Terrestrial water storage (TWS) is the total surface and subsurface water storage. It is difficult to measure terrestrial water storage changes (TWSC) over large scale areas, as there are different sources to measure and each source may faces lack of information. In-situ measurements are more accurate than any other method to determine different water sources storage changes but these methods cost lots of energy, time and money specially over large scale areas. Gravity Recovery and Climate Experiment (GRACE) project has been providing accurate TWSC data for all over the world since 2002 and these data have been used in many hydrological researches. The history of these researches proves that the GRACE is a reliable tool to estimate TWSC in different large scale areas around the world, thus it can be used in researches over Iran’s major river basins. This thesis main goal is to assess the GRACE accuracy based on correlation between in-situ and GRACE derived seasonal groundwater storage changes for Iran’s major river basins and also for Iran, using JPL-MASCONS RL06 as GRACE data and data derived from almost 10000 piezometers over study areas as in-situ data. The period of the study is 168 months, from October 2002 to September 2016. For estimating GRACE derived groundwater storage changes, soil moisture, snow equivalent water and surface water storage changes are removed from TWSC. Soil moisture and snow equivalent water values are derived from Global Land Data Assimilation System (GLDAS), also observed storage changes of dams are assumed as surface water changes. Obtained correlation coefficients (R) in order based on basin’s number for Iran’s six major basins are 0.70, 0.67, 0.86, 0.75, 0.19, 0.68 and obtained R for Iran is 0.79. The results show high correlation between in-situ and GRACE derived seasonal groundwater storage changes for 5 out of 6 river basins and also for Iran. In the next part of study effects of precipitation and consumption on groundwater storage changes have been illustrated as charts and based on results, the main reason for groundwater storage changes is the amount of consumption and precipitation doesn’t affect significantly on groundwater storage changes specially in large scale areas however some of the changes in next seasons should be considered as effect of precipitation in past seasons but it is not significant.
