Trade-offs between Hydropower and Irrigation Development and their Cumulative Hydrological Impacts

Full text

MK3
Optimising cascades
of hydropower
AGRICULTURE &
IRRIGATION
TRADE-OFFS
BETWEEN
HYDROPOWER AND
IRRIGATION
DEVELOPMENT AND
THEIR CUMULATIVE
HYDROLOGICAL
IMPACTS
A Case Study from the
Sesan River Basin
August 2013
Timo A. Räsänen. Olivier
Joffre, Paradis Someth and
Kummu Matti

Authors
Timo A. Räsänen (Aalto), Olivier Joffre (ICEM), Paradis Someth (ITC) and Kummu Matti (Aalto)
Produced by
Mekong Challenge Program for Water & Food Project 3 – Optimising cascades of hydropower for multiple use
Lead by ICEM – International Centre for Environmental Management
Suggested citation
Räsänen Timo A., Joffre Olivier, Someth Paradis and Kummu Matti. 2013. Trade-offs between Hydropower
and Irrigation Development and their Cumulative Hydrological Impacts. Project report: Challenge Program on
Water & Food Mekong project MK3 “Optimizing the management of a cascade of reservoirs at the
catchment level”. ICEM – International Centre for Environmental Management, Hanoi Vietnam, 2013
More information
www.optimisingcascades.org | www.icem.com.au
Image
Photo credit: T. Ketelsen
Project Team
Peter-John Meynell (Team Leader), Jeremy Carew-Reid, Peter Ward, Tarek Ketelsen, Matti Kummu, Timo
Räsänen, Marko Keskinen, Eric Baran, Olivier Joffre, Simon Tilleard, Vikas Godara, Luke Taylor, Truong Hong,
Tranh Thi Minh Hue, Paradis Someth, Chantha Sochiva, Khamfeuane Sioudom, Mai Ky Vinh, Tran Thanh Cong
Copyright
 2013 ICEM - International Centre for Environmental Management
6A Lane 49, Tô Ngoc Vân| Tay Ho, HA NOI | Socialist Republic of Viet Nam
This report was prepared as part of MK3 project on ‘Optimizing the Management of Cascades or Systems of
Reservoirs at Catchment Level”, which was part of larger project by the Challenge Program on Water & Food
between the years 2010-2013 (http://mekong.waterandfood.org/). The MK3 project was led by International
Centre of Environmental Management (ICEM) and this report was prepared in collaboration between Water
& Development Research Group of Aalto University in Helsinki, ICEM in Hanoi and Department of Rural
Engineering of Institute of Technology of Cambodia.

SUMMARY
The Mekong River basin is undergoing rapid demographic and economic development, which has led
to increased demands for food and energy production. The increased food and energy demand has
caused increased water withdrawals for agriculture and the extensive construction of hydropower
dams. The hydropower development is seen as a threat to regional food security as it will significantly
affect the productivity of aquatic ecosystems, which are a key source of nutrition in the region.
Recently, discussion has focused on implementing multi-purpose reservoirs to maximise benefits.
Hydropower reservoirs are large water storage facilities and they could potentially facilitate irrigation
and improve agricultural production. However, the benefits and negative impacts of such
development should be carefully assessed before implementation.
In this paper, we contribute to the discussion on the development of multi-purpose reservoirs by
assessing related water, food and energy trade-offs. We have achieved this aim by using the Sesan
River catchment, shared by Viet Nam and Cambodia, as a case study area and by developing a model-
based approach for the trade-off assessment. We evaluated whether the existing and planned
reservoirs have the potential to facilitate irrigation of rice, and we also looked at the effects on
hydropower production and flow regimes. The approach consisted of hydrological modelling,
agricultural land suitability assessment, crop water requirements assessment, and modelling of
hydropower and irrigation operations of multipurpose reservoirs on a catchment scale. The
assessment was first done on a case study hydropower cascade consisting of three reservoirs and
then expanded to the catchment scale to cover all 11 existing and planned hydropower projects.
We found that implementing irrigation did not reduce hydropower generation capacity by much, thus
suggesting that irrigation could be feasible. In the case study of the reservoir cascade, Yali-Sesan 3-
Sesan 3A, irrigation of 3,894 hectares, 6,490 ha and 9,086 ha reduced the annual hydropower
generation of Sesan 3A by 0.7 percent, 1.2 percent and 1.6 percent, respectively. Irrigation from Sesan
3A required a cascade operation with Yali and Sesan 3 to sustain adequate flows since Sesan 3A did
not by itself have sufficient storage capacity. When we expanded our analysis to the catchment scale
and studied seven reservoirs, the total irrigated area was 28,348 ha, which led to a 1.6 percent (209
Gigawatt hours) reduction in the total annual hydropower generation of nine projects that had a total
capacity of 13,056 GWh. Thus, on a catchment scale, the reduction of hydropower generation was
small compared to the irrigation potential the reservoirs provided.
The hydrological impacts of hydropower operations were significant. Hydropower operations
increased dry season (November-April) flows on average by 167 percent and decreased wet season
(May-October) flows on average by 11 percent in the lower reaches of Sesan River. Irrigation affected
dry season flows by reducing them, but the impacts were largely masked by the increased flows from
hydropower operations. Altogether, irrigating 28,348 ha resulted in annual water losses of 0.39 cubic
kilometres, corresponding to 1.9% of the annual flow of the Sesan River (20.5 km3). It was also
recognised that developing multipurpose reservoirs would lead to a highly managed river catchment
with transboundary water sharing issues and so, impacts on existing development in the catchment
should be carefully assessed.
We also found that the development of multipurpose reservoirs would lead to extensive land cover
changes. The existing and planned reservoir would inundate an area of 1,035 square kilometres and
the potential agricultural development would affect 280 km2. Altogether, the development would
affect 7 percent of the Sesan catchment area. It was also found that the planned Lower Sesan 2 and
Lower Sesan 3 reservoirs would inundate over 10,000 ha of land suitable for the cultivation of rice.
Although this study showed that irrigation from reservoirs would be technically feasible, the focus of
the assessment was on technical and hydrological aspects. Therefore, the study could not address
broader trade-offs related to multipurpose dams, such as impacts on aquatic ecosystems and social,
livelihood and land ownership aspects. Thus, the findings of this report call for further research on
multi-purpose dams to draw a more comprehensive picture of their benefits and negative impacts.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
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TABLE OF CON T E N T S
SUMMARY .......................................................................................................................................... 1
1 INTRODUCTION ........................................................................................................................... 3
1.1 Background ................................................................................................................................... 3
1.2 Objectives and approach .............................................................................................................. 4
2 SESAN RIVER BASIN .................................................................................................................... 4
3 METHODOLOGY .......................................................................................................................... 7
3.1 Catchment hydrology ................................................................................................................... 7
3.2 Hydropower operations ............................................................................................................... 8
3.3 Land suitability assessment .......................................................................................................... 8
3.4 Crop water requirement and irrigation schedule ......................................................................... 9
3.5 Irrigation potential from reservoirs .............................................................................................. 9
3.6 Assessment of irrigation expansion impacts on hydropower generation .................................. 10
3.7 Impact assessment of hydropower development and irrigation expansion on hydrology and
land cover............................................................................................................................................... 11
4 RESULTS .................................................................................................................................... 11
4.1 Catchment hydrology ................................................................................................................. 11
4.2 Hydropower operations ............................................................................................................. 12
4.3 Land suitability for irrigated rice ................................................................................................ 14
4.4 Crop water requirement............................................................................................................. 16
4.5 Irrigation potential from reservoirs ............................................................................................ 16
4.6 Trade-off between hydropower generation and irrigation ........................................................ 18
4.6.1 Yali - Sesan3 - Sesan 3A cascade ....................................................................................... 18
4.6.2 Basin wide trade-off .......................................................................................................... 19
4.7 Impact assessment ..................................................................................................................... 21
4.7.1 Hydrology .......................................................................................................................... 21
4.7.2 Land cover changes ........................................................................................................... 23
5 DISCUSSION .............................................................................................................................. 24
5.1 Lost agricultural land due to inundation .................................................................................... 24
5.2 Other measures for increasing agricultural production ............................................................. 24
5.3 Social and livelihood considerations .......................................................................................... 25
5.4 Further environmental impacts .................................................................................................. 25
5.5 Cumulative catchment scale environmental impacts ................................................................ 26
6 CONCLUSIONS ........................................................................................................................... 27
REFERENCES ..................................................................................................................................... 29

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
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1 INTRODUCTIO N
This report aims to assess alternative water management strategies for hydropower reservoirs and
the resulting impacts on hydropower generation and hydrology in the Sesan catchment. The strategy
investigated was the use of reservoirs for irrigation and hydropower generation. The study was
accomplished by selecting a case study reservoir for detailed analysis and then scaling up the
approach to the catchment to include nine [OR 11?] existing and planned reservoirs. This report
presents the methodology and the results. The report also raises important issues that need to be
considered before irrigation development. The report is intended to be a general assessment of the
trade-offs between hydropower generation and irrigated agriculture and not a development
suggestion per se for Sesan River catchment. It is important to note that this work focuses only on
technical and hydrological aspects, and other environmental, social, livelihood and economical
impacts were not assessed.
1.1 B A C K GR OU ND
The Sesan River catchment is part of the Mekong River basin, which is undergoing extensive
hydropower development (Grumbine and Xu, 2011; Stone, 2011). The hydropower development is
driven by demographics, human development, water and food security, economic integration and
climate change and also by new dam financiers and challenges in governance and decision making
(Grumbine et al., 2012). There are 36 large operational hydropower dams in the Lower Mekong basin
and there are plans to build 54 more by the year 2030 (MRC, 2011a). Furthermore, in the Upper
Mekong basin, there are five large operational dams and plans to build 24 more (Räsänen et al.,
2012).
Hydropower development on the Mekong will inevitably bring economic benefits and energy security
to the region, but it could adversely impact the region’s ecosystems, social systems, livelihoods and
even food security (ICEM, 2010; Stone, 2011; MRC, 2010). Major concerns have been raised about the
impact of dam construction on fish biodiversity and fisheries (Dugan, 2008; Dugan, et al.; 2010, Ziv, et
al., 2012), which are an important source of protein and livelihood for millions of people in the region
(Hortle, 2007; MRC, 2010; Baran and Myschowoda, 2009). Thus, hydropower development could
involve major trade-offs between several sectors such as energy, food and biodiversity, which are
closely interconnected but serve different purposes on different timescales.
The Mekong region has a monsoon climate with distinct dry and wet seasons, which means water
availability is uneven throughout the year (MRC, 2005). Water is abundant during the wet season and
scarce in the dry season. Water availability affects people’s livelihoods and the economy (MRC, 2010).
The agricultural sector is especially vulnerable to variations in water availability since as much as 80
percent of the region’s agriculture is rain-fed (Shimizu, et al., 2006). The most recent droughts that
affected agricultural production occurred in 1992, 1993, 1998, 1999, 2003, 2004, 2005 and 2010 (Te,
2007; MRC, 2011b; Qiu, 2010; Stone, 2010; MRC, 2010). The droughts were not always due to to
accumulated rainfall deficits, and were sometimes caused by variations in rainfall patterns. For
example, the 2004 drought was caused by an early end to the monsoon rains (Te, 2007). Furthermore,
the rainfall and flow regimes in the Mekong are known to be affected by the El Niño Southern-
Oscillation (ENSO) and most of the abovementioned droughts, except for 1993 and 1999, occurred
during El Niño years (Räsänen and Kummu, 2013). Inter-annual climate variability has also increased in
the Mekong (Räsänen et al., 2013).
Water deficits in the region have traditionally been solved using supplementary sources such as small
ponds, water accumulated in local land depressions (Shimizu et al., 2006) and irrigation systems
(MRC, 2010). Irrigation has been used varyingly in many regions to enable dry season agricultural
production, and in some areas, to stabilise wet season production and to produce a third crop in a
year (MRC, 2010). Currently, the irrigated area in the in the Lower Mekong basin is over 4 million
hectares and increasing (MRC, 2010).

