Predicting water flow in fully and partially saturated porous media: a new fractal-based permeability model

Hydrogeology Journal - Predicting the permeability of porous media in saturated and partially saturated conditions is of crucial importance in many geo-engineering areas, from water resources to...     

Improved flood susceptibility mapping using a best first decision tree integrated with ensemble learning techniques

Improving the accuracy of flood prediction and mapping is crucial for reducing damage resulting from flood events. In this study, we proposed and validated three ensemble models based on the Best First Decision Tree (BFT) and the Bagging (Bagging-BFT), Decorate (Bagging-BFT), and Random Subspace (RSS-BFT) ensemble learning techniques for an improved prediction of flood susceptibility in a spatially-explicit manner. A total number of 126 historical flood events from the Nghe An Province (Vietnam) were connected to a set of 10 flood influencing factors (slope, elevation, aspect, curvature, river density, distance from rivers, flow direction, geology, soil, and land use) for generating the training and validation datasets. The models were validated via several performance metrics that demonstrated the capability of all three ensemble models in elucidating the underlying pattern of flood occurrences within the research area and predicting the probability of future flood events. Based on the Area Under the receiver operating characteristic Curve (AUC), the ensemble Decorate-BFT model that achieved an AUC value of 0.989 was identified as the superior model over the RSS-BFT (AUC = 0.982) and Bagging-BFT (AUC = 0.967) models. A comparison between the performance of the models and the models previously reported in the literature confirmed that our ensemble models provided a reliable estimate of flood susceptibilities and their resulting susceptibility maps are trustful for flood early warning systems as well as development of mitigation plans.     

Investigating the impacts of typhoon-induced floods on the agriculture in the central region of Vietnam by using hydrological models and satellite data

Flooding associated with landing tropical cyclones (TCs) is one of the major natural hazards in the coastal region of Vietnam. Annually, approximately 5 or 6 TCs make landfall in Vietnam, bringing heavy rains and inducing flooding, particularly to the central coastal region because of its topography and geographic configuration. This study focuses on the modelling of typhoon-induced floods that have resulted in widespread damage to agriculture over the central Thua Thien Hue Province of Vietnam by coupling two well-known hydrological models, KINEROS2 and HEC-RAS (Daniel et al. in Open Hydrol J 5(1), 2011), and using GSMaP (Global Satellite Mapping of Precipitation) data as the satellite rainfall input. Landsat imagery and GIS are also used for mapping and analysing the inundated areas. The discharge and water level from the KINEROS2 and HEC-RAS models displayed acceptable results for the floods modelled from three selected typhoons; both the Nash–Sutcliffe simulation efficiency coefficient (NSE) and the coefficient of determination (R²) were greater than 0.6. The simulated inundation maps of these typhoon-induced floods were compared with those extracted from the Landsat imagery to assess consistency. The result revealed a similar spatial extension of the inundated agricultural areas. This information, together with the forecasted TC movements and associated rainfalls, will be helpful to plan methods for mitigating potential typhoon-induced flooding and damage, particularly damage to agricultural regions.     

The occurrence of cyanobacteria and microcystins in the Hoan Kiem Lake and the Nui Coc reservoir (North Vietnam)

The occurrence of increasing blooms of toxic cyanobacteria in freshwaters has received much attention due to the ability of many cyanobacteria to produce potent cyanotoxins. In this paper, the occurrence of dominant cyanobacteria and the concentration of microcystins (MCs) analysis were investigated monthly from July 2008 to April 2009 in the Hoan Kiem Lake and from February to April 2009 in the Nui Coc reservoir. Concentrations of intracellular MCs from water, bloom samples, and isolated strains were quantified by using high performance liquid chromatography (HPLC). During the study period, the microscopic examination of the phytoplankton samples showed the dominance of the genus Microcystis in the water environment of the Hoan Kiem Lake and the Nui Coc reservoir. The toxin analysis by HPLC demonstrated the presence of two MC variants: MC-LR and MC-RR in water samples. Total concentrations of the toxins in filtered samples from surface water ranged from non-detected to 0.91 μg L^?1 at Nui Coc reservoir and they ranged from 2.1 to 46.0 μg L^?1 at Hoan Kiem Lake. The results of the HPLC analysis confirmed the production of MCs in bloom samples (ranged from 115.9 to 184.6 μg L^?1 in the Hoan Kiem Lake and from 726.5 to 1116 μg L^?1 in the Nui Coc reservoir) and isolated strains of Anabaena sp. and Microcystis with the concentration of MC ranging from 152 to 396.2 μg g^?1 dry mass, respectively.     

