Decoupling flash drought frequency, intensification rate, and persistence across Iran’s heterogeneous hydrological basins

This study quantified three distinct characteristics of flash drought (FD) ​across Iran’s major hydrological basins during 2003–2023. The analysis used pentad-scale, 0–100 cm depth-averaged soil-moisture percentiles derived from ERA5-Land and GLDAS-2.1. Events were detected using a continuous pentad sequence when soil moisture declined from above the 40th to below the 20th percentile within four pentads at a mean rate of at least five percentile points pentad⁻¹. Robust-scaled long-term means of event rate, intensification rate, and restricted persistence were classified using partitioning around medoids (PAM). The analysis identified 7,044 rapid-onset pixel events; 1,188 (16.9%) continued beyond 90 days, and 88 right-censored candidates were excluded from completed-event persistence. Basin means ranged from 0.004 to 0.707 events pixel⁻¹ yr⁻¹, 12.33 to 17.72 percentile points pentad⁻¹ for intensification rate, and 33.56 to 54.78 days for restricted persistence. A three-group PAM typology (13, 12, and 5 basins) revealed substantive within-country heterogeneity, although its silhouette score (0.254) and bootstrap Jaccard values (0.591–0.627) indicate an exploratory rather than definitive partition. Five basin–metric trends had raw p < 0.05, but none remained significant after false-discovery-rate correction. These results demonstrate that recurrence, onset rapidity, and subseasonal persistence are related but non-interchangeable dimensions of FD behavior.