Lancang-Mekong River Basin Research Articles

Impacts of climate change and reservoir operation on streamflow and flood characteristics in the Lancang-Mekong River Basin

The Lancang-Mekong River Basin (LMRB) is one of the most important transboundary river basins in Asia. While climate change perturbs the streamflow and affects flood events, reservoir operation may mitigate or aggravate this impact. Therefore, quantitative assessment of the climate change impact and reservoir effect on the LMRB is a vital prerequisite for future hydropower development and environmental protection. This study aimed to estimate the variation of the streamflow and flood characteristics affected by climate change and reservoir operation within the LMRB. A reservoir module was incorporated into the Variable Infiltration Capacity (VIC) model to simulate the streamflow susceptible to the reservoirs. It was found that the reservoirs had a substantial influence on the streamflow during 2008–2016, when many reservoirs were constructed in the LMRB. The reservoirs across the Lancang River (the upper Mekong River located in China) reduced the annual average streamflow by 5% at Chiang Sean station (northern Thailand) in 2008–2016, whereas their influence became undetectable downstream of Vientiane station (northern Laos). The streamflow changes downstream of Mukdahan station at southern Laos (including the stations in Cambodia and southern Vietnam) were mainly attributed to the local reservoirs and climate change. Compared with the baseline period of 1985–2007, the upstream reservoir operation dramatically affected streamflow at the midstream stations with higher dry season streamflow (+15% to +37%), but lower wet season streamflow was less affected (−2% to −24%) in 2008–2016. Climate change increased the magnitude and frequency of the flood by up to 14% and 45%, respectively, whereas the reservoir operation reduced them by 16% and 36%, respectively. Our findings provide insights into the interaction between climate change and reservoir operation and their integrated effects on the streamflow, informing and supporting water management and hydropower development in the LMRB.

Predicted trends of soil erosion and sediment yield from future land use and climate change scenarios in the Lancang–Mekong River by using the modified RUSLE model

Soil erosion and sediments in the Lancang-Mekong River Basin as a result of climate change and changes in land use pose a threat to the existence of the riparian people, biodiversity and ecosystems. This study seeks to assess the annual soil erosion in terms of spatial distribution and the trends of sediment yield with the climate and land changes in future scenarios in 2030 and 2040 through the modified RUSLE model. Future lands were simulated by using the MLP artificial neural network and the Markov chain analysis. The future climate was examined by using the Max Planck Institute model, which showed a corrected bias and downscaled grid size under the Representative Concentration Pathways (RCPs). The simulated land use indicated that the forest areas were converted mostly to agricultural lands and urban areas. In the future, the average rainfall under all RCP scenarios is higher than that from the historical period. The R and C factors changed constantly, thereby affecting the soil erosion rate and sediment yield. The maximum erosion was estimated at approximately 21,000 and 21,725 t/km2/y under RCP8.5 in both years. Meanwhile, the results of sediment yield in 2030 and 2040 under RCP scenarios were much higher when compared to historical sediment data around 66.3% and 71.2%, respectively. Thailand's plateau, some parts of Cambodia and Laos PDR and the Mekong Delta are vulnerable to increase soil erosion and sediment yield. Measures to address these issues need to be planned to prepare and mitigate the possible effects, especially the loss of storage capacity in dams.

The transborder flux of phosphorus in the Lancang-Mekong River Basin: Magnitude, patterns and impacts from the cascade hydropower dams in China

