Abstract
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.
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5.1 Introduction
5.1.1 Study Area and Related Five Countries
Mainland Southeast Asia is located in the southeast corner of the Eurasian continent, surrounded by sea on its western, southern and eastern sides (Fig. 5.1). The five countries considered in this study are Myanmar, Thailand, Laos, Cambodia, and Vietnam, with a geographical location of 92°0′–109°30′ E and 5°30′–28°30′ N, covering a total area of about 1,938,700 km2.
Elevation and regional location of the study area of Mainland Southeast Asia (the red dot in the left-top figure shows the capital city of each country)
The elevation of the terrain of the region is low in the south and high in the north, with its north being closely connected with Southwest China. There are many mountains, rivers, and valleys in the area, for example, the Arakan Mountains in the west, the Hengduan Mountains in the middle, and the Truong Son Mountain ranges in the east. The highest mountain peak of above 5800 m in the study area is Mount Kaikabo in northwestern Myanmar (Dobby, 1959). Most of the rivers in this region flow from north to south, roughly in the same direction as the mountains. Famous rivers in the region are the Irrawaddy, the Salween, the Chao Phraya, the Mekong and the Red River (Fig. 5.2). The northern part of the study area has high mountains and deep valleys, and the terrain is mostly mountainous and hilly. Due to the rapid water flows, this region contains rich hydropower resources. In the southern region, the river valleys are open, the terrain is flat, and the water flow becomes slow. Consequently, sediment accumulates to form larger estuarine deltas and alluvial plains (Rigg, 2004).
River systems and the annual average temperature in the study area
Most of the region is located in the tropical monsoon climate zone which is not spatially homogeneous. It is under the joint influences of the Indian Summer Monsoon (ISM) and the Western North Pacific Monsoon (WNPM), which is significantly regulated by the El Niño Southern Oscillation (Chen et al., 2019; Räsänen et al., 2017; Wang et al., 2014). Most of the region has high temperatures throughout the year and is divided into dry and rainy seasons each year. The southwest monsoon prevails in the study area from May to October every year, and provides abundant precipitation (about 80% of the annual precipitation) to this region during the rainy season (Delgado et al., 2009, 2012; Yang et al., 2019). From November to May, the prevailing northeast monsoon causes dry weather and less rain in the dry season in most of the region (Nguyen-Le et al., 2015). The annual rainfall is about 500–5,400 mm, with an average magnitude of about 2183 mm, and the average annual temperature is −4–28 ℃ in the whole study area (Lutz et al., 2014).
The region has become one of the most dynamic and fastest economic regions globally (Rigg, 2004). In 2019, the population of the five countries reached 243 million, and the annual gross regional product (GDP) reached 926.9 billion USD. Among them, Vietnam has a population of 96.46 million, which is the most populous country in the region. Laos is the least populous country in the region, with only 7.17 million; Thailand has not only the largest GDP but also the largest GDP per capita. From 2000 to 2019, the GDP of the region increased fivefold, and the population increased by 18% (Table 5.1).
The economies of the region are in steady growth (Table 5.2). The GDP growth rates of Myanmar, Laos, Cambodia, and Vietnam are all above 6%. The study area’s economy has developed rapidly, but at the same time, the problem of regional imbalances still exists. Myanmar and Cambodia’s per capita GDP is less than US$ 2,000, while Vietnam and Laos are also in the ranks of lower-middle-income countries (Kumagai, 2015; MRC, 2011).
5.1.2 Data Used
In this chapter, the water resource and its use in the five countries (Myanmar, Thailand, Laos, Cambodia, and Vietnam) in the region are investigated. It is noted that the “blue” water (Mao & Liu, 2019) is mainly considered here while we do not consider “green” water resources in this study.
There is an uneven distribution of meteorological stations in the five countries, and there have also been different observation periods in these meteorological stations (Chen et al., 2018; Villafuerte & Matsumoto, 2015; Wang et al., 2016). Due to the scale of the analysis and data availability, different datasets may have different spatial resolutions, but we mainly focused on temporal aspects in the study. To ensure the water resources studies in the region, four datasets of precipitation and runoff were collected, and they were compared with the precipitation and runoff data from the Food and Agriculture Organization (FAO) of the United Nations (Ono et al., 2013; Sun et al., 2018; Yatagai et al., 2009, 2012). All the datasets have the same periods from 1981 to 2010. Three precipitation datasets were obtained from the Global Earth Observation for Integrated Water Resource Assessment (EartH2Observe), and the other one was from the Climatologies at high resolution for the earth’s land surface areas (CHELSA).
