IJERD – International Journal of Environmental and Rural Development (2023) 14-1

Research article
erd

Impact Assessment of New Dam Construction in Nam Ngum
Watershed on Electric Generation at Num Ngum 1 Dam
KOSHI YOSHIDA*
Graduate School of Frontier Sciences, The University of Tokyo, Chiba, Japan
Email: kyoshida@edu.k.u-tokyo.ac.jp

KEIGO NODA
Gifu University, Gifu, JapanTASK KATO
Tokyo University of Agriculture and Technology, Tokyo, JapanKEODUANGCHAI KEOKHAMPHUINational University of Laos, Vientiane, Laos
Received 14 January 2023

Accepted 25 May 2023 (*Corresponding Author)

Abstract Lao PDR is a landlocked country and around 90% of the country is located in
Mekong River Basin. Average annual rainfall is around 1,900 mm and 35% of the annual flowin the Mekong River is from Lao tributaries. Given the abundance of water resources in Laos,many hydropower dams have been constructed in the country. The generated electricity isexported to Thailand and other surrounding countries and has become an important means ofearning foreign currency for Laos. In particular, the Nam Ngum 1 dam has been in operation
since 1971 in the Nam Ngum watershed near Vientiane, the capital of Laos, and several
additional dams have been completed in this watershed since 2010. Hydropower has great
potential to boost the national income and raise living standards in Lao PDR. At the same time,hydropower reservoirs have a large number of potential cross-sector impacts, includingchanges in the downstream water environment. The purpose of the current study was toevaluate the impact of the construction of multiple dams in the Nam Ngum watershed on theriver flow regime, as well as on the hydropower generation of the preceding Nam Ngum 1
dam. In this study, the fully distributed TOPMODEL was developed and applied to analyze
the water flow in the Nam Ngum watershed with a spatial resolution of 1 km X 1 km.
Simulated river discharge and dam storage were in good agreement with observed data.
Subsequently, we investigated the impact of the construction of the Nam Lik 1/2, Nam Ngum
2, and Nam Ngum 5 dams, which were newly developed after 2010. Results indicated thatnew dam construction has positive effects in decreasing flood flow during the rainy season
and increasing discharge in the dry season, while electricity generation at the Nam Ngum1
dam also increased by 6.8%.
Keywords water resource development, hydropower, multi reservoirs, Laos

INTRODUCTION
Lao PDR is a landlocked country and around 90% of the country is located in Mekong River Basin.
Average annual rainfall is around 1,900 mm and 35% of annual flow in Mekong is from Lao
tributaries, so many hydropower dams have been constructed against the backdrop of abundant water
resources. The generated electricity was exported to Thailand or surrounding countries and has
become an important means of earning foreign currency for Laos. Electricity demand in Laos has
been growing at a high rate of more than 10% since 1999, both in terms of consumption and peak
power and according to the national power supply development plan, demand exceeded supply until
around 2014. To meet this growth in demand, the Lao government has decided to utilize its abundantⒸ ISERD112

IJERD – International Journal of Environmental and Rural Development (2023) 14-1hydropower resources, which are domestically produced energy, and to actively promote thedevelopment of power sources (Uematsu, 2019).
In particular, the Nam Ngum 1 Dam has been in operation since 1971 in the Nam Ngum
watershed near Vientiane, the capital of Laos, and several dams have been completed in thiswatershed since 2010. Hydropower has great potential to boost the national income and raise living
standards and create opportunities for the establishment of electricity-using industries in Lao PDR.
While hydropower reservoirs also have a large number of potential cross-sector impacts, includingchanges in downstream flows and water quality, dam safety, and resettlement of living people(Lacombe et al., 2014). Likewise, Kudo et al. (2013) assessed how to change the flow discharge into
Nam Ngum 1 reservoir due to new dam construction, however, impact on hydropower generation
was not obtained.
OBJECTIVE
The purpose of this study was to evaluate the impact of the construction of multiple dams in the Nam
Ngum watershed on the river flow regime, as well as on the hydropower generation of the preceding
Nam Ngum 1 dam.
METHODOLOGY
Study Area
The Nam Ngum watershed in Laos is a tributary of the Mekong River. The river length is about 415
km, and the catchment area is about 17,000 km2. The land use in the basin is paddy fields (16%),
forests and bushes (72%), and residential land (only 0.2%), making the basin still rich in nature. In
this watershed, several large-scale dams were constructed after 2010 to meet the increasing electricdemand of domestic and surrounding countries (Fig. 1).