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Trade-offs between hydropower and irrigation development and their cumulative hydrological
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There is general agreement among scientists that rice production, which often requires irrigation,
needs to be increased in Southeast Asia (Mukherji, et al., 2009). For example, the countries
considered in this study are highly food insecure (FAO, 2012; ESCAP/UN, 2009). Cambodia, especially,
has low productivity and could improve its agricultural production through irrigation (Yu and Fan,
2011). However, irrigation expansion is not the only way to increase agricultural productivity; drought
forecasting, improved irrigation efficiencies, development of drought management policies and
improving drought preparedness and mitigation could all play a role (Yu and Fan, 2011; Shimizu, et al.
2006; Te, 2007; Phengphaengsy and Okudaira, 2008; MRC, 2010).
Irrigation in the Mekong region requires water storage facilities, especially for dry season irrigation,
since water is unevenly distributed throughout the year. Hydropower development is rapidly
increasing the water storage availability in the basin, and this could potentially be used for irrigation if
the reservoirs are constructed to accommodate multiple purposes. It should be noted that planning
and designing a hydropower project for multiple uses may not completely alleviate its negative
environmental effects, but it could provide some benefits to society, livelihoods, and food security. A
multipurpose reservoir could contribute to more equitable sharing of resources and benefits.
1.2 O B J E C T IV ES A N D AP PR OA CH
This work has two main objectives: i. to estimate the impacts on hydropower generation when water
is abstracted for irrigation and ii. to estimate the cumulative hydrological impacts of hydropower
development and irrigation expansion across the Sesan River catchment. These objectives were
accomplished by establishing the baseline hydrology, estimating land suitability for irrigated rice along
the Sesan River, estimating the crop water requirements and, eventually, simulating hydropower
projects as multi-purpose reservoirs that facilitate irrigation. This approach allowed an assessment of
the trade-offs between irrigation and hydropower generation, as well as the evaluation of
hydrological impacts on a catchment scale. The trade-off analysis was done for a case study reservoir
with good irrigation potential and then expanded to the catchment level to include all existing and
planned hydropower projects. The impacts of hydrological changes were also discussed since
hydrology can affect aquatic ecosystems (Junk et al., 1989; Bunn and Arthington, 2002; Lamberts,
2008), which are important to millions of people in the Mekong region (MRC, 2010). The assessments
presented in this report are coarse scale estimates of technical and hydrological aspects.
2 SESAN RIVER B A S I N
Though the main focus of this report was the Sesan catchment and its development, the Srepok River
was also included in our study of hydrology since the Sesan River joins the Srepok River to form a
major tributary of the Mekong River (Figure 1). The Sesan catchment covers 18,684 km2 and the
landscape varies from the lowlands (60 metres to 300 m above sea level) of Cambodia in the west to
the highlands (300 m to 700 m above sea level) and the Annamite mountain range (1000 m to 2100 m
above sea level) of Viet Nam in the east. The Sesan River flows more than 440 km before joining with
the Srepok River. Before discharging the waters into Mekong, a third river, the Sekong, joins in and
the three together form the 3S River catchment (Figure 1).

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
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Figure 1. 3S sub-catchments of the Mekong Basin, with Sesan in the middle, Srepok in the south and
Sekong in the north. The map also shows the 11 hydropower projects studied in this
report and the main discharge stations used for calibrating the hydrological model.
The climate in the 3S region is characterised by distinct wet (May-Oct) and dry seasons (Nov-Apr)
(MRC, 2005; MRC, 2010). The beginning of the wet season is marked by the onset southwest
monsoon, which brings moisture and rainfall from East Sea. Later in the wet season, tropical cyclones
contribute to the rainfall. The dry season is characterised by drier air flowing in from the northeast.
The annual rainfall in the 3S catchment varies from 1,300 millimetres to more than 3,000 mm
depending on location, and averages 2,240 mm for the three catchments. The average annual
temperature ranges from 26 degrees Celsius in the lowlands to 18 °C in the more mountainous
regions. The distinct wet and dry seasons have created a river hydrology that has a pulsing nature, i.e.,
a flood pulse. This means that the flows are highest between June and November and lowest during
the dry season. For example, at Ban Kamphun, near the confluence of Sesan and Srepok (see location
in Figure 1), the average minimum monthly flow is 292 cubic metres per second in April (dry season),
while the average maximum monthly flow is 4,155 m3/s in August. More detailed descriptions of the
meteorology and hydrology of the 3S can be found in the Hydrology and Floor Control chapter.
The pulsing hydrology of the Mekong region has created aquatic ecosystems that are rich in
biodiversity and highly productive (Junk, et al., 2006; Lamberts, 2008). The 3S Rivers are no exception
and contain 329 fish species, which represent 42 percent of all the fish species recorded in the
Mekong. The Sesan River alone harbours 133 fish species. The 3S Rivers are also important for
migratory fish, and some 89 migratory species have been observed in the 3S Rivers, of which 54 have
been observed in the Sesan. These species provide an important source of protein and livelihoods.
Some 840,000 people live along the Sesan River and although it is unclear how many of them depend

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
6
on aquatic resources, the number is thought to be substantial. More recently, the abundance of fish
species and fish catches have been observed to decline in the 3S Rivers. For additional information
refer to the Fisheries and Environment Chapter.
Agriculture in the Sesan catchment has traditionally been based on shifting culture and on lowland
rainfed rice crops. However, cultivation patterns are changing rapidly. In Viet Nam, the area cultivated
with rubber and cassava grew at an annual rate of 8 percent between 2005 and 2009. Robusta coffee
was the other major crop in the basin. The expansion shifted cultivation away from rainfed rice to
these commercial crops, and led to the deforestation of newly reclaimed areas. Recently, in
Cambodia, the expansion of industrial plantations of rubber and cashews has become important, as
well as annual commercial crops such as cassava, soya and peanuts. As in Viet Nam, the expansion has
encroached on natural forest and, in Ratanakiri Province, forested area decreased by 40 percent
between 1997 and 2005. Agriculture expansion in Cambodia is driven by large plantations on private
land concessions, while the agriculture sector in Viet Nam is dominated by smallholder ownership.
Another major difference between the countries is the access to irrigation. Rainfed agriculture is most
common in Cambodia, with irrigation available for only 50 ha of rice and that too only in the dry
season. In Viet Nam, a multitude of small irrigation schemes supply 28,000 ha. Access to irrigation
allows for a double rice crop, with a first crop in the dry season (January to April) and a second crop in
the rainy season from (July to October). For additional information refer to the Agriculture and
Irrigation Chapter.
The Sesan, Sekong and Srepok Rivers are undergoing extensive hydropower planning and
construction. There are seven operational large hydropower projects on the upper parts of Sesan
catchment in Viet Nam (MRC, 2009; MRC, 2011a). There are plans to build four additional projects on
the lower reaches of the Sesan. In this study, we have focused on the 11 largest hydropower projects
on Sesan River and one project, the Lower Sesan 2, which is located below the confluence of the
Sesan and Srepok Rivers. The locations of the selected projects are shown in Figure 1 and their
technical characteristics are shown in Table 1.
The size and characteristics of the existing and planned hydropower projects vary greatly. The annual
energy production of the smallest projects (Prek Liang 1 and Prek Liang 2) is around 180 GWh, while
the larger projects (such as Yali) produce 3,700 GWh. The total energy production of the 11 projects
would be around 13,400 GWh. The active storage capacities of the projects vary from 3.4 million cubic
metres up to 948 mcm. The total active storage capacity of all projects on the Sesan River, excluding
Lower Sesan 2, will be 2,738 mcm, which corresponds to approximately 13 percent of the total annual
flow of the Sesan River. When all 11 hydropower projects are constructed, they would inundate
approximately 1,034 km2 of land, which corresponds 5.5 percent of the area of the Sesan catchment.
The existing reservoirs have already led to the resettlement of 17,431 people and the proposed
reservoirs would require at least 11,596 more people to be moved (the numbers of resettled people
are from BDC field dam data sheet 2 produced during the MK3 project work).
Table 1 Main characteristics of existing and planned hydropower projects in the Sesan catchment
(MRC, 2009).
Upper
Kontum
Plei
Krong
Yali
Se San
3
Se San
3A
Se San
4
Sesan
4A*
Se San
1
Prek
Liang 2
Prek
Liang 1
Lower
Sesan 3
Lower
Sesan 2
Commission year
2011
2008
2001
2006
2007
2009
2008
?
NA
NA
NA
2016
Active storage
[mcm]
122.7
948
779
3.8
4
264.2
7.5
3.4
180
110
323
379.4
Reservoir area
[km2]
7.4
53.3
65
3.4
8.5
58.4
1.7
10.6
11.9
7
414
394
Reservoir full
supply level [m]
1170
570
515
304
239
215
155.2
141
515
330
150
75
Reservoir minimum
supply level [m]
1143
537
490
303.2
238.5
210
150
140
496
310
147
74
Average flow [m3/s]
15.2
128
262
274
283
328.9
-
395
17.7
27.2
500
1304

