Introduction

Abstract

This chapter provides basic information on climate and water in the region, and introduces the framing, scope, process, and structure of the assessment.

1.1 The Region

The Lancang-Mekong River Basin (LMRB) is one of the most important transboundary river basins in the world, with a river length of 4,880 km and a total area of 795,000 km² (Fig. 1.1a) (Liu et al., 2022). The Lancang-Mekong River (LMR) originates from the Tibetan Plateau in the Qinghai Province in China. It flows from north to south through the Yunnan Province and the Tibet Autonomous Region, and is called the Lancang River within China. After entering the lower portion, the river is known as the Mekong River, and finally enters into the South China Sea. The Lancang-Mekong River is the 10th largest river in the world with an annual streamflow at the river mouth in the Mekong Delta of about 475 km³/a (Liu et al., 2022). The upper Lancang River Basin accounts for 21% of the total basin area, and water supply here mainly comes from rainfall and snowmelt. The lower Mekong River Basin is shared by Laos (accounting for 25% of the total basin area), Thailand (23%), Cambodia (20%), Vietnam (8%) and Myanmar (3%), while streamflow in the lower basin comes mainly from precipitation and upstream flow. On average, the countries’ share of water flows in the basin is: China, 16%; Myanmar, less than 2%; Laos, 35%; Thailand, 18%; Cambodia, 18%; and Vietnam, 11%.

Fig. 1.1

a The Lancang-Mekong River Basin (LMRB). b The dams and major streamflow gauging stations in LMRB

Located in the monsoon climate zone, the basin is affected alternately by the southwest monsoon and the northeast monsoon, resulting in the uneven precipitation distribution in time and space, and great volatility in the seasonal streamflow. The wet season (from June to November) is mainly controlled by the southwest monsoon rich in water vapour, and more than 80% of the precipitation is concentrated in this season. The dry season (from December to May) is mainly affected by the northeast monsoon, and from December to February is the cool season and from March to May is the hot season. In general, 75% of the total annual streamflow of the basin flows through the lower Mekong Delta from July to October, and affects the ecosystem and human activities in the downstream area with the rhythmic floods. The alternation of dry and wet seasons leads to seasonal reversal of streamflow in the lower Mekong basin: the river flows back into the Tonle Sap Lake (the largest lake in Southeast Asia) to be stored in wet seasons, while the Tonle Sap supplies the Mekong River in dry seasons. This is one of the most unique hydrological processes in the world (Wang et al., 2021).

The LMRB has complex natural conditions: the elevation difference in this basin is more than 5060 m from the river source in the Tibetan Plateau to the Mekong River estuary, with an average slope of 1.04‰. The northern part of the Lancang River Basin is an alpine valley with average altitude of 3,500–5,000 m; and the southern part is a wide valley with an altitude between 1,000 and 3,500 m. As for the upstream of the Mekong River Basin, Myanmar and the northern part of Laos have a large area of mountains. The terrain of midstream in Thailand and Laos is a transition region from mountain to plain. The downstream located in Cambodia and southern Vietnam is mostly plains. In addition, the downstream Mekong Delta has a large area of floodplains, including the central floodplain from Kratie town to the border of Vietnam, the Tonle Sap floodplain with the Tonle Sap Lake and surrounding tributaries, and the Vietnamese Mekong Delta floodplains.

Over 70 million people live in the LMRB. Since the Angkor period (approximately the ninth to fifteenth centuries) or even earlier times, the LMRB has fed a large population with abundant water resources. Until now, riparian countries still highly rely on this commonly shared river.

After the agricultural reforms in the late 1980s, Vietnam has become one of the largest rice exporters in the world, with 90% of the rice exported from the Mekong Delta. The rural economy based on rain-fed agriculture provides 65% of the economic income of the Mekong River Basin. At the same time, the LMRB is one of the most biologically diverse basins in the world, second only to the Amazon. Rich species diversity in the LMRB has created the world’s largest inland freshwater fishery, which provides a vital, and often only, source of animal protein for people in this basin. The residents in the lower Mekong River Basin depend on fish and other aquatic animals for 47–80% of their required protein intake, more than any other major basins in the world (Hecht et al., 2019; Hortle, 2007). The Tonle Sap Lake produces 60% of Cambodia’s fish catches and solves the survival problem of nearly 10 million people (Burbano et al., 2020).

