Water Resources in the Lancang-Mekong River Basin: Impact of Climate Change and Human Interventions

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

The LMRB (LMRB) has experienced significant climate change, particularly over the last 50 years. An increase in the annual precipitation but with significant seasonal differences in the changes, and a remarkable warming are observed over the Basin. The region also experienced more frequent extreme events, such as an increase in extreme precipitation, as well as hot days and warm nights, a decrease in cold days and cold nights, and a more frequent occurrence of droughts. The future climate over the Basin is projected to be continuous warming, which is most significant by the end of the twenty-first century. A general wetting is projected over the region with the spatial pattern of the projected annual total precipitation change show consistencies with the present day condition. Differences are found between the global and regional climate model projections in the precipitation, indicating the uncertainties existing in the projections, and also the importance of the model resolution in projecting future climate.

This chapter assesses surface water changes due to climate change and human activities, by particularly examining runoff and streamflow. Changes in the hydrological cycle due to climate change and human intervention can lead to diverse environmental impacts and risks. Fresh water is the agent that delivers many of the impacts of climate change on society. As the major component of freshwater systems, surface water has been significantly altered across basins in terms of spatial and temporal characteristics. The comprehensive understanding of the current status of surface water in the LMRB, such as the distributions and patterns of runoff changes across the Lancang-Mekong River Basin was completed through the high-resolution river network extraction and sophisticated hydrological models. Significant but different trends were found in the seasonal and annual runoff from the LMRB due to different reasons. Over the period of 1971–2010, the annual streamflow shows a general downward trend due to the continued enhancement of human activities. Runoff in the dry season is found to increase faster than the mean annual runoff. As for the spatial distribution, significant trends in streamflow were observed mainly in the middle basin and east of the lower basin. Superimposed on the substantial seasonal cycles is the noticeable lake shrinkage in recent years, especially the Tonle Sap Lake. Evidently decreased inundation was found in most years in the recent two decades from 2000 to 2018. An evident decreasing trend in runoff caused by climate change in the high correlation zone of the Tonle Sap Lake, mainly due to the precipitation decreasing, indicates that climate change contributed to the decrease in water level in the Tonle Sap Lake in addition to human activities. In addition to the decreases in the runoff, streamflow and water level in the Tonle Sap Lake, a significant (p < 0.05) downward trend in the baseflow was also found from 1980 to 2007. Unlike the historical changes in runoff, previous studies projected with high confidence an increasing trend for streamflow in the LMRB, regardless of the climate forcings and models used. However, the flow regime is highly susceptible to a variety of drivers, e.g., dam construction, irrigation expansion, land-use change and climate change. Substantial changes are expected in both annual and seasonal flow, along with a generally increasing trend. Although hydropower development exhibits a limited influence on total annual flows, it has the largest seasonal impact on streamflow, with an increase in the dry season and a decrease in the wet season, by outweighing those of the other drivers.

This chapter assesses human health risks of inorganic arsenic (As) from drinking well water and consumption of rice irrigated by high-As groundwater in the Mekong River Delta. Geogenic inorganic As (iAs) occurring at elevated levels in groundwater has been detected in more than 70 countries. Among mostly rural residents relying on groundwater for drinking, this exposure has resulted in negative health consequences including visible skin lesions, multiple internal organ cancers, numerous invisible non-cancer health effects such as cardiovascular diseases, and premature deaths. In the Mekong River Delta (MRD, defined by elevation <10 m above sea level in this book), As issues in groundwater have been documented as early as 1999 in Cambodia, with literature reporting its occurrence in Vietnam since 2005. Since the early 2000s, efforts have been made to test for As in about 100,000 wells from Cambodia, Laos, Vietnam and Thailand. Here, a combined dataset with a total of 94,768 unique As tests was analyzed to illustrate the spatial patterns and to assess the health risks of drinking well water As in Cambodia and in southern Vietnam. Although knowledge is far more limited, an attempt was also made to examine the potential health risks associated with iAs exposure from rice, a major staple for the MRD. Here, irrigation using highly As enriched groundwater for rice cultivation has expanded this environmental health problem from the hydrosphere (water) to the geosphere (soil) and, in turn, the biosphere (rice, and ultimately humans). Of 41,928 tests in Cambodia, 35.8% exceeded 10 μg/L, the WHO guideline value for drinking water As, while 21.5% exceeded 50 μg/L, the Cambodian drinking water standard. Of 52,858 tests in Vietnam, the exceedance rate for 10 μg/L, which is also the Vietnamese drinking water standard, is 10.0%. High As wells, regardless of whether it is relative to 10 or 50 μg/L, are located in proximity to the main course of the Mekong-Bassac Rivers, especially within a 5 km distance. The vast majority (>98%) of high-As wells are located in low-lying areas, i.e. <25 m elevation in Cambodia and <10 m elevation in Vietnam. High-As wells occur frequently at shallow depths (<70 m) across the MRD but also at deeper depths (300–500 m) in Vietnam. Due to the clustering of high As wells along the Mekong-Bassac Rivers, extreme human health tolls are identified in 11 districts of Cambodia and 3 districts of Vietnam with a population attributable fraction exceeding 0.1, meaning that >1 in every 10 adult deaths is solely due to drinking water As exposure. The annual excess deaths attributable to arsenic exposure alone is 1204 in Cambodia and 1486 in Vietnam, or 1 in every 27 adult deaths and 1 in every 78 adult deaths, respectively. In addition to uncertainties in bioavailability and toxicity of iAs in rice grains, soil and rice As data, especially rice As speciation data needed for risk assessment, are still limited in the MRD.