Nutrients are critical to the development of plant and animal life in the river ecological system. In recent years, more attentions have arisen on the impact of building hydropower dams to transborder nutrient flux in the Lancang-Mekong River Basin (LMRB). In this paper, phosphorus was taken as an example, which is considered as one of the most significant factors to limit biological activities in the river. The Soil and Water Assessment Tool (SWAT) was used to access the multisource loading pattern of Total Phosphorus (TP) under no-dam scenario, and various types of dam construction scenarios analyses were performed to investigate potential changes in the transborder TP flux from Lancang River Basin (LRB, Chinese part of LMRB) to lower LMRB (other parts of LMRB). The results showed that the Total Phosphorus load in the LRB is about 27,079 tons/year, of which 85% TP is collected from July to October. The annual TP export at the outlet of the LRB is 14,112 tons/year under no-dam scenario, which accounts for about 4.2% of the total TP loads in the lower LMRB. With seven to twenty-one dams included in experiments, TP flux at the outlet of the LRB decreased 50–59%, while the delivered ratio for the lower LMRB decreased 2.1–2.5%. As a summary, building dams has a significant impact on the TP transport pattern in the Chinese part of LMRB, but very limited effect in lower LMRB according to the modeling results, especially for the Mekong delta. Moreover, the existing dams (seven dams) seem to intercept much more phosphorus than those dams that are under construction.

Exploring synergies in the water-food-energy nexus by using an integrated hydro-economic optimization model for the Lancang-Mekong River basin

The Water-Food-Energy nexus study identifies developmental challenges and trade-offs present along the transboundary river basins. Intensive reservoir development for hydropower production impacts existing actors in the Lancang-Mekong River Basin. Concerns from these stakeholders highlight three major trade-offs that occur between hydropower and irrigation (HP-AG), hydropower and fisheries (HP-F), and irrigation and fisheries (AG-F). Dam construction has an impact not only on power production but also on ecosystems and ultimately the livelihood of people. In this study, we quantify the effects of reservoir operation on hydropower generation, irrigated crop production and fisheries yield in the Tonle Sap lake through a novel hydro-economic model at the whole basin scale. Our main finding is that trade-offs can be turned into synergetic opportunities. First, the dam operation can increase water availability for irrigation without severely harming hydropower production, raising irrigated crop revenue by 49% and reducing crop losses during droughts by 30%. Second, eco-friendly management increases fisheries yield by up to 75%, but decreases both irrigated crop production (−48%) and power production (−17%). Reservoirs can, therefore, benefit the whole basin by releasing more water in months with high irrigation demand (April and December) and by minimizing the adverse effects of flow fluctuations on the livelihood of farmers and fishers living downstream. Our results also reveal the overlooked trade-off between irrigated agriculture and fisheries. Cross-sectoral and transboundary partnerships should strengthen stakeholder participation in decision-making. Local solutions such as enhanced reservoir operation can respond to the broader global issue of natural resource trade-offs and sharing. Our alternative narrative enhances the dialogue about fair and efficient water use among Mekong riparian countries.

Spatial characteristics of total phosphorus loads from different sources in the Lancang River Basin

Lancang River, the upstream reach of Mekong River, is a hotspot region in the sustainable management of water resources and environment as it is currently facing the deterioration of aquatic ecosystems. Nutrient balance (i.e., Phosphorus) in the Lancang-Mekong River Basin has become a highly disputed issue in recent years due to the construction of cascade hydropower stations. However, the estimation of the total phosphorus (TP) load faces great difficulties and challenges due to the absent measured water quality data. This study estimates the TP load based on the social economic data, analyzes the spatial distribution of TP and the contribution of different TP sources in the Lancang River basin under the level of social-economic development in 2014. Results show that the annual average TP load in the Lancang River Basin is 1.6 × 104–3.9 × 104 tons, which is at a very low level compared with other large-scale basins in China. The TP load from natural soil erosion dominates all other sources, accounting for 69%, followed by agricultural production and fertilization. In general, the TP load increases from upstream to downstream, but heterogeneity also exits in different regions under the influence of various factors, such as rainfall intensity, soil properties and human activities. The results reveal a holistic picture of TP load in the Lancang River Basin, which could provide a new perspective on the trans-border international river management.

