| Summary: | Fraction of young water (<i>Fyw</i>) and mean transit time (MTT, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mover accent="true"><mi>τ</mi><mo>¯</mo></mover></semantics></math></inline-formula>) calculated from water isotope profiles are valuable information for catchment hydrological assessment, especially in anthropogenically impacted region where natural conditions may not be decisive to catchment hydrology. The calculation of <i>Fyw</i> and MTT were performed on three subsets of δ<sup>18</sup>O_H<sub>2</sub>O data collected at the Hanoi meteo-hydrological station, Red River, in three periods; 2002–2005, 2015, and 2018–2019. The mean (min and max) values of δ<sup>18</sup>O_H<sub>2</sub>O in rainwater over the three periods are, respectively, −5.3‰ (−11.0 and −1.2‰), −5.4‰ (−10.7 and −1.4‰), and −4.5‰ (−13.9 and 1.7‰). The corresponding values in river water are −8.4‰ (−9.8 and −6.9‰), −8.5‰ (−9.1 and −7.7‰), and −8.4‰ (−9.5 and −7.2‰), respectively. The mean of <i>Fyw</i> calculated from the δ<sup>18</sup>O_H<sub>2</sub>O data for different periods is 22 ± 9%, 10 ± 5%, and 8 ± 3%. Mean transit time is 4.69 ± 15.57, 1.65 ± 1.53, and 2.06 ± 1.87 years. The calculated <i>Fyw</i> (MTT) is negatively (positively) proportional to change in reservoir volume over the three periods, which is logical, since reservoirs tend to keep more water in the catchment and slower down water flow. The strong variation of <i>Fyw</i> and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mover accent="true"><mi>τ</mi><mo>¯</mo></mover></semantics></math></inline-formula>, two essential variables characterizing the catchment hydrology, represents an anthropogenic impact in the Red River system.
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