The Ca River basin has an area of 27,200 km2 distributed across the territories of two countries: Vietnam (65.2%) and Lao PDR (34.8%). Spatial and temporal variations in suspended sediment (SS) and dissolved nutrients (PO43-, NO3-, SiO2) were determined in two hydrological stations located along the Ca River 4–6 times per month in the rainy season and 1–4 times per month in the dry season, between the months of August 2017 and July 2018. A loading–discharge (L–Q) curve was used to analyze the correlation among water physicochemical parameters with seasonal river discharge. The results indicate that SS was higher in upstream flows compared to downstream flows, which is primarily due to erosion. Seasonal SSs and dissolved phosphate have an inverse correlation trend to that of dissolved silica. Results revealed that the concentration of phosphate and suspended sediments was higher in the rainy season than in the dry season. This finding proves that rain washes particulate matter from the surface runoff into the Ca River basin. Significant correlations between discharge and dissolved nutrient load were observed. This study provides useful information regarding variations of SS and water physicochemical parameters with seasonal water discharge in the Ca River.
For the majority of the world, most drainage basins are poorly gauged or simply ungauged. The estimation of the hydrological parameters in sub-catchments of large basins is a very difficult task but is frequently required in hydrological analyses. Estimating runoff from ungauged or sparsely gauged catchments is a serious challenge in developing countries like Vietnam. The Ca River Basin is the most extensive system in the North Central region of Vietnam but contains very limited hydrological gauging stations. Therefore, this study aims to investigate the extended applicability of a simulated hydrological Tank model at the Ca River Basin sub-catchments. The daily Tank model was calibrated in gauged sub-catchments: Quy Chau in northern-side and Hoa Quan in southern-side sub-catchments. The extended applicability of calibrated parameter sets was verified by data from the continuously gauged stations of Nghia Khanh, Dua, and Yen Thuong. According to the findings of this study, the use of parameters identified in small sub-catchments can be used to calculate the flow processes in similar, but larger catchments in the Ca River Basin. The simulated Tank model parameters can be applied to calculate discharge at ungauged locations within the study area for a variety of purposes.
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