ANNEX 2. BASELINE HYDROLOGY OF THE 3S BASIN

Transcription

1 ANNEX 2. BASELINE HYDROLOGY OF THE 3S BASIN MK3 Optimising cascades of hydropower HYDROLOGY & FLOOD CONTROL Sesan-Srepok-Sekong VMod hydrological modelling report August 213 Timo A. Räsänen

2 . Authors Produced by Suggested citation More information Image Project Team Copyright Timo A. Räsänen (Aalto) 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 Baseline hydrology of the 3S basin: Sesan-Sekong-Srepok VMod hydrological modelling 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, Cover image: Sesan River, Cambodia. Inside page: Fishermen in the Ochum 2 Reservoir, Cambodia (photo credit: T. Ketelsen) 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 213 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 211. 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. ii

3 3S Baseline hydrology: TABLE OF CONTENTS 1 INTRODUCTION THE 3S RIVER BASIN GEOGRAPHICAL DATA AND MODEL GRID METEOROLOGICAL DATA HYDROLOGICAL DATA MODEL SETUP AND RESULTS Model Setup Model calibration Model results Precipitation and temperature Discharge FINAL REMARKS REFERENCES iii

4 1 INTRODUCTION The hydrology of the Sesan river basin was modelled using the VMod distributed hydrological model. VMod is the hydrological component of the broader IWRM model (Koponen et al., 21). The VMod model setup was built for the 3S River basin which consists of three rivers, Sekong, Sesan and Srepok. It was calibrated to two of the river catchments, Sesan and Srepok. The main purpose of the hydrological model is to feed into the assessment of multiple-use options of the hydropower reservoir in the Sesan River catchment. The work was done as a part of the CGIAR Challenge MK3 project in spring 211.This report describes the modeling results and the data used to construct the model grid and run the model. VMod has been used earlier in the Mekong Region with good success, for example, by Räsänen et al. (212) and Lauri et al. (212). 2 THE 3S RIVER BASIN 3S refers to three rivers: Sekong in the north, Sesan in the middle and Srepok in the south (Fig 1). The 3S rivers form one of the largest tributaries of the Mekong. The 3S River basin is shared by Cambodia, Lao PDR and Vietnam, and has a total area of 78,529 km 2 it. The climate of the area is dominated by a monsoon climate that divides the year into a wet season from May to October, and a dry season from November to April. Most rainfall occurs in the wet season which has created a hydrological regime with annual flood pulse. The average annual rainfall in the region is 2,27 mm - this contributes approximately 16% to the total annual flow of the Mekong. The contribution of the Sesan River alone to the annual flow of Mekong is around 4%. Further catchment details are shown in Table 1. Figure 1: 3S River basin 2

5 Table 1 3S River Basin characteristics Basin Sekong Sesan Srepok 3S Area 28,765.7 km 2 18,684.2 km 2 31,79. km 2 78,529 km 2 Average outflow ~14 m 3 /s ~651 m 3 /s ~695 m 3 /s ~2,386 m 3 /s Extent EW 312 km Extent NS 53 km Outflow Mekong at Stung Treng Elevation range 8 2,4 m Average precipitation ,27 mm/a Average evaporation ,296 mm/a 3 G EOG RAPHICAL DATA AN D MOD EL GRID The model grid for the 3S River basin was constructed using the following data: - Digital elevation model (SRTM 9m) (Jarvis and Reuter, 28) - Land use (Land use map 24) (MRC, 211b) - Soil data (FAO Soil Map of the World) (FAO, 27) - 3S catchment boundary (MRC, 211b) - River and lake shore data (MRC, 211b) All geographic data was transformed to a 3 km resolution and the UTM 48N coordinate system. For the digital elevation model (DEM), an average elevation value from the raster values within the model grid box was used for grid box elevation. For soil and land use data, the most common class within the model grid box was selected for the grid box type. The 3S basin area was then extracted using catchment boundary data. The river data in the model grid was created from the above data by following steps: i. The elevation data was lowered by 1 m in grid boxes containing any part of the main channel of the Sekong, Sesan and Srepok rivers ii. The flow network was calculated using the modified DEM and lowest neighbour principle. The FAO soil data were simplified for model computation by reducing the number of soil classes in the FAO data. The reclassified soil types used in VMod are shown in Table 2. The resulting soil raster is shown in Figure 2Figure 2. The land use types were also simplified for model computation, by reducing the number of land use classes in the data. The reclassified land use is shown in Table 3. The resulting land use raster is shown in Figure 2 3