This chapter assesses water resource availability and use in the five countries in Mainland Southeast Asia (MSEA): Myanmar, Thailand, Laos, Cambodia, Vietnam. The total water resources in the region are estimated using a wide range of hydrometeorological data. Results show that the average annual runoff is about 1941.1 billion m³ in the region. Regarding spatial differences, rainfall and runoff in the southern coastal areas are generally higher than the ones in the central and northern inland areas, and the western coastal areas have more rainfall than the eastern coastal areas. Moreover, results indicate that the overall utilization rate of water resources in the region reached 9%, mainly used for hydropower development, agricultural irrigation, fishery and aquaculture, shipping and other aspects. Agriculture was the primary water user (about 92.2%) in the study area compared to industrial (about 3.6%) and domestic (about 4.2%) water users. The region is divided into different water resource zones, including 7 first-level water resources zones, 17 s-level water resources zones, and 138 third-level water resources zones. The division is done by considering the hydrology conditions, natural landforms, administrative divisions, and river systems in the study area. Particularly, results show that the seven first-level water resources regions are all transboundary basins, implying that the water resources management in the region needs the solid cooperation and overall planning of all countries. Results show that the total water demand in MSEA will reach 200, 208, and 225 billion m³ in 2025, 2030, and 2040, respectively. The prediction is obtained using the historical social and economic data. Social-economic developments are predicted to estimate the future water consumption. will assure a balance between the supply and demand of water resources in the study area, with asurplus of water resources supply ability.

Water, food, and energy resources are critical concerns to achieve the UN 2030 Sustainable Development Goals. However, achieving food, energy, and water security is under increasing pressure due to population and economic growth as well as climate change. Climate change affects the regional precipitation and discharge in both time and space scales. Rice consumption increased about 5 times during 1961–2017, and energy requirements increased with an annual growth rate of 5–6% between 1990 and 2010 at the global scale. This chapter studies the linkage of water-food and water-energy sectors as well as the nexus relationship in the Langcang-Mekong River Basin (LMR B). Agriculture is the main water consumer in LMRB, and expansion of irrigated cropland and agricultural intensification has significantly increased the irrigation water demand. The basin is an ideal location for developing and utilizing hydropower resources, and the hydropower potential is estimated at around 60,000 MW. Future climate change might decrease the regional hydropower potential, especially around the mainstream. Water demand for thermal power generation and fossil fuel extraction is increasing due to population growth and socio-economic development. Furthermore, biofuel production and crop planting areas both increased sharply in the Lower Mekong countries, especially in Vietnam and Thailand. Water, food, and energy resources are strongly connected in the Mekong River Delta. A nexus case study in the Mekong River Delta showed a strong connection among food, energy, and water systems. Rice yields will be vulnerable to extreme climate events, and the development of the energy sector will affect regional sustainability through nexus significantly. Specifically, the average total water withdrawal in 2050 was estimated to increase by 40% compared to that in the 2016 drought year and will be more than 3 times higher than the average withdrawal of 1995–2010.

Droughts and floods are the main threats to the Lancang-Mekong River Basin (LMRB). Drought mainly occurs during the dry season, especially in March and April, in the LMRB. The “dry gets drier” paradigm performs well in the LMRB, specifically in the Mekong Delta. Further, flood frequency and magnitude, which are determined by heavy rain, are also increasing in the LMRB. Droughts and floods show obvious seasonal and regional characteristics in the LMRB. The LMRB is a well-known rainstorm-flood basin. Floods in the LMRB are mainly caused by heavy rain. The LMRB is dominated by regional floods, and basin-wide floods rarely occur. From upstream to downstream, the flood peak and flood volume have shown increasing trends. Meanwhile, moving further downstream, the flood season ends later. In the upstream areas, floods are mainly concentrated in the period from July to October, with the highest probability of floods occurring in August. For the downstream areas, the flood season is from August to October. Climate change is one of the major factors affecting the LMRB’s droughts and floods. Global warming is an indisputable fact. Under global warming, extreme hydrological events show a tendency to increase. Climate models have suggested a future potential for increased flood frequency, magnitude, and inundation in the LMRB by 10–140%, 5–44% and 19–43%, respectively. Although the severity and duration of droughts are also increasing, the differences in drought indicators projected by different climate models are significant. Hydropower development was another major factor affecting droughts and floods in the LMRB. Large-scale hydropower development has drastically changed streamflow characteristics since 2009, causing increased dry season flow (+150%) and decreased wet season flow (−25%), as well as reduced flood magnitude (−2.3  to  −29.7%) and frequency (−8.2 to −74.1%). Large-scale reservoirs will have a profound impact on hydrological characteristics, droughts and floods, agriculture, fisheries, energy supply, and environmental protection in the LMRB. Coupling climate models and hydrological models is the main way to study the impact of climate change and reservoir operation in the LMRB. Climate change indirectly affects hydrological characteristics by affecting meteorological parameters, while reservoirs can directly change the propagation from meteorological extreme events to hydrological extreme events by releasing/storing water in different situations. Hydrological models are the link connecting and quantifying the coupled effects of climate change and reservoirs. More studies are needed to develop a comprehensive understanding of the future impacts of climate change and reservoir operation on extreme events in the LMRB, as well as adaptation and mitigation measures.