Influence of Tropical Cyclones of the South China Sea on the Variability of the Vietnamese Coastal Current Structure

The state of the natural environment of the marginal seas of the Northwest Pacific is largely controlled by the interaction of the atmospheric and hydrophysical processes. Tropical cyclones (typhoons), originating in the tropical zone of the Northwest Pacific and over the South China Sea basin, occupy a special place among atmospheric processes. The main destructive impact of typhoons falls on Southeast Asia. However, a significant number of are moving to the Russian Far East. The region of the South China Sea plays a significant role in the formation of tropical cyclones. This determines the importance of studying hydrometeorological processes not only in the Far East, but also in the South China Sea, and the need for cooperation between Vietnamese and Russian scientists. The main hydrodynamic structure of the western South China Sea is the Vietnamese Coastal Current (Western Boundary current), which depends not only on the seasonal monsoons but also on typhoons. The paper presents the results of joint Russian–Vietnamese studies of the dependence of the vertical structure of the Vietnamese Coastal Current on the Pacific tropical cyclones that form in the South China Sea. The study is done with numerical modeling. The period from April to June 1999 was used for modeling, provided with the necessary field data. The simulation results showed that, in general, the structure of water masses depends on the trajectories of tropical cyclones. In all cases considered, the Vietnamese Coastal Current is not a single flow, but represents a zone of eddy structures of different directions. An exception is the only situation in the condition of a tropical cyclone in the central region of the South China Sea when this current acquired the form of a single continuous flow directed from north to south only in the 200-m layer. The general patterns of changes in the dynamic structure of the Vietnamese Coastal Current for all the considered tropical cyclone trajectories include the following: areas with water transport in the northern direction prevail on the surface, while the rest of the water mass continues to flow generally in the southern direction. This transport of surface waters may be due to the influence of the emerging summer monsoon, and the rest of the water mass, which is less exposed to the still weak atmospheric processes of the monsoon type, continues to flow in the winter regime.

     

‘White cranes fly over black cranes’: The longue durée of rural–urban migration in Vietnam

The changing nature of rural–urban migration, in the context of transition economies of East and Southeast Asia, is usually interpreted as an outcome of structural transformation, while the longue durée of these migration flows tends to be neglected. Adopting a historical perspective, supported by the ’migration as resistance’ approach, this paper examines the changes in rural–urban migration dynamics in Vietnam from the 1950s to 2010 in relation to the country’s political and economic reforms (Ð?i M?i). Drawing on a case study of female migration between Phu Khe and Hanoi, it analyses how migrants resist and undermine state control over population mobility in order to pursue their movement and migration, and how they develop their migration networks over time. By doing so, the paper highlights a gradual process of the changing nature of rural–urban migration in which migrants themselves, through their resistance to governance of mobility and their establishment of migration networks prior to Ð?i M?i, play an important role.     

Modern climate of the Cát Tiên National Park (Southern Vietnam): Climatological data for ecological studies

A method of using the standard network weather station data for local ecosystem research is considered on the example of the modern climate of the Cát Tiên National Park (Southern Vietnam) and local climate change in 1980–2010. Special attention is focused on the environmental parameters, which play a role of the limiting factors. It is shown that the climate of Southern Vietnam responds with statistical significance to global climate change. Suggestions about the possible reactions of tropical monsoon forest ecosystem to climate change are given.     