For runoff data, three datasets from the Earth2Observe (Schellekens et al., 2017) and one dataset based on China’s transboundary Water Resources estimation were selected (Yan et al., 2019). The four runoff datasets were developed by researchers at the European Centre for Medium-Range Weather Forecasts (ECMWF), Universiteit Utrecht in the Netherlands, Universitat Kassel in Germany, and at the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences. More details can be found in Table 5.3.
To this end, the hydrology conditions, natural landforms, administrative divisions, and water systems in the region are considered together to divide the whole study area into different water resource zones in Sect. 5.2. In Sect. 5.3, the total amount of water resources (including precipitation and runoff) in the region is estimated. In Sect. 5.4, the overall utilization of water resources in the region is investigated, by considering different water use indicators. In Sect. 5.5, historical social and economic data are used, and the social-economic developments are predicted to estimate the future water consumption (including agricultural and domestic water demands) in the region, and the future balance between the supply and demand of water resources in the study area in further investigated.
5.2 Water Resource Zonation
To investigate the characteristics of water resources in various areas of MSEA, the whole region is divided into different water resource zones. The water resource zonation in the study area is carried out based on the administrative divisions, hydrometeorology conditions, natural landforms, and river systems of the region, referring to the relevant principles of China’s water resources zoning, and also combining the existing water resources development and utilization plans in the study (Xi et al., 2016; Zuo et al., 2018). To be specific, the hydrological and meteorological data were collected, based on the river-basin maps (Lehner & Grill, 2013), topographic maps, and administrative division maps of the region (GADM, https://gadm.org/download_country_v3.html/). The physical geography, human history and political environments of the study area were comprehensively considered to construct the water resource zones in the region (Chaowiwat et al., 2019; Eastham et al., 2008; Inomata & Fukami, 2008; Kravtsova et al., 2009; Lin et al., 2019; Minderhoud et al., 2019; Molle & Hoanh, 2011; Than & Maung, 2017). The division of water resource zones in this study area is different from the typical small and medium-scale divisions. It is necessary to consider the characteristics of the river basins and consider the political influence of transnational river basins.
Using the GIS-based method of watershed topography, the water resource zones of the whole region are divided into three levels, with mountainous areas and plains as zoning indicators. The specific division approaches and steps are explained as follows:
First-level water resource zones: There are differences in water resources, hydrometeorology, topography, and socio-economic conditions in different geographic areas. Existing large and medium-scale studies mainly used geographic areas as the basic water resource unit, and the basin areas of major rivers in the region are large. Therefore, the division of the first-level water resource zones mainly adopts the method of combining the geographical areas and the division of the river basin systems.
Second-level water resource zones: The river system map of the region and the first-level zoning map of water resources were overlaid in ArcGIS 10.2 software, and the second-level water resource zones were delimited in each first-level area. For the secondary districts with complex water systems or obvious differences in water resources, the independence and integrity of their water systems should be ensured, and the regionalization should mainly refer to the basin distribution, while considering the connection with the existing water resources regionalization results.
Third-level water resource zones: Based on literature for the quantitative definition of the spatial scope of mountains in China and the achievements of China Digital Mountain Map (Nan et al., 2016; Zhang et al., 2013), three indexes of altitude, slope and degree of fluctuation were selected to establish the quantitative definition index of mountains. Finally, it was divided into third-level water resource zones with mountainous areas and plains as basic units.
According to the above mentioned principles and methods, water resource zoning results in the region were obtained. According to different needs, the whole region was divided into first-level water resource zones, second-level water resource zones (Fig. 5.3), and third-level water resource zones (Fig. 5.4). There are 7 first-level water resource zones in the region, namely: Rakhine Mountain Water System, Andaman Bay Water System, Gulf of Thailand Water System, Mekong Water System, Beibu Gulf Water System, Central Vietnam Water System, and Pearl River Water System. There are 17 s-level water resource zones, namely: Rakhine Mountain River System, Irrawaddy River, Chindwin River, Sitang River, Salween River, Southern Andaman Gulf, Southwestern Gulf of Thailand, Chao Phraya River, Southeastern Gulf of Thailand River, Mekong Delta, Lower Mekong River, Middle Mekong River, Upper Mekong River, Red River, Ma River, Central Vietnam Water System, Zuojiang River. There is a total of 138 third-level water resource zones (Fig. 5.4). In alphabetical order, the 7 first-level water resource zones contain 6, 50, 25, 33, 9, 14 and 1 third-level water resource zones respectively. The specific division is shown in Table 5.4.
