Introduction: The occurrence of drought is a key characteristic of Iran's climate, observed in both humid and arid regions; its severity varies depending on shifts in climatic conditions relative to expected norms. Drought affects precipitation-derived moisture; soil moisture falls below the crop's actual requirements, resulting in crop damage. Hydrologists define drought as the condition where surface and groundwater levels fall below normal; the onset of drought leads to water shortages for human needs, resulting in socio-economic disruptionsFrom a climatological perspective, drought is a natural phenomenon that arises spontaneously due to shifts in weather patterns—specifically, a reduction in precipitation below normal levels—and its persistence leads to ecological and hydrological imbalances. Soil moisture responds to precipitation anomalies over the short term, whereas the response of groundwater, river flow, and water resource reserves to precipitation deficits is a long-term process. Thus, drought can be quantified; this method allows for the determination of the severity of both drought and wet periods. Sometimes, a brief drought lasting only a few weeks causes serious damage, while at other times, a prolonged drought can result in extensive harm. This indicates that the risk of drought-related losses can be significantly reduced through proper planning for water demands.

Materials and Methods: The calculation of the Standardized Precipitation Index for a given region is based on long-term precipitation data recorded over a selected period. The standardized precipitation for any given region and period has a mean of zero and a standard deviation of one; this index can also be used to identify wet periods. A drought is considered severe when the Standardized Precipitation Index (SPI) is -1 or lower, and the drought event concludes when the SPI becomes positive. Therefore, each drought event has a duration defined by its start and end, as well as the intensity for each month the event persisted. SPI series indicate that precipitation deficits have varying impacts on water resources. Data spanning thirty years from 13 stations in Gilan Province were used to calculate the Standardized Precipitation Index (SPI) and analyze dry and wet periods across 3-, 6-, 9-, 12-, 18-, 24-, and 36-month timescales. Furthermore, a Geographic Information System (GIS) was utilized for the spatial analysis of data and the validation of the obtained results. Subsequently, four stations were selected based on climatic similarity using cluster analysis, and estimates regarding the dry period were presented.

Discussion: Consequently, over short time scales (three-month periods), the Standardized Precipitation Index (SPI) values exhibit significant fluctuations; in other words, short-term dry and wet periods lack persistence and are heavily influenced by daily precipitation, whereas longer time scales (24- and 36-month intervals) show a much slower response to changes in precipitation. Furthermore, the frequency of long-term dry periods is significantly lower compared to 3- and 6-month time intervals. Based on the selected years and the Standardized Precipitation Index (SPI) for each station, maps showing the spatial distribution (via interpolation) of the index were calculated and plotted for each of the SPI periods under study. An analysis of maps derived from the SPI for 12-, 24-, and 36-month periods indicates that the highest levels of drought occur in the central to eastern parts of the province.

Conclusion: Consequently, the frequency and magnitude of fluctuations, as well as the intensity and duration of drought, are lower in short-term periods than in long-term periods. Following Manjil and Lahijan—which experienced the most severe 30-month dry spell (with a frequency of 3)—Qaleh Rudkhan exhibited the highest drought severity in the province, recording a 36-month dry spell with a frequency of 4. Maps illustrating the most severe 12-, 24-, and 36-month dry spells indicate that these conditions primarily affected the central to eastern regions. In fact, the plain regions—which receive the highest average rainfall and where agriculture is of great importance—are significantly affected by drought.

Objectives: Drought is considered a key characteristic of Iran's climate; it occurs in both humid and arid regions and can vary in severity depending on climatic diversity.The Standardized Precipitation Index (SPI) method was used to determine the severity and frequency of dry periods.A drought is considered severe when the Standardized Precipitation Index (SPI) is -1 or lower, and the drought event concludes if the SPI becomes positive. The SPI indicates that a precipitation deficit has varying impacts on water resources.