Integrated River Basin Management (IRBM) involves the integration of the multiple uses of water, the integration of multiple properties of water: water disaster, water resources, waterways, water environment, water ecology, water landscape and water culture, and the integration of water by space: upstream vs downstream, left bank vs right bank. The main problems of IRBM within the Lancang-Mekong River Basin includes flood disaster, navigation and its impact to basin cooperation, contradiction between development and protection, and public security in a framework of cooperation and integration. It has been a general concern for Mekong countries to manage water conservancy engineering and coordinate water supply, navigation, fishery, power generation, and water disaster management. All stakeholders put great emphasis on water conservancy engineering management in terms of basin planning, domestic and cross-border project construction, and cooperation mechanisms. In order to ensure the sustainable use of water resources, a series of continuously updated plans were proposed. Those plans set goals and provided measures for the rational and sustainable development of the resources in the basin, and meanwhile, it also put forward a mechanism to offset the adverse effects. The development of international navigation has deepened win-win cooperation, strengthened regional economic exchanges and tourism development, promoted regional prosperity among China, Laos, Myanmar and Thailand. The basin has abundant fishery resources and has the world’s third most diverse fish population, with 1,148 fish species, after the Amazon and Congo River Basins. Mekong countries have different needs for the development of fishery resources due to their different geographical locations and economic development, and thus very little cooperation in fisheries has been carried out among Mekong countries. The basin’s ecohydrological management involves environmental flow, water quality, soil erosion and sedimentation, aquatic organism and underground water protection. The current measures include enhancing monitoring, scientific assessment, rational regulation of water system, the establishment of natural reserves, and international cooperation. Climate change and construction of dams are both critical challenges faced by the basin in terms of ecohydrological management in the 21st century.

Integrated basin governance means integrated water governance taking basin as the spatial unit. It deals with rules of integrated water resources management, including the establishment of governance bodies, the definition of interests and roles of stakeholders, the principles and regulations of decision-making, and the arrangement of decision-making procedures. For trans-national basins, international cooperation for integrated basin governance is necessary that is mainly embodied by basin cooperation mechanisms. The implementation of international basin cooperation depends on a number of mechanisms. There are about fifteen cooperative mechanisms in the Mekong Region divided into two groups: intra-regional mechanisms (cooperation among Mekong countries) and mechanisms between Mekong countries and non-basin partners. MRC, GMS and LMC are the three most active mechanism. Within the Lancang-Mekong River Basin, each country has particular perspectives about international basin cooperation. China is very active in Basin cooperation and has invested a lot of resource in this regard, but is sensitive to the intervention from countries outside the region. Cambodia and Laos, with most territory located within the Basin and essential or even majority of foreign investment from China, are active to diversify their international cooperation while maintaining close cooperation with China. Most of the inflow of foreign investments into Myanmar comes from Asian countries, followed by European countries and the United States, and is influenced by its domestic political situation. Thailand has been a relatively stable recipient country of foreign investment for a long time and has benefited significantly, it has now become a donor country, playing an important leading role in basin cooperation. Vietnam’s foreign investment mainly comes from Japan, Korea, and ASEAN. Vietnam plays the leading role in environmental cooperation in Lower Mekong Cooperation with the United States, and has actively participated in the “One Decade of Green Mekong” initiative in Mekong-Japan cooperation. Some countries outside the basin, such as the United States, Japan, India, Korea, India and international organizations such as the World Bank and the Asian Development Bank, have significant influence on basin governance. Social participation in Lancang-Mekong River Basin governance plays a very important role. A variety of stakeholders, ranging from global network initiatives to local NGOs, from business enterprises to communities, have been actively engaging in the governance of the Lancang-Mekong River Basin. They have adopted different strategies (e.g., scientific research, capability building, policy advocacy, and citizen engagement) to exert influence on various issues such as climate change, biodiversity, hydropower development, and sustainable livelihood, revealing overlapping and interacting mechanisms of participation. The future trend of basin cooperation is more optimistic along with the consensus strengthening and capacity building, although there may be still some interferent brought by big power competition and interest disputations.