Evaluation of climate indices related to water resources in Iran over the past 3 decades

Climate change has significantly altered water resources globally. This study examines the effects of climate change on Iran's water resources using twelve climate indices developed by the Expert Team on Climate Change Detection and Indices (ETCCDI) from 1992 to 2022. Daily climate data were obtained from the Global Surface Summary of the Day (GSOD) database, which includes quality-controlled data from Iran's Meteorological Organization. The GSOD data undergo extensive automated quality control processes to eliminate random errors and ensure data reliability. Long-term trends were analyzed using linear regression, and spatial patterns were identified using K-means and hierarchical clustering methods. Analysis of 29 provincial capitals revealed significant trends in precipitation and temperature patterns. Temperature indices showed widespread increases, with 75.86% of cities experiencing significant rises in Mean daily mean temperature (TMm) (p < 0.05, up to 0.16 °C/year) and 96.55% showing increases in Mean daily maximum temperature (TXm). Heavy precipitation indices showed significant decreases (p < 0.05): 11.11% of cities for Annual Total Precipitation from Very Wet Days (R95pTOT) and 22.22% for Extremely Wet Days (R99pTOT). Annual Total Wet-Day Precipitation (PRCPTOT) showed spatial variation, with Rasht showing a significant increase (19.07 mm/year, p < 0.05), while Bojnourd exhibited a decrease (- 4.28 mm/year, p > 0.05). The Consecutive Dry Days (CDD) index showed a significant increase in 14.81% of the cities, with Khorramabad experiencing the highest increase of 4.4 days/year (p < 0.05). Fraction of days with above average temperature (TXgt50p) increased significantly in over 70% of the cities. Study limitations include incomplete data for Karaj and Qom, and missing precipitation records for Kermanshah and Sari. These findings reflect significant impact of climate change on Iran's climate patterns, indicating increased water scarcity. Effective water resource management and climate adaptation are crucial.