Schematic diagram of first- and second-level water resource zones in the study area
Schematic diagram of third-level water resource zones in the study area
5.3 Water Resources Estimation
5.3.1 Precipitation and Runoff
By comparing with the average annual precipitation data released by FAO, the appropriate precipitation data sources for each country were selected, with the same periods from 1981 to 2010. Specifically, the CHELSA data was used in Myanmar, the Universität Kassel data was used in Thailand, the Universiteit Utrecht data was used in Vietnam and Cambodia, and the ECMWF data was used in Laos (Karger et al., 2017; Schellekens et al., 2017).
The Pearson-III (P-III) probabilistic distribution was used for the hydrological frequency analysis here, as a basis of estimating the statistical characteristics (mean value, coefficient of variation (Cv), coefficient of skewness (Cs)) of the annual precipitation in each country. After that, the annual precipitation design values in each country under guarantee rates of 20, 50, 75, and 95%, respectively, were obtained. The annual precipitation design values of the five countries are shown in Table 5.5.
The annual average rainfall has a value range of 1500–2100 mm in the region. The rainfall in the summer and autumn seasons (May to October) is 1423 mm, accounting for 77% of the annual rainfall; the rainfall in winter and spring (November to May) accounts for 23% of the annual rainfall. Myanmar has the largest annual average rainfall, with an annual value of 2071 mm; Thailand has the smallest annual average rainfall, with an annual value of 1574 mm. Specifically, the rainiest months in Myanmar are from June to August, accounting for 58% of annual precipitation. The rainy months of precipitation in Thailand, Laos, Cambodia and Vietnam are concentrated from July to September, accounting for 39–47% of annual precipitation (Fig. 5.5).
Seasonal distribution of precipitation in the study area
The four runoff datasets released by FAO were used to find the most suitable runoff data for each country in the region. To be specific, the runoff data from the Universiteit Utrecht was used in Myanmar and Vietnam, the runoff data from the Universitat Kassel was used in Thailand and Laos, and the runoff data from ECMWF was used in Cambodia. The runoff depth in the study area was calculated here and it had a seasonal distribution throughout the year (Fig. 5.6). The runoff from June to October can account for 70% of the annual runoff, and the runoff from February to April is small, accounting for about 8% of the annual runoff.
Seasonal distribution of runoff depth in the study area
It can be seen from Fig. 5.7 that the distribution pattern of rainfall and runoff depth in the study area is roughly the same in areas with low elevation but is different in areas with high elevation. The rainfall and runoff depth in the southern coastal area of the study area are generally higher than that in the central and northern areas. The western coastal area of the study area has more rainfall and more abundant water resources than the eastern coastal area. The annual rainfall in most regions is more than 1500 mm. The annual rainfall is above 3000 mm in the mountains of the Thanai River in northern Myanmar, the Rakhine coast in western Myanmar, the Irrawaddy Delta, the southeast coast of Myanmar, the west of the Malay Peninsula in Thailand, and the southwest coast of Cambodia. The annual runoff depth in these areas is more than 1500 mm, with the most abundant water resource quantity in the region.
Spatial distribution of a precipitation and b runoff depth in the study area
5.3.2 Water Resources
The Pearson-III (P-III) probabilistic distribution was used again for the runoff frequency analysis, as a basis for estimating the statistical characteristics (mean value, Cv, Cs) of the annual runoff in each country. The annual runoff design values in each country under different guarantee rates are shown in Table 5.6. The average annual runoff in the region is 1941.1 billion m3, and the amount of water resources can be seen to be relatively abundant. It can be seen from Table 5.6 that the country with the largest amount of water resources is Myanmar, which has an average annual runoff of 1,015 billion m3; Thailand’s annual average runoff is 232.6 billion m3; Laos’ annual average runoff is 203.3 billion m3; Cambodia’s annual average runoff is 141.5 billion m3; Vietnam has an average annual runoff of 348.7 billion m3.
