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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.Reconstructions of Tibetan Plateau streamflow over the last millennia reveal close associations with dry season vegetation and major population shifts in Southeast Asia.

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. Key Points 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

Dams have fragmented rivers and threatened aquatic biodiversity globally. However, the findings regarding dam impacts on riverine macroinvertebrates vary across regions and taxa. We conducted a global meta-analysis to quantify the effects of dams on macroinvertebrate assemblages (i.e. species richness and abundance) based on 3849 data points extracted from 54 publications. Responses of macroinvertebrates to dams varied among climatic zones, dam altitudes, dam sizes (i.e. height), downstream distances from the dams, and taxonomic groups. The overall effect size of dams on macroinvertebrate richness was negative, while that of dams on abundance was positive but varied among different dam types. Richness reductions were most pronounced in cold regions and high-altitude regions and were least pronounced in tropical regions and low-altitude regions, while abundance increases were more obvious in tropical regions and low-altitude regions. Macroinvertebrate richness reduction and abundance increase were coupled (i.e. when the richness slightly decreased, the increase in abundance was more significant, and vice versa) under the influence of dams across different climatic zones, altitudes, dam heights, and downstream distances from the dams. Furthermore, different taxonomic groups responded variably to dams, with stoneflies (Plecoptera), caddisflies (Trichoptera) and true bugs (Hemiptera) being the most sensitive groups (i.e. significant reduction in richness) among the taxa examined. Macroinvertebrate richness reductions were primarily attributed to changes in downstream substrate composition (i.e. from coarse to fine substrates), while abundance increases were potentially caused by replacements among taxa at downstream sites. Collectively, our results contribute to improving the prediction of the effects of dams on riverine macroinvertebrate assemblages and are valuable for guiding assessment and monitoring of river ecosystems, as well as sustainable dam development, planning and restoration.

Lower Mekong Basin (LMB) experiences a recurrent drought phenomenon. However, few studies have focused on drought monitoring in this region due to lack of ground observations. The newly released Climate Hazards Group Infrared Precipitation with Station data (CHIRPS) with a long-term record and high resolution has a great potential for drought monitoring. Based on the assessment of CHIRPS for capturing precipitation and monitoring drought, this study aims to evaluate the drought condition in LMB by using satellite-based CHIRPS from January 1981 to July 2016. The Standardized Precipitation Index (SPI) at various time scales (1–12-month) is computed to identify and describe drought events. Results suggest that CHIRPS can properly capture the drought characteristics at various time scales with the best performance at three-month time scale. Based on high-resolution long-term CHIRPS, it is found that LMB experienced four severe droughts during the last three decades with the longest one in 1991–1994 for 38 months and the driest one in 2015–2016 with drought affected area up to 75.6%. Droughts tend to occur over the north and south part of LMB with higher frequency, and Mekong Delta seems to experience more long-term and extreme drought events. Severe droughts have significant impacts on vegetation condition.

Tonle Sap Lake is the largest freshwater lake in Southeast Asia. Water development infrastructures are increasingly being constructed in the Lancang–Mekong River Basin, which is a major concern considering its potential impact on Tonle Sap Lake. This study aimed to investigate variations in the area of the lake and discuss their possible linkage to runoff alterations in the Lancang River (Upper Mekong) by comparing runoff at the Yunjinghong hydrological station before and after significant changes in runoff trends that occurred in 2008. First, four commonly used water body extraction methods (MNDWI, NDWI, NDVI, and EVI) were compared and MNDWI was found to provide a better and more stable performance. Based on MOD09A1 data, MNDWI was used to extract the water area of the lake from 2000 to 2014, and characteristics of variations in the area before and after 2008 were analyzed. The water area of Tonle Sap Lake displayed an overall decreasing trend, and specifically decreased by 8.3% during the flood season and by 1.5% on average during the dry season after 2008. Seasonal variations in the water area of Tonle Sap Lake were dominantly influenced by runoff from the Mekong River. Compared with the period 2000–2007, runoff at Yunjinghong station were increased during the dry season (20.74%) and decreased during the flood season (34.25%) between 2008 and 2014. Changes in upstream runoff contributed to runoff at the Stung Treng station in the lower Mekong River by 6.17% (dry season) and −2.41% (flood season). Evidently, the operation of dams in the Lancang River does not primarily account for the area decrease of Tonle Sap Lake during the flood season. In contrast, runoff increase during the dry season mitigates the area decrease of Tonle Sap Lake to a certain extent.

1. China is Asia's most important upstream riparian country, sharing 110 rivers and lakes with 18 downstream countries. Consequently, China's management of transboundary water resources must consider both environmental and geopolitical risks. 2. The major threats to and conflicts over international rivers in China revolve around biotic homogenisation due to the installation of transport links, water allocation, water pollution, alteration of natural flow patterns and disruption of fisheries due to the installation of hydropower dams, and droughts and floods exacerbated by climate change. Because these problems have an international component, they fall under China's Peaceful Rise strategy, mandating that transboundary conflicts be resolved amicably as part of the overarching goal of increasing regional economic growth with as little conflict as possible. 3. Science-backed policy is more likely to result in long term, mutually agreeable solutions; the results of applied ecological research have already resulted in a number of mitigation measures, including setting operational thresholds to reduce the downstream impact of dams, designating protected areas along key river stretches where dams cannot be installed (one dam in a critical location has been cancelled), and the installation of terrestrial protected-area networks. 4. Synthesis and applications. Applied ecology will continue to play an important role in the diagnosis and resolution of environmental threats to China's transboundary waters. More importantly, applied ecology can inform the development of a transboundary environmental compensation mechanism and regional consultative mechanisms that support informed, cooperative decision-making for China and its riparian neighbours.

