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The imbalance in global streamflow gauge distribution and regional data scarcity, especially in large transboundary basins, challenge regional water resource management. Effectively utilizing these limited data to construct reliable models is of crucial practical importance. This study employs a transfer learning (TL) framework to simulate daily streamflow in the Dulong-Irrawaddy River Basin (DIRB), a less-studied transboundary basin shared by Myanmar, China, and India. Our results show that TL significantly improves streamflow predictions: the optimal TL model achieves an average Nash-Sutcliffe efficiency of 0.872, showing a marked improvement in the Hkamti sub-basin. Despite data scarcity, TL achieves a mean NSE of 0.817, surpassing the 0.655 of the process-based model MIKE SHE. Additionally, our study reveals the importance of source model selection in TL, as different parts of the flow are affected by the diversity and similarity of data in the source model. Deep learning models, particularly TL, exhibit complex sensitivities to meteorological inputs, more accurately capturing non-linear relationships among multiple variables than the process-based model. Integrated gradients (IG) analysis further illustrates TL’s ability to capture spatial heterogeneity in upstream and downstream sub-basins and its adeptness in characterizing different flow regimes. This study underscores the potential of TL in enhancing the understanding of hydrological processes in large-scale catchments and highlights its value for water resource management in transboundary basins under data scarcity.

Within the context of the Belt and Road Initiative (BRI) and the China-Myanmar Economic Corridor (CMEC), the Dulong-Irrawaddy (Ayeyarwady) River, an international river among China, India and Myanmar, plays a significant role as both a valuable hydro-power resource and an essential ecological passageway. However, the water resources and security exhibit a high degree of vulnerability to climate change impacts. This research evaluates climate impacts on the hydrology of the Dulong-Irrawaddy River Basin (DIRB) by using a physical-based hydrologic model. We crafted future climate scenarios using the three latest global climate models (GCMs) from Coupled Model Intercomparison Project 6 (CMIP6) under two shared socioeconomic pathways (SSP2-4.5 and SSP5-8.5) for the near (2025–2049), mid (2050–2074), and far future (2075–2099). The regional model using MIKE SHE based on historical hydrologic processes was developed to further project future streamflow, demonstrating reliable performance in streamflow simulations with a validation Nash-Sutcliffe Efficiency (NSE) of 0.72. Results showed that climate change projections showed increases in the annual precipitation and potential evapotranspiration (PET), with precipitation increasing by 11.3% and 26.1%, and PET increasing by 3.2% and 4.9%, respectively, by the end of the century under SSP2-4.5 and SSP5-8.5. These changes are projected to result in increased annual streamflow at all stations, notably at the basin’s outlet (Pyay station) compared to the baseline period (with an increase of 16.1% and 37.0% at the end of the 21st century under SSP2-4.5 and SSP5-8.5, respectively). Seasonal analysis for Pyay station forecasts an increase in dry-season streamflow by 31.3%–48.9% and 22.5%–76.3% under SSP2-4.5 and SSP5-8.5, respectively, and an increase in wet-season streamflow by 5.8%–12.6% and 2.8%–33.3%, respectively. Moreover, the magnitude and frequency of flood events are predicted to escalate, potentially impacting hydropower production and food security significantly. This research outlines the hydrological response to future climate change during the 21st century and offers a scientific basis for the water resource management strategies by decision-makers.

The transboundary influence of environmental change is a critical issue in the Lancang-Mekong region. As the largest river-connected lake in the lower Mekong, the ecological change and influence of Tonle Sap Lake have received widespread attention and discussion, especially after 2008, when the hydrological regime of the Lancang-Mekong River mainstream underwent distinct changes. However, the linkage and coupling mechanism between the lake riparian environment and mainstream water level change are still unclear. In this study, the interannual spatiotemporal changes in land cover in the Tonle Sap Lake riparian zone (TSLRZ) and their relationship with mainstream water levels were analysed. The results showed that the expansion of farmland was the most notable change in 1988–2020. After 2008, the land cover changes intensified, manifested as accelerated farmland expansion, intensified woodland fragmentation and significant water body shrinkage. Furthermore, the responses of the water body, degraded land, wasteland and grassland areas to the mainstream water levels weakened after 2008. Evidently, the land cover changes in the TSLRZ in the last 30 years were less related to the mainstream water level change than to local reclamation and logging. These results can offer a new scientific basis for the transboundary influence analysis of hydrological change.

