The Mekong River Basin (MRB) in Southeast Asia is among the world’s ten largest rivers, both in terms of its discharge and sediment load. The spatial and temporal resolution to accurately determine the sediment load/yield from tributaries and sub-basin that enters the Mekong mainstream still lacks from the large-scale model. In this study, the SWAT model was applied to the MRB to assess long-term basin hydrology and to quantify the sediment load and spatial sediment yield in the MRB. The model was calibrated and validated (1985–2016) at a monthly time step. The overall proportions of streamflow in the Mekong River were 34% from surface runoff, 21% from lateral flow, 45% from groundwater contribution. The average annual sediments yield presented 1295 t/km2/year in the upper part of the basin, 218 t/km2/year in the middle, 78 t/km2/year in the intensive agricultural area and 138 t/km2/year in the highland area in the lower part. The annual average sediment yield for the Mekong River was 310 t/km2/year from upper 80% of the total MRB before entering the delta. The derived sediment yield and a spatial soil erosion map can explicitly illustrate the identification and prioritization of the critical soil erosion-prone areas of the MR sub-basins.
The Tonle Sap Lake (TSL) Basins of the Lower Mekong are one of the world’s most productive ecosystems and have recently been disturbed by climate change. The SWAT (Soil & Water Assessment Tool) hydrological model is utilized to investigate the effect of future climate scenarios. This study focused on two climate scenarios (RCP2.6 and RCP8.5) with three GCMs (GFDL-CM3, GISS-E2-R-CC, and IPSL-CM5A-MR) and their impact on the hydrological process and extremes in the Sen River Basin, the largest tributary of the TSL basin. The annual precipitation, surface runoff, lateral flow, groundwater flow, and total water yield are projected to decrease in both the near-future (2020–2040) and mid-future period (2050–2070), while actual evapotranspiration is projected to increase by 3.3% and 5.3%. Monthly precipitation is projected to increase by 11.2% during the rainy season and decrease by 7.5% during the dry season. Two climate models (GISS and IPSL model) lead to decreases in 1-day, 3-day, 7-day, 30-day, and 90-day maximum flows and minimum flows flow. Thus, the prediction results depend on the climate model used.
Climate change alters hydrological cycles and streamflow regimes at the local, regional and global levels. In this study, we aimed to assess the change in water balance change and hydrological extremes in the Prek Thnot River Basin of the Lower Mekong in Cambodia through a hydrological model (SWAT) under the two climate change scenarios (RCP2.6 and RCP8.5) following three different GCMs. An ensemble of 3 GCMs included GFDL-CM3, GISS-E2-R-CC and IPSL-CM5A-MR models and was applied to a well-calibrated SWAT model through climate change factors. Annual precipitation under RCP2.6 likely decreases by 0.1–0.5% for the near future (2021–2040) and mid-future (2051–2070) and decreases by 0.2–1.3% under RCP8.5. The decrease in precipitation will lead to reductions in water yield by 1–4% (RCP2.6) and 2–5% (RCP8.5). However, peak flow is expected to increase, while the low flow was projected to decrease (1–2% for RCP2.6 and 8–9% for RCP8.5). The study further found that high flow events will increase in both magnitude and frequency. The finding highlights water resources management issues in the Prek Thnot River Basin, including the frequency of future flood events.
Drought is an inevitable consequence of climate variability and is pervasive across many regions all over the globe. This study aims to explore the relationship of drought and to evaluate the adaptivity and sensitivity in the Prek Thnot River Basin, Cambodia. The drought indicators were used between the occurrence of droughts based on Standardized Precipitation Index (SPI) and Standardized Runoff Index (SRI) to assess the impact in 8 stations in its base months (June, August, and November) for each time scale (1-month, 3-month, and 9-month). The results indicated that 1998 and 2005 were the most critical years. Furthermore, the dry event could be seen in each basin mostly in June and August as a state in drought frequency as in all stations. The outcomes indicated that most stations experienced drought conditions. The small amount of rainfall occurs over these stations during the year indicated, and they had a high frequency of drought. The drought conditions during the period could have many impacts on population living livelihood, especially on sanitation, economy while the most damaging negative impact is on crop production in the regions, given Cambodians’ dependence on agricultural resources.
A modified piano key weir with a rounded nose and a parapet wall (MPKW) can improve the discharge capacity significantly compared to a standard piano key weir. However, the optimum of the inlet/outlet width ratio (Wi/Wo) on the discharge efficiency of MPKW is still not investigated numerically. The present work utilized the numerical modeling to investigate and analyze the effects of the inlet/outlet key width ratios on the hydraulic characteristics and discharge capacity of the MPKW. To validate the numerical model with the experimental data, the results indicate that the average relative error is 2.96%, which confirms that the numerical model is fairly well to predict the specifications of flow over on the MPKW. Numerical simulation results indicated that the discharge capacity of the MPKW can be improved up to 8.5% by optimizing the Wi/Wo ratio ranging from 1.53 to 1.67 even if the other parameters of the MPKW keep unchanged. A big Wi/Wo ratio generally leads to an increase in discharge capacity at low heads and a little effect on the discharge efficiency at high heads. The discharge efficiency of the inlet and outlet crests increases up to 9.6% for high heads, while discharge efficiency of the lateral crest decreases up to 23.5% compared with the reference model. The findings of the study revealed that the intrinsic influencing mechanism of the Wi/Wo ratio on the discharge performance of MPKWs.
Flood is a water-related disaster that causes negative impacts on Human and societies. In Cambodia, flood has become more severe and frequent in most regions, especially, in the floodplain areas. This study aims to simulate the flood hazard index and vulnerability located in the Sen River Basin using the Analytical Hierarchy Process (AHP) method and Geographical Information System (GIS). GIS was applied for the estimated Flood Hazard Index (FHI) in which seven parameters were selected: Flow accumulation, Elevation, Density Drainage, Rainfall Intensity, Slope, Soil Type, and Land Use. Additionally, the relative importance of each parameter of physical factors has been compared in the pairwise matrix 7×7 to gain the weight value by AHP Method. The Flood Hazard Zones was classified into five classes from Very Low to Very High and were observed as Very low 6175 Km2, Low 4094 Km2, Medium 2002 Km2, High 818 Km2, and Very High 859 Km2. The FHI was verified with the flood hazard map from global surface water and Flood history from Google Earth Engine. Consequently, the results were found to be reasonable and adequate, revealing a realistic representation of hazards on the corresponding flood hazard map.
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