An investigation on spatial and temporal trends in frost indices in Northern Iran

In this study, the trends of frost indices were investigated using the Mann-Kendall nonparametric test at 90% probability level. Frost indices, namely number of frost days (nFDs), number of frost-free days (nFFD), last spring frost (LSF), first fall frost (FFF), and growth season length (GSL), were obtained using minimum temperatures for 12 synoptic stations in the Caspian Sea coast, northern Iran. The statistical period of data ranged from 2000 to the end of 2018. Results showed an upward trend line for the nFDs index in western parts of the study area (Guilan province and part of Mazandaran province, with a significant downtrend at Rasht station as well). The nFDs index had a downward trend line slope in the eastern parts (Golestan province and part of Mazandaran province). Results of nFDs index at Gorgan station revealed a significant upward trend for the nFDs index. The nFFD index results were contrary to those of the nFDs index. The LSF index also presented a downtrend in the western part (Guilan province). Positive trend slopes were observed for the LSF index in the eastern regions (Golestan province and most of Mazandaran province). This index showed a significant uptrend at Astara station. Thus, the results of the LSF index indicate that LSF will occur earlier in the western regions and later in the eastern regions than previous periods. The FFF index also had a downtrend in the western part (the whole Guilan province and most of Mazandaran province), and the eastern part (Golestan province and a small part of Mazandaran province) experienced an upward slope. It should be noted that the FFF index had no significant trends in the study area. The uptrend and downtrend of this index indicate later and earlier occurrence, respectively, of FFF than normal situations. Finally, the GSL index, which represents the length of plant growth period, had a downtrend in the middle areas (Mazandaran province). The western (Guilan province) and the eastern (Mazandaran province) regions also had upward slopes. Also, no significant trends were observed for this index at all the stations.