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
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Upper
Kontum
Plei
Krong
Yali
Se San
3
Se San
3A
Se San
4
Sesan
4A*
Se San
1
Prek
Liang 2
Prek
Liang 1
Lower
Sesan 3
Lower
Sesan 2
Design flow [m3/s]
30.5
367.6
424
486
500
719
-
319
17.7
27.2
500
2119.2
Rated head [m]
904.1
31
190
60.5
21.5
56
-
32
168
153
58.5
26.2
Installed capacity [MW]
250
100
720
260
96
360
-
90
25
35
243
480
Mean annual
energy [GWh]
1056
417
3658
1224
475
1420
-
480
186
189
1977
2312
* Sesan 4A is a re-regulating dam for Sesan 4 and does not have power generating units
3 METHODOLOGY
The assessment of the trade-offs between hydropower generation and irrigated agriculture was based
on six components: A. hydrological modelling, B. land suitability assessment, C. crop water
requirement modelling D. hydropower baseline modelling, E. assessment of irrigation potential and F.
trade-off modelling (Figure 2). Hydrological modelling provided inputs such as precipitation and
temperature to the land suitability assessment and crop water requirement modelling. It also
provided river discharge inputs to the hydropower baseline modelling. The land suitability assessment
and crop water requirement modelling provided the potentially irrigable land and irrigation water
volumes, which could be used to assess the irrigation water demand for each reservoir. For defined
irrigation water volumes, the trade-offs were then modelled and compared against the hydropower
baseline. The last component was the cumulative hydrological impact assessment, where the
hydrological impacts were assessed against the natural hydrology generated by the hydrological
modelling. A general description of the methodology and data used in each component is given
below and a more detailed description of the methods can be found in the Hydrology & Flood control
and Agriculture & Irrigation chapters, and the Annex supporting this Chapter.
Figure 2. Methodological approach for modelling the trade-offs in hydropower generation and
irrigated agriculture
3.1 C A T C HM EN T HY DR O LO G Y
Catchment hydrology was modelled using a distributed hydrological model, VMod, which is the
hydrological part of the broader IWRM model (Koponen, et al., 2010). The model is based on a gird
presentation of the catchment as well as a river network model. Each grid cell of the catchment model
can be defined to have different land use and soil characteristics. The model computes the
hydrological processes (for example, precipitation, interception, infiltration, evapotranspiration,
surface flow, soil water storage and flows to streams or neighbouring grid cells) within each grid cell
according to the parameterization of land use and soil characteristics.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
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The river network model routes the flow from smaller streams to larger streams and finally to the
outflow point of the catchment, while considering the given river channel characteristics. The model is
driven by meteorological data defining the precipitation and potential evaporation (PET). Various
methods can be used to calculate the PET. The model uses also spatial interpolation to produce field
variables from the point measurements of meteorological variables. The model is calibrated and
validated against the measured river flows at selected locations. The model’s grid presentation allows
the extraction of several hydrological variables from any grid cell in the basin. The model has been
used previously for the Mekong region by Räsänen, et al. (2012) and Lauri, et al. (2012).
In this study, the model was setup and calibrated for the 3S River basin, with main focus being the
Sesan River catchment. We used soil type data and soil characteristics from the Food and Agriculture
Organisation (2001, 2007), land use maps from the MRC (2011d), land use characteristics from
Hageman (2002), soil hydraulic characteristics from Saxton and Rawls (2006) and daily precipitation,
temperature and discharge data from the MRC (2011c). Temperature data were supplemented with
reanalysis data from NOAA (2011). The model was calibrated against four discharge measurements
stations (see locations Figure 1) for the period 2001 to 2007, and also for the 11 existing and future
dam sites using the announced average discharge from the MRC (2009) (a more detailed description
of the methodology is presented in the Hydrology and Irrigation chapter).
3.2 H Y D R O P OW ER O PE R AT IO N S
Hydropower operations were modelled using the generalised dynamic programming tool, CSUPD
(Labadie, 2003). CSUDP is an optimization tool that uses discrete Dynamic Programming with
Successive Approximations (DPSA) to solve sequential decision problems such as those presented by
hydropower cascade operations. The model is a general tool and therefore, requires users to define
the problem to be solved. While modelling hydropower projects, the user has to define the elevation-
volume relationship of each reservoir, the energy production characteristics of the power plant and
an objective function defining the desired operation. When the problem has been defined and
objective function set up, the model optimises the operations by minimising or maximising the
objective function. The user can additionally impose restrictions during the optimisation process, such
as on water levels or release demands. An example of the outcome of the optimisation process would
be maximised energy production, with related water releases and reservoir water levels within the
desired limits of operation. Dynamic programming is a popular method for solving various water
resource management problems (Labadie, 2004; Rani and Moreira, 2010) and it has been used
previously in the Mekong region by Räsänen, et al. (2012).
In this study, the CSDUP model was setup for 11 hydropower projects on the Sesan River. For baseline
modelling, the projects were considered individually and not as a cascade since the projects have
different owners and the projects do not generally co-operate. For the model setup, we used inflows
to each reservoir from the Digital Elevation Model, which is a calibrated hydrological model from
Jarvis and Reuter (2008). We used dam and power plant characteristics from the MRC (2009) for
defining the elevation-volume-surface area of the reservoirs and the energy production
characteristics. The model was run to maximise the total energy production of each dam, beginning
with the most upstream dam, for the period 2001 to 2007. The model was run on a weekly time step
and with no restrictions for releases and reservoir water levels due to a lack of data. Further
refinement of the model would require more input data, such as planned operational goals. Still, the
modelling approach here gives a reasonable estimate of the future operations of the 11 hydropower
projects (a more detailed description of the methodology is presented in the supporting Annex).
3.3 L A N D S UI TA B I L I T Y AS SE SS M E N T
Land suitability for irrigation was determined using a Land Use Evaluation Tool (LUSET) developed by
the International Rice Research Institute (CGIAR-CSI, 2006), which was previously used in Viet Nam
(Yen, et al., 2006). The model is based on two inputs interfaces – the crop requirements and the land
unit information modules. For each land unit (a 250 m by 250 m grid cell), the suitability for growing

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
9
particular crops is determined based on the characteristics of the land units and the crop
requirements, according to a pre-defined suitability determined by climate (temperature and rainfall),
terrain (slope and drainage) and several soils characteristics. For each crop parameter, three
suitability classes were defined: highly suitable, moderately suitable and marginally suitable. The crop
parameters were then tested for each land unit. A suitability value ranging from 0 to 100 was
calculated for each land unit characteristic: temperature, water and terrain (including terrain and soil
characteristics). An overall suitability value (combining temperature, water and terrain) was obtained,
which was used to classify the land unit as S1 (a score greater than 85, or highly suitable), S2 (score
between 60 and 85, or moderately suitable), S3 (score between 40 and 60, or marginally suitable) or N
(score less than 40, or not suitable). When one of the parameters of the land unit, such as water,
temperature or terrain, were not suitable, the land unit was classified as not suitable. In the last step,
each land unit was geo-referenced and the land suitability for the crops was visualised spatially across
the Sesan basin.
Land use suitability for irrigated rice in the catchment was tested by taking into account the potential
for irrigable land along the main river, where water is available in the dry season. Buffer zones of 1 km
and 5 km on each side of the river, where the slope is below 2 percent, were tested for suitability for
dry season irrigated rice. The width of the buffer zone was decided based on a GIS analysis of the
flatness of the terrain along the Sesan River, with 5 km from the river being the maximum width of
the flat area. The MRC’s GIS database was used for land unit characteristics. Soil physical and chemical
properties were based on soil properties of the Sesan basin in Viet Nam. Climate data was generated
by a distributed hydrological model, VMod (see Hydrology and Floor Control). Crop characteristics
from Sys, et al. (1993) were updated based on consultations with local experts to reflect local
practices for planting dates and crop durations (a more detailed description of the methodology is
presented in the Agriculture and Irrigation).
3.4 C R O P W AT ER R E Q U IR EM EN T AN D I RR IG AT IO N S C H E D UL E
The water requirements of the rice crops between 2001 and 2007 was calculated using the FAO’s
CROPWAT. Water requirements and irrigation requirements were estimated using soil, climate and
rice crop data. FAO CROPWAT allows the development of irrigation schedules for different water
management conditions and irrigation practises under both rainfed and irrigated conditions. Crop
water requirements were calculated based on the Crop Evapotranspiration - Guidelines taken from
Allen, et al. (1998).
The rice crop water requirements and irrigation schedules were evaluated at two locations: Site 1 in
Cambodia (N 13.862, E 106.616, 108 m) and Site 2 in Viet Nam (N 14.444, E 107.900, 600 m). The sites
are supposed to be in the high suitability class for paddy rice cultivation. In this study, the soil type at
Site 1 was assumed to be sandy loam and the type at Site 2 was assumed to be loam. Daily rainfall and
temperature were used as basic climate data and were estimated using the VMod model. Effective
rainfall was calculated using the USDA Soil Conservation Service. Reference evapotranspiration was
calculated by the FAO Penman-Monteith method using minimum and maximum temperatures. Two
rice varieties (medium variety of 125 days for wet season rice and earlier variety of 105 days for dry
season rice) were selected for estimating the rice crop water requirements. For irrigation scheduling,
it was assumed that there is no irrigation for wet season (rainfed) rice and the irrigation practise for
dry season rice is a seven day rotation of 100 mm of standing water depth (a more detailed
description of the methodology is presented in the supporting Annex).
3.5 I R R I G A TI ON P O T E NT IA L FR OM RE S E R VO IR S
The irrigation potential of each reservoir was defined by three factors: i. potential irrigable areas as
defined by land suitability assessment, ii. water allocation from the dry season water budget and iii.
the crop water requirements. The land suitability assessment was used to limit the maximum irrigable
area for each reservoir. The dry season water budget, defined as the sum of the total dry season flow
volume and reservoir’s active storage volume, was used to define three different water allocation

