Hydrological Processes and Sediment Dynamics in Climatically Transitional Watersheds under Climate Change and Human Activities

Climate change and intensified human activities have profoundly altered hydrological processes in watersheds worldwide. Soil erosion and sediment yield are key indicators of watershed hydrological responses, with far-reaching consequences for water resources, landscape evolution, ecosystem functioning, and socio-economic sustainability. While most existing studies focus on rainfall-driven water erosion, hydrological processes in climatically transitional regions remain less well understood. In such regions, the alternating and combined effects of multiple erosive agents can increase ecological vulnerability and amplify erosion and sediment yield risks. Here, we present a long-term investigation of hydrological and sediment dynamics in representative wind-water erosion crisscross watersheds in northern China. These watersheds are located within an arid and semi-arid climatic transition zone and feature a complex geomorphic setting comprising aeolian sand landscapes and loess hilly terrain. This combination gives rise to distinct hydrological behaviors. Multi-temporal analyses show that precipitation and runoff are similar between the two geomorphic units. In contrast, the loess hilly region exhibits significantly higher annual sediment yields and a much stronger seasonal concentration, with sediment transport being highly episodic and dominated by a few flood events during the rainy season. Attribution analyses indicate that watershed topography, soil texture, and landscape complexity jointly control spatial variability in sediment yield. In particular, the complex composition of underlying surfaces promotes the formation of hyperconcentrated floods, during which peak suspended sediment concentrations frequently exceed 300 kg m^-3. River discharge-sediment hysteresis analyses further demonstrate that ecological restoration has reduced flood process complexity and sediment source variability. However, hyperconcentrated flows remain the dominant driver of sediment production, sustaining a high erosion risk. Futhermore, we observe an increasing alternation of drought and flood extremes in this region, pointing to growing hydrological instability. Under future scenarios of intensified climate extremes, such variability is likely to further amplify erosion and sediment yield risks. These findings highlight the importance of integrated monitoring of soil and water processes across contrasting geomorphic units to improve erosion risk assessment and watershed management under global change.