Dynamic characteristics of sandbar evolution in the lower Lancang-Mekong River between 1993 and 2012 in the context of hydropower development

Hydropower development activities in the Lancang-Mekong River basin have become increasingly intense in recent decades, with potential impacts on river landforms and even the evolution of the estuarine delta. River sandbars are very sensitive to changes in runoff and sediment concentration; therefore, to analyze the dynamic characteristics of sandbar evolution in the lower Lancang-Mekong River (i.e., Mekong River), this study compared the dynamic parameters of the river before and after dam construction, using multiple regression equations based on data from historical remote sensing imagery, runoff, channel width, and gross domestic product (GDP) per capita. The results show that the sandbars have shrunk significantly since 2005, being consistent with the shrinkage of the Mekong delta, which suggests that the shrinkage of the two landforms may be dominated by the same factors. The direct factors causing this shrinkage phenomenon are a relative increase in erosive forces and a decrease in sediment sources simultaneously, which is specifically due to the slight increases of stream power and runoff shear stress, damming, sand mining activities downstream, urbanization and others. However, compared to climate changes, GDP-related human activities that manipulate these direct factors are likely a more significant driving force behind the changes in sandbar size. When the GDP per capita of the downstream basin was less than 1,614 USD/yr, the increase in the intensity of human activities led to an increase in sandbars. Conversely, when the GDP per capita exceeded 1,657 USD/yr beginning in 2005–2006, the sandbars shrank. This study reminds us that the impact of human activities on the evolution of downstream fluvial landforms is increasing in the lower Mekong River.

Satellite-Based Operational Real-Time Drought Monitoring in the Transboundary Lancang–Mekong River Basin

Existing gauging networks are sparse and not readily available in real-time over the transboundary Lancang–Mekong River (LMR) basin, making it difficult to accurately identify drought. In this study, we aimed to build an operational real-time Lancang–Mekong drought monitor (LMDM), through combining satellite real-time data and the Variable Infiltration Capacity (VIC) hydrological model at a 0.25° spatial resolution. Toward this, three VIC runs were conducted: (1) a 60-year (1951–2010) historical simulation driven by Princeton’s global meteorological forcing (PGF) for yielding ‘normal’ conditions (PGF-VIC), wherein the VIC was calibrated with 20-year observed streamflow at six hydrological stations; (2) a short-period (2011–2014) simulation to bridge the gap between the historical and the real-time modeling; (3) the real-time (2015–present) simulation driven by bias-corrected satellite data, wherein the real-time soil moisture (SM) estimate was expressed as percentile (relative to the ‘normal’) for drought monitoring. Results show that VIC can successfully reproduce the observed hydrographs, with the Nash–Sutcliffe efficiency exceeding 0.70 and the relative bias mostly within 15%. Assessment on the performance of LMDM shows that the real-time SM estimates bear good spatial similarity to the reference, with the correlation coefficient beyond 0.80 across >70% of the domain. In terms of drought monitoring, the LMDM can reasonably reproduce the two recorded droughts, implying extreme droughts covering the Lower LMR during 2004/05 and widespread severe 2009/10 drought across the upper domain. The percentage drought area implied by the LMDM and the reference is close, corresponding to 66% and 60%, 43% and 40%, and 44% and 36% for each typical drought month. Since January 2015, the LMDM was running in an operational mode, from which the 2016 unprecedented drought was successfully identified in Mekong Delta. This study highlights the LMDM’s capability for reliable real-time drought monitoring, which can serve as a valuable drought early warning prototype for other data-poor regions.

澜湄合作机制下的跨境水资源分配模式研究

澜湄合作机制下的跨境水资源分配模式研究

Emergency water supply decision-making of transboundary river basin considering government–public perceived satisfaction

The emergency supply of transboundary water resources is a prominent problem affecting the social and economic development of basin countries. However, current water supply decisions on transboundary water resources may ignore the psychological perception of multi-stakeholders, and the evolution of emergencies increases the uncertainty of decision making. Both factors would lead to the low acceptance of water-related decisions. Utility satisfaction, perceived losses, and quantity satisfaction were selected in this paper to identify the perceived satisfaction of upstream governments, downstream governments, and the public, respectively, over multiple decision-making stages. A modeling framework combining prospect theory and the multi-stage multi-objective programming methodology was then developed to measure the perceived satisfaction of different stakeholders in a watershed under emergency. A two-stage NSGA-II and TOPSIS based approach was adopted to find the optimal compromise solution to solve the model. The framework was applied in the Lancang–Mekong River basin to provide suggestions to decision makers. Upstream decision makers must choose a moderate proportional fairness degree when making emergency decisions to maximize the perceived satisfaction of all stakeholders. Meanwhile, the perceived loss of downstream countries with low water demand should be considered first in the formulation of emergency water supply plans. Furthermore, although water supply from upstream countries can improve perceived water quantity satisfaction of downstream publics, additional actions must still be taken to change the traditional concepts of the public.

