Lower Mekong Basin Research Articles

Variations of Surface and Subsurface Water Storage in the Lower Mekong Basin (Vietnam and Cambodia) from Multisatellite Observations

In this study, we estimate monthly variations of surface-water storage (SWS) and subsurface water storage (SSWS, including groundwater and soil moisture) within the Lower Mekong Basin located in Vietnam and Cambodia during the 2003–2009 period. The approach is based on the combination of multisatellite observations using surface-water extent from MODIS atmospherically corrected land-surface imagery, and water-level variations from 45 virtual stations (VS) derived from ENVISAT altimetry measurements. Surface-water extent ranges from ∼6500 to ∼40,000 km 2 during low and high water stages, respectively. Across the study area, seasonal variations of water stages range from 8 m in the upstream parts to 1 m in the downstream regions. Annual variation of SWS is ∼40 km 3 for the 2003–2009 period that contributes to 40–45% of total water-storage (TWS) variations derived from Gravity Recovery And Climate Experiment (GRACE) data. By removing the variations of SWS from GRACE-derived TWS, we can isolate the monthly variations of SSWS, and estimate its mean annual variations of ∼50 km 3 (55–60% of the TWS). This study highlights the ability to combine multisatellite observations to monitor land-water storage and the variations of its different components at regional scale. The results of this study represent important information to improve the overall quality of regional hydrological models and to assess the impacts of human activities on the hydrological cycles.

A cautionary tale about the potential impacts of gated culverts on fish passage restoration efforts

Combined fishway-culvert facilities are common in many tropical river systems in South East Asia, but little is known regarding their mutual passage effectiveness. We investigated the lateral passage of Lower Mekong Basin fish in Lao PDR, through a combined cone fishway-sluice gate culvert facility between the Mekong River and an adjacent wetland. Fish abundance and species richness were assessed during the beginning of the 2014 wet season at three locations within the fishway-culvert facility: (i) the bottom of the fishway (i.e. downstream end of the facility); (ii) the top of the fishway; and (iii) the top of the culvert (i.e. upstream end of the facility). Neither total fish abundance nor species richness varied significantly among the three locations while the wetland remained at levels where the culvert was only partially inundated and the culvert gate was fully open. However, part-way through the study the culvert became completely inundated and the culvert gate had to be partially closed to protect the downstream fishway. During this period of partial gate closure, fish abundance and species richness became significantly lower at the top of the culvert than at the two fishway locations. This suggests that sluice gate culverts are most effective at facilitating the lateral movement of fish in tropical river systems when they are designed and operated appropriately for the local hydrological conditions, and their gates remain fully open.

Synthetic aperture radar and optical remote sensing image fusion for flood monitoring in the Vietnam lower Mekong basin: a prototype application for the Vietnam Open Data Cube

ABSTRACT Flood monitoring systems are crucial for flood management and consequence mitigation in flood prone regions. Different remote sensing techniques are increasingly used for this purpose. However, the different approaches suffer various limitations, including cloud and weather effects (optical data), and low spatial resolution and poor colour presentation (synthetic aperture radar data). This study fuses two data types (Landsat and Sentinel-1) to overcome these limitations and produce better quality images for a prototype flood application in the Vietnam Open Data Cube (VODC). Visual and quantitative evaluation of fused image quality revealed improvement in the images compared with the original scenes. Ground-truth data was used to develop the study flood extraction algorithm and we found a good agreement between our results and SERVIR Mekong (a joint initiative by the US agency for International Development (USAID), National Aeronautics and Space Administration (NASA), Myanmar, Thailand, Cambodia, Laos and Vietnam) maps. While the algorithm is run on a personal computer (PC), it has a clear potential to be developed for application on a big data system.