Fig. 1 Nam Ngum watershed and dams’ location
Nam Ngum 1 Hydroelectric Power Plant is located on the Nam Ngum River about 90 km northof Vientiane, the capital of Laos, and its reservoir has active storage of 4,700 MCM (million m3),
which started operation in 1971. As the main power source for the metropolitan area, its generation
capacity was 155 MW until 2017. The utilization rate of the Nam Ngum 1 power plant was initially
66%, but the Nam Song and Nam Leuk power plants developed in 1995 and 2000, respectively,
increased the flow rate by an average of 65 m3/s and 15 m3/s, respectively, and the utilization rate
has increased to 74%. After 2018, the capacity was improved to 235 MW by the JICA project(Uematsu, 2019).
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Nam Ngum 2 Hydroelectric Power Plant was completed in 2011, which is located about 100
km north of Vientiane, the capital of Laos, and is the second power plant on the Nam Ngum River.
The Nam Ngum 2 Power Project is a concrete surface impermeable rockfill dam with a dam heightof 181 m. The installed capacity is 615 MW, and 100% of the power be exported to Thailand. Its
basin area accounts for 67% of the Nam Ngum 1 power plant basin area, having an effective storage
capacity of 2,620 MCM. Nam Ngum 5 was completed in 2012, with a dam height of 99 m. With acatchment area of 413 km2 and an effective storage capacity of 251 MCM, the project is smaller thanthe Nam Ngum 2 power project (Kudo et al., 2013).
The Nam Lik 1/2 Power Project started operation in 2011, having 100 MW power generationand 101 m height concrete surface impermeable dam in the middle reaches of the Nam Lik River, atributary of the Nam Ngum River. With a medium-sized active storage capacity of 540 MCM, thisdam is expected to smooth the flow conditions in the lower reaches of the dam throughout the year
and increase the dry season flow.
Rainfall-Runoff Model
To evaluate the river water flow, a distributed water cycling model was developed and applied toanalyze the water balance in the basin. TOPMODEL was employed for the rainfall-runoff analysis.
Such a distributed model can include the spatial distribution of topography, land use, and soil
characteristics. Therefore, TOPMODEL is widely used for hydrological characteristic analysis, water
management, water quality analysis, and future forecasting. TOPMODEL was proposed by Beven
and Kirkby (1979) based on the contributing area concept in hill slope hydrology. This model isbased on the exponential transmissivity assumption, which leads to a topographic index ln (a/To/tanb),
where a is the upstream catchment area draining across a unit length, To is the lateral transmissivity
under saturated conditions, and b is the local gradient of the ground surface. Figure 2 illustrates theconceptual structure of the water cycle as estimated by TOPMODEL. For details, refer to Yoshida etal. (2017).

Fig. 2 TOPMODEL structure
Dam Operation Model
Large scale dams having quite huge water storage capacity makes a significant effect to control riverwater discharge (Hanasaki et al., 2003). Normally, the dam reservoir stores water in the rainy season
and releases water in the dry season so that downstream river discharge becomes stable compared to
the variation of natural river flow. In the Nam Ngum watershed, several dams have more than 250
MCM capacity which is mostly used for electric generation. In this study, a simple dam storage modelⒸ ISERD

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was applied to describe the river discharge stabilization effect (Juthithep et al., 2014). The storage ofa reservoir Vres (m3) can be calculated by the following equation.
Vrest+∆t=Vrest+(Qint-Qoutt)×∆t(1)

Qoutt=Qpwt+Qspillt(2)

Qpwt =Qpwmax×(Vrest/Vresmax)(3)

Where Qin is the inflow of a reservoir (m3/s) calculated by flow accumulation along the upstream
river network, Qout is the outflow of a reservoir (m3/s), Qpw is the turbine discharge (m3/s), Qpw is thespillway discharge (m3/s), Vresmax is active storage(m3) and Qpwmax is maximum turbine flow (m3/s).
1 Dam Case and 4 Dam Case Scenarios
In this study, we used the data from 2002-2010. During this period, only the Nam Ngum 1 plant was
operating, so first we checked model performance under the 1dam case scenario. And then, 4 dams’
case was also simulated by using the same input data and compared with the 1 dam case scenario toinvestigate the impact of newly developed dams on downstream river flow and hydropower
generation at Nam Ngum 1 dam, considering Nam Lik 1/2, Nam Ngum 2, and Nam Ngum 5, whichwere newly developed after 2010. To evaluate the hydropower generation at Nam Ngum 1 dam, therelation between turbine discharge and hydropower generation was used in Fig. 3.