The great river systems originating from the Tibetan Plateau are pivotal for the wellbeing of more than half the global population. Our understanding of historical ranges and future changes in water availability for much of Southeast Asia is, however, limited by short observational records and complex environmental factors. Here we present annually resolved and absolutely dated tree ring-based streamflow reconstructions for the Mekong, Salween and Yarlung Tsangpo rivers since 1000 ce, which are supplemented by corresponding model projections until 2100 ce. We show a significant positive correlation between streamflow and dry season vegetation indices over the Indochinese Peninsula, revealing the importance of the Tibetan Water Tower for the functioning and productivity of ecological and societal systems in Southeast Asia. The streamflow variability is associated with low-frequency sea-surface temperature variability in the North Atlantic and North Pacific. We find that streamflow extremes coincide with distinct shifts in local populations that occurred during medieval times, including the occupation and subsequent collapse of Angkor Wat from the eleventh to the sixteenth century. Finally, our projections suggest that future streamflow changes will reach, or even exceed, historical ranges by the end of this century, posing unprecedented risks for Southeast Asia.

Satellite-based precipitation products have been widely used to estimate precipitation, especially over regions with sparse rain gauge networks. However, the low spatial resolution of these products has limited their application in localized regions and watersheds. This study investigated a spatial downscaling approach, Geographically Weighted Regression Kriging (GWRK), to downscale the Tropical Rainfall Measuring Mission (TRMM) 3B43 Version 7 over the Lancang River Basin (LRB) for 2001–2015. Downscaling was performed based on the relationships between the TRMM precipitation and the Normalized Difference Vegetation Index (NDVI), the Land Surface Temperature (LST), and the Digital Elevation Model (DEM). Geographical ratio analysis (GRA) was used to calibrate the annual downscaled precipitation data, and the monthly fractions derived from the original TRMM data were used to disaggregate annual downscaled and calibrated precipitation to monthly precipitation at 1 km resolution. The final downscaled precipitation datasets were validated against station-based observed precipitation in 2001–2015. Results showed that: 1) The TRMM 3B43 precipitation was highly accurate with slight overestimation at the basin scale (i.e., CC(correlation coefficient) = 0.91, Bias= 13.3%). Spatially, the accuracies of the upstream and downstream regions were higher than that of the midstream region. 2) The annual downscaled TRMM precipitation data at 1 km spatial resolution obtained by GWRK effectively captured the high spatial variability of precipitation over the LRB. 3) The annual downscaled TRMM precipitation with GRA calibration gave better accuracy compared with the original TRMM dataset. 4) The final downscaled and calibrated precipitation had significantly improved spatial resolution, and agreed well with data from the validated rain gauge stations, i.e., CC= 0.75, RMSE(root mean square error) = 182 mm, MAE(mean absolute error) = 142 mm, and Bias= 0.78% for annual precipitation and CC= 0.95, RMSE= 25 mm, MAE= 16 mm, and Bias= 0.67% for monthly precipitation.

The Lancang-Mekong River has attracted much attention from researchers, but the cooperation on water issues in this river basin has been limited, even after the establishment of the Mekong River Commission (MRC). Cooperation on water resources has been determined as one of the key priority areas in the Lancang-Mekong Cooperation Mechanism, but there are no details of targets. In order to establish the priorities of water cooperation under the mechanism, we adopted nine categories to classify the objectives of 87 water cooperation events based on the ‘Lancang-Mekong Water Cooperative Events Database’ from 1995 to 2015. Based on the occurrence of cooperative events, cooperative objectives, cooperative scales, and approaches to cooperation, we conducted statistical, correlation, and text analyses. Our analyses indicated the following results: under the impact of economic conditions inside and outside the river basin, full cooperation appeared more difficult than bilateral and multilateral cooperation. Each of the partners adopted different preferences for cooperation targets. Cooperation with more definite objectives was easier to establish than cooperation with broader and more complex objectives. The potential objectives for water cooperation were navigation, hydropower, joint management, data sharing, flood control and water use. Because hydropower development is controversial, and because water cooperation is avoided by most existing regional cooperation mechanisms due to its complexity, we suggest the following priority areas for water cooperation in the Lancang- Mekong River Basin. 1) Navigation and flood control/drought relief are attractive objectives for all the riparian countries across the whole watershed. 2) Data sharing should be a priority for cooperation in the watershed due to its laying the foundation for the equitable and reasonable utilization of transboundary waters. 3) Hydropower is an objective best implemented mainly through bilateral cooperation, and on tributaries.

