low flow levels at the outlet of the NEL region (supplementary water storage
provided by the new South Sudanese hydropower dams). The total hydropower generated will be multiplied by
10 (compared to the current situation) with all the identified hydropower projects. The comparison between
scenarios 5 and 7 gives the same results. The hydropower production is mainly shared by the two countries
crossed by the Nile River: Uganda (65%) and South Sudan (30%).
• Comparison between scenarios 3 and 5 (scenario 3 + try to develop large irrigation areas) shows that an
increase in irrigation of +50% has very few impacts on the water resources quantities, except for the
supplementary storage required (+71%). Moreover, whereas the model tried to irrigate 610,000 ha, it appears
that only 510,000 ha can be irrigated 4 years out of 5, because of the environmental flow constraint limiting the
supplementary storage capacities on some of the smaller tributaries. There is no significant impact on the
hydropower production and this conclusion will remain the same when the hydropower development level is
large (comparison between scenario 4 and 7).
• Comparison between scenarios 5 and 6 (scenario 5 + try to develop potential irrigation areas) shows that a huge
irrigation increase (+650%) has a big impact on water resources quantities, especially during dry years.
Moreover, once again, whereas the model tried to irrigate 24,000,000 ha, it appears that only 4,000,000 ha can be
irrigated 4 years out of 5, because of the environmental flow constraints all over the NEL region. And it is only
possible to irrigate those 4,000,000 ha with a very high increase in water storage capacities (+534%). Also, this
huge increase in irrigation has a significant impact on hydropower production (-14%). The comparison between
scenarios 7 and 8 emphasizes this conclusion (-21% of hydropower production). The position of Uganda in the
NEL region, as being the upstream country benefiting from the regulated flows of the Nile River, gives it an
advantage for irrigation development. The other countries surrounding the reservoirs of Lake Victoria and Lake
Albert also have access to this regulated resource, but less irrigable land potential inside the NEL region than
Uganda. Therefore, Uganda would obtain 60% of the 4,000,000 ha irrigated of scenarios 6 and 8.
• Comparison between scenarios 3 and 9 (scenario 3 + increase in temperature of 6°C) shows the very high
impact that climate change could have on the NEL region water resources quantities and associated water uses.
For instance, the average outflows of Lake Victoria would decrease by 42%. Hydropower and irrigation areas
decrease in Uganda and South Sudan whereas the other countries increase their storage to sustain the same levels
of hydropower and irrigation production.
Tab. 3. Key results of the simulations of the scenarios.
It is clear from the above analysis that, despite the fact that the current level of development of the NEL region
waters is low, there is already a conflict between irrigation water use and environment. Moreover, the
competition could increase and also affect the hydropower water use if the irrigated area increases up to around
500,000 ha. It is also clear that the conflict between irrigation water use and environment will limit the irrigated
area well below the estimated potential of 24,000,000 ha in the NEL region, but this limit will depend on the
environmental constraints to be chosen or maybe on conflict with other water uses such as hydropower or
navigation.
2.2 Understanding of the complex NEL region water system
The use of the NEL-BPM can also facilitate the understanding of the complex NEL region water system. Fig. 4
shows a simplified water balance of the NEL region, for scenario 0 (current situation), which highlights the
importance of the major lakes and wetlands whose water balances are also presented (the NEL-BPM only
provides the estimated net evaporation over the Sudd; the distinction between rainfall and evaporation for the
Sudd is therefore only indicative in Fig. 4).
Fig. 4. Schematic representation of the NEL region water balance.
While it makes sense to focus on the NEL region water system because its characteristics, especially the roles of
the lakes and wetlands, make it a coherent scale, the simulated scenarios also have impacts outside of the NEL
region, especially for the downstream countries Sudan and Egypt. This is very clear when one compares the
different NEL region outflows in Tab. 3. For instance, the comparison between scenarios N and 5 shows that the
development of 510,000 ha in the NEL region and supply of the 2030 NEL region population reduces the NEL
region average outflows by 44 m3/s (-9%). It is also worth noting that the net water abstractions in scenario 5
total 175 m3/s in the NEL region. The buffer effect of the NEL region lakes and wetlands therefore attenuates the
effects of abstractions: on average, for 1 m3 abstracted in the NEL region, around 0.25 m3 are taken from the
NEL region outflows.
