Assessing Nonlinear Responses of Flash Drought Characteristics to Elevation in the Western Himalayan Catchment

Flash droughts are rapidly developing soil moisture deficits that intensify within weeks and pose growing threats to agriculture, ecology, hydropower, water, and food security. Their behaviour in Himalayan basins remains poorly understood due to complex topographic features, highly heterogeneous soil moisture & land-atmosphere energy fluxes, and the coexistence of snow-fed & rain-fed hydrological regimes. Addressing to this gap and supporting Sustainable Development Goals (SDGs) related to zero hunger (SDG 2), clean water availability (SDG 6), and climate action (SDG 13), this study investigates flash drought characteristics and their elevation dependence in the Indian Indus Basin. Multi-source daily root-zone soil moisture (RZSM) datasets from ERA5, SMAP, FLDAS_CA, and GLDAS (versions 2.1 and 2.2) bilinearly interpolated at 0.1° resolution were evaluated against in-situ measurements from the NGARI network of the International Soil Moisture Networks (ISMN), which identified the ERA5-GLDAS2.1-GLDAS2.2 ensemble as the most reliable RZSM data (R² = 0.7003). The validated RZSM data were converted to 8-day (octad) means, and soil moisture percentiles were computed using an empirical Weibull distribution. The RZSM percentile octads were then used to identify flash droughts in the basin. Flash drought events were detected when percentiles declined from ≥40 to ≤20 within three octads at an intensification rate ≥6.5 percentiles per octad, and persisted below the 20th percentile up to eleven octads. From these events, the mean annual and seasonal onset speeds, durations, severities, and frequencies were quantified. Piecewise linear regression with breakpoints selected using the change in Akaike information criterion (ΔAIC) revealed distinct elevation-dependent regimes. Mean annual flash drought severity, frequency, and duration increased from low to mid-elevation zones (up to ~2000 m) and declined toward higher elevations. Mean annual onset speed was maximum (~17-30 percentile/octad) at low elevations, indicating rapid soil moisture depletion under strong atmospheric demand, whereas higher elevations exhibited slower onset (~6.5-17 percentile/octad), likely due to snowmelt-driven soil moisture replenishment and reduced evaporative demand. Similar elevation dependence regimes of flash drought characteristics were observed seasonally, with maximum frequency (~10-15%) and onset speed (~15-20 percentile/octad) in the monsoon, but the highest duration (~6.5-10 octads) and severity (~100-150) in the post-monsoon. These non-linear elevation responses highlighted the critical role of topography in modulating flash drought evolution in the complex Himalayan basin. This study presents the first elevation-based characterization of flash droughts and demonstrates the value of high-resolution reanalysis-based soil moisture data for enhancing flash drought monitoring in data-scarce mountain basins.