Large-scale variations of global groundwater from satellite gravimetry and hydrological models, 2002–2012

Groundwater storage is an important parameter in water resource management, land-surface processes and hydrological cycle. However, the traditional instruments are very difficult to monitor global groundwater storage variations due to high cost and strong labor intensity. In this paper, the global total terrestrial water storage (TWS) is derived from approximately 10 years of monthly geopotential coefficients from GRACE observations (2002 August–2012 April), and the groundwater storage is then obtained by subtracting the surface water, soil moisture, snow, ice and canopy water from the hydrological models GLDAS (Global Land Data Assimilation System) and WGHM (WaterGAP Global Hydrology Model). The seasonal, secular and acceleration variations of global groundwater storage are investigated from about 10 years of monthly groundwater time series. Annual and semiannual amplitudes of GRACE–WGHM and GRACE–GLDAS are almost similar, while WGHM groundwater results are much smaller. The larger annual amplitude of groundwater variations can be up to 80 mm, e.g., in Amazon and Zambezi Basins, and the smaller annual amplitude of groundwater variations is less than 10 mm, e.g., in Northern Africa with larger deserts. The annual and semi-annual phases agree remarkably well for three independent results. In the most parts of the world, the groundwater reaches the maximum in September–October each year and the minimum in March–April. The mean trend and acceleration of global groundwater storage variations are 1.86 mm/y and − 0.28 mm/y2 from GRACE–GLDAS, and 1.20 mm/y and − 0.18 mm/y2 from GRACE–WGHM, respectively, while the WGHM model underestimates the trend and acceleration. Meanwhile the GRACE–GLDAS is generally closer to in-situ observations in Illinois and satellite altimetry. Therefore, the GRACE–GLDAS provides the relatively reliable data set of global groundwater storage, which enables to detect large-scale variations of global groundwater storage.