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
10
scenarios based on the water availability during the dry season. The three scenarios would allocate 3
percent, 5 percent or 7 percent of the dry season water budget for irrigation. The crop water
requirements determined how many hectares could be irrigated in each of the scenarios. Thus, the
irrigation potential of each reservoir was based on land suitability, water availability and water
demand characteristics.
3.6 A S S E SS ME NT O F I RR IG AT IO N EX P A N SI ON I M P A C T S ON H YD R O P O W ER
G EN E R A TI ON
Assessing the impacts of irrigation expansion on hydropower generation was based on the CSUDP
model, which was also used for hydropower baseline simulation (see chapter 3.2). The water
abstracted for irrigation was added to the hydropower baseline model so that the water was not
available for hydropower generation. Thus, the optimisation process of CSUDP maximised the
hydropower generation of each project according to the inflows to the reservoirs and the water
abstraction from reservoirs.
In the assessment, a single hydropower project that had high irrigation potential was selected for a
case study. The impacts of irrigation on hydropower generation were examined under the three
scenarios, where we allocated 3 percent, 5 percent or 7 percent of the dry season water budget for
irrigation. In the simulations for each scenario, we abstracted the relevant water volumes from the
case study reservoir and optimised the operation of the hydropower project using CSUDP. The
simulation did not consider irrigation from upstream reservoirs. The effect of irrigation on
hydropower generation for each scenario was compared with the hydropower baseline situation. The
assessment was done on weekly and annual scales. All simulation scenarios are listed in Table 2.
The impacts of irrigation expansion were then assessed on a catchment scale by scaling up the 5
percent water allocation scenario to all existing and planned reservoirs that could be used for
irrigation in the Sesan catchment. The cumulative impacts of irrigation expansion on hydropower
generation were assessed for each hydropower project using the methods presented earlier for the
case study reservoir.
Table 2. Simulation scenarios for assessing the impacts of irrigation expansion on hydropower
generation and the cumulative impacts of hydropower development and irrigation
expansion on catchment hydrology.
Scenario
Description
A1. Natural
Natural river flows without hydropower and
irrigation development
B1. Hydropower baseline (case study hydropower project)
Only hydropower operations
B2. 3% water allocation for irrigation (case study
hydropower project)
Hydropower operations and 3% of dry season
water budget for irrigation
B3. 5% water allocation for irrigation (case study
hydropower project)
Hydropower operations and 5% of dry season
water budget for irrigation
B4. 7% water allocation for irrigation (case study
hydropower project)
Hydropower operations and 7% of dry season
water budget for irrigation
C1. Hydropower baseline (full hydropower development)
Only hydropower operations
C2. 5% water allocation for irrigation (full hydropower
development)
Hydropower operations and 5% of dry season
water budget for irrigation from nine
reservoirs

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Trade-offs between hydropower and irrigation development and their cumulative hydrological
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3.7 I M P A CT A SS E S S ME NT O F H Y D RO PO WE R D EV EL OP M E NT AN D I R R I G AT IO N
E X P A N S IO N ON H Y D R OL O G Y AN D LA N D C O VE R
The impacts of catchment scale hydropower development and irrigation expansion on river flows
were assessed at three locations: downstream of Yali, Lower Sesan 3 and Lower Sesan 2. In the impact
assessment, full hydropower development (scenario B1) and catchment scale irrigation development
(scenario C2) were compared with the natural river flows without development (scenario A1) (Table
2). The impact assessment on flows was done on a weekly time scale using CSUDP simulation results.
The simulations did not include a percolation component in the water balance equations and
therefore, the total water loss due to irrigation was estimated separately. We used an average
percolation rate of 8.8 percent water volume diverted to the fields, as observed by Phengphaengsy
and Okudaira (2008). We assumed that the percolated water would return back to river within the
same year it was released.
Assessing the impacts of irrigation expansion on land cover was done through the use of satellite
images (Google, 2012) and land use maps from the MRC (MRC, 2011d), which were used previously in
the hydrological model. The impact assessment involved visually identifying the major land uses of the
areas downstream of each reservoir that could be impacted by irrigation. The exact locations of the
potential irrigation sites could not be defined in this study, therefore this assessment is qualitative
and the impacts for land cover are only suggestive.
4 RESULTS
4.1 C A T C HM EN T HY DR O LO G Y
The hydrological model VMod reproduced the measured flows remarkably well for the Sesan River,
despite the scarcity of data used to drive the model. The model was calibrated using four locations on
a daily time step for the period 2001 to 2007, with the main calibration point being the lowest flow
measurement station at Ban Kamphun (Figure 1). The model flows were also compared with the
annual average flows at 11 existing or future dam sites. The model was able to explain the majority of
the variation in daily flows (Figure 3) at the lowest calibration point of Ban Kamphun (Figure 1). The R2
between measured and simulated flows at Ban Kamphun was 0.87. The water balance error between
the simulated and measured flows at four calibration sites varied between -9 percent and +7 percent
and averaged +3.3 percent. The water balance error at 11 dam sites was on average 5 percent. The
calibration results are presented in more detail in Hydrology and Flood Control. The measured flows
between 2001 and 2007 were already affected by the Yali hydropower project, which started
operating in 2001. This could be observed as increased flows and daily variability at the Voeun Sai
flow measurement station on the Sesan River and further downstream (see Hydrology and Flood
Control). However, the main calibration and validation site at Ban Kamphun was not much affected by
Yali because of significant water volumes at Ban Kamphun. The hydrological model also produced
precipitation and temperature maps for land suitability assessment. The annual average precipitation
and temperature of the 3S catchments are presented in Figure 4 and more detailed hydrological
characteristics of the Sesan catchment can be found in the supporting Annex and Hydrology and Flood
Control chapter.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
12
Figure 3 Measured and simulated discharge at the lowest measurement station, Ban Kamphun,
below the confluence of Sesan and Srepok Rivers. The location of the measurement site is
shown in Figure 1.
Figure 4 Simulated spatial distribution of annual average A) precipitation and B) temperature of the
3S catchments
4.2 H Y D R O P OW ER O PE R AT IO N S
The hydropower operations of 11 hydropower projects on the Sesan River were simulated using
CSUDP. The simulation was meant to maximise the annual energy production of each hydropower
project. The simulated annual mean energy production matched the actual energy production (MRC,
2009) of most projects, with a few exceptions (
0
1000
2000
3000
4000
5000
6000
7000
8000
Jan-01
Jan-02
Jan-03
Jan-04
Jan-05
Jan-06
Jan-07
[m3/s]
Measured
Simulated

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
13
Table 3 and more detailed results are presented in the supporting Annex). For example, the energy
production of Sesan 1, Prek Liang 1 and Prek Liang 2 were overestimated in the simulations, likely due
to inaccuracies in the data used in the simulations.
The weekly energy production of Yali and Lower Sesan 2 are shown in
Figure 5. The weekly hydropower generation pattern of each plant followed the hydrological regime,
i.e., the hydropower generation was significantly higher in the wet season than in the dry season.
More detailed results are presented in the supporting Annex.
Table 3 Simulated and announced annual mean energy production of 11 hydropower projects.
Announced
(MRC, 2009)
[GWh]
Simulated
[GWh]
Difference
[%]
Upper kontum
1056.4
1059.4
0.3
Plei Krong
417.2
497.5
16.1
Yali
3658.6
3858.2
5.2
Se San 3
1224.6
1229.8
0.4
Se San 3A
475
454.9
-4.4
Se San 4
1420.1
1480.8
4.1
Se San 1
479.7
642.5
25.3
Prek Liang 2
186
238
21.8
Prek Liang 1
189
314.6
39.9
Lower Sesan 3
1977
1692.3
-16.8
Lower Sesan 2
2311.8
2222.9
-4
TOTAL
13395.4
13691
2.2
0
20
40
60
80
100
120
140
160
1
5
9
13
17
21
25
29
33
37
41
45
49
[Gwh/week]
[week]
2002
2003
2004
2005
2006
A. Yali
0
10
20
30
40
50
60
70
80
90
100
1
5
9
13
17
21
25
29
33
37
41
45
49
[Gwh/week]
[week]
2002
2003
2004
2005
2006
B. Lower Sesan 2

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Trade-offs between hydropower and irrigation development and their cumulative hydrological
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Figure 5. Examples of simulated weekly energy production of hydropower projects: A) Yali and B)
Lower Sesan 2
4.3 L A N D S UI TA B I L I T Y FO R IR RI G A T ED R IC E
Based on the land suitability assessment, the size and suitability of the potential irrigable land within a
5 km buffer varies greatly between the reservoirs (Figure 6 and Table 4) (see also the Agriculture and
Irrigation chapter). The Upper Kontum reservoir has a large potential area for downstream irrigation
(Error! Reference source not found.) of the plain around Kontum city. However, most of this area is
already developed with several irrigation schemes, and a similar observation can be made
downstream of the Pleikrong reservoir. When we consider a 5 km buffer, the Upper Kontum,
Pleikrong, Sesan 3A, Sesan 4 and Sesan 4A reservoirs have very large potential irrigable areas, but
these results have to be treated with caution since micro-topography will probably reduce the
available area. In addition, population density is low around Sesan 3A, Sesan 4 and Sesan 4A.
Downstream of Lower Sesan 2 and Lower Sesan 3 reservoirs, the terrain presented only a limited
amount of moderately suitable areas. However, since rainy season irrigation is already present
downstream of Lower Sesan 3, there is some potential for dry season irrigation through the
modification and rehabilitation of existing irrigation infrastructure (more detailed results are
presented in the Agriculture and Irrigation chapter).
0
20
40
60
80
100
120
140
160
1
5
9
13
17
21
25
29
33
37
41
45
49
[Gwh/week]
[week]
2002
2003
2004
2005
2006
A. Yali
0
10
20
30
40
50
60
70
80
90
100
1
5
9
13
17
21
25
29
33
37
41
45
49
[Gwh/week]
[week]
2002
2003
2004
2005
2006
B. Lower Sesan 2

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
15
Figure 6. Estimated potentially suitable land for irrigated rice according to a land suitability
assessment with 5 km buffer from the Sesan River.
Table 4 Estimated potentially suitable land for irrigated rice downstream of each reservoir in Sesan
catchment according to a land suitability assessment with 5 km buffer from the Sesan River.
Reservoir
Highly
suitable for
irrigated rice
[ha]
Moderately
suitable for
irrigated rice
[ha]
Marginally
suitable for
irrigated rice
[ha]
Upper Kontum
1948
48533
4771
Plei krong
0
2817
0
Yali
0
1209
0
Sesan 3
0
559
0
Sesan 3A
242
10099
300
Sesan 4
282
3192
410
Sesan 4A
20
6192
6636
Lower Sesan 3
0
9329
41949
Lower Sesan2
0
2033
6052
TOTAL
2492
83963
60119