Estimation of Soil Erosion and Sediment Yield in the Lancang–Mekong River Using the Modified Revised Universal Soil Loss Equation and GIS Techniques

The Lancang–Mekong River basin, as an important transboundary river in Southeast Asia, is challenged by rapid socio-economic development, especially the construction of hydropower dams. Furthermore, substantial factors, such as terrain, rainfall, soil properties and agricultural activity, affect and are highly susceptible to soil erosion and sediment yield. This study aimed to estimate average annual soil erosion in terms of spatial distribution and sediment deposition by using the revised universal soil loss equation (RUSLE) and GIS techniques. This study also applied remote sensing and available data sources for soil erosion analysis. Annual soil erosion in most parts of the study area range from 700 to 10,000 t/km2/y with a mean value of 5350 t/km2/y. Approximately 45% of the total area undergoes moderate erosion. Moreover, the assessments of sediment deposition and erosion using the modified RUSLE and the GIS techniques indicate high sediment erosion along the flow direction of the mainstream, from the upper Mekong River to the Mekong Delta. The northern part of the upper Mekong River and the central and southern parts of the lower Mekong River are the most vulnerable to the increase in soil erosion rates, indicating sediment deposition.

Projection of temperature change and extreme temperature events in the Lancang–Mekong River basin

The Lancang–Mekong River basin (LMRB) is under increasing threat from global warming. In this paper, the projection of future climate in the LMRB is explored by focusing on the temperature change a...

Variations in terrestrial water storage in the Lancang-Mekong river basin from GRACE solutions and land surface model

Measurement of regional water resource variations is significant for understanding water cycle and water management. This is especially true for transboundary river basins. This study investigated the terrestrial water storage dynamics of the Lancang-Mekong River basin, which has a total area of 795,000 km2 and distributed between six countries in Southeast Asia, based on the terrestrial water storage anomalies (TWSA) from the Gravity Recovery and Climate Experiment (GRACE) satellites and the Global Land Data Assimilation System (GLDAS) model. A spatial weighted centroid analysis concept from geographical information science was introduced to track the movement of terrestrial water storage centroid of the basin. And the response of terrestrial water storage dynamics to climate variations throughout the basin was examined by quantifying the correlations of TWSA with a multi-scalar drought index. Results show that during 2003 to 2016, the upstream basin (Lancang River basin) experienced slight decreasing trends (ranging from −2.45 to 0 km3/yr revealed by five different GRACE solutions), and the downstream basin (Mekong River basin) generally did not express statistically significant trends; five GRACE solutions are more consistent in the downstream basin than the upstream basin, and the mascons-based TWSA processed at Jet Propulsion Laboratory (JPL) produced quite different results with the other four solutions. Generally, there are good correlations between GRACE observations and GLDAS model. The TWSA is 1–2 month lagged correlated with precipitation, evapotranspiration, and runoff anomalies, because water from precipitation may take a long period to loss from evapotranspiration and infiltrate into the soil. In general, water storage dynamics are closely correlated with climatic dry and wet conditions at middle-long periods (6–12 months) in the basin, and the correlation coefficients in wet season are generally higher than those in dry season. However, a different pattern was found since 2010: in the 2010–2016 period, correlations between TWSA and the multi-scalar drought index in Lancang River basin reduced relative to the 2003–2009 period but became stronger in Mekong River basin after 2009. The results of this study highlight the differentiation of water storage dynamics’ response to the climate variation between upstream and downstream of Lancang-Mekong basin in the period 2010–2016 and the possibility of examining basin storage disturbed by human activities in the basin.

