Lake Urmia is the second largest saltwater lake in the world, and it plays an important role in the agricultural area in northwestern Iran. The Urmia lake basin has influenced by climate change, specifically the rise in temperatures in the last two decades. Thermal growing season indices are among the most important indicators for analyzing the reactions of vegetation to climate change. Thermal growing season, theoretically speaking, is a time for plants in which they can grow, and it is often defined based on a temperature threshold (which, in this study, is 5 ºC). Partaking of modified non-parametric Mann-Kendall test, the present research investigated the trends in thermal growing season indices in the Urmia lake basin during three decades (1996-2025). In the forecast period (2016-2025), the temperature outputs of the RegCM model under scenario RCP4.5, coupled with the CLM4.5 level and the single-dimensional lake model, were used after the proper bias correction. Non-parametric tests such as the Buishand range test, Buishand’s U, and the Pettitt test were conducted to determine change points. The results of the study indicate significant trends in the early ending of growing season and the shortening of the growing season in the northeastern and central parts of the basin. These results also suggest that both indices have significant change points in the central part of the basin in 2014 and the northeastern part of the basin in 2013. No significant trend was observed at the beginning of the growing season in the whole basin.
