南方红壤区典型水土流失治理小流域的洪水径流泥沙特征

为探究南方红壤区经长期水土流失治理小流域的水沙特征，该研究收集长汀县朱溪河小流域2017—2020年降雨及洪水水沙数据，通过冗余分析、多元逐步回归方程、含沙量-流量滞回曲线等方法进行分析。结果显示：（1）流域年洪水径流深和泥沙量分别为282.30～892.50 mm和35.80～179.50 t/km²，洪水事件的产沙模数集中在0～20.0 t/km²，但总泥沙量由大于5.0 t/km²的少数事件决定；（2）降雨量、30 mim的最大雨强和降雨侵蚀力是影响洪水径流泥沙的主要降雨特征，对径流、泥沙变化的解释度分别为68.99%和49.28%，通过主要径流特征估算泥沙量、平均含沙量和最大含沙量，拟合优度达0.624～0.870；（3）洪水事件共出现6种含沙量-流量滞回关系，其中线型出现频率（55%）最高，该类事件中含沙量随流量的变化具有分阶段特征，临界含沙量约为0.1 g/L。经过长期的水土流失治理，红壤区小流域的洪水泥沙量普遍较低，且主要受径流量影响，洪水事件的滞回关系表明流域的泥沙供应通常处于持续少量的状态，研究结果有助于揭示红壤区土壤侵蚀的发展趋势。

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.