With the rapid urbanization and the population explosion, water resource conflicts in the LMRB are increasing. At the same time, the uneven distribution of precipitation has also exacerbated the problem of water disputes. All in all, the two important issues facing the LMRB are how to tackle increasing extreme events under climate change and how to manage water under increasing pressure from rapidly growing demands. The lower part of the basin is mainly located in the plains and deltas with flat terrain, which is vulnerable to flood disasters. Meanwhile, the increasing drought incidents also threaten the water security of the basin. According to the Emergency Events Database (EM-DAT, https://www.emdat.be/), the LMRB has recorded 173 floods and 23 droughts between 1990 and 2016, affecting 148.5 million people and causing a total of 61.4 billion US dollars of economic losses.

In order to tackle the increasing frequency and intensity of extreme events and meet the increasing energy demands in the LMRB, a large number of reservoirs have been constructed in the past decades. Before 2008, the basin was one of the least affected major river basins by human activity in the world with the effective reservoir capacity accounting for only 2% of the annual streamflow. By the end of 2021, the total storage capacity of the 103 reservoirs under operation in the basin had reached a staggering number of 100.3 km³, accounting for 23% of the annual streamflow (Fig. 1.1b, according to GMDD, the Greater Mekong Dam Database, https://wle-mekong.cgiar.org/maps/). Among these dams, 23 are located in China, producing 18,081 MW of electricity annually, while 80 dams are located downstream generating 15,034 MW of electricity annually (Hecht et al., 2019). These reservoirs have brought huge social and economic benefits to the countries in the basin, including mitigating extreme events, increasing energy supply, improving river navigation conditions, and ensuring agricultural irrigation (Yun et al., 2021). On the other hand, reservoir expansion has also aroused many criticisms. For example, reservoir operation changes the streamflow and affects the flood characteristics in the river, which might affect the aquatic ecosystem and vegetation distribution (Yang et al., 2019). Further, decline in river network connectivity due to dams may hinder the migration and reproduction of fish and lead to a decline in food security (Anh et al., 2018). Also, the interception of sediment by the reservoirs may reduce the supply of soil nutrients and increase the erosion of the Mekong Delta (Schmitt et al., 2019).

1.2 Background and Context

The complex climate in the LMRB is of high spatiotemporal variability, shifting from plateau climate at the upper basin to temperate monsoon and tropical monsoon climates in the middle and lower basin. Tropical cyclones mainly influence the basin during the wet seasons, and it can partly cause the second peak of seasonal streamflow in September–November (Chen et al., 2019). The incursion of tropical cyclones into the LMRB is a major factor in the development of regional flood events (MRC, 2015). Tropical cyclones also play a vital role in mobilizing sediment of the Mekong River (Darby et al., 2016), where the river delta is threatened by land subsidence (~1.6 cm yr⁻¹) and sea level rise (Erban et al., 2014).

Over 80% of the people live close to the river, making the lower basin one of the world’s largest inland fisheries (Ziv et al., 2012). There is increasing vulnerability of riparian countries to floods, which tends to cause fatalities and property damage, especially for those who live on the margins of economic development (MRC, 2015).

Under future climate change conditions, more frequent precipitation brought by the intensifying water cycle will greatly change the streamflow. Meanwhile, large-scale hydropower development would also profoundly change the way people live in this basin. In order to adapt to the changing environment and requirements of the society, a number of questions have been raised in recent decades which need to be dealt with properly. The important concerns include (1) trends of regional climate change in the past and future, (2) water resources change in terms of quantity and quality, (3) water usages for various sectors and their linkage to food and energy security, (4) impacts of climate change and dam construction on water-related hazards, (5) transboundary river management and governance. To address these concerns, it is necessary to comprehensively assess the combined impacts of climate change and human interventions on water resources in the LMRB.

1.3 Motivation and Framing of the Assessment

The LMRB is extremely sensitive to climate change. The warming rate here is higher than the mean global warming rate (Liu et al., 2022). Despite rich water resources (~8,000 m³/cap/yr), the high temporal and spatial variabilities in runoff create frequent seasonal droughts. In the past few decades, the hydrological system within the LMRB has been significantly influenced by climate change, consequently exacerbating extreme events, e.g., droughts and floods. The climate change and human intervention induced impacts on water have been projected to be intensified in the near future, bringing unprecedented threats to human societies and ecosystems. To this point, we proposed this report entitled “Water resources assessment in the Lancang-Mekong River Basin: Impact of climate change and human interventions” to support socio-economic development through sustainable use of water by providing accurate and updated information on climate and water resource changes presented in a consistent way. It provides implications to support decisions and stakeholders at all levels.