Fig. 3 Relation between turbine discharge and hydropower generation at Nam Ngum 1
RESULTS AND DISCUSSION
By using the proposed model, river flow at Pakkanhoung (catchment area of 14,300 km2) station was
simulated from 2002 to 2010. The first 5 years of data were used for parameter calibration, and thelatter 4 years of data were used for validation. Parameters were calibrated by the try-and-error method
to maximize the Nash-Sutcliffe efficiency (NSE). Solid line and plot in Fig. 4 show the calculated
and observed river discharge at Pakkanhoung station under 1 dam case. Estimated NSE were 0.59
and 0.50 in the calibration and validation periods respectively. Model performance can be evaluatedas “satisfactory” if NSE > 0.50 (Moriasi et al., 2007). The reason for the relatively low accuracy isthat there is no station in a relatively high-elevation area. The dashed line in Fig.4 shows riverdischarge under 4 dam cases. Due to the flow regulation effect of multiple dam reservoirs in upstream,
positive effects to decrease in flood flow in the rainy season and to increase discharge in the dry
season were obtained. Especially, minimum discharge in the dry season increased from 221 m3/s to
264 m3/s and it contributed to generating more electricity. Figure 5 shows the comparison betweenⒸ ISERD

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calculated and observed monthly storage at Nam Ngum 1 reservoir for 2002-2004. The correlation
coefficient between the calculated and observed values is R = 0.65. Calculated storage ranging morethan 3,000 MCM shows relatively good agreement, however, the calculated values are slightly
overestimated when storage became less than 3,000 MCM. Figure 6 shows the annual hydropower
generation with 1 dam case and 4 dam cases. Under the 1 dam case, annual hydropower generation
varied from 857.8 -1077.6 GWh. Under the 4-dam case, annual hydropower generation was increased
by 6.8% due to the flow regulation effect of newly developed dams upstream, however, the incrementin 2004 was only 3.2%. The year 2003 was a drought year so dam storage at the end of rainy season
in 2003 was relatively small. That is why additional release discharge was insufficient to increasehydropower generation in the dry season of 2004. The electricity demand for air conditioners
increases especially during the dry season when the weather becomes very hot, but the amount ofelectricity generated decreases due to a decrease in the flow of rivers. In 2002, 30% of domestic
power consumption was imported, but the import price from Thailand was 10% higher than the priceof electricity exported by the Lao PDR, resulting in a loss of foreign currency earned through exports.
Under the Independent Power Producer (IPP) concession of new dam construction projects, most of
the generated electricity is exported to Thailand. However, in Vientiane city capital of Laos also,
electricity demand increases by around 10% per year, so even a small amount of increment in Nam
Ngum 1 hydropower dam can contribute to reducing the electricity import from Thailand.

Fig. 4 Calculated and observed river discharge at Pakkanhoung station

Fig. 5 Calculated and observed monthly storage at Nam Ngum 1 reservoirⒸ ISERD

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Fig. 6 Estimated annual hydropower generation under 1 Dam and 4 Dam cases
CONCLUSION
In this study, the fully distributed TOPMODEL was developed and applied to analyze the water flowin the Nam Ngum watershed to assess the impact of newly developed dams on downstream river
flow and hydropower generation in the Nam Ngum 1 dam. Simulated river discharge and dam storage
were in good agreement with observed data, and then, we investigated the impact of dam construction
considering NamLik1/2, NamNgum2, and NamNgum5, which were newly developed after 2010. As
a result, new dam constructions have positive effects to decrease flood flow in the rainy season andto increase discharge in the dry season, and electricity generation at the Nam Ngum1 dam also
increased by 6.8%. In this simulation, the same simple reservoir model was used in 1 dam and 4 damcases due to the lack of data, however, normally reservoir operation rule curve became more complex
under multiple dam operations. Therefore, the dam operation model should be improved by using
the actual rule curve and river flow data after 2010.
ACKNOWLEDGEMENTS
This research was supported by JST SICORP Grant Number JPMJSC20E3, and the MEXT/JSPS
Grant-in-Aid for Scientific Research (KAKENHI) no.20KK0346 and no. 23H02324.
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