The amount of water resources in each river basin was calculated based on the water resources division of the region (Table 5.7). Among the seven first-level water resource zones, the watershed with the largest amount of water resources is the Andaman Bay water system, with an average amount of annual water resources of 951 billion m3, followed by the Mekong River system, with an average amount of annual water resources of 432 billion m3. The Pearl River system has the least amount of water resources, with an average annual amount of only 8.7 billion m3. Furthermore, our estimated water resources in this study show a strong agreement with the FAO dataset.
5.4 Water Utilization
The development and utilization of water resources in each country in the region were studied. The data on water use released by FAO, the World Bank, the Statistical Yearbook of various countries and the National Bureau of Statistics of China in recent years were collected for this study. Based on these data, the historical water use data for the five countries were estimated. The current situation of water resource utilization in the region was investigated, including the total amount and proportion of agricultural water, industrial water and domestic water.
5.4.1 Water Use Analysis
The region is densely covered with water systems, with the Irrawaddy River, Salween River, Chao Phraya River, Mekong River and Red River distributed in the territory, and water resources are therefore rich. Based on the FAO data, during the baseline year of 2005, the per capita water resources of Myanmar, Cambodia and Laos were higher than the world average of around 9000 m3 per capita. The per capita water resources of Thailand and Vietnam were about half of the world average; the water resources utilization rate in the region ranged from 2 to 26% and varies greatly (Hasson et al., 2013; Räsänen et al., 2017). In general, the utilization rate of water resources in Myanmar, Laos and Cambodia is relatively low, only between 2 and 3%. The utilization rate of water resources in Thailand and Vietnam exceeded 26 and 23% respectively. On the whole, the utilization of water resources in the region reached 9%. Due to the differences in the economic development levels, natural geographical conditions and political environment of these countries, the extent of and approaches to the utilization of rivers within these countries are different. In general, the water resources in the five countries of the region are mainly used for hydropower development, agricultural irrigation, fisheries, and shipping (Ziv et al., 2012).
5.4.2 Water Supply
As shown in Fig. 5.8, agriculture is the main water user in the region, and the proportion of industrial and domestic water is relatively small. Based on the FAO data, during the baseline year of 2005, the total amount of water resources utilized in the region is 178.00 billion m3, of which industrial water accounts for 3.58%, agricultural water utilization is 92.21%, and domestic water utilizes only 4.21%. The total utilization of water resources in Myanmar is 33.00 billion m3, of which industrial accounts for 1%; agricultural water utilizes about 89%, and the share of domestic water is about 10%. The total utilization of water resources in Thailand is 57.31 billion m3, of which industrial water and domestic water account for a similar percentage (4.8%). Agricultural water utilizes the maximum amount of water resources with almost 90.4%. The total utilization of water resources in Cambodia is 2.18 billion m3, of which industrial water accounts for only 1.5%; agricultural water utilizes 94.0%; and domestic water consumes 4.5%. The total utilization of water resources in Laos is 3.49 billion m3, of which industrial water and domestic water consume only 4.9 and 3.7%, respectively. The agricultural water utilizes maximum water resources with 91.4% of the total usage. The total utilization of water resources in Vietnam is 81.86 billion m3, of which industrial water shares 3.7%, agricultural water accounts for 94.8%, and domestic water is only 1.5%.
Water utilization in the baseline year of 2005 in the study area
5.4.3 Major Water Use Indicators
It can be seen from Table 5.8 that the per capita comprehensive water consumption of Myanmar, Thailand and Vietnam exceeds 700 m3; the per capita comprehensive water consumption of Laos is about 607 m3, which is higher than China’s per capita water consumption of 432 m3 in 2005; while the per capita water consumption of Cambodia is only 162 m3. In terms of per capita domestic water consumption, Vietnam, Cambodia and Laos consume less water, while the per capita domestic water consumption of Myanmar and Thailand exceed 100 m3. In terms of irrigation water indicators, with the exception of Cambodia, the average irrigation water in other countries is above 40 m3/acre. The industrial water consumption of Thailand and Cambodia is less than 100 m3 per thousand USD, while the other three countries use more than 100 m3 per thousand USD. From the perspective of GDP water consumption indicators of thousand USD, Thailand uses the least water consumption of 302.8 m3, followed by Cambodia’s 346.7 m3. The other three countries all consume more than 1,200 m3 of water. Among them, Myanmar has the largest water consumption of 2769.2 m3. According to the GDP of thousand USD by water use indicators, Thailand has the highest water use efficiency among the five countries during the baseline year, while Myanmar has the lowest.
