CATCHMENT HYDROLOGY AND ITS MODELING

Sekong Sesan Srepok VMod hydrological modeling report CATCHMENT HYDROLOGY AND ITS MODELING MK3 Optimising cascades of hydropower HYDROLOGY & FLOOD CONTROL In Nam Theun-Nam Kading (Lao PDR) and in Sesan (Cambodia and Vietnam) River catchments November 2013 Timo A. Räsänen and Matti Kummu

Sekong Sesan Srepok VMod hydrological modeling report Author Produced by Suggested citation More information Image Project Team Copyright Timo A. Räsänen and Matti Kummu Mekong Challenge Program for Water & Food Project 3 Optimising cascades of hydropower for multiple use Lead by ICEM International Centre for Environmental Management Timo A. Räsänen and Matti Kummu. 2013. Catchment hydrology and its modeling in Nam Theun-Nam Kading (Lao PDR) and in Sesan (Cambodia and Vietnam) River Catchments. Report for the 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, 2012 www.optimisingcascades.org www.icem.com.au Cover image: Pond in the Nam Theun -Nam Kading Basin (Photo PJ Meynell). Inside page: View of the Nam Theun- Nam Kading Basin (photo credit: PJ Meynell) 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 2013 ICEM - International Centre for Environmental Management 6A Lane 49, Tô Ngoc Vân Tay Ho, HA NOI Socialist Republic of Viet Nam Acknowledgements Information and ideas for this project were collected during a site visit by the team in 2011. We would like to acknowledge the kind support from Electricity Vietnam (EVN) in visiting their field office at the Lower Sesan 2 dam site. Rainfall and stream flow data were from Mekong River Commission Secretariat sources. Funding for this project was from the Mekong river component of the CGIAR Challenge Program on Water and Food.

TABLE OF CONTENTS 1. INTRODUCTION...3 2. HYDROLOGY OF SESAN AND NAM THEUN-NAM KADING CATCHMENTS...4 3. MEASURED IMPACTS OF EXISTING HYDROPOWER PROJECTS ON RIVER FLOWS OF SESAN AND NAM THEUN-NAM KADING RIVERS...7 4. HYDROLOGICAL MODELING OF NAM THEUN-NAM KADING AND SESAN CATCHMENTS...8 REFERENCES... 11 2

1. INTRODUCTION This chapter introduces catchment hydrology and its modeling in two case studies conducted within the project MK3 Optimising the Management of a Cascade of Reservoirs on Catchment Level. The two case study areas were the Nam Theun-Nam Kading River catchment (Figure 1A) in Lao PDR, and the Sesan River catchment (Figure 1B) shared by Cambodia and Vietnam. The project was part of the CGIAR Challenge Program on Water & Food in the Mekong during the years 2011 2013. In the assessment of water resource development, a good understanding of catchment hydrology is necessary. However, in many cases hydrological data are scarce and often inadequate. This was the case in the Nam Theun-Nam Kading and Sesan catchments, the case study areas for several assessments in the MK3 project. Therefore, in order to fill some of the data gaps, a distributed hydrological model called VMod (Koponen et al., 2010) was used to simulate the catchment hydrology and to provide the necessary hydrological inputs for the assessments. The data from hydrological model was used, for example, in agroecological zoning (see E-Book chapter on Land Use Suitability), trade-off assessment of multipurpose reservoirs (see E-Book chapter on Trade Offs) and environmental flow assessment (see E-Book chapter on e-flow) and in the assessment of fish pass possibilities for Lower Sesan 2 dam (see E-Book chapter on Fish Passage). VMod has been used earlier in the Mekong region with good success by Räsänen et al. (2012), Lauri et al. (2012), and Darby et al. (2013). This chapter presents the main hydrological characteristics of the Nam Theun-Nam Kading and Sesan catchments (Section 2), how the existing hydropower projects have altered river flows (Section 3) and how the two catchments were modelled using a hydrological model to supplement the scarce measured hydrological data (Section 4). The hydrological modeling of the catchments is also presented in more detail in Annexes 1 and 2. The paper is not intended to be a comprehensive analysis but instead aims to give the reader an introduction to the hydrology and its modeling in these case study catchments. 3