Conclusion
The NEL-BPM is a decision support tool that is able to assess the impacts of different water resources
management and development trajectories on the quantities of water in the NEL region.
As for all models, the NEL-BPM outputs are subject to inaccuracy, caused by data scarcity, water system
complexity, the large scale of the analysis and the necessity to keep the model as simple as possible, notably for
the involvement of stakeholders who typically have a broad range of non-technical backgrounds. Further
improvements of the NEL-BPM could concern the detail of its architecture, the improvement of the calibration
of the reservoirs’ functioning during the wet period (1961-1965), rainfall-runoff modelling for some of the
tributaries (including the torrents between Lake Albert and Mongalla) and of course the simulation of more
scenarios allowing notably for a sensitivity analysis on the numerous assumptions listed in section 1.5.
While all the physical and hydrological details are necessary for a reliable and detailed representation of the NEL
region water system’s functions, some of the ultimate criteria relevant to the decision making process are
monetary. Costs and benefits are easy enough to interpret and understand, and are directly relevant to all
stakeholders. This was advanced in the NEL-MSIOA through coupling the NEL-BPM with an economic model.
The conclusion was that scenario 7 was the most profitable and plausible one. Scenario 7 consists of developing
510,000 ha of irrigation and the generation of 37,000 GWh; it requires the supplementary storage of around 3
km3 spread over the NEL region and respects some relatively stringent environmental flow rules.
However, further water resources management and development activities in the NEL region are constrained by
the cost and potential impact of new projects and by the potential reduction of flows due to climate change,
which could increase variability and impose more conservative management for storage.
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1.
The Authors
J. Verdonck is an Integrated Water Resources Management (IWRM) expert with both an engineering MSc degree from
Ecole Polytechnique in Paris, and a specialized MSc degree in IWRM from ENGREF in Paris/Montpellier (France). He has
10 years of experience in IWRM and has worked for The WorldBank, for the French “Institut de Recherche pour le
Développement” (IRD), and currently for the consultancy company BRL Ingénierie. J. Verdonck has been involved in a
number of IWRM studies in Africa and has especially focussed on two aspects: (i) institutional aspects related to water
management, and notably related to the institutional design of river basin organizations, and (ii) more technical aspects
related to the elaboration and use of water resources planning models. J. Verdonck is based in Kenya and currently mainly
working on studies for NBI and LVBC, after having mainly worked on the Niger River, Zambezi River and Lake Chad
basins during the last 10 years.
E. Olet is a Civil Engineer with an MSc degree in hydraulic engineering from the UNESCO-IHE Institute for Water
Education in Delft (Netherlands) and a BSc Degree in civil engineering from Makerere University in Kampala (Uganda). He
is a registered engineer with the Engineers Registration Board in Uganda and a Member of the Uganda Institution of
Professional Engineers. His speciality is water resources infrastructure planning in river basins, hydropower planning,
watershed management, flood and drought control, design of hydraulic structures, construction supervision, engineering
audits, environmental and social impact assessment and economic and financial analysis of engineering projects. E. Olet has
14 years experience working in Government, consultancy and in multi-national river basin organisations. He presently works
as program officer in charge of water resources development, at the NELSAP, an investment program of the NBI. Prior to
that, he worked with Norplan (U) Ltd - consulting engineers and planners, where he was head of the water resources division.
He also worked as project engineer in the water for production section of the Ministry of Water and Environment in Uganda.
E. Olet was the project manager for the NEL Water Resources Development Project, under which the NEL-MSIOA, which
provided material for this article, was undertaken.