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Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
16
4.4 C R O P W AT ER R E Q U IR EM ENT
The annual dry and wet season irrigation demand in the Upper Sesan catchment averaged 16,000 m3
per ha and the demand in the Lower catchment averaged 14,800 m3 per ha (Figure 7). The slightly
lower irrigation demand in Lower Sesan catchment resulted mainly from the different meteorological
conditions within the basin (Figure 4). The irrigable areas in Lower Sesan catchment were on average
warmer and drier than in the Upper Sesan catchment. The weekly irrigation patterns for dry and wet
season rice are shown in Figure 7A and the annual irrigation volumes are shown in Figure 7B. The
annual irrigation demands in the Upper and Lower Sesan catchments were lowest in 2002 (15,600 m3
per ha and 13,500 m3 per ha, respectively) and highest in 2003 (16,400 m3 per ha and 15,500 m3 per
ha, respectively). Thus, the inter-annual variation was relatively small. More detailed results of crop
water requirements are presented in the supporting Annex.
Figure 7. Estimated irrigation abstraction water requirements for upper (Viet Nam) and lower
(Cambodia) Sesan catchment on A) weekly and B) annual scale for transplanted irrigated
dry and wet season rice
4.5 I R R I G A TI ON P O T E NT IA L FR OM RE S E R VO IR S
In terms of water availability for irrigation, all reservoirs except Upper Kontum have large dry season
water budgets. However, only the Pleikrong, Yali, Sesan 4, Lower Sesan 3 and Lower Sesan 2 have
active storages large enough to store and regulate water for irrigation (Table 5). The Yali reservoir has
little suitable land for irrigation and could not support irrigation directly downstream. Instead, it could
be used to provide water to the Sesan 3A, which has significant potential for irrigation. Irrigation from
Sesan 3A would thus be based on a cascade operation with the Yali and Sesan 3 dams.
In terms of land area suitability and water availability, the Plei Krong, Sesan 4, Lower Sesan 3, Lower
Sesan 2 and the Yali - Sesan 3 - Sesan 3A cascade have the highest potential for downstream
irrigation. The total potential irrigable area for the Pleikrong is 2,817 ha, for Sesan 4 is 3,474 ha, for
the Lower Sesan 3 is 9,329 ha, and for the Lower Sesan 2 is 2,033ha. The Yali - Sesan 3 - Sesan 3A
cascade irrigates 10,341 ha (Table 5). More detailed results based on the land suitability assessment
and land suitability are presented in the Agriculture and Irrigation chapter. The Sesan 3A dam already
has large irrigated areas right below it (Figure 8) and, therefore, it provides a realistic case study site
for estimating the impacts of downstream irrigation on hydropower generation. For our assessment,
we assumed a cascade operation between Yali - Sesan 3- Sesan 3A since the Sesan 3A by itself does
not have sufficient storage and needs to co-ordinate with the upstream dams. Three scenarios were
considered: 3 percent, 5 percent or 7 percent dry season water budget allocation that irrigated 3,894
ha, 6,490 ha and 9,086 ha, respectively.
0
500
1000
1500
2000
2500
1
5
9
13
17
21
25
29
33
37
41
45
49
[m3/ha]
[week]
Cambodia
Vietnam
A
0
4000
8000
12000
16000
20000
2002
2003
2004
2005
2006
Avg.
[m3/ha]
[year]
Cambodia
Vietnam
B.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
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Table 5 Total irrigation potential of each reservoir and potentials according to different dry season
water budget allocations. Dry season water budget refers to the sum of reservoir active
storage and cumulative dry season (Nov-Apr) discharge
Reservoir
Active
storage
[mcm]
Dry Season
water
budget
[mcm]
Suitable area
for irrigated
rice
[ha]
Potentially
irrigated
area with 3%
water
allocation
[ha]
Potentially
irrigated
area with 5%
water
allocation
[ha]
Potentially
irrigated
area with 7%
water
allocation
[ha]
Upper Kontum
122.7
180
50,481
360
600
840
Plei krong
948
1,412
2,817
2,817
2,817
2,817
Yali
779
1,947
1,209
1,209
1,209
1,209
Sesan 3
3.8
1,231
559
559
559
559
Sesan 3A
4
1,293
10,341
2,586
4,311
6,035
Sesan 4
264.2
1,726
3,474
3,452
3,474
3,474
Sesan 4A
7.5
1,527
6,212
3,054
5,091
6,212
Lower Sesan 3
323
2,510
9,329
4,706
7,843
9,329
Lower Sesan2
379.4
6,704
2,033
2,033
2,033
2,033
TOTAL
2,831.6
86,455
20,777
27,937
32,508
Figure 8. Sesan 3A dam and downstream irrigated areas in Viet Nam. The yellow line is the border
between Viet Nam and Cambodia (Source of satellite image: Google Earth)

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Trade-offs between hydropower and irrigation development and their cumulative hydrological
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18
4.6 T R A D E - OFF B E T W E EN H Y D R O P OW ER G EN ER A T I O N A ND I R R I G A T I ON
4 . 6 . 1 Y a l i - S es a n 3 - S es an 3 A c as ca de
The annual hydropower generation of Sesan 3A varied in the baseline (B1) simulation between 424.7
and 482.1 GWh (Table 6). The simulations with water abstraction from the Sesan 3A reservoir for
irrigation in scenarios B2 (3,894 ha), B3 (6,490 ha) and B4 (9,086 ha) reduced the annual hydropower
generation from the B1 hydropower baseline by 0.6 percent to 0.7 percent, 1 percent to 1.2 percent
and 1.4 percent to 1.7 percent, respectively. The impact of irrigation on hydropower generation was
slightly larger in the dry years of 2004 and 2005. The hydropower generation of Yali and Sesan 3 was
not affected and their operations did not differ from the baseline scenario.
The reduction of hydropower generation occurred mainly during dry season (Figure 9). Dry season
hydropower generation was reduced by irrigation scenario B2 (3,894 ha) by 1.8 to 2.4 percent,
scenario B3 (6,490 ha) by 3.1 percent to 4.0 percent and B4 (9,086 ha) by 4.3 to 5.6 percent. The
largest reductions in hydropower generation occurred at the start of the irrigation period, between
weeks 19 and 48, when the water abstraction from the reservoir was the greatest. During week 19,
the average reduction in hydropower generation for the three irrigation scenarios was between 5
percent and 12 percent, and during the week 48, the reduction was between 6.5 percent and 15.1
percent (Figure 9).
Table 6 Annual and dry season average hydropower generation of Sesan 3A in scenarios B1:
Hydropower baseline; B2: 3 percent water allocation; B3: 5 percent water allocation and; B4:
7 percent water allocation for irrigation and corresponding reductions in hydropower
generation. The respective irrigated areas for scenarios B2, B3 and B4 were 3,894 ha, 6,490
ha and 9,086 ha.
B1. Baseline
B2. 3% allocation
B3. 5% allocation
B4. 7% allocation
[GWh]
[%]
[%]
[%]
Annual 2002
468.8
-0.6
-1.0
-1.4
Annual 2003
482.1
-0.7
-1.1
-1.6
Annual 2004
424.7
-0.7
-1.2
-1.7
Annual 2005
429.5
-0.7
-1.2
-1.7
Annual 2006
465.1
-0.7
-1.2
-1.6
Dry season 2002
150.5
-1.8
-3.1
-4.3
Dry season 2003
148.8
-2.2
-3.6
-5.0
Dry season 2004
162.7
-1.9
-3.2
-4.5
Dry season 2005
132.1
-2.4
-4.0
-5.6
Dry season 2006
146.1
-2.2
-3.6
-5.0

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
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Figure 9 Weekly average hydropower generation of Sesan 3A during B1: hydropower baseline;
irrigation scenarios B2: 3 percent water allocation; B3: 5 percent water allocation and; B4: 7
percent water allocation (shown with lines) and corresponding weekly reduction in
hydropower generation (shown with columns). The respective irrigated areas for scenarios
B2, B3 and B4 were 3,894 ha, 6,490 ha and 9,086 ha.
4 . 6 . 2 B a s i n wi de t r ad e - of f
The average annual hydropower production of nine hydropower projects in scenario C1: hydropower
baseline was 13,056 GWh (Table 7). The irrigation expansion in scenario C2: 5 percent water
allocation resulted in a 1.6 percent reduction in total annual hydropower generation. The 1.6 percent
reduction corresponds to a 209 GWh loss in annual hydropower generation. The largest impacts were
experienced at the downstream projects Sesan 1 and Lower Sesan 3 due to accumulating water
abstraction for irrigation along the river. On average, the hydropower generation of Sesan 1
decreased by 3.2 percent and Lower Sesan 3 by 3.4 percent, which correspond to 20.5 GWh and 55.6
GWh, respectively. Lower Sesan 2 was less affected as it receives water also from Srepok River. The
total irrigated area in scenario C2 was 28,348 ha.
The majority of the reductions in hydropower generation occurred in the dry season (Table 7 and
Figure 10). On average, dry season hydropower generation fell to 4,241 Gwh from 4,427 GWh, which
correspond to a 4.2 percent reduction. Here again, the largest impacts were experienced at the
downstream projects Sesan 1 and Lower Sesan 3, where dry season reductions averaged 7.5 percent
and 8.6 percent, respectively. The largest short-term reductions happened at the start of dry season
irrigation, in week 48, when the baseline energy production fell by 10.1 percent.
0
5
10
15
20
25
30
0
2
4
6
8
10
12
14
16
1
5
9
13
17
21
25
29
33
37
41
45
49
Hydropower reduction [%]
Hydropower generattion [GWh/week]
Week
Reduction in B2. 3% allocation
Reduction in B3. 5% allocation
Reduction in B4. 7% allocation
Production in B1. Baseline
Production in B2. 3% allocation
Production in B3. 5% allocation
Production in B4. 7% allocation

MEKONG CPWF| Optimising cascades of hydropower (MK3)
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Table 7 Annual and dry season average hydropower generation of nine hydropower projects in
scenarios C1: hydropower baseline and C2: 5 percent water allocation for irrigation. The
corresponding reductions in hydropower generation and respective irrigated areas for each
reservoir are also presented.
Baseline annual
average
hydropower
generation
Irrigated
area
Change in annual
average
hydropower
generation
Baseline dry season
average
hydropower
generation
Change in dry
season average
hydropower
generation
[GWh]
[ha]
[%]
[GWh]
[%]
Upper
Kontum
1,057
600
-1.9
398
-4.1
Plei
Krong
497
2,817
-1.4
149
-3.7
Yali
3,850
0
-0.6
1,333
-1.6
Sesan 3
1,228
0
-0.6
405
-1.6
Sesan 3A
454
6,490
-1.7
148
-5
Sesan 4
1,478
3,474
-2
449
-6.1
Sesan 4
A
-
5,091
-
-
-
Sesan 1
641
0
-3.2
271
-7.5
Lower
Sesan 3
1,634
7,843
-3.4
636
-8.6
Lower
Sesan 2
2,218
2,033
-1.3
638
-4.3
TOTAL
13,056
28,348
-1.6
4,427
-4.2
Figure 10 Weekly average hydropower generation of nine hydropower projects in scenario C1:
hydropower baseline and in scenario C2: 5 percent water allocations for irrigation (shown
with lines). The corresponding weekly reduction in hydropower generation is also presented
(shown with columns). The total irrigated area in scenario C2 was 28,348 ha.
0
2
4
6
8
10
12
14
16
18
20
0
50
100
150
200
250
300
350
400
450
500
1
5
9
13
17
21
25
29
33
37
41
45
49
Reduction in hydropower generation [%]
Hydropower generation [GWh]
Week
Reduction in C2. 5% allocation
Production C1. Baseline
Production in C2. 5% allocation