2. HYDROLOGY OF SESAN AND NAM THEUN- NAM KADING CATCHMENTS The Nam Theun-Nam Kading River is a tributary of the Mekong and is situated largely in a mountainous area and in higher elevations (Figure 1A). The area of the Nam Theun-Nam Kading catchment is 14,890 km 2. The Sesan River is part of a larger Sekong-Sesan-Srepok tributary system and together these rivers form one of the largest tributaries of the Mekong (Figure 1B). The western parts of the Sesan catchment are largely in Cambodian lowlands but the eastern parts of the catchment are in the Vietnam s Central Highlands. The area of the Sesan catchment is 18,684 km 2. Figure 1. Maps of the case study areas in the Mekong Basin: A) Nam Theun-Nam Kading catchment in Lao PDR and B) Sesan catchment in Cambodia-Vietnam, and existing hydropower projects and hydrological measurement stations referred in this paper. The hydrology of the region, where the two case study catchments lie, is dominated by a monsoon climate with distinct wet (May Oct) and dry (Nov Apr) seasons (MRC, 2005, MRC, 2010). During the wet season the monsoon brings moisture and rainfall from the South China Sea and during the dry season rainfall is generally scarce or non-existent. Strong links have been found with the Western North Pacific component of the Asian summer monsoon (Delgado et al., 2012). Examples of typical intra-annual rainfall distributions are displayed in Figure 2A, which shows monthly rainfall from three stations in the Nam Theun-Nam Kading catchment. The intra-annual temperature variation in the study catchments is relatively low as shown in Figure 2B with an example of average temperature in Nam Theun-Nam Kading catchment. The temperatures are a few degrees higher during the boreal summer than in the boreal winter. The onset of monsoon rainfall in June July slightly lowers the temperatures in the boreal summer. The rainfall and temperature patterns in Nam Theun-Nam Kading catchments are relatively similar but of 4

course some local differences exist due to different locations in the Mekong Basin and elevation differences between the catchments. Figure 2. Measured rainfall and temperature patterns in the Nam Theun-Nam Kading catchment: A) average monthly rainfall distribution from three stations (see locations in Figure 1) and B) Average daily temperature distributions from one station (Theun-Hinboun dam site; See location in Figure 1). The average annual rainfall at CS03 was 1020 mm, at Theun-Hinboun dam site 1543 mm and at P170501 2315 mm. The monsoon climate in the region creates river flow regimes with a pulsing nature: During May, when the wet season starts, the flow levels start to rise: they peak in August-September and decline back to dry season flow levels around November; the lowest water levels are experienced in April. Figure 3 shows examples of flow regimes from the Theun-Hinboun dam site in NTNK catchment (see location in Figure 1A) and from the Ban Kamphun in Sesan-Srepok catchments (see location in Figure 1B). Figure 3 also shows how the flow regimes in upper stretches of the river (Figure 3A) generally have a greater number of distinct flood peaks than in the lower stretches of the river (Figure 3B) where the flow levels are higher. The flow time series from Theun-Hinboun dam site show further and interestingly high flood peaks in late wet season, especially in 2002 and 2011, but similar late-season flood peaks can also be seen in 2006, 2007, 2009 and 2010. The NTNK area is known to be affected by late wet season tropical cyclones; the 2002 and 2011 flood events resulted from cyclones passing by (see E-Book chapter on Flood Control Challenges). Regarding the discharge at Theun-Hinboun dam in Figure 3B, it is important to note that the Nam Theun 2 hydropower project (see location in Figure 1) started its operation in 2010; it affected flows during its construction and operation phases. Similarly, in the Sesan catchment, hydropower operations started in Vietnam in 2001 when the Yali project (see location, Figure 1) began its operation. 5

