Monitoring the Meteorological and Hydrological Drought Trend in Tajan watershed, Mazandaran province

Drought as a climatic phenomenon is considered as one of the most important natural hazards, which is known for its slow onset and may have multiple effects on hydrology, agriculture and socio-economic conditions. Drought monitoring includes its severity, spatial distribution, and duration, and becomes an essential input for establishing drought risk management and mitigation plans. Many drought indicators have been introduced and applied in recent decades in regions with different climatic characteristics.

In this study, the meteorological and hydrological drought of Tajan watershed in Mazandaran province was evaluated and monitored using standardized precipitation index (SPI) and standardized streamflow index (SSI). For this purpose, firstly, the data of six stations Rain gauges and eight hydrometric stations were used in the period of 1990-2020, after removing the statistical deficiencies and completing the data of rainfall, flow rate, homogeneity and normality of the data. Finally, the meteorological and hydrological drought process was analyzed with the mentioned two indicators using Mann-Kendall test and Sen slope estimator in MAKESENS software.

The results showed that the SPI index has the highest response to rainfall on a one-month scale, so that its lowest value was at Kordkheil station with a value of -3.31, the most severe drought that occurred in April 2011. On the other hand, droughts have occurred in all rain gauge stations and their intensity has increased over time. According to the SSI index, hydrological drought has also occurred in all hydrometric stations with different intensities. The trend of these indicators also shows the increase in intensity and frequency of drought with the passage of time in the studied watershed.

In Darabkola and Kordkheil stations, the SPI index has the highest number of years of occurrence and the highest intensity of drought. In Darabkola station, the frequency of moderate, severe and very severe droughts are relatively high. Very, very severe drought in Edareh Sari station has the highest amount in terms of frequency, which indicates that there is the greatest risk of drought occurrence in the plains and plains of the basin. The results of the trend with the Mann-Kendall test indicate an increase in drought and a decrease in rainfall in the Tajan watershed. Due to climate changes, increase in demand due to increase in population and decrease in rainfall, the occurrence of drought in the future may occur with more frequency and intensity, which can cause great damage to agriculture and other parts of the region. The SSI index in hydrometric stations indicates the decrease in flow and the occurrence of hydrological drought in the basin.