In classical severity–duration–frequency (SDF) analysis, the dependence between drought characteristics is not effectively considered. The present study aims to propose the SDF relationships of streamflow drought in the source area of the Yellow River (SAYR) using a copula-based approach. Comparison of multiple time-varying threshold levels and the integration and elimination of drought events were considered. Selection of marginal probability distribution and copula-based joint probability distribution was properly conducted with multiple means. Copula-based joint and conditional probabilities were computed. The findings support carrying out integration and elimination processing on the preliminarily identified streamflow droughts through a run analysis with a time-varying threshold level of the 80% quantile of daily streamflow. The Gaussian copula was selected as the optimal model for constructing bivariate joint probability distribution, with generalized extreme value and log-normal as the suitable marginal probability distributions of streamflow drought duration and severity. The proposed copula-based SDF relationships of streamflow drought events can provide more critical information in addition to univariate frequency analysis, benefitting from the joint and conditional probabilities. The multivariate probabilistic analyses can effectively consider the connection and interaction between drought characteristics, while conditional probability distribution allows analyzing the impact of one drought characteristic on another. The results also indicate a relatively high risk of streamflow drought with short duration and low severity in the region, requiring effective drought-mitigation strategies and measures.

Exploring the future ecosystem service value (ESV) of the upper–middle Yellow River Basin is of great significance to enhancing its ecological security and capacity. This is in response to the strategy for the ecological protection and high-quality development of the Yellow River Basin. In this study, the land use change from 2000 to 2020 was analyzed quantitatively. The land use pattern in 2035 was predicted using Cellular Automata and Markov models under business as usual (BAU), ecological protection (EPS), and high urbanization (HUS) scenarios. The future ESV was estimated and the impact of land use changes on the regional ESV was identified. The results indicate that the study area experienced a reduction (~12,139 km²) in cultivation and an expansion (~10,597 km²) of built-up land from 2000 to 2020. In 2035, under the BAU scenario, the area of construction land and water would expand by 24.52% and 11.51%, respectively, while the area of grassland and unused land would decrease by 18,520 km² and 2770 km², respectively. Under the EPS scenario, the area of forests, grasslands, and water would increase by 16.57%, 10.59%, and 4%, respectively. Under three different scenarios, the regional ESVs are estimated at from CNY 2475 to 2710 billion, while grasslands contribute the largest part accounting for from 57.98% to 59.21%. These findings could help to guide land development and protection through regional ecological construction.