investigation of climate variability impacts on multilayer aquifers (case study: gorgan plain)

in many arid and semi-arid regions, groundwater resources exploitation is inevitable to meet water demands. this paper aims to study the response of phreatic and confined aquifers in a multilayer groundwater system to climate variability. to achieve this aim, gorgan plain multilayer aquifer has been investigated using different meteorological and hydrological drought indices (di, pni, spi and gri) and the correlation analysis method. 19 rain gauge stations, 19 observation wells and 19 piezometric wells data with the time period of 38, 38 and 14 years, respectively, has been used. the spi showed better performance in monitoring of climate variability and detecting drought events, so it used to compare with gri. also, the results indicate difference between aquifers response to meteorological droughts. in this case, long-term 18-month and 48-month spi indices are the most correlated parameters to the groundwater in both phreatic and confined aquifers and the effect of fluctuations in rainfall remained up to 15 months and 5 months in each zone, respectively. this reflects this fact that, if the principles of sustainable exploitation be followed, confined aquifer has a high reliability andonly when prolonged drought or wet condition occurs, it could be affected.