Hydrological changes and vertical crustal deformation in south India: Inference from GRACE, GPS and absolute gravity data V.M. Tiwari ⇑ , N. Srinivas, B. Singh CSIR-National Geophysical Research Institute, Hyderabad, India article info Article history: Received 17 September 2013 Received in revised form 26 February 2014 Accepted 7 March 2014 Available online xxxx Keywords: GRACE GPS Absolute gravity Hydrological change Vertical deformation abstract Monsoon rainfall over Indian subcontinent causes large hydrological changes that deform the earth on varied time scale. The seasonal hydrological mass changes are in the range of 20–50 cm of equivalent water height over southern India, which causes vertical deformation of 1–2 cm. We compare the defor- mation computed from GRACE mass signal with that of height changes from continuous GPS data from two locations in south India and find that the amplitude and phases of seasonal vertical deformation derived from both (GPS and GRACE) are consistent, indicating that hydrological effects are major cause of periodic deformation in the region. This supports the earlier deduction that GRACE data can be utilized to remove hydrological effects from GPS data. High precision absolute gravity values measured near the GPS location and groundwater levels measured in the boreholes corroborate the space based observa- tions of hydrological changes and vertical deformation. GPS and GRACE data also indicate inter-annual variation caused due to rainfall variability, signifying that hydrological effects must be removed before deriving any long term vertical deformation trend. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Indian subcontinent receives a large amount of rainfall during Indian summer monsoon season and partly during winter mon- soon period. These rainfalls are the major source of hydrological mass changes. It is now possible to determine the magnitude of re- gional hydrological mass variations from Gravity Recovery and Cli- mate Experiment (GRACE) satellite data, which further allow deriving crustal deformation due to loading and unloading (Wahr et al., 1998). Time varying gravity field models derived from GRACE satellite are extensively utilized to determine hydrological water mass variations, which are in agreement with hydrological models (Ramillien et al., 2005; Rodell et al., 2007; Tiwari et al., 2009; Tiwari et al., 2011). The GRACE derived seasonal hydrological mass changes and associated deformations compare convincingly with GPS observations (Davis et al., 2004; Bevis et al., 2005). Although, deformations derived from GRACE data do not always agree with GPS measurements (e.g. Van Dam et al., 2007); in general GRACE and GPS observations are quite consistent (e.g. Davis et al., 2012 and reference therein). GRACE-based vertical deformations are uti- lized to derive tectonic motions in active regions e.g. Himalaya (Fu and Freymueller, 2012). Previous GPS studies in the Indian shield indicate very low intraplate tectonic deformation trend (Mahesh et al., 2012). On the other hand, seasonal and inter-annual hydro- logical changes appear to dominate in the southern Indian shield region (Tiwari et al., 2011) and likely to produce seasonal and in- ter-annual vertical deformations. The Southern Indian shield is an assemblage of varied hard rock formations, like basalt, granu- lite, granite-greenstone and meta-sediments (Naqvi and Rogers, 1987). Surface water storage contributes a sizeable part of the total water storage in south India and the groundwater is stored in the upper few meters of the weathered zone, fracture and cavities of the hard rock terrain. These geological and hydrogeological situa- tions are quite different from those places where GPS and GRACE derived deformations have been compared so far.Therefore, it is interesting to compare GRACE-derived deformation with GPS data over hard rock terrain of the Indian shield region. High-precision (±1–2 micro-Gal accuracy) repeat gravity observations using FG5 Absolute Gravimeter (AG) in the study region can provide primary information on mass redistributions related to hydrological load- ing and vertical deformations (Tiwari and Hinderer, 2011; Tiwari et al., 2006). Since, AG measurements are based on high-precision length and times standards (Niebauer et al., 1995), they can pro- vide useful constraint and valuable input to vertical deformation estimates (Van Camp et al., 2011). Hence, combination of AG or other ground gravity data, and GPS data can provide estimate of mass variations which can be compared with the GRACE data (de Linage et al., 2008). In this paper, time variable gravity changes from GRACE satellite are utilized to derive the hydrological http://dx.doi.org/10.1016/j.pepi.2014.03.002 0031-9201/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +91 4027012840; fax: +91 4027171564. E-mail address: vmtiwari@ngri.res.in (V.M. Tiwari). Physics of the Earth and Planetary Interiors xxx (2014) xxx–xxx Contents lists available at ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi Please cite this article in press as: Tiwari, V.M., et al. Hydrological changes and vertical crustal deformation in south India: Inference from GRACE, GPS and absolute gravity data. Phys. Earth Planet. In. (2014), http://dx.doi.org/10.1016/j.pepi.2014.03.002