Precipitation and Flow Variations in the Lancang–Mekong River Basin and the Implications of Monsoon Fluctuation and Regional Topography

In the Lancang–Mekong River Basin, monsoon fluctuation affects profoundly the spatial and temporal distributions of precipitation, which is the major cause of the uncertainty in hydrological processes and water resources. This study investigated the implications of monsoon fluctuation and regional topography on precipitation variation in the Lancang–Mekong River Basin, and it examined the potential link between monsoons and river flow. The results indicated that the fluctuations of the western North Pacific summer monsoon (WNPSM) and the Indian summer monsoon (ISM) played different roles in precipitation variation. The areas in which precipitation variation was found synchronous with the WNPSM were much larger than those associated with the ISM. Significant positive correlation was found between precipitation during June–September (JJAS) and the WNPSM index (WNPMI) and the ISM index (IMI) over 29.3% and 12.8% of the basin, respectively, and that these areas were distributed mainly on the left and right bank downstream, respectively. A strong (weak) WNPSM was found to increase (decrease) moist westerlies that caused excess (deficient) orographic precipitation through the interaction with the Annamite Mountains. During strong (weak) WNPSM years, observed river flow during JJAS at the Mukdahan, Pakse, and Stung Treng stations in the downstream area were 8.0% (5.0%), 8.2% (12.6%), and 12.1% (19.5%) higher (lower) than the mean, respectively, showing that downstream river flow is modulated by the WNPSM intensity. These findings could support long-term hydrological predictions, and be beneficial for optimal flood control and water resource utilization in the basin.

Water benefits sharing under transboundary cooperation in the Lancang-Mekong River Basin

Rapid economic development and population growth in the Lancang-Mekong River Basin makes it imperative for water resources to be used fairly and efficiently among all stakeholders. This paper proposes an integrated model framework to assess the water benefits sharing schemes among the five stakeholders in the basin, namely, China, Thailand, Laos, Cambodia, and Vietnam. Three objectives are considered for this assessment—hydropower (HP), eco-index (EI), and agricultural water use (AW). The hydrologic model, optimization model, and cooperative game theory methods are jointly used to analyze the water benefits of stakeholders under different scenarios. According to the model, cooperation can bring more incremental benefits in dry years when water resources are scarce. Three cooperative game theoretical methods, namely, the Shapley value, the Gately point, and the Nash–Harsanyi solution, are employed to find the possible benefit sharing solutions. The results show that China and Laos need to operate their reservoirs for downstream countries’ beneficial uses under cooperation, and thus, compensation by reallocating the incremental benefit from cooperation is necessary no matter which method is used. It is found that all of the future damming plans would enhance the basin-level cooperation in most cases. In particular, according to the analysis, damming in the basins of the Sesan, Srepok, and Sekong rivers would have more effect than damming in the upper Lancang-Mekong River Basin after 2030. Based on some verified assumptions, the results of this paper can offer real-world preferences, but the uncertainties need to be further explored.

Effects of Hydrologic Conditions and Reservoir Operation on Transboundary Cooperation in the Lancang–Mekong River Basin

AbstractMultiple stakeholders in transboundary river basins would benefit from cooperation, but how and why stakeholders can achieve this cooperation remains difficult to determine. This paper prop...

Integrating precipitation zoning with random forest regression for the spatial downscaling of satellite‐based precipitation: A case study of the Lancang–Mekong River basin

Downscaling satellite‐based precipitation to fine scales is crucial for deepening our understanding of global hydrologic cycles and water‐related issues. In this study, a novel approach that integrates precipitation zoning with random forest regression is proposed for the spatial downscaling of satellite‐based precipitation. Precipitation zoning is delineated through iterative rotated empirical orthogonal function (REOF) analyses of ground‐ and satellite‐based precipitation observations. Random forest regression is applied to link the satellite‐based precipitation to 1‐km‐resolution predictors such as latitude (Lat), longitude (Lon), elevation, aspect, slope, and the normalized difference vegetation index (NDVI). The accuracy of the resultant downscaled precipitation is evaluated based on five statistical evaluation indices. The performance of the proposed approach is exemplified in the Lancang–Mekong River basin, taking Tropical Rainfall Measuring Mission (TRMM) 3B43 and Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks–Climate Data Record (PERSIANN–CDR) products acquired in 2001 (wet year), 2005 (normal year), and 2009 (dry year) as the databases. The results show that seven precipitation subregions can be roughly distinguished in the study basin. Zoning‐based downscaling outperforms non‐zoning‐based downscaling in terms of accuracy, resulting in statistically significant reductions in root mean square error (RMSE) and mean absolute error (MAE) of 1–17% across the entire basin. Among the selected predictors, the variables Lat and Lon are the most important for precipitation estimation, whereas the remaining variables have lesser and subregion‐dependent importance. The proposed approach is promising for generating high‐spatial‐resolution precipitation data in regions with sparse ground‐based observations and differentiated climatic regimes.

