Seasonal and trend analysis of TWS for the Indo-Gangetic plain using GRACE data

The GRACE satellite mission data is used to study Earth's mass variability on the Earth surface. The major component to the surface mass variability is changing terrestrial water storage (TWS). The GRACE mission data (level 3, release 5, i.e. RL05, time span January 2003 to December 2014) from the three agencies, i.e. CSR, GFZ and JPL, are used to analyse the TWS dynamics for the Indo-Gangetic plain which is one of the most water-stressed regions in the world. The dynamics of TWS is expected to have periodic behaviour due to the seasonal pattern. Because of the vast study area, significant seasonal pattern changes are prevalent in the region. Hence, the entire study area was classified on the basis of seasonal behaviour. For the classification of the study area, a Fast Fourier Transform (FFT) was applied to get the dominant frequency which represents the seasonality. The intensity of most dominant frequency varies spatially from a grid to grid and it actually increases from the upper Indo-Gangetic plain to the lower Indo-Gangetic plain. This intensity variation was used to classify the study area. Further, trend analysis and time series models were established for each classified region. The trend analysis shows that the TWS is depleting at the rate of (3.91 +/- 0.23) cm/month for the region near the Himalayan foot plain, Delhi and Haryana. For the other classified regions, the rate of depletion is (2.75 +/- 0.55) cm/month and(2.58 +/- 0.56)cm/month. The trend analysis further identified a major drought that hit the country (i.e. India) in the year 2009.