6 Table 2. Reclassified soil classes used in VMod Model class Title Explanation 1 Water Permanent water body 2 Acrisols Subsurface accumulation of clays, low base saturation 3 Histosols Organic material 4 Argic Argic/Ochric horizon, sand on top, clay below 5 Ferrasols Deep strongly weathered soils 6 Alluvia Permanent or temporary wetness 7 Lithosols Limited soil development 8 Cracking Hard when dry, plastic when wet Table 3. Reclassified land use classes in VMod Class number Title 1 Water 2 Decidious forest 3 Evergreen forest 4 Shrub and grassland 5 Irrigated agriculture 6 Agriculture 7 floodplain 8 Urban 9 Glacier Figure 2. Processed soil (left) and land use (right) grids for the hydrological model of 3S River basin. 4

7 4 METEOROLOGICAL DATA The model used ground-based rain gauge and temperature measurements and also NCEP Reanalysis II temperature data to force the model. The measurements are from the MRC hydrological database (MRC, 211a). The NCEP Reanalysis II dataset is a global dataset with 2.5 degree resolution (NOAA, 211). The NCEP Reanalysis II data was downloaded from the NOAA online database and converted for model use. Figure 3 shows the data points used in the 3S model. For forcing the hydrological model, Vmod, the following meteorological data was used: - precipitation (daily) - minimum temperature (daily) - maximum temperature (daily) - incoming solar radiation (daily) Figure 3: The meteorological data points used in the hydrological model VMod. 5 H YDROLOGICAL DATA Flow and water level measurements were available from the Mekong River Commission s hydrological database (MRC, 211a) for several locations in 3S. However, several measurements stations were found to be unreliable. The unreliable stations were identified by visual inspection of data, water balance calculations and model parametrisation. The stations which were expected to have the most reliable data were selected for the model calibration (see Figure 3). The flow statistics and daily flow measurement data from three calibration sites are shown in Table 4 and Figure 4. In addition, average flows from existing and planned hydropower project sites from the MRC hydropower database were used for calibration. The locations of dam sites and related average flows were taken from Mekong River Commission s hydropower database (MRC, 29). 5

8 Figure 3. Flow measurement sites used in the calibration of the hydrological model VMod Ban Kamphun Andaung meas Ban Don m3/s /1/21 1/1/22 1/1/23 1/1/24 1/1/25 1/1/26 1/1/27 Figure 4. Daily flows at Ban Kamphun, Andaung Meas, Ban Don (see locations in Figure 3). Table 4: Flow statistics in Ban Kamphun, Andaung Meas and Ban Don (see locations in Figure 3). Location Average annual flow [m 3 /s] Average minimum monthly flow (April) [m 3 /s] Average maximum monthly flow (August) [m 3 /s] Ban Kamphun Andaung Meas Ban Don

9 6 MODEL SETUP AND RESULTS 6.1 M O D E L S E T U P The VMod model was setup using the model grid presented in Chapter and weather data presented in Chapter 4. The parametrisation of soil hydraulic characteristics was based on FAO s Lecture Notes on Major Soils of the World (FAO, 21) and the soil water calculator developed by Saxton and Rawls (Saxton and Rawls, 26). The soil water calculator was used to obtain soil hydraulic properties which were then used in the model parametrisation. For precipitation and temperature measurements, elevation correction factors were used. Generally rainfall intensities increase and temperatures decrease (adiabatic cooling) towards higher elevations. The model adjusts the measured rainfall and temperature according to grid box elevation and elevation correction factor. The correction for precipitation was +.5 mm/meter of elevation; for temperature the correction factor was -.6 C/meter of elevation. For evaporation computation, the Hargreaves-Shamani method was used. 6.2 M O D E L C A L I B R A T I O N The model was calibrated for the Sesan and Srepok rivers using observed flows from Ban Kamphun and Ban Don (21-27), and Andaung Meas (21-25). In addition, the model flows were calibrated for flows at 11 existing or planned dam sites. Separate model validation runs were not executed due to the lack of data and poor data quality. 6.3 M O D E L R E S U L T S P r e c i p i t a t i o n a n d t e m p e r a t u r e The spatial distribution of interpolated annual average rainfall and temperature in 3S catchments is shown in Figure 5. The daily average precipitation and temperatures of two selected sites, Location 1 and Location 2 (see Figure 5A), are shown in Figure. 6. Annual values of these two locations are also summarised in Table 5. Figure 5. Spatial distribution of annual average A) precipitation and B) temperature of 3S catchments and locations for data extraction points for further analyses. 7