Ranking of CMIP5 GCM Skills in Simulating Observed Precipitation over the Lower Mekong Basin, Using an Improved Score-Based Method

This study assessed the performances of 34 Coupled Model Intercomparison Project Phase 5 (CMIP5) general circulation models (GCMs) in reproducing observed precipitation over the Lower Mekong Basin (LMB). Observations from gauge-based data of the Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE) precipitation data were obtained from 1975 to 2004. An improved score-based method was used to rank the performance of the GCMs in reproducing the observed precipitation over the LMB. The results revealed that most GCMs effectively reproduced precipitation patterns for the mean annual cycle, but they generally overestimated the observed precipitation. The GCMs showed good ability in reproducing the time series characteristics of precipitation for the annual period compared to those for the wet and dry seasons. Meanwhile, the GCMs obviously reproduced the spatial characteristics of precipitation for the dry season better than those for annual time and the wet season. More than 50% of the GCMs failed to reproduce the positive trend of the observed precipitation for the wet season and the dry season (approximately 52.9% and 64.7%, respectively), and approximately 44.1% of the GCMs failed to reproduce positive trend for annual time over the LMB. Furthermore, it was also revealed that there existed different robust criteria for assessing the GCMs’ performances at a seasonal scale, and using multiple criteria was superior to a single criterion in assessing the GCMs’ performances. Overall, the better-performed GCMs were obtained, which can provide useful information for future precipitation projection and policy-making over the LMB.

Developing Land Use Land Cover Maps for the Lower Mekong Basin to Aid Hydrologic Modeling and Basin Planning

This paper discusses research methodology to develop Land Use Land Cover (LULC) maps for the Lower Mekong Basin (LMB) for basin planning, using both MODIS and Landsat satellite data. The 2010 MODIS MOD09 and MYD09 8-day reflectance data was processed into monthly NDVI maps with the Time Series Product Tool software package and then used to classify regionally common forest and agricultural LULC types. Dry season circa 2010 Landsat top of atmosphere reflectance mosaics were classified to map locally common LULC types. Unsupervised ISODATA clustering was used to derive most LULC classifications. MODIS and Landsat classifications were combined with GIS methods to derive final 250-m LULC maps for Sub-basins (SBs) 1–8 of the LMB. The SB 7 LULC map with 14 classes was assessed for accuracy. This assessment compared random locations for sampled types on the SB 7 LULC map to geospatial reference data such as Landsat RGBs, MODIS NDVI phenologic profiles, high resolution satellite data, and Mekong River Commission data (e.g., crop calendars). The SB 7 LULC map showed an overall agreement to reference data of ~81%. By grouping three deciduous forest classes into one, the overall agreement improved to ~87%. The project enabled updated regional LULC maps that included more detailed agriculture LULC types. LULC maps were supplied to project partners to improve use of Soil and Water Assessment Tool for modeling hydrology and water use, plus enhance LMB water and disaster management in a region vulnerable to flooding, droughts, and anthropogenic change as part of basin planning and assessment.

Uncertainty assessment of streamflow projection under the impact of climate change in the Lower Mekong Basin: a case study of the Srepok River Basin, Vietnam

AbstractUncertainty assessment of future projection of streamflow is of the essence for an effective formulation of water resources management and planning adaptive to climate change. The aim of this study is to investigate the uncertainty in streamflow projection under the climate change impact in the Srepok River Basin. Uncertainty associated with emission scenarios (RCP2.6, RCP4.5 and RCP8.5), General Circulation Models (GCMs) (CanESM2, CNMR‐CM5 and HadGEM2‐AO), statistical downscaling methods (delta change method, quantile mapping and SDSM), and hydrological models (ANN, HEC‐HMS and SWAT) is examined. The results showed the largest uncertainty source of the streamflow projection is the GCM simulations, followed by the statistical downscaling methods, hydrological models and emission scenarios. In addition, the use of hydrological models has a considerable impact on uncertainty in the simulations of dry seasonal streamflow. Generally, the present study highlighted the importance of using multi‐GCMs in the studies on hydrological impact of climate change.