Satellite-based products with high spatial and temporal resolution provide useful precipitation information for data-sparse or ungauged large-scale watersheds. In the Lower Lancang-Mekong River Basin, rainfall stations are sparse and unevenly distributed, and the transboundary characteristic makes the collection of precipitation data more difficult, which has restricted hydrological processes simulation. In this study, daily precipitation data from four datasets (gauge observations, inverse distance weighted (IDW) data, Tropical Rainfall Measuring Mission (TRMM) estimates, and Climate Hazards Group InfraRed Precipitation with Stations (CHIRPS) estimates), were applied to drive the Soil and Water Assessment Tool (SWAT) model, and then their capability for hydrological simulation in the Lower Lancang-Mekong River Basin were examined. TRMM and CHIRPS data showed good performances on precipitation estimation in the Lower Lancang-Mekong River Basin, with the better performance for TRMM product. The Nash-Sutcliffe efficiency (NSE) values of gauge, IDW, TRMM, and CHIRPS simulations during the calibration period were 0.87, 0.86, 0.95, and 0.93 for monthly flow, respectively, and those for daily flow were 0.75, 0.77, 0.86, and 0.84, respectively. TRMM and CHIRPS data were superior to rain gauge and IDW data for driving the hydrological model, and TRMM data produced the best simulation performance. Satellite-based precipitation estimates could be suitable data sources when simulating hydrological processes for large data-poor or ungauged watersheds, especially in international river basins for which precipitation observations are difficult to collect. CHIRPS data provide long precipitation time series from 1981 to near present and thus could be used as an alternative precipitation input for hydrological simulation, especially for the period without TRMM data. For satellite-based precipitation products, the differences in the occurrence frequencies and amounts of precipitation with different intensities would affect simulation results of water balance components, which should be comprehensively considered in water resources estimation and planning.

The southwest alpine gorge region is the major state base of hydropower energy development in China and hence planned many cascading hydropower stations. After the reservoir impoundment, the intense water level fluctuations under the interaction of cascade dams operating and the mountainous flooding, usually cause bank collapse, landslide and debris flow hazards. The Xiaowan reservoir (XWR), for example, as the ‘dragon head’ meg reservoir located in the middle mainstream of Lancang River, have resulted in a series of geohazards during its building and operating. In this study, we investigated the number and surface area of collapses and landslides (CLs) occurred in the water level fluctuations zone (WLFZ) of XWR using remote sensing images of Gaofen-1 and Google Earth; evaluated the CLs susceptibility using information value method. The results presented that the total WLFZ area of 87.03 km2and 804 CLs masses with a total area of 1.98 km2were identified in the riparian zone of XWR. CLs mainly occurred at an elevation of 1190–1240 m, and the CLs density increased with an increase in altitude. The WLFZ with a slope gradient of 25°–45° is the main CLs distribution area that accounts for more than half of the total CLs area. The susceptibility assessment revealed that high and very high susceptibility zones are generally distributed along zones with an elevation of 1210–1240 m, a slope degree of 25°–45° and a slope aspect perpendicular to the direction of Lancang River. Furthermore, these susceptible zones are close in distance to the dam site and tend to be in the riparian zones with the formation lithology of Silurian strata. These results provide a valuable contribution to prevent and control geohazards in the XWR area. Moreover, this study offers a constructive sample of geohazards assessment in the riparian zone of large reservoirs throughout the mountains of southwest China.

