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This paper presents the results of the assessment of achieving sustainable development of clean water and sanitation (SDG6) goals for the Srepok river basin by the research team at the Water Resources Institute (WRI). The assessment results of some key indicators including “Proportion of the population using safely managed drinking water services 6.1.1”, “Proportion of the population using safe managed sanitation services 6.2.1a”, “Proportion of bodies of water with good ambient water quality 6.3.2”, and “Proportion of transboundary basin area with an operational arrangement for water cooperation 6.5.2” are achieved at rather high levels (83-100%) compared with the target set by the United Nations for those in 2030. Moreover, the “Current progress of implementing integrated water resources management” (6.5.1) in Vietnam in general and the Srepok river basin in particular, is still slow, according to results obtained from using the questionnaire survey developed by UNEP, as 55% (high average level). In addition, the “Increasing level of water stress (indicator 6.4.2)” globally poses a challenge for each country and river basin in the process of implementing the goal to improve this issue.

The purpose of this study was to apply numerical modeling to assess the water quality in Nam Trieu and Lach Tray coastal estuaries. The wind reanalysis data from the Weather Research and Forecasting (WRF) Model were used to simulate waves and hydrodynamic regimes with a definition of 14 km within the longitude range from 105oE to 120oE and latitude range from 5° to 22° in the study area. The calibration and validation results for the wind–wave, hydrodynamic regimes showed that the performance of the 2D hydrodynamic model is extremely good, with a Nash coefficient ranging from 0.89 to 0.92, a root–mean–square error (RMSE) observation standard deviation ratio (RSR) varying from 0.25 to 3.6, and a percent bias (PBIAS) from −4.2% to 0.2%. The calibrated 2D hydrodynamic model was combined with the Ecolab model to evaluate and simulate the spreading of water quality parameters (biochemical oxygen demand (BOD), NH4+-N, and total suspended solids (TSS)). The water quality parameters BOD (16.5–22.5 mg/l), NH4+-N (0.003–0.009 mg/l), and TSS (29.63–40.8 mg/l) were smaller than the parameters of the national technical regulation on coastal water quality and surface water quality in March and August. The assessment results of water quality an industrial zone can help in management, planning, and decision–making to reduce wastewater discharge.

This study presents an effort to develop a new conceptual rainfall-runoff model based on a proposal of storage channel concept. The model implicitly solves the flow equations and was successfully calibrated and validated by applying it to a lowland catchment in the central province of Vietnam for comparison with the observed data. Moreover, the applicability of the model was also tested by comparing its results to those computed by a well-known model in Japan, the ordinal Tank model developed by Sugawara. From both the performance statistics and the hydrographs of the model results, it has been found that the model integrating a proper discharge control device provides satisfactory estimation of runoff discharge. The full model structure has totally twelve parameters to be calibrated. Sensitivity analysis for these parameters shows that they contribute effects to both the magnitude and the shape of the runoff hydrographs. Discharge control devices installed at the channel's outlets have a high advantage when considering the effects of time-varying boundary conditions at the channel′s outlets, in contrast some traditional lumped rainfall-runoff models using fixed discharge coefficients may not. The use of water control devices has made the model possible to consider the effect of tidal regime in lowland catchments while also play an important part in the model toward the shape, peak and timing of the runoff hydrographs as well as express the nonlinearity of the rainfall-runoff process. In short, the newly developed model has several good advantages over lumped hydrological models. Furthermore, the new model is basically able to analyze both runoff and flow movement in river basins that include mountainous, hilly and lowland tidal areas.