Evolution Pattern and Frequency Coupling Relationship of Runoff and Sediment Under Changing Environmental Conditions: A Case Study of the Kuye River Basin in the Loess Plateau

Abstract

Conducting a comprehensive investigation into the interplay between runoff and sediment discharge in a dynamic environment is essential to uphold aquatic ecosystems and promote sustainable regional development of high quality. Initiated since the 1950s, extensive soil and water conservation projects in the middle reaches of the Loess Plateau, China, have resulted in a substantial reduction in runoff and sediment discharge. This reduction has prompted alterations in the dynamics between runoff and sediment discharge, impacting the frequency of their interactions. Analyzing measured data from the Kuye River Basin (Kuye Basin) in the middle reaches of the Yellow River for the period 1956 to 2019, this study investigates the evolving patterns of runoff and sediment discharge. Utilizing the Copula function, we analyzed the joint probability distribution characteristics of runoff and sediment discharge, aiming to identify the factors influencing their evolution patterns. The findings revealed a substantial downward trend (p < 0.01) in both monthly and annual scales for runoff and sediment discharge in the Kuye Basin. Additionally, two abrupt changes were observed in 1979 and 1996, demonstrating multi‐timescale periodicity. Based on the mutation points, the study period was divided into three phases: P1 (1956–1978), P2 (1979–1995), and P3 (1996–2019). The optimal model for the joint distribution of runoff and sediment discharge is the Clayton Copula in both P1 and P2, while in P3, the optimal model shifts to the Gumbel Copula. Over the transition from P1 to P3, the runoff and sediment discharge in each recurrence period experienced a significant decrease, and the asynchronous probability of abundance encounters for runoff and sediment discharge at different design frequencies increased. Human activities, such as soil and water conservation measures, have influenced the variations in runoff and sediment discharge at different stages. With the passage of time, the effectiveness of current measures has approached saturation, resulting in cumulative values of nearly 28 × 109 m3 for runoff and 4.4 × 109 t for sediment discharge, respectively. These research findings lay the groundwork for a deeper comprehension of the evolving dynamics between runoff and sediment discharge in the basin and contribute to the formulation of effective policies for soil erosion control.