The transboundary Kherlen River Basin, a temperate grassland basin spanning China and Mongolia. While recurrent droughts in recent decades have substantially altered global vegetation patterns, how these changes feed back to modulate hydrological processes, particularly hydrological drought, constitutes a critical knowledge gap. A novel Comprehensive index, the Comprehensive Hydrological Drought Assessment Index (CHDAI), was developed to bridge this gap. The CHDAI synthesizes surface runoff, soil moisture, groundwater, and snowmelt to provide a holistic measure of hydrological response. Its utility was demonstrated by characterizing hydrological droughts (2000–2020) in the Kherlen River Basin, The regulatory influence of vegetation dynamics (measured by kNDVI) on drought was then quantified, and the causal pathways through which kNDVI affects the CHDAI were rigorously identified using structural equation modeling (SEM). Results reveal a large-scale exacerbation of hydrological drought in the Kherlen River Basin, with 97.15% of its area experiencing a significant drying trend (P < 0.05). This was manifested through increased drought duration (0.26 yr⁻¹), frequency (0.01 yr⁻¹), and intensity (-0.19 yr⁻¹) across 84.42% of the region, in the central and eastern sectors. SEM revealed that vegetation influences hydrological drought through spatially heterogeneous pathways. At the basin scale, kNDVI exhibited a weak negative association with CHDAI, while in the middle and lower reaches, this association became more pronounced, highlighting the spatial complexity of vegetation-hydrology interactions. • Novel CHDAI integrates runoff, soil moisture, groundwater & snowmelt, improving drought assessment. • 97.15% of the Kherlen River Basin shows significant drying, coupled with basin-wide vegetation greening (kNDVI 0.005 yr⁻¹). • Vegetation greening directly/indirectly worsens hydrological drought, with spatiotemporal heterogeneity across reaches. • Structural equation modeling reveals soil moisture as the key hydrological driver of CHDAI in the transboundary basin. • CHDAI supports transboundary water management and agro-pastoral sustainability under climate change.
Bai et al. (Thu,) studied this question.