Figure 3. Discharge at A) Theun-Hinboun dam site in Nam Theun-Nam-Kading catchment and at B) Ban Kamphun in Sesan-Srepok catchment. The discharge at the Theun-Hinboun site represents the inflow to the reservoir and is calculated using reservoir spillway, turbine releases and water levels. The discharge at Ban Kamphun is based on direct water level measurements. The flow regimes with annual flood pulse have created highly diverse and productive aquatic ecosystems in the study region (see e.g. Junk et al., 2006, Lamberts, 2008). For example, the Mekong is one of the world s richest inland fisheries which produce livelihoods and food security for millions of people (MRC, 2010, Baran and Myschowoda, 2009, Hortle, 2007). But the ongoing hydropower development will alter the hydrological regimes (Lauri et al., 2012, Räsänen et al., 2012) and affect the aquatic ecosystems (Ziv et al., 2012, Stone, 2011, Grumbine et al., 2012, Baran and Myschowoda, 2009). The climate and the flow regimes in the study region have been reported to become more variable during the recent decades. For example Delgado et al. (2010) found that flow variability in the Mekong has increased during the 20 th century and the likelihood of extreme floods has increased during the last half of the 20 th century. Delgado et al. (2012) linked the increased variability to variability of the Western North Pacific monsoon. Räsänen and Kummu (2013) studied the role of El Niño Southern Oscillation (ENSO) on the hydrology of the Mekong and found that ENSO strongly modulates the annual flood pulse of the Mekong and that the ENSO-hydrology relationship have strengthened in the post 1980 period. In addition, a palaeoclimatological study by Räsänen et al. (2013) found that the inter-annual hydrometeorological variability of the past 50 years in the Mekong was highest compared to the past 700 years and that the increase in variability is at least partly associated with ENSO. The results from Räsänen et al. (2013) thus suggest that inter-annual alternations between very dry and very wet years have increased. The ENSO activity is also known to have increased during the recent decades and a research by Li et al. (2013) suggest that it is a result of the global warming. 6

3. MEASURED IMPACTS OF EXISTING HYDROPOWER PROJECTS ON RIVER FLOWS OF SESAN AND NAM THEUN-NAM KADING RIVERS Both study catchments, Nam Theun-Nam Kading and Sesan, have existing hydropower projects at the time of writing, but their impacts on flow regimes are neither well documented nor published. The average hydrological impacts of Theun-Hinboun dam (see location in Figure 1A) on the flow regime of Nam Theun River are shown in Figure 4A; the impacts of hydropower development on the Vietnamese side of the Sesan catchment on the flow regimes in Voeun Sai in Cambodia (see location in Figure 1A) are presented in Figure 4B. The hydrological impacts in Figure 4 are based on measured data. The Theun-Hinboun dam is a hydropower project with inter-basin water transfer. The project has a relatively small reservoir and thus small regulating capacity but it diverts the water from Nam Theun River to Nam Hinboun River for turbines (see Figure 1A). Therefore, especially the dry season flows have decreased in the Nam Theun River Figure 4A shows the inflow to the reservoir and the release to Nam Theun River during the period 2003 2006. The inflow to the reservoir has been calculated using known water levels plus turbine and spill way releases in the reservoir. Currently the situtation in Nam Theun river is different, however. In 2010 the Nam Theun 2 hydropower project started its operation which reduced the flows in Nam Theun River even further. Nam Theun 2 is also a hydropower project with interbasin water transfer to Se Bang Fai River (see Figure 1A). The construction of Nam Theun 2 required the Theun-Hinboun dam company to built an additional reservoir in Nam Ngouang River (see Figure 1A) to provide the necessary dry season flows for the Theun-Hinboun hydropower project. No published data or reports exist on how these three hydropower projects have affected the flow regimes of Nam Theun-Nam Kading, Nam Hinboun and Se Bang Fai Rivers. The flow regime of the Sesan River has been affected by hydropower development in the upper reaches of the river in Vietnam. The impacts of the Yali hydropower project in Vietnam on the flow regimes in Voeun Sai Cambodia are shown in Figure 4B (see locations in Figure 1B). The Voeun Sai flow measurement site is situated several hundred kilometers downstream of the Yali dam but it clearly shows changes in dry season flows. The daily variability in dry season flows have increased and on some occasions the flow levels also increased. The comparison in Figure 4B has been done with data from the pre- (1994 1997) and post- (2002 2005) Yali construction period and therefore can t be considered entirely accurate, as the compared years may be different in their flow behavior. The number of hydropower projects later increased in the upper reaches of Sesan (see Figure 1B) and their downstream hydrological impacts have not been well studied or at least no studies have been published. 7