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
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21
4.7 I M P A CT A SS ES S M E NT
4 . 7 . 1 H y d r ol og y
The hydropower operations in scenario C1: baseline hydropower generally affected seasonal flow
regimes by increasing dry season flows and decreasing wet season flows. The degree of seasonal
change was dependent on the regulating capacities, i.e., the active storage of each reservoir. The
impact on flow regimes is presented for three locations: immediately downstream of Yali, Lower
Sesan 3 and Lower Sesan 2 dams.
The largest relative flow changes in scenario C1: baseline hydropower was caused by Yali, which had
the largest regulating capacity compared to average flow at the dam site (Table 8 and Figure 11). For
example, the operation of Yali increased the monthly average minimum flow, which occurred in
March, by 267 percent, while the Lower Sesan 3 and Lower Sesan 2 increased the flows by 227
percent and 73 percent, respectively. Similarly, the Yali decreased the monthly average maximum
flow, which occurred in August, by 40 percent, while the decrease at Lower Sesan 3 and Lower Sesan
2 were 23 percent and 15 percent, respectively. The relative changes at Lower Sesan 2 are smaller
largely because it also receives waters from the Srepok River.
The average maximum flood peaks decreased at all three comparison sites. The seven day maximum
flood at Yali was reduced by 36 percent, at Lower Sesan 3 by 33 percent and at Lower Sesan 2 by 6
percent (Table 8 and Figure 11). The comparisons between the dams are not entirely accurate as the
downstream dams include the impacts of upstream dams. Overall, the baseline hydropower
operations created a new flow regime in the Sesan that reduced the amplitude of the annual flood
pulse. More detailed hydrological impacts of hydropower baseline operations are presented in the
supporting Annex.
The inclusion of irrigation scenario C2 (see irrigation areas per reservoirs in Table 7) in the simulations
resulted in relatively small changes in the flow regimes compared to the changes caused by
hydropower baseline operations. Irrigation mainly reduced dry season flows in a cumulative manner
along the Sesan River. The annual flows reduced by 0.4 percent downstream of Yali, 2.2 percent
downstream of Lower Sesan 3 and 1.0 percent downstream of Lower Sesan 2. The reduction in dry
season flows downstream of Lower Sesan 3 was 8.6 percent. The flow reductions did not include
possible return flows back to river through percolation and ground water movement. The annual
average water volume abstracted for irrigating 28,348 ha was 0.43 km3, which is 2.1 percent of the
annual average flow of Sesan (20.5 km3). The annual water abstraction ranged from 0.42 km3 (2002)
to 0.45 km3 (2003). If percolation was taken into account and considered to return back to river in the
same year, the annual water losses averaged 0.39 km3, which corresponded to a 1.9 percent of the
Sesan River’s annual average flow (with a percolation rate of 8.8 percent, as mentioned in
Phengphaengsy and Okudaira (2008).

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Trade-offs between hydropower and irrigation development and their cumulative hydrological
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22
Table 8 The impacts of scenarios C1: hydropower baseline and C2: 5 percent water allocation on A1:
natural hydrology at three locations (see projects in Table 1, locations in Figure 1 and
irrigated areas per project in Table 7. The total irrigated area in scenario C2 was 28,348 ha.
Yali
Lower Sesan 3
Lower Sesan 2
A1
Average
discharge
C1
Impact of
baseline
C2
Impact of
baseline +
irrigation
A1
Average
discharge
C1
Impact of
baseline
C2
Impact of
baseline +
irrigation
A1
Average
discharge
C1
Impact of
baseline
C2
Impact of
baseline +
irrigation
Period
[m3/s]
[%]
[%]
[m3/s]
[%]
[%]
[m3/s]
[%]
[%]
7-day
max.
1,463
-36.4
-36.4
3,045
-32.8
-32.8
6,059
-5.6
-5.6
7-day
min.
26
352.3
342.6
49
342.9
290.9
160
111.4
94.8
Jan
71
93.4
90.1
134
79.8
64.0
426
25.1
19.6
Feb
51
177.4
176.2
96
151.5
129.9
307
47.5
39.9
Mar
41
267.0
261.7
75
227.2
198.6
236
72.9
63.6
Apr
49
238.1
233.8
86
187.4
162.2
234
88.5
80.5
May
115
64.9
62.2
217
48.3
43.0
504
26.4
23.2
Jun
198
25.6
24.4
443
1.8
1.2
934
2.1
1.7
Jul
403
-9.7
-9.7
845
-19.3
-19.7
1,579
-7.7
-7.9
Aug
818
-40.3
-40.4
1,649
-22.9
-23.0
3,446
-14.7
-14.7
Sep
758
-18.1
-18.2
1,515
-10.2
-10.3
3,818
-4.1
-4.1
Oct
436
-5.5
-5.5
838
-4.6
-4.6
2,267
-1.7
-1.7
Nov
185
1.5
-0.1
349
2.2
-5.3
1,075
0.7
-1.9
Dec
109
24.3
22.0
208
20.3
8.3
661
6.4
2.3
ANNUAL
269
0.2
-0.4
538
0.2
-2.2
1,291
0.1
-1.0

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
23
Figure 11 Cumulative impacts of scenario C1: hydropower baseline and C2: 5 percent water
allocation for irrigation on the discharge every month immediately downstream of A)
Yali, B) Lower Sesan 3 and C) Lower Sesan 2 dams
4 . 7 . 2 L a n d c ov er ch an ge s
The existing reservoirs and reservoirs under construction, namely the Upper Kontum, Plei Krong, Yali,
Sesan 3, Sesan 3A, Sesan 4 and Sesan 4A, would inundate approximately 198 km2 of land. If all
planned reservoirs, namely Sesan 1, Prek Liang 1, Prek Liang 2, Lower Sesan 3 and Lower Sesan 2,
were constructed, the total inundated area would be approximately 1,035 km2, which corresponds to
5.5 percent of the land area of the Sesan basin (18,684 km2). The two largest reservoirs, Lower Sesan
3 and Lower Sesan 2, will inundate 414 km2 and 394 km2, respectively. All reservoirs are in the Sesan
basin, except for a part of the Lower Sesan 2 reservoir located in the Srepok basin.
If irrigation expansion occurs downstream of Plei Krong, Sesan 4, Lower Sesan 3, Lower Sesan 2 and
the Yali-Sesan 3-Sesan 3A cascade, further land cover changes would occur. The potential changes
downstream of these hydropower projects are shown in Table 9.
0
100
200
300
400
500
600
700
800
900
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
[m3/s]
A1. Natural
C1. Baseline
A.
0
200
400
600
800
1000
1200
1400
1600
1800
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
[m3/s]
B.
0
500
1000
1500
2000
2500
3000
3500
4000
4500
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
[m3/s]
C.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
24
Table 9 Potential land cover changes due to irrigation expansion downstream of hydropower
projects
Project
Potential land cover changes
Plei Krong
Downstream areas are already used for agriculture and the land cover changes
due to further development would be small
Sesan 3A
Downstream areas have agricultural lands on the south bank of the river, and a
mosaic of deciduous forests and shrub and grasslands on the north bank. Thus,
development on the south bank on areas already under agricultural use would
lead to less land cover change than on the north bank.
Sesan 4
Downstream area is a mosaic of agricultural, shrub and grasslands and deciduous
forests. Thus, development would lead to reduction of forest and shrub patches.
If the re-regulating dam Sesan 4A is used for irrigation, it would further reduce
shrub lands and deciduous forest that cover areas downstream of the dam. The
area downstream of Sesan 4A on the Cambodian side is densely covered in
deciduous forests.
Lower Sesan 3
Downstream areas have scattered agricultural lands, deciduous forests and
degraded forests. Thus, development on lands not already in agricultural use
could lead to deforestation.
Lower Sesan 2
Downstream areas have large areas of deciduous forests but also scattered
agricultural lands. Thus, development may very likely lead to deforestation.
5 DISCUSSION
This study looked at the feasibility of using hydropower reservoirs for improving food security by
increasing the area planted to irrigated rice. The study focused on the technical and hydrological
aspects, therefore, it is necessary to discuss other impacts that could result from the development.
5.1 L O S T A GR IC UL T U R AL L AN D DU E T O IN U ND AT I O N
The findings of this study seem to suggest that irrigation from large hydropower reservoirs will
increase agricultural potential, but this conclusion may be unwarranted. For example, the Lower
Sesan 3 and Lower Sesan 2 have a downstream irrigation potential for rice of 9,329 ha and 2,033 ha,
respectively (highly and moderately suitable land classes), but the reservoirs themselves would
inundate 16,450 ha and 730 ha (highly and moderately suitable land classes). In fact, the Lower Sesan
3 would inundate all (10,710 ha) of the highly suitable land for irrigated rice on the Cambodian side of
the Sesan basin (Figure 6). Therefore, the Lower Sesan 3 and Lower Sesan 2 would also have negative
impacts by reducing land availability in the Sesan catchment and these impacts should be considered
in the trade-off analysis.
5.2 O T H E R ME AS UR E S FO R IN CR EA S I N G AG RI C U L T U R A L PR OD UC T I O N
This study focused on improving agricultural production by increasing irrigated areas. At the same
time, it is necessary to consider whether irrigation is the right development tool. For example,
drought forecasting, improved irrigation efficiencies, development of drought management policies
and improving drought preparedness can play an important role in agricultural productivity (Yu and
Fan, 2011; Shimizu, et al., 2006; Te, 2007; Phengphaengsy and Okudaira, 2008; MRC, 2010). It is likely
that developing the agricultural sector may require all of the above-mentioned measures.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
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25
5.3 S O C I A L A ND L I V E LI HO OD C ON SI D ER AT IO NS
The size and scope of the development measures considered in this study are determined by several
factors such as policies, financiers, institutions and markets. It is important to study these factors
since they will determine the beneficiaries of development. For example, if large-scale irrigation is not
designed properly, it may benefit only large scale landowners and not local smallholders or poorer
residents (for example, Ratner, et al. (2007) address the influence of built structures on local
livelihoods in Cambodia). Irrigation expansion may lead to increased cash crop production, which does
not necessarily alleviate poverty or improve food security. Therefore, the distribution of the benefits
depends greatly on the design, planning and management of irrigation schemes as well as on
governance and institutional arrangements.
The development of large-scale infrastructure may lead to various other direct and indirect social and
livelihood impacts. The most direct impact would be the resettlement of the population. For example,
the existing reservoirs on the Sesan River have already necessitated the relocation of 16,296 people,
and the construction of the Lower Sesan 3 and Lower Sesan 2 would require 10,844 more people to
be moved (MRC, 2009). In total, hydropower development on the Sesan would affect 27,140 people.
The resettlement would inevitably affect social conditions and livelihoods and the management of
these implications would be a major challenge. The representative of HydroLancang, China stated in
his presentation at the second Challenge Program on Water and Food forum on Water, Food and
Energy in Hanoi that resettlement of people is the most challenging part of commissioning a new
hydropower project (personal note in 2nd CPWF Forum on Water, Food and Energy, Hanoi,
14/11/2012). Indirect social and livelihood impacts of hydropower development have been
experienced previously in the Sesan catchment. For example, upstream hydropower development has
led to irregular flooding and degradation of water quality, which in turn have caused material, crop
and income losses and the loss of lives (Wyatt and Baird, 2007; Baird, et al., 2002; McKenney, 2001).
5.4 F U R T H E R EN VI R O N M E NT AL I MP AC T S
Hydropower and irrigation development would have several environmental impacts in addition to the
direct hydrological and land cover impacts presented in Sections 4.7.1 and 4.7.2. The dams and the
resulting land cover changes would affect biodiversity and the geomorphology of the catchment.
Generally, dams affect aquatic biodiversity indirectly through the fragmentation of river systems and
altered flow regimes (Dugan et al., 2010; Nilsson, et al., 2005; Nilsson and Berggren; 2000, Junk, et al.
1989; Bunn and Arthington, 2002; Dugan, 2008; Lamberts, 2008; Baran and Myschowoda, 2009; Ziv,
et al., 2012). For example, the Sesan River is rich in fish biodiversity but the fish abundance has
already been observed to decline. The reasons for decline have been attributed to upstream
hydropower development. The decline in fish abundance is significant to the people living in the
region as fish is a major source of protein. Hydropower development on the 3S Rivers will also affect
fish abundance in the larger Mekong Basin. For example, the construction of Lower Sesan 2 has been
estimated to reduce the fish biomass of the Mekong basin by 9 percent (Ziv, et al., 2012).
Hydropower and irrigation development would affect the geomorphology of the river catchment by
sediment trapping, causing rapidly fluctuating flow regimes and increased soil erosion due to
agriculture. The sediment trapping efficiencies (TE) of hydropower reservoirs in the Mekong have
been estimated by Kummu, et al. (2010). For the Sesan catchment, the estimated total TE for existing
and planned reservoirs was 85 percent to 95 percent, while for individual hydropower projects, the TE
estimates varied between 35 percent and 93 percent. The rapidly fluctuating flow regimes due to
hourly and diurnal hydropower operations may lead to bank erosion and cause changes in river
geomorphology. Converting forested land or land permanently covered by vegetation into agricultural
land may affect sediment fluxes through increased soil erosion. Bare or heavily tilled soils are more
susceptible to erosion by heavy rains and thus, could lead to increased sediment transport within the
catchment. Therefore, sediment trapping, fluctuating flow regimes and soil erosion would affect
sediment fluxes in the river, which may lead to negative impacts such as reservoir siltation, soil

