An approach for runoff and sediment nexus analysis under multi-flow conditions in a hyper-concentrated sediment river, Southwest China.

Nexus of runoff and sediment in watercourses is important for sustainable watershed management, especially in rivers with hyper-concentrated sediments. However, complex relationships between runoff and sediment were not fully understood. In this research, Mann-Kendall (MK) and moving t-test methods were used to confirm the suitable temporal range based on time series data of precipitation in the Anning River Basin from 1971 to 2017. At the same time, the month of typical precipitation (i.e., the largest annual average precipitation) was identified. A model of runoff and sediment over two stations from 2010 to 2015 were constructed respectively based on copula analysis method. Synchronous and asynchronous probabilities of runoff and sediment were analysed from the temporal and spatial aspects separately. The encounter probabilities of runoff and sediment over multiple periods and at two stations were analysed. The sediment distribution was illustrated under multiple runoff distribution conditions. The encounter probability of runoff and sediment distribution changed greatly, which was principally due to employment of reservoir operation, soil and water conservation measures. The variability of the joint probability was much obvious in tributary regions than that of the mainstream areas of Anning River basin. The results could be adopted to improve the understanding of interactive relationships between runoff and sediment and provide a significant references for regional management of soil and water.