Global IWRM Ideas and Local Context: Studying Narratives in Rural Cambodia

This article investigates how the “constructivist turn” in public policy and international political economy informs the interaction of global ideas and local practice in water governance. We use the implementation of ideas associated with Integrated Water Resources Management (IWRM) in the Lower Mekong river basin. This article provides some explanation of the attitudes in the villages in Cambodia due to the Sesan 2 Dam, which would see the relocation of thousands of people, damage fisheries, and inflict high coping costs on villagers. Based on 24 in-depth interviews with villagers, commune heads and local community leaders, we find diverse narratives which transcend the “pro or anti” dam narrative. We find four narrative types—myths, stories, noise and informed opinion, which relate to each other in degrees of social meaning and ideational force. Of these, the first two are more likely to be useful in terms of mobilization and policy-making. This typology provides a framework for analysis of social change in the studied villages and other contexts of policy translation. We should state that these four types are not separate from each other but are linked along two axis which together conscribe the four types of narratives outlined.

Interacting land use and soil surface dynamics control groundwater outflow in a montane catchment of the lower Mekong basin

Groundwater contribution to streamflow sustains biodiversity and enhances ecosystem services, especially under monsoon-driven climate where stream baseflow is often the only available water resource during the dry season. We assessed how land use change influences streamflow and its groundwater contribution in a small headwater catchment subject to shifting cultivation in Montane Southeast Asia. Continuous time series of rainfall, reference evapotranspiration, groundwater level, stream discharge and electrical conductivity (EC) of surface and groundwater were monitored from 2002 to 2007. With the rainfall-runoff model GR4J, we investigated temporal changes in the hydrological behaviour of the study catchment to verify consistencies with observed land use change. An EC-based hydrograph separation method allowed estimating the groundwater contribution to 104 stormflow events. Mean soil surface crusting rates corresponding to each of the nine land uses identified in the catchment were determined using 236 standard 1-m2 micro-plots. Mean plant cover for each land use was assessed in 10 × 10-m2 plots. Bedrock topography and soil layers’ structure were assessed by electrical resistivity tomography to determine pathways of subsurface storm flows. Our results indicate that an increase in the catchment's areal percentage of fallow from 33% to 71% led to a decrease in the annual runoff coefficient from 43% to 26%. The concurrent reduction of soil crusting rate over the catchment, from 48% to 30%, increased rainwater infiltration. Consecutively, groundwater contribution to storm streamflow increased from 83% to 94%, highlighting the protective role of a dense vegetation cover against flash floods. The overall reduction of the annual basin water yield for inter-storm streamflow from 450 to 185 mm suggests that the potential gain in groundwater recharge was offset by the increased root water uptake for evapotranspiration, as confirmed by the drop in the groundwater level. This analysis illustrates how two different land uses with opposite impacts on soil permeability (i/ extensive soil surface crusting under annual crops resulting in limited runoff infiltration or ii/ fallow regrowth promoting both infiltration and evapotranspiration) both inhibit groundwater recharge. The maintenance of strips of fallow buffers between annual crop plots can slow down runoff and locally promote infiltration and groundwater recharge while limiting evapotranspiration.

Impact of reclamation on the environment of the lower mekong river basin

Study RegionIn the lower Mekong River Basin, the watershed development, such as reclamation, has been rapidly going on.The reclamation is expected to cause many problems on this important watershed environment. Study FocusThe objective of this research is to quantitatively clarify the impact of reclamation on the watershed environment of the lower Mekong River Basin. New hydrological insightsThe locations of reclamation areas were extracted using MNDWI, NDVI and NDSI derived by Landsat data. As a result, the 49 reclamation areas covering approximately 95% of all 52 reclamation areas determined by visual extraction, were extracted.Then, the multiple logistic regression model was constructed to find the tendency of the occurrence of reclamation and reproduce the occurrence of reclamation.The test of goodness of fit, such as Hosmer-Lemeshow test and Nagelkerke coefficient, shows high adaptability (R2 = 0.89) of the probability model.The probability of occurrence of reclamation could be explained by the distance from the Phnom Penh city and the distance from the river channels.Moreover, the existence of reclamation areas was incorporated into the flood-inundation model and total phosphorus transportation model changing the value of the elevation and the phosphorus loading respectively.Consequently, the inundation water level increases about 3.8 m and total phosphorus concentration increases about 0.71 mg/L around Phnom Penh in case of the reclamation of 10% of all inundation areas of Phnom Penh and Kandal province.