5.5 Water Demand Prediction
At present, there is a lack of studies on future water demand in the region, which is also a difficult issue (Alcamo et al., 2003, 2007; Hejazi et al., 2013; Rosegrant & Cai, 2002). For example, the simulation results of domestic, agricultural and industrial water demand based on the global hydrological model LPJML, provided by the ISIMIP2B project, were far lower than the current actual water consumption levels in these countries (Pokhrel et al., 2021). Therefore, we re-estimate the future water demand for the region adopting a different approach with the aim to obtain higher accuracy. The future water demand of the region was predicted by taking the five countries as water supply and water demand calculation units. By collecting the social and economic data of the study area, the social and economic development of the five countries from 2025 to 2040 is predicted, including the prediction of population and GDP and other social and economic indicators. On this basis, the water demand situation of each industry in the future is predicted, the predicted annual available water supply was calculated, and the balance between supply and demand of water resources was analyzed.
5.5.1 Social Development Prediction
The social development prediction is mainly aimed at predicting the future population and social economy. The population prediction is based on the World Bank data from 1990 to 2019, with 2019 as the baseline year, and the Malthusian model is used to predict the population from 2020 to 2040. The prediction model was generated using 70% of the data set (from 1991 to 2015) as training and the remaining 30% (from 2016 to 2019) as test data. Three groups of 1991–2015 (25 samples), 2001–2015 (15 samples), and 2006–2015 (10 samples) were selected respectively. The population data in different periods were fitted to the Malthusian prediction model (Prentiss et al., 2018) of the countries’ total population in the region. The average relative error between the models was compared, and the optimal sequence were selected. After comparative analysis, it was determined that Myanmar, Thailand, Cambodia, and Vietnam used the average population growth rate data on a 10-year time scale, whilst Laos used the average population growth rate data on a 15-year time scale (see Table 5.9).
Comparing the population prediction results obtained by using the Malthusian method with the population prediction data released by the United Nations World Population Outlook WPP2019, the model here showed a more reasonable range of goodness-of-fit. The prediction results are shown in Table 5.10. It is predicted that the total population of the five countries in the region will reach 260.65 million in 2030, and 271.88 million in 2040.
The GDP prediction is based on data from 1990 to 2019, with 2019 as the baseline year, using the SPSS25 software to establish a time series prediction model for the five countries’ GDP and each industrial structure. The prediction results are shown in Table 5.11. It is predicted that the total GDP of the five countries in the region in 2030 will reach US$1386.06 billion, and the total GDP in 2040 will reach US$1789.29 billion. The per capita GDP is expected to rise from US$3802 in 2019 to US$6581 in 2040.
5.5.2 Agricultural Water Demand Prediction
The main water demand is agricultural irrigation. According to relevant domestic and foreign norms and standards, the irrigation water quota per area unit of cultivated land can be calculated under the guarantee rate of 50, 75, and 95%. The total amount of irrigation water is directly proportional to the amount of irrigated cultivated land. The agricultural irrigation water on a certain area of arable land can be determined by the following formula:
where \({{{W}}}_{{{n}}}\) represents the total amount of irrigation water in year t; \({{{S}}}_{{{N}}}\) is the area of arable land in year t; \({{{D}}}_{{{N}}}\) is the irrigation water consumption per unit area.
Figure 5.9 shows the prediction of the future irrigation area in the study area. In recent years, the agricultural production in the study area has gradually developed, and the planting of crops has increased. Based on the development of various crops in recent years and the agricultural development plan of the study area, combined with the irrigated area data of the inter-departmental impact model comparison project ISIMIP, the irrigated area of the five countries in the region is predicted. The irrigation water index is an important factor in the prediction of agricultural water demand. According to the future economic development of these countries, referring to the water quota and water resource bulletin of the same economic development level, the prediction level of the annual irrigation water index in the study area is predicted.