Figure 4. Measured hydrological impacts of existing hydropower projects in Nam Theun-Nam Kading and Sesan catchments: A) Average inflow to Theun-Hinboun reservoir and average reservoir release to Nam Theun River during 2003-2006; B) Comparison of flow changes at Vouen Sai in Cambodia before construction of Yali dam in Vietnam (1994-1997) and after (2002-2005). These two study catchments would provide a good opportunity for further studies on the hydrological impacts of hydropower projects. However this would require a willingness by the hydropower companies and government agencies to share their hydrological and operatio data publicly. Downstream impacts on hourly, daily and seasonal water levels should be studied further to understand their implications for riparian ecosystems, people and their livelihoods. 4. HYDROLOGICAL MODELING OF NAM THEUN- NAM KADING AND SESAN CATCHMENTS For modeling we used the VMod distributed hydrological model (Koponen et al, 2010; Räsänen et al, 2012; Lauri et al, 2012). In the modeling of Nam Theun-Nam Kading, the model was based on 1x1 km grid representation of the catchment. The Sesan model was based on the grid size of 3x3 km. In the modeling of Sesan, the adjacent Sekong and Srepok rivers were included as together these rivers form a major tributary (the so-called 3S ) of the Mekong. Nam Theun-Nam Kading was modelled with finer spatial resolution due to more complex karst mountain topography. The model grid cells had individual information on catchment characteristics such as soil type, land cover, elevation, slope, and river network. In each grid cell the model computed precipitation, evaporation, infiltration, soil moisture and water flows to adjacent grid cells or streams (see more in Koponen et al, 2010). The model was driven by daily measured rainfall and temperature data, and calibrated against daily measured flows in the rivers as well as known annual flows in existing and future dam sites. The modeling period was 2003 2007 in the Nam Theun-Nam Kading catchment and 2001 2007 in the Sekong-Sesan-Srepok catchment. These modeling periods were determined by the availability and quality of hydrological data (precipitation and temperature) for driving the model and of river flow data for model calibration. More on methodology can be found in Annex 1 and Annex 2. The calibration of the model proved to be successful in replicating the flows in the Nam Theun-Nam Kading, Sesan and Srepok Rivers despite the relatively scarce data used to drive the model. For example, in Nam Theun-Nam Kading the model was able to reproduce 71% of the measured daily flow variability at Theun-Hinboun dam site, but the success of the model varied in different years. For example, the model performance was higher in 2003 2005 when the model was able to replicate 84 89% of the daily flow variability. The measured and computed flow at Theun-Hinboun dam can be seen in Figure 5. In the Sesan catchment, the model was able to reproduce 87% of the measured daily flow variability in the Ban Kamphun measurement station after the confluence of Sesan and Srepok Rivers (Figure 6). The modelled annual flow in eleven existing and future dam sites in Sesan was on average within 5% from the measured flow volume. Examples of model results from Sesan are shown in Figure 5 and more detailed results can be found in Annex 1 and Annex 2. The modeling results from Sesan in Figure 6 show the basic hydrological characteristics of the modelled catchment. The annual rainfall varied between 1300 mm and 3000 mm within the catchment (Figure 6A). The highest rainfall areas were found to be in the northern parts of the Sekong-Sesan-Srepok catchment and in more mountainous areas in the east. The temperature variations were found to largely follow the topography of the catchment: lowlands in the west were warmer and mountainous areas in the east cooler (Figure 6B). The measured and computed discharges illustrate clearly the distinct feature of the hydrology in the region (Figure 6B): During May, when the wet season starts, the flow levels start to rise, 8

peak in August-September, decline back to dry season flow levels around November and the lowest water levels are experienced in April. Such a flow regime can be called a monomodal flood pulse (see e.g. Junk et al., 2006). The hydrological modeling of the case study catchments was a somewhat complicated task. Data was scarce and both rivers were affected by hydropower construction and/or operations. During the modeling process several measured data time series were found to be suspicious in quality and had to be discarded. Due to data limitations, the model for Sesan had to be setup for the period when the Yali hydropower project had already started operating. However, great care was taken to confirm reliability of the model results. In the end, the model results can be considered adequate for conducting further hydrology- and water resource-related analyses in the study catchments but understanding the limitations of the model and its results is necessary when using the model outputs. Figure 5. Measured and computed flow at the Theun-Hinboun dam site in Nam Theun-Nam Kading catchment (see location in Figure 1) from the period 2003 2007. 9

Figure 6. Hydrological model outputs from Sekong, Sesan and Srepok River catchments from the period 2001 2007: A) Annual average precipitation; B) Annual average temperature; C) Measured and computed discharge at Ban Kamphun (see location in tile A). 10

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