Response to Comments on "Designing river flows to improve food security futures in the Lower Mekong Basin".

Sabo et al presented an empirically derived algorithm defining the socioecological response of the Tonle Sap Dai fishery in the Cambodian Mekong to basin-scale variation in hydrologic flow regime. Williams suggests that the analysis leading to the algorithm is flawed because of the large distance between the gauge used to measure water levels (hydrology) and the site of harvest for the fishery. Halls and Moyle argue that Sabo et al's findings are well known and contend that the algorithm is not a comprehensive assessment of sustainability. We argue that Williams' critique stems from a misunderstanding about our analysis; further clarification of the analysis is provided. We regret not citing more of the work indicated by Halls and Moyle, yet we note that our empirical analysis provides additional new insights into Mekong flow-fishery relationships.

Comment on "Designing river flows to improve food security futures in the Lower Mekong Basin".

The designer flow regime proposed by Sabo et al (Research Articles, 8 December 2017, p. 1270) to support fisheries in the Lower Mekong Basin fails to account for important ecological, political, and economic dimensions. In doing so, they indicate that dam impacts can be easily mitigated. Such an action would serve to increase risks to food and livelihood futures in the basin.

Comment on "Designing river flows to improve food security futures in the Lower Mekong Basin".

Sabo et al (Research Articles, 8 December 2017, p. 1270) use sophisticated analyses of flow and fishery data from the Lower Mekong Basin to design a "good" hydrograph that, if implemented by planned hydropower dams, would increase the catch by a factor of 3.7. However, the hydrograph is not implementable, and, if it were, it would devastate the fishery. Further, the analyses are questionable.

Historical and Operational Monitoring of Surface Sediments in the Lower Mekong Basin Using Landsat and Google Earth Engine Cloud Computing

Reservoir construction and land use change are altering sediment transport within river systems at a global scale. Changes in sediment transport can impact river morphology, aquatic ecosystems, and ultimately the growth and retreat of delta environments. The Lower Mekong Basin is crucial to five neighboring countries for transportation, energy production, sustainable water supply, and food production. In response, countries have coordinated to develop programs for regional scale water quality monitoring that including surface sediment concentrations (SSSC); however, these programs are based on a limited number of point measurements and due to resource limitations, cannot provide comprehensive insights into sediment transport across all strategic locations within the Lower Mekong Basin. To augment in situ SSSC data from the current monitoring program, we developed an empirical model to estimate SSSC across the Lower Mekong Basin from Landsat observations. Model validation revealed that remotely sensed SSSC estimates captured the spatial and temporal dynamics in a range of aquatic environments (main stem of Mekong river, tributary systems, Mekong Floodplain, and reservoirs) while, on average, slightly underestimating SSSC by about 2 mg·L−1 across all settings. The operational SSSC model was developed and implemented using Google Earth Engine and Google App Engine was used to host an online application that allows users, without any knowledge of remote sensing, to access SSSC data across the region. Expanded access to SSSC data should be particularly helpful for resource managers and other stakeholders seeking to understand the dynamics between surface sediment concentrations and land use conversions, water policy, and energy production in a globally strategic region.

