Background The evolution of rainfall-runoff relationships in changing environments is an important reference to local water resources management, disaster prevention and mitigation. Thus, it is essential to further reveal the spatial and temporal variations of rainfall-runoff relationships in the Minjiang River basin and identify the main driving factors. The study on the rainfall-runoff relationship of the Minjiang River basin is beneficial to understand the changes in the hydrological cycle under the changing environment, to ensure the economic development of the Minjiang River basin, and to provide scientific basis for soil and water conservation.

Methods Based on rainfall and runoff data from 1960 to 2019, the TFPW-MK method was applied to detect the rainfall-runoff trend at different temporal scales from inter-annual and intra-annual levels, including flood and non-flood season, the first rainy season and second rainy season, maximum rainfall(runoff) month and minimum rainfall(runoff) month. The method was also applied at different spatial scales including the three major tributaries of the upper Minjiang River and the mainstream. Combined with Sen trend method, the causes affecting the changes of rainfall-runoff relationship including climate change, land use change and reservoir engineering construction were analyzed.

Results 1) The inter-annual rainfall and runoff of Minjiang River basin and its tributaries were all insignificantly increasing. 2) Except for the tributary Jianxi, Minjiang River and other tributaries had a significant upward trend of rainfall-runoff in the non-flood season; the trend changes within the flood season were clearly differentiated, with a tiny decrease in rainfall-runoff in the first rainy season and a more significant increase in rainfall-runoff in the second rainy season. 3) The trend of extreme rainfall-runoff was not consistent, the minimum monthly rainfall changed insignificantly while the minimum monthly runoff increased significantly, and the maximum monthly rainfall basically remained the same while the maximum monthly runoff showed a decreasing trend.

Conclusions The inter-annual rainfall-runoff trends in the Minjiang River basin vary consistently under changing environments, but large variation within the year. The differentiated trend in the flood season is due to that typhoon activity and westerly path under the climate change lead to the increase of rainfall in the second rainy season, and the runoff also increased accordingly. And the inconsistent of extreme rainfall-runoff is mainly due to the influence of flood storage and refill operation of the reservoirs during the year. Therefore, the divergence of trends between the first and second rainy season is mainly influenced by the rainfall of different synoptic systems; the variation of extreme rainfall-runoff trends is mainly influenced by the construction of large reservoir projects.