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.

Southwest China’s Yunnan province is evolving into one of the world’s largest hydro-power-producing regions. It already rivals the world’s largest hydro-producing nations. However, five of Yunnan’s six basins are international and therefore its hydropower development is of great academic and geopolitical interest. While the implementation of large projects on Yunnan’s three large rivers (Jinsha, Mekong and Nu) is relatively well studied, hydropower development outside these three main streams is hardly known. Here, we identified 128 large hydropower projects (≥50 MW) having a capacity of 16.5 GW, along with another 16.4 GW of other types of power generation, neither of which has been discussed in the academic literature yet. The paper utilizes a powershed approach to study the rapid hydropower development underway in Yunnan, both in its implication and challenges (at basin and administrative level) as well as in its trade-offs within the broader electricity context. Yunnan’s power generation and consumption patterns are characterized by diverging interests of local/provincial usage and export utilization. Within the province, the largest (hydro-) power users are energy/electricity intensive industries, which themselves have strong impacts on land use changes. Yunnan is also evolving as a major power exporter, already in 2013 exporting about one-third of its generated electricity mainly to Guangdong’s Pearl River Delta. We see a need for a critical revision of those existing generation and consumption paradigms, which includes a rethinking of major development modes, both in terms of future hydropower generation and utilization projects as well as export obligations.

The Mekong River, a global freshwater biodiversity hotspot, has suffered from intensive barrier construction, resulting in major challenges in safeguarding its fauna. Here, we provide a comprehensive evaluation of the impacts of river barriers on the distribution of 952 fish species in the Mekong River Basin. Our analysis revealed that 93% of Mekong fish species analysed suffer from habitat fragmentation, and species with larger habitat range requirements experienced higher river fragmentation impacts. Sub-basins along the main channel in the Lower Mekong had high values of species richness but relatively high barrier impacts. Across all migration types, freshwater-resident migratory (potamodromous) fishes were affected by the greatest levels of habitat fragmentation (Fragmentation Index, 42.45 [95% confidence interval, 38.61–46.62]). Among all International Union for Conservation of Nature conservation status categories, Critically Endangered species experienced the highest habitat fragmentation index (34.48 [19.46–53.52]). Among all barrier types in the Mekong, small dams and sluice gates contributed more to habitat fragmentation than large dams. While the effects of existing individual large dams on habitat fragmentation and fish distribution in the Mekong Basin are greater than for small barriers, the cumulative impacts of small barriers are greater; hence, basin-wide connectivity planning is needed for more effective conservation. Fish species in the Mekong River experience strong habitat fragmentation due to human-built barriers, with small dams and sluice gates having the greatest impact, according to analyses of barrier and fish occurrence data from the Mekong River.

Evaluating long-term changes in precipitation resources is important for accurate hydrological evaluation and forecasting, water security and rational allocation of water resources. For this purpose in the Xinjiang Habahe area, tree-ring specimens were collected from Picea obovata , Larix sibirica , and Betula platyphylla to establish a tree-ring width chronology, which was used to analyse a correlation with the average temperature and precipitation per month for 1958–2016. Based on correlation coefficients for monthly temperature and precipitation with the chronology of tree-ring widths, radial tree growth was mainly restricted by precipitation, and tree-ring width chronology was significantly correlated with overall precipitation from the previous July to the next June ( r  = 0.641, P  < 0.01). The above results were used to establish a transformation equation, and the overall precipitation from the previous July to the following June from 1800 to 2016 in Habahe was reconstructed after adjusted degrees of freedom, and obtain an explanatory rate of the variation up to 41.1% (40.0%). In addition to the reliability of the reconstructed values, the stability of the conversion function was determined via the “leave-one-out” method, which is commonly used in research on tree rings, and by cross-checking the conversion function with the reduced error value (RE), product mean test ( t ), with a sign test (ST). During the last 217 years, there were nine dry periods: 1803–1829, 1861–1865, 1872–1885, 1892–1905, 1916–1923, 1943–1954, 1961–1966, 1973–1981, and 2005–2011; and 12 wet periods: 1830–1834, 1836–1860, 1866–1871, 1886–1891, 1906–1915, 1925–1930, 1934–1942, 1955–1960, 1967–1972, 1982–1996, 2000–2004, and 2012–2016. Comparisons of the reconstructions for neighboring regions and a spatial correlation analysis showed that the reconstructed sequence of the present precipitation data better represented the changes in precipitation in Habahe. Additionally, a power spectrum analysis revealed that precipitation over the past 217 years in Habahe Province exhibited 2–5 years of quasiperiodic variation. A power spectrum analysis and wavelet analysis indicated that El Niño-Southern Oscillation influenced the precipitation cycles. This reconstruction provides more information on high-frequency precipitation, which is an important supplement to the existing tree-ring reconstruction of precipitation in the study area. The reconstruction of regional high-resolution precipitation changes over the last several hundred years provides unique, important data for understanding regional differences in climate at the decadal-centennial scale.