Comparing fishway designs for application in a large tropical river system

River infrastructure poses a serious threat to diverse and productive fish stocks in many tropical river-floodplain systems; particularly the Lower Mekong River, where the fisheries are vital for food security. Dams and weirs block fish migration pathways and prevent access to feeding, spawning or nursery habitat. Fishways are becoming increasingly important for mitigating the effects of barriers; however, knowledge regarding their effectiveness for the biodiverse tropical river systems is still scant. This study examined the effectiveness of differing low-cost fishway designs for rehabilitating degraded floodplain fisheries in the Lower Mekong Basin (LMB) in Laos: (1) vertical slot; (2) submerged orifice — 150 mm square opening; and (3) submerged orifice — 300 mm square opening. Day and night in situ field experiments were undertaken to compare the abundance, biomass, species richness and size range of fish able to pass through each design with relatively low drops between pools (i.e. 150 mm each) and low water velocities (i.e. 1.71 ms−1). Passage of a total of 73 species was supported by the fishway designs at a similar abundance, biomass, species richness and size range of fish, during both the day and night; although, the vertical slot design supported a different suite of fish species to that of the other two designs during the day. This suggests that each of these fishway designs could be successfully used to support the rehabilitation of fisheries in the LMB and potentially other large tropical river systems with relatively diverse migratory fish communities and variable hydrological characteristics. However, the vertical slot provides greater design and operational flexibility over the submerged orifice designs particularly in tropical systems with inherently variable hydrology. The final fishway design choice ultimately depends on the fish species and size classes being prioritised for restoration and the unique hydrological characteristics of the site.

An Open-Boundary Locally Weighted Dynamic Time Warping Method for Cropland Mapping

This paper proposes an open-boundary locally weighted dynamic time warping (OLWDTW) method using MODIS Normalized Difference Vegetation Index (NDVI) time-series data for cropland recognition. The method solves the problem of flexible planting times for crops in Southeast Asia, which has sufficient thermal and water conditions. For NDVI time series starting at the beginning of the year and terminating at the end of the year, the method can separate the non-growing season cycle and growing season cycle for crops. The non-growing season cycle may provide some useful information for crop recognition, such as soil conditions. However, the shape of the growing season’s NDVI time series for crops is the key to separating cropland from other land cover types because the shape contains all of the crop growth information. The principle of the OLWDTW method is to enhance the effects of the growing season cycle on the NDVI time series by adding a local weight to the growing season when comparing the similarity of time series based on the open-boundary dynamic time warping (DTW) method. Experiments with two satellite datasets located near the Khorat Plateau in the Lower Mekong Basin validate that OLWDTW effectively improves the precision of cropland recognition compared to a non-weighted open-boundary DTW method in terms of overall accuracy. The method’s classification accuracy on cropland exceeds the non-weighted open-boundary DTW by 5–7%. In future studies, an open-boundary self-adaption locally weighted DTW and a more effective combination rule for different crop types should be explored for the method’s best performance and highest extraction accuracy for cropland.

Tradeoff analysis between electricity generation and ecosystem services in the Lower Mekong Basin

The Mekong River is the largest freshwater fishery and the third most bio-diverse river system in the world. Two of 11 planned mainstream hydropower projects, Xayaburi and Don Sahong, are nearly completed and a third project proposal, Pak Beng, has been submitted by the Lao PDR government for consideration. This paper builds on previous studies and examines the tradeoffs (between water use, food security supply and energy production) for the proposed mainstream hydropower projects in the Lower Mekong Basin (LMB).The paper concludes that the forecast loss of capture fisheries, sediment/nutrients and social mitigation costs measured as Net Present Value (NPV at 10% discount rate) are greater than the benefits from electricity generation, improved irrigation and flood control. The paper also forecasts huge negative economic impacts for Cambodia and Vietnam in contrast to previous Mekong River Commission’s (MRC) conclusions that all countries will benefit from hydropower development.The paper recommends reassessing the economic impacts of hydropower development using full environmental cost accounting. It also recommends that a new LMB energy strategy be developed taking into account less hydropower income than previously anticipated, updated forecasts for LMB power demand and anticipated technology developments for improved energy efficiency & renewable energy (especially solar which is now competitive with hydropower).

