Spatial and temporal variabilities of suspended sediment and dissolved nutrients in the Ca River basin, North Central Vietnam

The Ca River basin has an area of 27,200 km(2) 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.