The Study of the Periods of Drought and its Impact on Groundwater Resources of the Gorgan Roud River Basin

IntroductionDrought, with its gradual, tranquil, and crawling occurrence, is one of the most important natural disasters that affects various aspects of human life. This phenomenon, as a disastrous climatic phenomenon, directly affects communities through changes in their access to water resources. One of the most important effects of drought on water resources is the reduction and drop in groundwater aquifers and the decrease in river flow rates. The hydrological drought, with the effects of periods of atmospheric drops, affects the sources of groundwater or surface water supplies such as river flows, reservoirs, lakes, and groundwater. Therefore, the effect of the rainfall shortage on the components of the hydrological system such as soil moisture, river flow, surface of reservoirs, and groundwater is seen after a long time. Climatological drought with a time lag in one place leads to a hydrological drought which, consequently, leads to water stress. Determining the starting and ending dates of droughts, their severity, continuity, spatial distribution, assessment, and quantification is one of the most important issues in the study area. Accordingly, the main objective of this research was to determine the extent of the continuity of meteorological and hydrological droughts and the relationship between them.
MethodologyThe Gorganroud River watershed forms 48% of Golestan Province with an area of ​​11393.1 km2. It is located in the geographical range of '36 ° 36 'to 37 ° 37' the northern latitude and '00 ° 54 'to '29 ° 56 the eastern altitude. It is located in the national scale of the Gorgan River basin in the north of the country. From the south east to the eastern Alborz, from the east to mount Aladagh and mount Glydiyah, from the north to the Atrak basin, and from the west to the Caspian Sea and the Gharasso basin. The Gorganroud River has 17 main branches that are connected in different parts and, ultimately, flood the Caspian Sea. The basin is used as a forest in the south and east, but in the north and west, alluvial plains are exploited in agriculture and pasture. In order to study droughts, the standardized precipitation index (SPI) and standardized water-level index (SWI) were used. The data used in this study was extracted from 16 meteorological stations and 31 piezometric wells, with a common statistical period of 30 years (1362-1392). To analyze the drought's trend, seven scales of 1, 3, 6, 12, 24, and 48 were used in a monthly and annual scales. In this study, to reconstruct the statistical errors and homogenize the data, acorrelation and normal ratio methods were used. Then, the SPI and SWI indices and the quantitative analysis of droughts of basin were used to evaluate the trend of rainfall and underground water in different spatial and temporal scales.
ResultAccording to the calculations and checking the map of the annual extent of meteorological drought, the western and eastern regions of the basin were affected by drought more than other regions. In addition, according to the map of the annual extent of the groundwater droughts, the southwestern, western, and northern parts were affected by drought more than other regions. Considering the duration of the meteorological drought, the northeastern, western, and southwestern parts had longer durations than other regions. Considering the duration of the groundwater drought, the northern, southwestern, and central parts of the basin had the longest duration.