Risk assessment and quantitative attribution of changes in water-sediment relationships by combining the Copula function and the Budyko hypothesis

Changes in external environmental conditions include surface conditions and climate, the process of water and sediment transport also exhibit non-steady-state characteristics, nonlinear and low-correlation characteristics, leading to a more complex interaction mechanism between water and sediment transport. It is worth exploring whether the water-sediment relationship is coordinated. The study was conducted on the changes and causes of water-sediment relationships in the Taohe River Basin in this paper. The results showed that: (1) The water and sediment transport in the Taohe River have significantly decreased. The annual average runoff and sediment discharge have decreased by 25.41% since 1987 and by 72% since 1995 respectively. (2) There is a positive correlation between water and sediment in the Taohe River. After 2009, the situation with less water and more sediment has changed to a situation with more water and less sediment. The water-sediment relationship became increasingly coordinated. (3) The water and sediment exhibit a symmetrical joint distribution characteristic, and the Frank-Copula function has the best fitting effect. The water-sediment synchronization and asynchronous probabilities are 85.62% and 14.38%, respectively, and the water-sediment synchronization probability is significantly greater than the water-sediment asynchronous probability. (4) Climate change has respectively led to a reduction of 24% to 47% in runoff and a decrease of 5% in sediment discharge, while that of human activities is 53% to 76% and 95% respectively. Human activities are the dominant factor causing these reductions, and they have a greater impact on the reduction of sediment discharge. The results provide scientific basis for the regulation and disaster prevention of water and sediment, and the management of water resources. • The patterns and correlations of water and sediment changes are revealed. • The marginal and joint distributions of water and sediment have both been constructed. • The risks of abundance and scantiness encountering of water and sediment have been assessed. • The partial differential equations for attribution have been constructed under six Budyko hypotheses.