Runoff and sediment characteristics of flood events in a typical soil and water conservation watershed in the red soil region of Southern China

Runoff and sediment characteristics can be the important indicators on the soil erosion. However, it is still unclear on the runoff and sediment variations of the watershed in the red soil region of southern China, especially after a continuous soil erosion control. This study aims to investigate the characteristics of runoff and sediment in the small watershed after the long-term soil and water conservation in the red soil region of southern China. The Zhuxi watershed in Changting County Fujian Provence was taken as the study area to collect the rainfall and hydrological data from 2017 to 2020. A systematic analysis was implemented to determine the runoff and sediment relationships of the flood events using redundancy analysis (RDA), multiple stepwise regression equations, and sediment concentration (C)-discharge (Q) hysteresis loop methods. The results showed that: 1) The monthly average runoff and sediment concentration were consistent with the trend of monthly rainfall changes during the study period. The annual flood runoff depth and sediment yield were 282.30-892.50 mm and 35.80-179.50 t/km², respectively. The sediment modulus of the flood events was distributed mainly in 0-20.0 t/km². But the total flood sediment yield were determined by a few flood events with the sediment modulus greater than 5.0 t/km². 2) Rainfall (P), the maximum rainfall intensity of 30 min (I₃₀), and rainfall erosivity (R) were the main rainfall characteristics that affected the runoff and sediment. However, the contribution rates of rainfall characteristics to the variations of runoff and sediment were 68.99% and 49.28%, respectively. It infers that the rainfall characteristics were more closely related to the runoff indicators than to the sediment indicators. The runoff was the most important influencing factor of sediment. Thus, the great fitting was achieved in the R² range of 0.624-0.870 with estimating the sediment yield (S), average sediment concentration (C_m), and the maximum sediment concentration (C_max) of flood events using main runoff indicators, including the runoff depth (H), average discharge (Q_m), and peak flow (Q_max). 3) There were six types of C-Q hysteresis loops in the flood events with the sediment modulus greater than 0.1 t/km². Among them, the occurrence frequencies of the single-valued and complex loop were 55% and 22%, respectively. The counterclockwise loop was more appeared in the flood events with the small sediment modulus. The highest occurrence frequency was found in the single-valued loop. The variation of C with Q in the single-valued flood events was characterized by the phased method. The critical C value was about 0.1 g/L, as the C-Q relationship changed. The sediment carrying capacity of runoff was significantly changed at a sediment concentration of about 0.1 g/L. Moreover, the single-valued loops were also dominated in the C-Q hysteresis loop for the large-scale flood event in the watershed. There were the low sediment yields of the flood events and the great influence of the runoff on the flood sediment in the watershed of the red soil region after a long-term soil erosion control. The sediment supply of the flood events, including the large-scale flood events, in the watershed was generally in a continuous and limited state. The findings can provide the much more comprehensive understanding of the runoff and sediment processes in the watershed with continuous soil and water conservation, in order to reveal the development of soil erosion in the red soil region of southern China.