Assessing cumulative impacts of the proposed Lower Mekong Basin hydropower cascade on the Mekong River floodplains and Delta – Overview of integrated modeling methods and results

Approach, methods, and results from a comprehensive integrated water resources modeling framework for the Lower Mekong Basin (LMB) are presented in this paper. The modeling suite was specifically tailored to the known conditions and data availability at key locations within the LMB and more particularly within the floodplains of Cambodia and the Mekong River Delta of Vietnam. Model simulation output was used to project cumulative changes to hydraulic, sediment, and water quality parameters likely to occur in this relatively unique system under multiple scenarios of hydropower development in the LMB. While substantial changes in river flow regimes are predicted as a result of hydropeaking operations especially when combined with dry-season drawdowns to maximize power production, the multiple-scenario analysis indicated that overall, the largest changes are expected to be in loss of sediment and nutrient transport and reduction of water quality due to increased salinity intrusion into areas of the Mekong Delta.

Regional Assessment of Groundwater Recharge in the Lower Mekong Basin

Groundwater recharge remains almost totally unknown across the Mekong River Basin, hindering the evaluation of groundwater potential for irrigation. A regional regression model was developed to map groundwater recharge across the Lower Mekong Basin where agricultural water demand is increasing, especially during the dry season. The model was calibrated with baseflow computed with the local-minimum flow separation method applied to streamflow recorded in 65 unregulated sub-catchments since 1951. Our results, in agreement with previous local studies, indicate that spatial variations in groundwater recharge are predominantly controlled by the climate (rainfall and evapotranspiration) while aquifer characteristics seem to play a secondary role at this regional scale. While this analysis suggests large scope for expanding agricultural groundwater use, the map derived from this study provides a simple way to assess the limits of groundwater-fed irrigation development. Further data measurements to capture local variations in hydrogeology will be required to refine the evaluation of recharge rates to support practical implementations.

Designing river flows to improve food security futures in the Lower Mekong Basin.

Rivers provide unrivaled opportunity for clean energy via hydropower, but little is known about the potential impact of dam-building on the food security these rivers provide. In tropical rivers, rainfall drives a periodic flood pulse fueling fish production and delivering nutrition to more than 150 million people worldwide. Hydropower will modulate this flood pulse, thereby threatening food security. We identified variance components of the Mekong River flood pulse that predict yield in one of the largest freshwater fisheries in the world. We used these variance components to design an algorithm for a managed hydrograph to explore future yields. This algorithm mimics attributes of discharge variance that drive fishery yield: prolonged low flows followed by a short flood pulse. Designed flows increased yield by a factor of 3.7 relative to historical hydrology. Managing desired components of discharge variance will lead to greater efficiency in the Lower Mekong Basin food system.

RCP8.5-Based Future Flood Hazard Analysis for the Lower Mekong River Basin

Climatic variations caused by the excessive emission of greenhouse gases are likely to change the patterns of precipitation, runoff processes, and water storage of river basins. Various studies have been conducted based on precipitation outputs of the global scale climatic models under different emission scenarios. However, there is a limitation in regional- and local-scale hydrological analysis on extreme floods with the combined application of high-resolution atmospheric general circulation models’ (AGCM) outputs and physically-based hydrological models (PBHM). This study has taken an effort to overcome that limitation in hydrological analysis. The present and future precipitation, river runoff, and inundation distributions for the Lower Mekong Basin (LMB) were analyzed to understand hydrological changes in the LMB under the RCP8.5 scenario. The downstream area beyond the Kratie gauging station, located in the Cambodia and Vietnam flood plains was considered as the LMB in this study. The bias-corrected precipitation outputs of the Japan Meteorological Research Institute atmospheric general circulation model (MRI-AGCM3.2S) with 20 km horizontal resolution were utilized as the precipitation inputs for basin-scale hydrological simulations. The present climate (1979–2003) was represented by the AMIP-type simulations while the future (2075–2099) climatic conditions were obtained based on the RCP8.5 greenhouse gas scenario. The entire hydrological system of the Mekong basin was modelled by the block-wise TOPMODEL (BTOP) hydrological model with 20 km resolution, while the LMB area was modelled by the rainfall-runoff-inundation (RRI) model with 2 km resolution, specifically to analyze floods under the aforementioned climatic conditions. The comparison of present and future river runoffs, inundation distributions and inundation volume changes were the outcomes of the study, which can be supportive information for the LMB flood management, water policy, and water resources development.