Temporal changes in meteorological-hydrological drought (Case study: Guilan Province)

Drought is known as one of the environmental phenomena and it is also considered as a direct impact of climatic changes that can occur in any geographical area. 

In this study, monthly and annual rainfall data of 17 rain gauge stations belonging to the Meteorological Organization from 1972 to 2017 in Guilan Province have been used to investigate the severity, duration, and extent of drought. To evaluate and compare the capabilities of meteorological and hydrological drought, the standardized precipitation index (SPI) and streamflow drought index (SDI) were calculated (Lui et al., 2012) for the whole province in the periods of three, six, nine, 12-month, and annual scale depending on the capability of the index, severity, and duration of drought spells. 

To investigate the extent of drought, zoning maps were prepared using the inverse distance weighting method. According to statistics and information obtained from a statistical period from 1972 to 2017, the amount of the three-month SPI index is determined as a wet period. The results of drought severity zoning in Guilan Province showed that the SPI values in six, nine, and 12-month time scales were categorized as dry periods. In most of the cities of the study province, the drought was observed with the highest probability and had positive changes in the area of drought extent. 

The maps obtained from SPI interpolating in four statistical periods showed that the value of standardized precipitation index has increased in 2017 compared with 1972, and most cities in the studied province are covered by more drought-prone areas. In addition, the main drought in the region is observed in the meteorological stations located in the south and southeast of the region. According to the SDI values, it can be concluded that most of the river gauge stations are without or with moderate drought.