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
26
degradation and physical changes on the riverbank and aquatic habitats. The changes in sediment
fluxes may be significant for the larger Mekong basin since the 3S Rivers tributary system is one of the
major sources of sediments for the Mekong (Kummu, et al., 2010). Soil erosion and reservoir siltation
may be reduced by soil conservation practises and by designing dams to avoid as much sediment
trapping as possible.
The expansion of agricultural areas will inevitably affect terrestrial ecosystems due to the clearing of
forests and other natural vegetation. For example, in Ratanakiri province in Cambodia, forested areas
declined by 40 percent between 1997 and 2005 (see chapter on Agriculture and Irrigation.
5.5 C U M U L A TI VE C AT C HM EN T SC AL E E NV I R O NM EN TA L I MP AC TS
This study focused on 11 existing and planned large hydropower projects and their ability to increase
of agricultural area by 28,348 ha. The study did not consider impacts of pre-existing agriculture and
smaller irrigation dams on the Upper Sesan catchment. The Agriculture and Irrigation chapter notes
that over 250,000 ha are under cultivation on the Upper Sesan catchment, while over 130,000 ha
have been cultivated in the Stung Treng and Ratanakiri provinces in the Lower catchment. The total
existing irrigated area on the Upper and Lower catchments is not known but, as a lower bound, it is
estimated at least 28,000 ha of irrigated rice exists on the Upper catchment and 35 ha exists in the
Ratanakiri province of the Lower catchment. It is possible that other crops are also being irrigated.
The Upper Sesan catchment also has a large number of smaller irrigation dams that cumulatively
affect the hydrological regime and sediment flux. These small dams also form barriers to fish
migration. Thus, the findings of this study represent only a part of the potential impacts.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
27
6 CONCLUSION S
The study was prepared as part of the MK3 project, which was part of the Challenge Program on
Water & Food in the Mekong. The aim was to assess the impact of expanding irrigation by using water
from hydropower reservoirs. The main objectives were: i. to estimate the impact of water abstraction
for irrigation on hydropower generation and ii. to estimate the cumulative hydrological impacts of
catchment scale hydropower development and irrigation expansion on the Sesan River catchment.
These objectives were accomplished by estimating the potential for irrigable land for each reservoir
and the crop water requirements, and using a modelling approach to simulate catchment hydrology,
hydropower operations and irrigation. The simulations were done for a case study reservoir and then
expanded to the catchment scale to include irrigation from seven reservoirs. For the irrigation
scenarios, two annual crops were considered: irrigated dry season rice and wet season rice that
requires supplementary irrigation.
We found that several reservoirs had high potential for downstream irrigation. We selected the Yali-
Sesan 3-Sesan 3A cascade for study and found that dry and wet season irrigation of 3,894 ha, 6,490 ha
and 9,086 ha of rice from Sesan 3A resulted annually in average reductions of hydropower generation
of 0.7 percent, 1.2 percent and 1.6 percent, respectively. The reductions were experienced mostly
during the dry season. Irrigation from Sesan 3A required a cascade operation with upstream dams
Sesan 3 and Yali to sustain adequate flows as the Sesan 3A does not have the necessary storage
capacity.
Scaling up irrigation to catchment scale resulted in dry and wet season irrigation of 28,348 ha from
seven reservoirs. Catchment scale irrigation reduced the total annual average hydropower generation
of nine hydropower projects by 1.6 percent. Without irrigation, the projects generated 13,056 GWh
annually, and irrigation reduced the capacity by 209 GWh, mostly during the dry season. The effects
of irrigation on power generation were cumulative, such that the Lower Sesan 3, which is furthest
downstream, experienced the highest losses. Thus, the impact of irrigation expansion on catchment
scale hydropower generation was found to be modest.
Hydropower development affected hydrology significantly. Hydropower operation of 11 simulated
hydropower projects increased dry season (November-April) flows on average by 167 percent and
decreased wet season (May-October) flows on average by 11 percent below the baseline flow of the
Lower Sesan 3 dam. This reduced the amplitude of the annual flood pulse. Irrigating 28,348 ha
required an average basin-wide annual water abstraction of 0.43 km3. This led to water losses of 0.39
km3, which corresponds to 1.9 percent of the annual average flow of the Sesan River. Water
abstraction occurred mainly during dry season. For example, irrigation reduced dry season flows by
8.6 percent downstream of the Lower Sesan 3 dam. Together, hydropower development and
irrigation decreased wet season flows by 12 percent and increased dry season flows by 153 percent,
which would be significant if the impacts of irrigation had not been largely masked by the impacts of
hydropower operations. Water use in irrigation is largely consumptive and needs to be considered
incremental to other water usage in the catchment.
During the assessment process, it was found that the development of multipurpose reservoirs could
lead to extensive land cover changes. The existing and planned reservoirs would inundate 1,035 km2
and agricultural development would affect 280 km2. Altogether, the impacted area of the
development would be equal to 7 percent of the Sesan catchment area.
The findings suggested that agricultural potential could be increased through irrigation from
hydropower reservoirs, but it was also found that the reservoirs inundated significant areas of land
that would be suitable for irrigated rice. For example, Lower Sesan 3 could potentially irrigate 9,329
ha but the reservoir itself would inundate 16,450 ha of agriculturally suitable land. Over 10,710 ha of
that land is considered highly suitable for rice. Therefore, losses of agricultural land due to inundation
should be considered in the trade-off analysis.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
28
Overall, the study found that it is technically possible to develop irrigation using hydropower
reservoirs without significantly affecting hydropower generation. The catchment hydrology would,
however, be seriously disturbed by the hydropower operations. The study also found that developing
hydropower and irrigation would lead to a highly managed river catchment, and could cause inter-
projects and trans-national water sharing issues.
This study focused only on technical and hydrological aspects and therefore, it cannot be solely used
to draw bigger picture conclusions about trade-offs. It is important to study further environmental,
social and livelihood implications related to hydropower and irrigation development. There could be
further impacts on catchment geomorphology, terrestrial and aquatic ecosystems, on the living
conditions of local people and on food security. In addition, social, political and economic aspects
such as land ownership, communal arrangements for existing land, and the institutions needed for
managing the possible irrigation schemes need to be considered. Thus, further research on multi-
purpose dams would draw a more comprehensive picture of the benefits and negative impacts of
these dams.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
29
REFERENCES
ALLEN, R. G., PEREIRA, L. S., RAES, D. & SMITH, M. 1998 Crop Evapotranspiration - Guidelines for
Computing Crop Water Requirements. Irrigation and Drainage, Paper 56. Food and
Agriculture Organization (FAO) of the United Nations, Rome.
BAIRD, I., BAIRD, M., CHEATH, C. M., SANGHA, K., MEKRADEE, N., SOUNITH, P., BUN NYOK, P., SARIM,
P., SAVDEE, R., RUSHTON, H. & PHEN, S. 2002. A Community-Based Study of the Downstream
Impacts of the Yali Falls Dam Along the Se San, Sre Pok and Sekong Rivers in Stung Treng
Province, Northeast Cambodia. Se San Protection Network Project, Partners for development
(PFD), Non Timber Forest Products project (NTFP), Se San District Agriculture, Fisheries and
Forestry Office, Stung Treng District Office, Cambodia. 81 pp.
BARAN, E. & MYSCHOWODA, C. 2009. Dams and fisheries in the Mekong Basin. Aquatic Ecosystem
Health & Management, 12, 227-234.
BUNN, S. & ARTHINGTON, A. 2002. Basic principles and ecological consequences of altered flow
regimes for aquatic biodiversity. Environmental Management, 30, 492-507.
CGIAR-CSI 2006. LUSET - Land Use Evaluation Tool. Available at:
http://csi.cgiar.org/CGIARGeoSpatialTools.asp#LUSET.
DUGAN, P. 2008. Mainstream dams as barriers to fish migration: International learning and
implications for the Mekong. Catch and Culture, Fisheries Research and Development in the
Mekong Region, Mekong River Commission, 14, 9-15.
DUGAN, P., BARLOW, C. & AGOSTINHO, A. 2010. Fish migration, dams, and loss of ecosystem services
in the Mekong Basin. Ambio, 39, 344-348.
ESCAP/UN 2009. Sustainable Agriculture and Food Security in Asia and Pacific. ESCAP, United Nations,
Bangok.
FAO 2001. Lecture Notes on the Major Soils of the World. World Soil Resources Reports, 94, FAO.
FAO 2007. Digital Soil Map of The World, FAO.
http://www.fao.org/geonetwork/srv/en/metadata.show?id=14116.
FAO 2012. The State of Food Insecurity in the World 2012. Economic growth is necessary but not
sufficient to accelerate reduction of hunger and malnutrition. Rome FAO. Rome FAO.
GOOGLE 2012. Google Earth satellite images, www.earth.google.com, Accessed 3.10.2012.
GRUMBINE, R. E., DORE, J. & XU, J. 2012. Mekong hydropower: drivers of change and governance
challenges. Frontiers in Ecology and the Environment, 10, 91-98.
GRUMBINE, R. E. & XU, J. 2011. Mekong Hydropower Development. Science, 332, 178-179.
HAGEMAN, S. 2002. An Improved Land Surface Parameter Dataset fo Global and Regional Climate
Models. Max-Planck Institute for Meteorology, Report 336.
HORTLE, K. 2007. Consumption and the yield of fish and other aquatic animals from the lower Mekong
Basin. Mekong River Commission Technical Paper 16, Mekong River Commission, Vientiane.
http://www.mrcmekong.org/assets/Publications/technical/tech-No16-consumption-n-yield-
of-fish.pdf. Accessed January 2012.
ICEM 2010. MRC Strategic Environmental Assessment (SEA) of Hydropower on the Mekong
Mainstream. Hanoi, Viet Nam.
http://www.mrcmekong.org/assets/Publications/Consultations/SEA-Hydropower/SEA-Main-
Final-Report.pdf Accessed June 2012.
JARVIS, A. & REUTER, H. 2008. Hole-filled SRTM for the globe Version 4. CGIAR-CSI SRTM 90m. CGIAR-
CSI SRTM 90m database http://srtm.csi.cgiar.org. Accessed April 2010