Dam removal is becoming an effective approach for aquatic biodiversity restoration in damming river in order to balance the aquatic ecosystem conservation with large-scale cascade damming. However, the effects of dam removal on fish communities in Asian mountainous rivers, which are dominated by Cypriniformes fishes, are still not well known. To determine whether dam removal on a mountainous river benefit restoration of fish diversity, we investigated the response of fish assemblage to dam removal using a before- after-control-impact design in two tributaries of the Lancang River (dam removal river: the Jidu River, and control river: the Fengdian River). Fish surveys were conducted one year prior to dam removal (2012) and three years (2013–2015) following dam removal. We observed rapidly and notably spatio-temporal changes in fish biodiversity metrics and assemblage structure, occurring in the Jidu River within the first year after dam removal. Overall, fish species richness, density and Shannon-Wiener diversity all increased immediately in above- and below-dam sites, and maintained a stable level in subsequent years, compared to unchanged situation in the control river. All sites in the Jidu River experienced shifts in fish composition after dam removal, with the greatest temporal changes occurred in sites below- and above- the former dam, resulting in a temporal homogenization tendency in the dam removed river. These findings suggest that dam removal can benefit the recovery of habitat conditions and fish community in Asian mountainous rivers, but the results should be further evaluated when apply to other dammed rivers since the dam age, fluvial geomorphology and situation of source populations could all affect the responses of fish assemblages.

Construction of river infrastructure, such as dams and weirs, is a global issue for ecosystem protection due to the fragmentation of river habitat and hydrological alteration it causes. Accurate river barrier databases, increasingly used to determine river fragmentation for ecologically sensitive management, are challenging to generate. This is especially so in large, poorly mapped basins where only large dams tend to be recorded. The Mekong is one of the world's most biodiverse river basins but, like many large rivers, impacts on habitat fragmentation from river infrastructure are poorly documented. To demonstrate a solution to this, and enable more sensitive basin management, we generated a whole‐basin barrier database for the Mekong, by training Convolutional Neural Network (CNN)–based object detection models, the best of which was used to identify 10,561 previously unrecorded barriers. Combining manual revision and merged with the existing barrier database, our new barrier database for the Mekong Basin contains 13,054 barriers. Existing databases for the Lower Mekong documented under ∼3% of the barriers recorded by CNN combined with manual checking. The Nam Chi/Nam Mun region, eastern Thailand, is the most fragmented area within the basin, with a median [95% CI] barrier density of 15.53 [0.00–49.30] per 100 km, and Catchment Area‐based Fragmentation Index value, calculated in an upstream direction, of 1,178.67 [0.00–6,418.46], due to the construction of dams and sluice gates. The CNN‐based object detection framework is effective and potentially can transform our ability to identify river barriers across many large river basins and facilitate ecologically‐sensitive management. Convolutional Neural Networks applied to satellite images to measure river fragmentationMore than ten thousand unrecorded barriers identified in Mekong Basin, SE Asia's largest river71.7% of sub‐basins in the Mekong Basin are impacted by river barriers, the Chi‐Mun Basin is the most fragmented region Convolutional Neural Networks applied to satellite images to measure river fragmentation More than ten thousand unrecorded barriers identified in Mekong Basin, SE Asia's largest river 71.7% of sub‐basins in the Mekong Basin are impacted by river barriers, the Chi‐Mun Basin is the most fragmented region

The precipitation regime of the low latitude highlands of Yunnan in Southwest China is subject to the interactions between the East Asian Summer Monsoon and the Indian Summer Monsoon, and the influence of surface orography. An understanding of changes in its spatial and temporal patterns is urgently needed for climate change projection, hydrological impact modelling, and regional and downstream water resources management. Using daily precipitation records of the low latitude highlands over the last several decades (1950s–2007), a time series of precipitation indices, including annual precipitation, number of rainy days, mean annual precipitation intensity, the dates of the onset of the rainy season, degree and period of precipitation seasonal concentration, the highest 1-day, 3-day and 7-day precipitation, and precipitation amount and number of rainy days for precipitation above different intensities (such as ⩾10 mm, ⩾25 mm and ⩾50 mm of daily precipitation), was constructed. The Trend-Free Pre-Whitening Mann-Kendall trend test was then used to detect trends of the time series data. The results show that there is no significant trend in annual precipitation and strong seasonal differentiation of precipitation trends across the low latitude highlands. Springs and winters are getting wetter and summers are getting drier. Autumns are getting drier in the east and wetter in the west. As a consequence, the seasonality of precipitation is weakening slightly. The beginning of the rainy season and the period of the highest precipitation tend to be earlier. In the meantime, the low latitude highlands has also witnessed less rainy days, more intense precipitation, slightly longer moderate and heavy precipitation events, and more frequent extreme precipitation events. Additionally, regional differentiation of precipitation trends is remarkable. These variations may be associated with weakening of the East Asian summer monsoon and strengthening of the South Asian summer monsoon, as well as the “corridor-barrier” effects of special mountainous terrain. However, the physical mechanisms involved still need to be uncovered in the future.

