Exploration of the runoff-sediment relationship in Lianshui watershed in south Jiangxi of China under the condition of a single rainstorm

The runoff-sediment relationship is one of the most important aspects to reflect the variation of runoff and sediment. There is a significant impact of the rainstorm on the runoff collection and sediment transport in the watershed. It is a high demand to explore the runoff-sediment relationship under the condition of a single rainstorm in the control of soil and water loss. Heretofore, most research was focused mainly on the northern region of China. It is still lacking in the southern region with frequent rainstorm and flood. Taking the Lianshui watershed located in southern Jiangxi Province as the research area, this study aims to launch the project of forest rehabilitation from slope agriculture, large-scale orchard development, and urbanization construction over the past 30 years. The underlying surface was changed significantly. The characteristics of runoff and sediment were analyzed to determine the relationship between them under different flood types and periods using a cluster, discriminant, and Spearman correlation analysis, according to the hydro-meteorological and land use data from 1965 to 2020 under the condition of a single rainstorm. The hysteretic relationship was then established between the runoff and sediment. The results showed that: 1) The 120 rainstorm floods in the watershed were divided into three types, with the occurrence frequency from the high to the low as Type III, Type I, and Type II, where the proportion were 63.3%, 32.5%, and 4.2%, respectively. The smallest eigenvalue was found in the runoff and sediment transport of Type III flood. The longest flood duration was in the Type I flood. The other eigenvalues were in the middle level. The Type II flood presented the largest eigenvalue, except for the middle-level flood duration. There was a highly correlated runoff depth and sediment transport in the three flood types. The Spearman correlation coefficients were 0.515, 0.800, and 0.481, respectively. 2) Five periods (1965-1975, 1976-1985, 1986-1995, 1996-2005, and 2006-2020) were divided by the whole study period. There was the highest average value of runoff depth and flood duration in 1986-1995, whereas, the average value of peak flow reached the highest in 1976-1985. The eigenvalue of sediment transport in 1996-2005 and 2006-2020 increased significantly, compared with the previous three periods, while the sediment transport capacity of unit runoff raised significantly. 3) The proportion of runoff-sediment loop curves in the type of the clockwise, counterclockwise, positive figure-eight, reverse figure-eight, and compound type were 18.3%, 17.5%, 26.7%, 15.8%, and 21.7%, respectively, among the 120 rainstorm floods. Meanwhile, there was a great variation in the proportion of runoff-sediment loop curves under different flood types. The highest proportion in Type I, Type II and Type III flood were the compound (33.3%), clockwise (40.0%), and positive figure-eight type (30.3%), respectively. However, the highest proportion of counterclockwise and figure-eight type curves was found in the period of 1996-2005 and 2006-2020, respectively, which was closely related to the changes in the location and quantity of sediment sources caused by the large-scale development of orchards in the watershed. The proportion of compound type curve reached the maximum in 1986-1995, due mainly to the high average value of runoff depth and flood duration. The finding can provide the scientific basis for the management and regulation of runoff and sediment in the watershed, in order to implement soil and water conservation measures.