Aim: This study separates the roles of temperature and precipitation in the variability of snow water equivalent (SWE) in Iran, estimates the temperature threshold of the transition from precipitation to temperature control, and tests whether the shares of dry, warm and warm–dry snow droughts have changed.
Methods: November–April ERA5-Land fields were analysed for 4,764 snow-domain cells (0.1°) over water years 1981–2025. Detrended partial correlation and LMG relative importance quantified the independent controls, segmented regression with block bootstrap estimated the temperature sensitivity threshold, and 15-year moving windows traced the temporal transition. Snow-drought years (SWEI ≤ −0.5) were classified with the standardized precipitation and temperature indices (SPI, STI), and changes in type shares were tested with logistic regression and the Pettitt test.
Results: Snow-season temperature increased by 0.67 °C per decade and peak SWE decreased by 10.3 mm per decade, whereas precipitation showed no significant trend. Temperature dominates SWE variability in almost all cells below 2,000 m, while precipitation dominates only above 3,200 m. The temperature sensitivity of SWE increases up to a threshold of 3.6 °C [2.4–4.5 °C], and the precipitation-to-temperature transition occurs at −3.2 °C. The share of warm and warm–dry droughts rose from 45.6% in 1982–2003 to 74.1% in 2004–2025, and the odds of a drought being warm-driven increased by about 8% per year (p = 0.001), whereas dry droughts slightly declined.
Conclusion: Iran’s snow droughts have shifted from a precipitation-deficit regime to a temperature-driven regime, implying a structural, warming-related decline of mountain water storage that should be incorporated into water-resource planning.