Correlating mass physical properties with ALOS reflectance spectra for intertidal sediments from the Ba Lat Estuary (northern Vietnam): an exploratory laboratory study

Characterization of the sediment composition of tidal flats and monitoring of their spatiotemporal changes has become an important part of the sustainable management of coastal environments. To accurately classify sediments through remote sensing, a comprehensive understanding of sediment reflectance spectra is indispensable. The present laboratory-based study explores the performance of the high spatial resolution (10?×?10 m) Advanced Land Observing Satellite (ALOS) launched in 2006. Relationships between reflectance spectra (bands 1 to 4) and four typical mass physical properties were investigated under wet and dry experimental conditions for intertidal sediments sampled near the Ba Lat Estuary in northern Vietnam. Reflectance in the near-infrared region corresponding to ALOS band 4 (0.76–0.89 μm) was found (1) to have a strong negative correlation with sand content (dry wt%) under both wet and dry conditions (linear correlation coefficient r?=?–0.7859 and –0.8094, respectively), (2) to increase with decreasing relative water content (%) in a given sediment type (r?=?–0.7748 to –0.9367 for mud, sandy mud, muddy sand, and sand), (3) to have a positive correlation with organic matter content (r?=?0.7610 and 0.6460 under wet and dry conditions for contents >0.20 dry wt%), and (4) to be insignificantly correlated with mineral composition assessed in terms of contents (wt%) of quartz, clay minerals, and mica group minerals. Positive relationships between reflectance and water content for the pooled data of all sediment types (r?=?0.6395) or organic matter content contrast with previous findings, and can be attributed to close interrelationships between these properties and the predominance of sand content as controlling factor of reflectance. This study clarifies that ALOS band 4 provides the most useful imagery for intertidal monitoring because its reflectance, as simulated using the laboratory data, shows the strongest correlation with sand content. In a next step, these experimental findings should be verified by identifying the reflectance relationships at satellite image scales, and also considering the effects of other tidal flat features on reflectance, such as microtopography and biological surface characteristics.     

An Application of the Multi-Physics Ensemble Kalman Filter to Typhoon Forecast

This study examines the roles of the multi-physics approach in accounting for model errors for typhoon forecasts with the local ensemble transform Kalman filter (LETKF). Experiments with forecasts of Typhoon Conson (2010) using the weather research and forecasting (WRF) model show that use of the WRF’s multiple physical parameterization schemes to represent the model uncertainties can help the LETKF provide better forecasts of Typhoon Conson in terms of the forecast errors, the ensemble spread, the root mean square errors, the cross-correlation between mass and wind field as well as the coherent structure of the ensemble spread along the storm center. Sensitivity experiments with the WRF model show that the optimum number of the multi-physics ensemble is roughly equal to the number of combinations of different physics schemes assigned in the multi-physics ensemble. Additional idealized experiments with the Lorenz 40-variable model to isolate the dual roles of the multi-physics ensemble in correcting model errors and expanding the local ensemble space show that the multi-physics approach appears to be more essential in augmenting the local rank representation of the LETKF algorithm rather than directly accounting for model errors during the early cycles. The results in this study suggest that the multi-physics approach is a good option for short-range forecast applications with full physics models in which the spinup of the ensemble Kalman filter may take too long for the ensemble spread to capture efficiently model errors and cross-correlations among model variables.     

东南亚断裂构造的演化

本研究项目是采用诸如地质学解释、影像判读和地球物理探测等多种方法相结合进行的。在所得结果的基础上参考了越南国内外同行的一些资料编制了东南亚、越南及邻区的断裂构造图．其比例尺分别为1：4百万和1：1百万。分析所得结果显示出东南亚断裂构造演化的下列情况：1)在现今地质结构方面东南亚是欧亚岩石圈板块的东南部分．由一条消减带围绕．这条消减带的伸展从Myanmer开始,通过Nicobar,Java Timor直到东菲律宾。东南亚被Song Hong(即红河)断裂,Three Pagodas断裂和Hainam-Natuna断裂等2级断裂系统分成3个微板块。2)在早新生代．东南亚是分为5个微板块的。它们的分界断裂中有2个一级断裂(中央东海扩张带和Lupar-Kuching消减带)和3个二级断裂(即上述3个)。3)上述绝大多数二和三级断裂从晚新生代起活化且继承了从早新生代即已发生和发展了的二、三级断裂,但在某些条件下．运动方向却完全变成了相反,尤其是走滑运动的方向。我们的研究结果表明：在这一地区内,盆地、隆起、岩浆侵入、褶皱和局部断裂等构造的形成都取决于这些沿着一、二级走滑断裂的微板块运动。