3.1 GRACE data sets
In the present study, GRACE is used to evaluate the variations in groundwater storage (satellite based approach). GRACE mission was started on 17th march 2002 and it was a collective work of German Aerospace Center (GAC) and National Aeronautics and Space Administration (NASA). The main objective of the mission is tracking of water from the space. The GRACE satellites functioning mainly depend on the gravity variations of the earth. The gravity is not constant everywhere on the globe. Gravity of the globe varies with the position. It is high at poles and low at the equator. It varies from place to place very slightly. In this mission, two satellites are used to detect the gravity anomalies. These two space crafts are 220 km apart from each other and 500 km apart from the earth. These two are connected with the K-Band microwave ranging system.
The two satellites experience the gravity pull of the various places of globe. Due to this gravitation the distance between them changes which are stored as gravity anomalies data. Here is the brief explanation of the working principle. For example, if these two satellites approaching the huge mass of mountains, the first satellite experiences the gravity pull of mountains. Due to this the distance between them increases. After some time, the both satellites experience the gravity pull of the mountains (the mountains position is exactly between the satellites). Due to this the distance between them decreases. After some time, the trailing satellite will experience the gravity pull. Due to this, the distance between them again increases. The ranking system perceives very small distance changes occur between the two spacecraft. It can sense the changes as small as 10 micrometers despite of the distance between them is 220 km. These two space crafts are connected to the GPS such that scientists identify the position of them over the globe. The two satellites experience the gravity pull of the various places of globe. Due to this gravitation the distance between them variations which are stored as gravity anomalies data.
The satellites cover an area of ~160,000 km2 and observe a specific place once in a month (Rodell et al. 2009). The global water includes surface water (reservoirs), snow, soil moisture and groundwater. In the current study we neglect snow and surface water. Because there are no snowfall records in the study area and reservoir in the study site have very less area such that the satellites cannot track. The data is collected as terrestrial water storing anomalies with respect to the space and time i.e.in the specified time and at the specified place, the global water storing variations get collected. The terrestrial water storing variations data are accessible from this link http://gracetellus.jpl.nasa.gov/data/. From that web page I downloaded RL05 monthly mass grids of land. There are CSR, JPL and GFZ data sets of TWS available. The data is for 10×10pixels and relative to average time baseline from January 2004 to December 2009 (information about this time mean baseline explained in GRACE data download page). The arithmetic mean of these three data sets gives accurate values of terrestrial water storing variations (Sakumura et al. 2014). To minimize signal error scaling coefficients should be multiplied to the obtained data. The downloaded TWS variations are given as equivalent thickness of water. All the data sets processed through software Panoply 4.10.3.
Equivalent thickness of water: The gravity field variations observed relative to the time mean baseline is converted to surface mass variations in the particular area (kg/m2). Most of the time variable variations are related to water. They divided this mass change with water density (1000 kg/m3). The output is called as Equivalent depth or thickness of water (cm or m).