This paper seeks to quantify the social and economic impact of resettlement based on the physiographic element changes post relocation. We focus on communities affected by the Nuozhadu hydropower project, the largest existing hydropower project on the mainstream of the Upper Mekong River. Soil and meteorological data were collected from the Soil Spatial Database and the China Terrestrial Ecological Information Spatial Meteorology Database, while social and economic data were collected via field surveys. We have three major conclusions: (1) Communities will be relocated to a new climate and new elevation, moving from a north tropical climate zone under 700 m to a subtropical climate zone above 700 m. (2) Physiographic element changes due to relocation will reduce household economic income. After relocation, the annual family income of the Shidaimao group decreased by 62%; the annual family income of the other 5 study groups (Lasa, Hani, Nochangchangyi, Mengsa, and Dawazi) dropped by 65%. (3) Communities relocated across the study township are 61.1% less connected with their former relatives after relocation while family-to-family free labor exchange, a previous community norm, decreased by 91%. China’s dam resettlement compensation system focuses on the loss of economic resources after relocation. However, this study finds that the physiographic elements of the relocation sites are an important driver of ensuring economic growth and stability after relocation. As a result, we recommend more attention be paid to physiographic continuity when designing relocation models.

Transboundary water, more competitive utilization and uncertain availability under the globalization trend, the issue of its apportionment which directly impacts national benefits of each riparian state is becoming one of the important topics in the world. Water is scarce in China, the most important upstream state in Asia, and this task has to be thought over in the coming future. Based on “International Freshwater Treaties Database” (1820–2007) by Oregon State University, and publications and reports on transboundary water utilization and management since 1999, 28 indicators of water apportionment adopted in 49 international treaties and cases in 1864–2002 are divided into 6 types, the spatial and temporal characteristics of the adopted indicators are analyzed in order to find the key indicator(s) of transboundary water apportionment. The major results include: the major adopted indicators, have significant differences among 5 regions/continents, the indicators at rank first and second place in the developed region (North America and Europe) according to the adopted times are “keeping minimum water flow” and “mean annual runoff”, but in the developing region (Asia, Africa and South America), the ranking order of the above two indicators is reversed; the major adopted indicators in the watersheds with insufficient water are “mean annual runoff” and “keeping minimum water flow”, the ones in the watersheds with sufficient water are “keeping minimum water flow” and “maximum water intake”; the international treaties signed from the first phase to the fourth phase, the developing process shows a progress of “fewer-increasing a lot-decreasing rapidly-equation basically”, the regional distribution of the treaties shifts mainly from the developed region to the developing one, especially to Asia and Africa; the major adopted indicators shifts from “keeping minimum water flow” and “mean annual runoff” in 1864–1945, to “keeping minimum water flow” and “maximum water intake” in 1946–1971, then to “hydraulic facility operation” and “mean annual runoff” in 1972–1991, and finally to “keeping minimum water flow” and “mean annual runoff” in 1992–2002, the process shows similar a loop. Finally, the key indicator on transboundary water apportionment can be determined as “keeping minimum water flow”.

The environmental impact assessment (EIA) of cascade dams building in international rivers has been widely discussed in China and ASEAN countries since the 1990s. In Southwest China, all the great mountainous rivers have been the major state base for large-scale hydropower development. Among these rivers, the environmental change and response of the watercourse under the cascade dams building in the upper Mekong (called Lancang River in China) has been the focus in recent 20 years. In this paper, the Lancang River, which has over 25 years of large-scale hydropower development, was chosen as a case study for establishing the affected evaluation indicators system and its regression model of runoff and sediment, determining the indicators weight by the hierarchy analysis method and factor analysis method, and setting up the quantitative evaluation models of indicators affected level based on the “marginal efficiency” principle. Using these methods and model established, the influence degree of runoff and sediment in the Lancang River from 1986 to 2007 were assessed. The major results are: (1) the impact of sediment transport change by the cascade development is much higher than that of the runoff change; (2) the years’ number with different impact levels respectively are 72.7% as the “smallest” level, 18.2% as “smaller”, and 9.1% as “general”; (3) the change process of runoff and sediment shows a “U-shaped” pattern, which indicates the balance of sediment change because of the interaction of sediment reduction by Manwan reservoir storage and the increase by the Dachaoshan dam construction.

