Inelastic and elastic storage properties and daily hydraulic head estimates from continuous global positioning system (GPS) measurements in northern Iran

This study uses land measurements from the continuous global positioning system (GPS) covering the Gorgan Plain in northern Iran, over the period 2006-2016. The measurements show long-term displacements of about 84 cm downward, 17 cm northward, and 26 cm eastward relative to the International Terrestrial Reference Frame, and seasonal amplitudes of 25.19, 2.23, and 0.81 mm along vertical, north-south, and east-west directions, respectively, in the Gorgan confined-aquifer system (GCAS). Results indicate that the periodic signals in both vertical and horizontal displacements are consistent with seasonal head fluctuations in the GCAS, but the changes in horizontal components appear much later compared to the vertical component. The inelastic skeletal storativity is computed from GPS measurements to be as much as 12 times greater than the elastic value. Moreover, the daily hydraulic heads reconstructed from well and GPS data jointly are consistent with head measurements from piezometers. Furthermore, the irreversible component of groundwater-storage (GWS) variations incorporated from dewatering of compacted aquitards (0.62 m(3)/m(2)) is computed to be 9.5 times greater than its recoverable component associated with aquifer units (0.0655 m(3)/m(2)). This study indicates that quantification of inelastic storativity and irreversible GWS in those aquifers of significant subsidence is essential for sustainable water resources management.