Variability and trend detection of water discharge over the past 60 years in the Vietnamese Hieu River Basin

Abstract This paper presents an in-depth analysis of the variability and trend detection of water discharge in the Hieu River Basin over the period from 1/1/1961 to 31/12/2020. Firstly, the wavelet analysis is implemented to investigate the flow variability at different time scales. Using time series of daily flow at Quy Chau and Nghia Khanh, the periodicities of flow are examined, showing that periodic oscillations in flow mainly occurred at 1 year, from 2 to 4 years, and from 4 to 8 years. Secondly, discharge variability is investigated, revealing an increase from January to September (excluding June) and a decrease from October to December. At Quy Chau, flow increased seasonally and annually by 0.19 and 0.06 m3/s, respectively, while its values at Nghia Khanh decreased of up to −0.50 m3/s. Thirdly, temporal trends of flow are assessed using Mann–Kendall and Sen's slope estimator, and sequential Mann–Kendall test. Results show flow decreases during 1961–1969, 1973–1976, and 1996–2020 and increases in 1969–1973 and 1976–1996. Temporal patterns of flow at Quy Chau and Nghia Khanh demonstrated synchronization across different time scales. Relationship between flow and rainfall is discussed to investigate insights into their relationship.