10 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec [mm] [mm] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec [mm] [mm] MEKONG CPWF Optimising cascades of hydropower (MK3) A. Precipitation at Location A. Precipitation at Location A. Precipitation at Location A. Precipitation at Location Figure. 6 Daily average A, B) precipitation and C, D) temperature at Location 1 and Location 2 in the Sesan Catchment. The locations are shown in Figure 5A. The elevations at Location 1 and 2 are 16 masl and 6 masl respectively. Table 5. Annual average precipitation, temperatures, potential evapotranspiration and actual evapotranspiration at Location 1 and Location 2 in the Sesan Catchment. The locations are shown in Figure 5A. Location 1 Location 2 Difference Precipitation [mm] 1,951 1, Annual average temperature [C] Annual average maximum temperature [C] Annual average minimum temperature [C] Annual potential evaporation [mm] 2,339 2, Annual evaporation [mm] 1,338 1,

11 m3/s m3/s MEKONG CPWF Optimising cascades of hydropower (MK3) D i s c h a r g e Figure 7, Figure 8 and Figure 9 show the computed flows compared to measured flows in three measurement points, Ban Kamphun, Andaung Meas, Ban Don. Table 6 summarises flow statistics of computed and measured flows and Table 7 shows the computed and MRC hydropower database flows at 11 dam sites MEASURED COMPUTED /1/21 1/1/22 1/1/23 1/1/24 1/1/25 1/1/26 1/1/27 Figure 7. Measured and computed flow at Ban Kamphun 5 45 MEASURED /1/21 1/1/22 1/1/23 1/1/24 1/1/25 1/1/26 1/1/27 Figure 8. Measured and computed flow at Andaung Meas 9

12 m3/s MEKONG CPWF Optimising cascades of hydropower (MK3) 3 25 MEASURED COMPUTED /1/21 1/1/22 1/1/23 1/1/24 1/1/25 1/1/26 1/1/27 Figure 9. Measured and computed flow at Ban Don Table 6. Flow statistics for computed and measured flows Ban Kamphun Andaung Meas Ban Don Lumphat Measured average flow m 3 /s 1, Computed average flow m 3 /s 1, Ratio R Ban Kamphun Andaung Meas Ban Don Lumphat Measured max. flow m 3 /s 6,25 4, ,19 Computed max. flow m 3 /s 7,11 3,226 2,427 4,47 Ratio Ban Kamphun Andaung Meas Ban Don Lumphat Measured min. flow m 3 /s Computed min. flow m 3 /s Ratio

13 Table 7. Flow statistics for computed and MRC hydropower database flows at dam site Upper Kontum Plei Krong Yali Sesan 3 Se San 3A Se San 4 MRC HP database m 3 /s Computed m 3 /s Ratio Se San 4A Lower Sesan 3 Lower Sesan 2 Prek Liang 2 Prek Liang F INAL REMARKS A distributed hydrological model, VMod, was setup for simulating the hydrology of the 3S River basin with a main focus on the Sesan River. The hydrological model performed remarkably well considering the scarcity and quality of the available data which did not allow a full calibration and validation procedure to be undertaken. The calibration results are, however, relatively accurate. For example the R 2 at the lowest calibration point was.87 and the water balance error in 11 mainstream locations of Sesan River was on average 5%. A certain degree of cautiousness regarding the model s reliability is still warranted. The forcing data were found to be insufficient and of poor quality. Despite these deficits the model results are considered adequate for further assessments in MK3 project. 11

14 REFERENCES FAO, 21. Lecture Notes on the Major Soils of the World. World Soil Resources Reports, 94, FAO. FAO, 27. Digital Soil Map of The World, FAO. Jarvis, A. and Reuter, H., 28. Hole-filled SRTM for the globe Version 4. CGIAR-CSI SRTM 9m. CGIAR- CSI SRTM 9m database Accessed April 21 Koponen, J., Lauri, H., Veijalainen, N. and Sarkkula, J., 21. HBV and IWRM Watershed Modeling User Guide. MRC Information and Knowledge management Programme, DMS Detailed Modeling Support for the MRC Project. Lauri, H. et al., 212. Future changes in Mekong River hydrology: impact of climate change and reservoir operation on discharge. Hydrol. Earth Syst. Sci., 16(12): MRC, 29. Hydropower database. Mekong River Commission, Vientiane Lao PDR. MRC, 211a. Hydrological database. Mekong River Commission, Vientiane Lao PDR. MRC, 211b. Mekong River Commission data, Vientiane, Lao PDR. NOAA, 211. NCEP-DOE Reanalysis 2 database. Nationa Oceanic and Atmospheric Administration, Earth System Research Laboratory, Physical Sciences Division, Boulder Colorado, USA. Accessed 211. Räsänen, T.A., Koponen, J., Lauri, H. and Kummu, M., 212. Downstream hydrological impacts of hydropower development in the Upper Mekong Basin. Water Resources Management, 26(11): Saxton, K.E. and Rawls, W.J., 26. Soil Water Characteristic Estimates by Texture and Organic Matter for Hydrologic Solutions. Soil Science Society of America journal, 7(5):