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
30
JUNK, W., BAYLEY, P. & SPARKS, R. 1989. The flood pulse concept in river-floodplain systems
In: Dodge D (ed.) Proceedings of the international large river symposium (LARS). Canadian Special
Publication of Fisheries and Aquatic Sciences, 106, 110–127.
JUNK, W. J., BROWN, M., CAMPBELL, I. C., FINLAYSON, M., GOPAL, B., RAMBERG, L. & WARNER, B. G.
2006. The comparative biodiversity of seven globally important wetlands: a synthesis.
Aquat.Sci, 68, 400-414.
KOPONEN, J., LAURI, H., VEIJALAINEN, N. & SARKKULA, J. 2010. HBV and IWRM Watershed Modelling
User Guide. MRC Information and Knowledge management Programme, DMS – Detailed
Modelling Support for the MRC Project. http://www.eia.fi/index.php/support/download.
KUMMU, M., LU, X., WANG, J. & VARIS, O. 2010. Basin-wide sediment trapping efficiency of emerging
reservoirs along the Mekong. Geomorphology, 119, 181-197.
LABADIE, J. 2003. Generalised dynamic programming package: CSUDP. Documentation and user
guide, version 2.44. http://modsim.engr.colostate.edu/csudp.shtml. Accessed 16 Sep 2010
LABADIE, J. 2004. Optimal operation of multireservoir systems: State of the art review. Journal of
Water Resources Planning and Management 130, 93-11.
LAMBERTS, D. 2008. Little Impact, Much Damage: The Consequences of Mekong River Flow
Alterations for teh Tonle Sap Ecosystem. In: KUMMU, M., KESKINEN, M. & VARIS, O. (eds.)
Modern Myths of The Mekong. Water & Development Publications - Helsinki University of
Technology.
LAURI, H., MOEL, H. D., WARD, P., RÄSÄNEN, T., KESKINEN, M. & KUMMU, M. 2012. Future changes in
Mekong River hydrology: impact of climate change and reservoir operation on discharge.
Hydrol. Earth Syst. Sci., 16, 4603-4619.
MCKENNEY, B. 2001. Economic Valuation Of Livelihood Income Losses And Other Tangible
Downstream Impacts From The Yali Falls Dam To The Se San River Basin In Ratanakiri
Province, Cambodia. NGO Forum on Cambodia, Phnom Penh, Cambodia. 21 pp.
MRC 2005. Overview of the Hydrology of the Mekong Basin. Vientiane Lao PDR: Mekong River
Commission.
MRC 2009. Hydropower database. Vientiane Lao PDR: Mekong River Commission.
MRC 2010. State of the Basin Report 2010. Vientiane, Lao PDR.
http://www.mrcmekong.org/assets/Publications/basin-reports/MRC-SOB-report-2010full-
report.pdf Accessed June 2012: Mekong River Commission.
MRC 2011a. Assessment of Basin-wide Development Scenarios - Main Report. Mekong River
Commission (MRC). Vientiane, Lao PDR. http://www.mrcmekong.org/assets/Other-
Documents/BDP/Assessment-of-Basin-wide-dev-Scenarios-MainReport-2011.pdf Accessed
June 2012.
MRC 2011b. Flood Situation Report, November 2011, Technical Paper No. 36. Mekong River
Commission, Vientiane Lao PDR.
http://www.mrcmekong.org/assets/Publications/technical/Tech-No36-Flood-Situation-
Report2011.pdf.
MRC 2011c. Hydrological database. Vientiane Lao PDR: Mekong River Commission.
MRC 2011d. Mekong River Commission data, Vientiane, Lao PDR.
MUKHERJI, A., FACON, T., BURKE, J., DE FRAITURE, C., FAURÈS, J.-M., FÜLEKI, B., GIORDANO, M.,
MOLDEN, D. & SHAH, T. 2009. Revitalizing Asia’s irrigation to sustainably meet tomorrow’s
food needs. International Water Management Institute, Colombo, Sri Lanka and Food and
Agriculture Organization of the United Nations, Rome, Italy.

MEKONG CPWF| Optimising cascades of hydropower (MK3)
Trade-offs between hydropower and irrigation development and their cumulative hydrological
impacts –A case study from Sesan River
31
NILSSON, C. & BERGGREN, K. 2000. Alterations of riparian ecosystems caused by river regulation.
BioScience, 50, 783-792.
NILSSON, C., REIDY, C., DYNESIUS, M. & REVENGE, C. 2005. Fragmentation and flow regulation of the
world’s large river systems. Science, 308, 405-408.
NOAA 2011. NCEP-DOE Reanalysis 2 database. Nationa Oceanic and Atmospheric Administration,
Earth System Research Laboratory, Physical Sciences Division, Boulder Colorado, USA.
http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis2.html. Accessed 2011.
PHENGPHAENGSY, F. & OKUDAIRA, H. 2008. Assessment of irrigation efficiencies and water
productivity in paddy fields in the lower Mekong River Basin. Paddy Water Environment, 6,
105-114.
QIU, J. 2010. China drought highlights future climate threats. Nature, 465, 142-143.
RANI, D. & MOREIRA, M. 2010. Simulation-optimization modelling: A survey and potential application
in reservoir system operations. Water Resources Management, 24, 1107-1138.
RATNER, B. D., RAHUT, D. B., KÄKÖNEN, M., HAP, N., KESKINEN, M., YIM, S., SUONG, L. &
CHUENPAGDEE, R. 2007. Influence of built structures on local livelihoods, Case studies of
road development, irrigation and fishing lots. Technical Assistance to the Kingdom of
Cambodia for the Study of the Influence of Built Structures on the Fisheries of the Tonle Sap.
Asian Development Bank, TA 4669-CAM, http://users.tkk.fi/u/mkeskine/BS-livelihoods.pdf.
RÄSÄNEN, T. A., KOPONEN, J., LAURI, H. & KUMMU, M. 2012. Downstream hydrological impacts of
hydropower development in the Upper Mekong Basin. Water Resources Management, 26,
3495-3513.
RÄSÄNEN, T. A. & KUMMU, M. 2013. Spatiotemporal influences of ENSO on precipitation and flood
pulse in the Mekong River Basin. Journal of Hydrology, 476, 154-168.
RÄSÄNEN, T. A., LEHR, C., MELLIN, I., WARD, P. J. & KUMMU, M. 2013. Paleoclimatological perspective
on river basin hydrometeorology: case of the Mekong. Hydrol. Earth Syst. Sci., 17, 2069-2081.
SAXTON, K. E. & RAWLS, W. J. 2006. Soil Water Characteristic Estimates by Texture and Organic
Matter for Hydrologic Solutions. Soil Science Society of America journal, 70, 1569-1578.
SHIMIZU, K., MASUMOTO, T. & HAI PHAM, T. 2006. Factors impacting yields in rain-fed paddies of the
lower Mekong River Basin. Paddy Water Environment, 4, 145-151.
STONE, R. 2010. Severe Drought Puts Spotlight on Chinese Dams. Science, 327, 1311.
STONE, R. 2011. Mayhem in the Mekong. Science, 333, 814-818.
SYS, I., VAN RANST, E., DEBAVEYE, J. & BEERNAERT, F. 1993. Land Evaluation Part III -Crop
requirement. Belgium General Administration for Development Cooperation.Agricultural
Publications No. 7.
TE, N. 2007. Drought Management in the lower Mekong Basin. 3rd Southeast Asia Water Forum. Kuala
Lumpur, Malaysia.
WYATT, A. B. & BAIRD, I. B. 2007. Transboundary impact assessment in the Sesan River Basin: the case
of the Yali Falls Dam. Water Resources Development, 23, 427‐442.
YEN, B. T., PHENG, K. S. & HOANH, C. T. 2006. LUSET User's guide. International Rice Research
Institute: 15p.
YU, B. & FAN, S. 2011. Rice production response in Cambodia. Agricultural Economics, 42, 437–450.
ZIV, G., BARAN, E., NAM, S., RODRIQUEZ-ITURBE, I. & LEVIN, S. 2012. Trading-off fish biodiversity, food
security, and hydropower in the Mekong River Basin. PNAS, 109, 5609-5614.