This report provides a comprehensive, up-to-date picture of the current state of knowledge based on published articles and recent research from the author team. New evidence of past, present and projected future changes in climate and water resources is based on many independent scientific analyses from observations and simulations using models.

The report is an assessment similar to the Intergovernmental Panel on Climate Change (IPCC) assessment report. It is not a review or a textbook of climate and water sciences, but is based on the published scientific and technical literature available. Underlying all aspects of the report is a strong commitment to assessing the science comprehensively, without bias and in a way that is relevant to policy but not policy prescriptive.

1.4 Approach and Processes

Like many other environmental issues, climate change and water resources are complex, which poses a challenge to provide authoritative scientific evidence for policy makers to take actions. Over the past decades, it became clear that scientific assessment is a powerful tool to meet this challenge. It is particularly useful in reaching a consensus among a group of experts when there are diverse and sometimes contradictory evidences from a variety of indicators and perspectives, which can be demonstrated by the success of IPCC assessment reports.

This assessment followed the essential principles used in the above-mentioned global assessments. Specifically, we tried to involve experts who are active researchers and come from different countries as authors and review editors. Further, this assessment focuses on summarizing and evaluating the existing literature published in peer-reviewed journals, although occasionally official governmental documents and reports from regional and international organizations were also mentioned.

The assessment was designed and edited by Deliang Chen, Junguo Liu and Qiuhong Tang, and managed by Yuehan Dou and Kai Wang. A group of lead authors was appointed to lead each theme (chapter), and to invite and engage contributing authors to contribute to specific aspects of the assessment. When an expert on a specific topic was missing during the assessment process, an additional expert was invited to act also as contributing author. An important step in the process is the multiple reviews of the assessment. While the lead authors constantly reviewed the writings of the lead authors and contributing authors for their chapters, the editors commented on the drafts in several phases of the project. Finally, the complete chapter drafts were reviewed by review editors. The whole process took three years to complete.

1.5 Structure of the Report

This report consists of a short introduction and 8 thematic chapters covering climate change, surface water change, arsenic pollution, water utilization, water-food-energy nexus, water related hazards, water management, and water governance. In order to facilitate the accessibility of the findings of this report for a wide readership and to enhance their usability for stakeholders and users, each thematic chapter has an executive summary (abstract) highlighting major findings within the chapter. These executive summaries (abstracts) can be particularly useful for local government and stakeholders for water management towards sustainability.

Introduction (This Chapter): This chapter provides basic information on climate and water in the region, and introduces the framing, scope, process, and structure of the assessment.

Climate variability and climate change: Past and future (Chap. 2): This chapter assesses climate change in the past decades and projects future changes until the end of this century by using observed records and model simulations.

Surface water (Chap. 3): This chapter analyzes river network geometric features, assesses past and future changes in runoff, baseflow, and discharge, reveals the dynamics of the inundation area and turbidity in the Tonle Sap Lake.

Arsenic in Hydro-Geo-Biospheres of the Mekong River Delta: Implications for human health (Chap. 4): This chapter investigates arsenic cycling in Hydro-Geo-Biospheres in the Mekong River Delta and assesses the environmental impacts of groundwater arsenic as well as its health effects and exposure from drinking water and food. It also provides policy recommendations for arsenic mitigation.

Water utilization and the link to food and energy (Chaps. 5 and 6): These 2 chapters assess the water demand and utilization in the basin. It covers the relevant aspects from irrigation, hydropower generation, domestic water uses within the water-food-energy nexus.

Water hazards: drought and flood (Chap. 7): This chapter describes the characteristics of water related hazards including drought and flood in the basin. The impacts of climate change and human interventions on flood and drought are assessed to support the local risk mitigation and adaptation.

River basin management and governance (Chaps. 8 and 9): These chapters summarise the tradeoff between economic development and resource conservation in the basin, and present the major challenges for water resources management and governance. It also highlights the importance of international cooperation for transboundary water management.

A graphic presentation of the structure and contents of all chapters is provided in Fig. 1.2.

Fig. 1.2

The structure and contents of the chapters