Water Cooperation Priorities in the Lancang-Mekong River Basin Based on Cooperative Events Since the Mekong River Commission Establishment

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.

Hydrological Simulation Using TRMM and CHIRPS Precipitation Estimates in the Lower Lancang-Mekong River Basin

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.

Reconstruction of annual runoff since CE 1557 using tree-ring chronologies in the upper Lancang-Mekong River basin

In this study, with the use of a multiple linear regression approach, the tree-ring chronologies of eight sampling sites in the upper Lancang-Mekong River Basin were developed to provide a 449-year (CE 1557–2005) reconstruction of the annual runoff, thus placing recent changes in annual runoff into a long-term context. These eight tree-ring chronologies have recently been archived in publicly available databases through the International Tree-Ring Data Bank. Reconstruction results showed a good correlation coefficient of 0.662 (n = 39, p-value < 0.01) between the reconstructed and the observed annual runoff. The adjusted coefficient (R2) for the degrees of freedom is 42.3%, which meets the precision requirements of reconstruction. The reconstructed runoff displays a trend toward more moist conditions: there were 37 extremely wet years and 23 extremely dry years, exceeding the mean ±1 standard deviation, during the past 449 years. Empirical mode decomposition (EMD) was used to fully analyze and understand the multi-scale variation of the reconstructed runoff. Six intrinsic mode function (IMF) components with different scales were obtained and the sum of all components can be reverted to the original variable sequence. The first and second IMF mainly reflect the change characteristics of the interannual scale of reconstructed sequence. Both are likely controlled by the Quasi-Biennial Oscillation (QBO) and the El Niño-Southern Oscillation (ENSO), respectively. The third IMF showed a 10–13 year scale fluctuation, which is very similar to the solar activity of an 11-year cycle. The fourth and fifth IMF mainly represents multidecadal and centennial oscillations, and they have shown coherent variations with predecessors’ reconstructed May-September precipitations in the same time domain. The lowest frequency component (the residue IMF) represents the trend term of the original signal.

How bacterioplankton community can go with cascade damming in the highly regulated Lancang-Mekong River Basin.

Rivers make vital contributions to the transport of water, sediment and nutrients from terrestrial to marine ecosystems. However, many large rivers worldwide are suffering from dam regulation. Increasing attention has been paid to bacterioplankton communities since they are highly responsive to river alterations and may influence biogeochemical processes. Here, a comprehensive study was conducted in the highly regulated Lancang-Mekong River Basin to address the question of how bacterioplankton communities respond to cascade damming. The results showed that dam constructions increased nutrient concentrations and threatened water quality in cascade reservoirs. Bacterioplankton cell abundance was reduced by damming, and α-diversity was inhibited in cascade reservoirs. Fortunately, however, river ecosystems were resilient after the remarkable disturbance caused by damming. Moreover, bacterioplankton community composition was significantly altered by cascade dams, including a shift in the dominant phylum from r-strategists to k-strategists. Meanwhile, according to GeoChip analysis, the functional composition of bacterioplankton was less affected than taxonomic composition. In addition, geographic and environmental features both followed a distance-decay relationship with community and functional composition, but the local environment condition was the dominant driver in the Lancang River. Therefore, the impoundments of cascade dams had significant impacts on bacterioplankton communities and more attention should be paid to the potential ecological consequences of river regulation.