Prediction of irrigation area in the study area
According to the prediction results of irrigation area and water consumption, the annual water demand is predicted using the quota method. The total agricultural water demand in the region will reach 180.8 billion m3 in 2025, 1865 billion m3 in 2030 and 2004 billion m3 in 2040 (see Fig. 5.10 and Table 5.12).
Prediction of irrigation water quota in the study area
5.5.3 Domestic Water Demand Prediction
Residential water demand is divided into urban water demand and rural water demand. The calculation indexes include population, urbanization rate, and water consumption quota. Natural population growth and changes in water quotas are the principal indicators that affect domestic water consumption. Referring to the research reports and regulations of relevant regions at home and abroad, the future domestic water quota was determined. Considering the future economic development status and water-saving technology level of various countries, it was assumed that there will be no major changes in the daily water consumption per capita in cities and rural areas in each country, and the urbanization rate of the region will keep increasing in the future. Therefore, the prediction results of the water quota index indicate that for the countries of the region from 2025 to 2040, the residential water quota will keep increasing in the future, which is similar to the trend of the continuous development of the total population of various countries, and the total increase in residential water consumption indicators will be relatively small (Table 5.13).
Based on the population prediction data and the results of water quota indicators, the quota method can predict the domestic water demand in the future. The total domestic water demand in the region is expected to be 8.66 billion m3 in 2025 and 9.07 billion m3 in 2030. The specific prediction results are shown in Table 5.14.
5.5.4 Water Resources Balance and Open Issues
The total water demand of the region in 2025, 2030, and 2040 is obtained by predicting water demand for life, agriculture and industry (Fig. 5.11). Among them, the total water demand in 2025 is estimated to be 20.27 billion m3, 208.03 billion m3 in 2030, and 225.83 billion m3 in 2040. In general, the water demand in the region indicates an upward trend. Among them, Myanmar and Vietnam will have larger water demand growth, while Cambodia, Laos, and Thailand will have relatively small water demand growth in the future.
Prediction results of total water demand in the study area
In summary, a water demand prediction model was established based on the combination of socio-economic development trends and historical water consumption data of various industries, and the social water consumption in future scenarios was estimated. The current water supply and water conservancy project planning for the water supply prediction was further investigated. Finally, the relationship between water demand and water supply was analyzed. The results of the balance of supply and demand are shown in Table 5.15. The amount of water supply shows that Myanmar will have the maximum water surplus in 2025, 2030, and 2040 with 19.42, 22.82, and 14.8 billion m3, respectively. In contrast, Cambodia will have the minimum water surplus in 2025, 2030, and 2040 with 0.94, 1.23, and 0.63 billion m3, respectively. Vietnam, Laos, and Thailand will also have a reasonable amount of water surplus (within a range) to 2040.
Considering that climate will likely change in the region significantly (Supari et al., 2020; Tangang et al., 2020; Weiss, 2009), the effective surplus of water supply will play a significant role in the future water supply in the region. It is projected that the annual mean temperature in the region will increase between 1.5 and 3.3 °C (SSP2-4.5; medium emissions scenario) by 2100 (Almazroui et al., 2020; Gutiérrez et al., 2021). Accordingly, the annual mean precipitation is also expected to increase under all scenarios (Almazroui et al., 2020). The drier season is projected to have more impact on Vietnam and Thailand (Weiss, 2009). This will eventually increase the water stress and influence water supply in some parts of the region, especially in Thailand and Vietnam. In addition, the projection of the models indicates an increase in the number of extreme precipitations in the future, while the total amount of precipitation will be reduced (Lorenzo & Kinzig, 2020; Supharatid et al., 2021). Hence, this region may face more drought events and flood disasters in the future. In this context, Cambodia, Laos and Thailand will be more at risk due to lower water surplus in future. Taking full advantage of the existing water supply facilities and considering future water conservancy projects, the water resources in the region can be effectively used, and the total water supply in the five countries is more significant than their water demands.
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Zhang, S., Sang, YF., Qiu, T., Zhu, Y., Sajadi, P., Chan, F.K.S. (2024). Water Resource Availability and Use in Mainland Southeast Asia. In: Chen, D., Liu, J., Tang, Q. (eds) Water Resources in the Lancang-Mekong River Basin: Impact of Climate Change and Human Interventions. Springer, Singapore. https://doi.org/10.1007/978-981-97-0759-1_5
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