JOINT RESEARCH May 2019) May – HYDROLOGICAL IMPACTS FINAL REPORT FINAL FINAL REPORT FINAL OF THE LANCANG HYDROPOWER CASCADE ON 2018 (July DOWNSTREAM EXTREME EVENTS Mekong River Commission, Lancang-Mekong Water Resources Cooperation Center China Institute of Water Resources and Hydropower Research International Water Management Institute October 2019 This document has been prepared by the following authors: Dr. Janejira Chuthong Chief Hydrologist Mekong River Commission Secretariat (MRCS) Dr. Hui Liu Senior engineer China Institute of Water Resources and Hydropower Research (IWHR) Dr. Fengran Xu Professor of engineering China Institute of Water Resources and Hydropower Research (IWHR) Dr. Dongsheng Cheng Professor of engineering Lancang-Mekong Water Resources Cooperation Center (LMWRCC) Dr. Wenhai Zhang Engineer Lancang-Mekong Water Resources Cooperation Center (LMWRCC) Dr. Mansoor Leh Researcher – Water Accounting International Water Management Institute (IWMI) Dr. Guillaume Lacombe Senior Researcher – Water Resources and Hydrology International Water Management Institute (IWMI) With contributions from the teams as listed in Annex 1 Document History Version Revision Description Issue date Issued by 1 0 Draft Report April 2019 Janejira Chuthong – MRCS Hui LIU and Fengran Xu -IWHR Guilaume and Mansoor – IWMI 2 0 Final Draft Report August 2019 Janejira Chuthong – MRCS Anoulak Kittikhoun - MRCS Hui LIU and Fengran Xu -IWHR Mansoor – IWMI 3 0 Final Report (un-edited) October 2019 Janejira Chuthong – MRCS Anoulak Kittikhoun - MRCS i | P a g e Table of Contents Executive Summary .................................................................................................................................... xi 1 Introduction ..................................................................................................................................... 1 1.1 Background ........................................................................................................................... 1 1.2 Objectives .............................................................................................................................. 2 1.3 The scope of the Joint Research ........................................................................................... 3 1.3.1 Comparative analysis of the droughts of 2009-2010 and 2012-2013 .................... 3 1.3.2 Analysis of extreme drought of 2015-2016 ............................................................. 3 1.3.3 Analysis of the flash flood of December 2013 ......................................................... 4 2 Data and methodology .................................................................................................................... 5 2.1 Data collection ...................................................................................................................... 5 2.2 Methodology ......................................................................................................................... 6 2.3 Working mechanism and proposed work plan ..................................................................... 8 2.4 Expected outputs .................................................................................................................. 9 2.5 Role and responsibility .......................................................................................................... 9 3 Field and exchange visits ............................................................................................................... 11 3.1 Joint Visit to Lancang River, China, 22-26 September 2016 ............................................... 11 3.2 Joint Visit to the Mekong River, Nakhon Phanom Province, Thailand, 13-14 December 2017 ..................................................................................................................................... 15 4 Profile of the Lancang-Mekong Basin ........................................................................................... 21 4.1 General Geography ............................................................................................................. 21 4.2 Existing Hydropower Dams in China ................................................................................... 23 5 Comparative analysis of the droughts of 2009-2010 and 2012-2013 .......................................... 25 5.1 Research Scope ................................................................................................................... 25 5.2 Hydrological Process during the Two Events ...................................................................... 27 5.3 Drought Analysis ................................................................................................................. 29 5.3.1 Data ........................................................................................................................ 29 5.3.2 Methodology .......................................................................................................... 29 5.3.3 Meteorological Drought ........................................................................................ 31 5.3.4 Hydrological Drought ............................................................................................. 44 5.3.5 Comparison of Meteorological and Hydrological drought .................................... 47 5.4 Effect of Water Supplement of Lancang Hydropower Cascade on the Lower Reaches .... 48 5.4.1 Impact on the Mekong Mainstream Flow ............................................................. 48 5.4.2 Impact on the Mekong Mainstream Water Level ................................................. 50 5.4.3 Impact on the Mekong Mainstream Water Volume ............................................. 51 5.5 Discussion ............................................................................................................................ 54 ii | P a g e 6 Analysis of extreme drought of 2015-2016 .................................................................................. 55 6.1 Background ......................................................................................................................... 55 6.2 Implementation of the emergency water supplement from the Lancang River ................. 56 6.3 Analysis of cause of the drought in the Lancang- Mekong Basin ........................................ 58 6.3.1 Rainfall and inflow discharge to the Lancang Basin ................................................ 58 6.3.2 Hydrological Condition at the end of Wet Season 2015 ....................................... 59 6.3.3 Drought in the Mekong Basin ................................................................................ 61 6.4 Influence of Lancang cascade reservoir operation on dry season volume of the Mekong River ..................................................................................................................................... 67 6.4.1 Annual volume of the Lancang River ....................................................................... 67 6.4.2 Impact of cascade dams on dry season volume of the Mekong River ..................... 67 6.5 Hydrological influence of the emergency water supplement to the Mekong River ............ 68 6.5.1 Influence on discharge of the Mekong River ........................................................... 68 6.5.2 Influence on water level of the Mekong River ......................................................... 71 6.5.3 Influence on volume of the Mekong River .............................................................. 72 6.6 Net contribution of the emergency water supplement to discharge of the Mekong River 76 6.7 Flow propagation along the Mekong mainstream .............................................................. 79 6.8 Salinity variation in the Mekong Delta................................................................................. 83 7 Analysis of the respective hydrological impacts of climate variability and hydropower operation ....................................................................................................................................................... 86 7.1 Materials and Methods ....................................................................................................... 86 7.2 Hydrological Model Setup ................................................................................................... 89 7.3 Results and Discussion ........................................................................................................ 90 7.3.1 Rainfall .................................................................................................................... 90 7.3.2 Observed Flows ...................................................................................................... 91 7.3.3 Model Calibration and Validation Performance .................................................... 92 7.3.4 Comparison of Observed and Simulated Flows ..................................................... 94 7.3.5 Discussion ............................................................................................................... 97 7.4 Model Limitations and Potential Improvements ................................................................ 98 7.5 Analysis of the flash flood event in December 2013 .......................................................... 99 7.5.1 Rainfall over the Lower Mekong Basin in December 2013 ................................... 99 7.5.2 Water