The influence of human activities on rainfall-runoff relationships at different time scales in the Minjiang River Basin

For water management and ecological security, it’s essential for identifying the influence of human activities on the rainfall-runoff relationships. The univariate and bivariate approaches were used to examine evolution of the rainfall-runoff relationship in Minjiang River Basin and tributaries at different time scales. The former approach with integrated Trend Free Prewhitening Mann–Kendall (TFPW-MK) and three mutation methods, and the latter approach with double mass curve (DMC) where the break points were determined by three mutation methods reasonably and with copula, were applied based on rainfall and runoff data during 1960–2019. The influence of human activities including land use and reservoir construction on the rainfall-runoff relationship were analyzed. Results showed that: (1) The changes of rainfall-runoff relationship at different time scales were different. The runoff changes during non-flood seasons and the minimum month in most tributaries were more obvious than the rainfall, while the runoff over inter-annual and flood season were basically unchanged; (2) The intra-annual changes of rainfall-runoff relationship were mainly due to reservoir construction. The sudden step in the cumulative capacity of the large and medium reservoirs basically corresponded to change points of the double mass curve approach (DMC) in the non-flood season and the monthly minimum; The significant increase in runoff during low-flow period and the notable reduction in maximum monthly runoff in mainstream were due to the reservoir regulation; (3) The combination of univariate and bivariate approaches can distinguish the impact of climate change and human activities on rainfall-runoff relationship, and identify clearly the influence of human activities.