Abstract: A fundamental element of sustainable development is that humans live within nature’s biological capacity. Quantifying this, however, remains a significant challenge for which there are many emerging tools. The concept of the Ecological Footprint is one such accounting tool for comprehensive assessment of the status of sustainable development, based on integration of resource consumption and land capacity, reflecting the human impact on the environment. A region’s development is defined as unsustainable when the Ecological Footprint surpasses the biological capacity. In this paper, the Ecological Footprint concept was applied in assessing the development of Yunnan Province, China in a period between 1988 and 2006. The results showed that the Ecological Footprint per capita in Yunnan rose from 0.854 gha in 1988 to 2.11 gha in 2006. Ecological deficit, defined as when the human demand on the land surpasses the regions biological productive capacity, emerged in 1991 and quickly increased from 0.02 gha in 1991 to 1.05 gha in 2006. The increase in the ecological deficit is primarily a result of the rapid increase in population and consumption level. To achieve sustainable development in Yunnan, production and consumption rates need to be modified.

Abstract: The Mekong supports one of the richest inland fisheries in the world, with many of the fish migrating long distance to spawn. Little is known about the fisheries and migration strategies of the Upper Mekong whilst it is supposed that many fish species move between the Lower and Upper Mekong. Most likely, natural fish migration in the river has been altered by dam construction across the mainstream of the Upper Mekong. In this paper, the interconnectivity of fish species between different sections of the Mekong and negative impacts of dams on migratory fish are studied. Of the 162 fish species in the Upper Mekong and the 869 species in the Lower Mekong, 61 species are common. Results show that there is no significant difference at order level between the UM and LM. Similarity coefficients are used to evaluate interconnectivity at species, genus and family levels among four different sections of the Upper Mekong with each other and with the Lower Mekong as a whole. The highest similarity is found between the middle and lower reach of the Upper Mekong at species and genus levels and the middle and upper reach at family level. Of the eight cascade dams, Mengsong Dam in planning is considered as the biggest threat to migratory fish from the Lower Mekong and should be particularly concerned.

Abstract: The longitudinal range-gorge region (LRGR) in Southwestern China, characterized by longitudinal mountain ranges and deep valleys, includes the basins of four major international rivers: the Yuanjiang-Red, Lancang Mekong, Nujiang Salween and Irrawaddy. This region is classified as one of the world’s biodiversity hotspots, and provides an important ecological and economic corridor linking China and Southeast Asian countries. Over the past half century, it has served as a resource base for timber and minerals needed to fuel economic development, which resulted in rapid and drastic changes in ecosystem and species diversity. Proposed and ongoing development programs, such as China’s Great Western Development campaign, Greater Mekong Subregional Economic Cooperation (GMS) and China-ASEAN free trade zone development (“10+1”), threaten to bring unprecedented disturbance to the region’s ecosystems. Present and emerging threats to eco-security have caught tremendous attention worldwide. Therefore, studies on such problems are critical for enhancing ecosystem health and transboundary eco-security. This paper indicates several multi-disciplinary and cross-sector studies on transboundary resources in this region that will meet three major national needs: 1) identifying core scientific issues of ecological development and infrastructure construction in high plateau and mountainous areas for the Western Development campaign; 2) developing maintenance mechanisms and control methodologies for transboundary eco-security and resource base development; 3) providing scientific grounds for multi-lateral diplomacy, trade and economic cooperation, and conflict resolution as part of China’s opening-up to south Asia. The key subjects to be solved include dynamics and major drive forces of this area, ecological effects caused by major projects construction, and transboundary eco-security and its controlling. The research projects proposed in this article will develop theories on ecosystem change and transboundary eco-security, and provide a scientific basis for national and international development strategies.

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