Multi-scale meteorological and hydrological drought severity in a high-altitude Himalayan basin: An intensity–duration–frequency approach using SPI and SSI (1970–2016)

In high-altitude basins, snow and glaciers influence precipitation effectiveness by storing winter snowfall and regulate streamflow through delayed meltwater release, leading to distinct seasonal and hydrological drought responses. This study applies an Intensity–Duration–Frequency (IDF) framework to quantify meteorological and hydrological droughts in the Astore Basin, Pakistan using long-term precipitation and streamflow data (1970–2016). The Standardized Precipitation Index (SPI) and Standardized Streamflow Index (SSI) were calculated at 1-, 3-, 6-, and 12-month time scales to characterize drought severity, duration, and frequency. Frequency analysis was conducted to estimate precipitation and streamflow deficits for return periods ranging from 2 to 100 years and drought durations between 1 and 11 months. Results show that drought severity and duration increase systematically with both time scale and return period. For short-term droughts (SPI-1/SSI-1), deficits are confined to 3 months for a 2-year return period and increase to 5 months for return periods of 5–100 years, with precipitation deficits reaching ~66 mm and streamflow deficits ~330 m³/sec for extreme events. Medium-term droughts (SPI-3/SSI-3 and SPI-6/SSI-6) exhibit longer durations of 7–9 months for 50–100 year return periods, with cumulative precipitation deficits of ~188 mm (SPI-3) and ~564 mm (SPI-6), and corresponding streamflow deficits of ~1256 m³/sec (SSI-3) and ~907 m³/sec (SSI-6). Long-term droughts (SPI-12/SSI-12) represent persistent basin-scale water stress, with durations extending up to 11 months and maximum deficits of ~443 mm for precipitation and ~1623 m³/sec for streamflow at the 100-year return period. Across all time scales, the highest drought severity occurs during pre-monsoon and winter seasons, reflecting limited precipitation, high agricultural water demand, and delayed hydrological response. These results demonstrate the importance of multi-scale deficit-based IDF analysis for quantifying drought risk in snow- and glacier-fed basins and supporting long-term water resource management.