A short note on the natural and anthropogenic variations in the water storage changes at Visakhapatnam, Andhra Pradesh, India
We analyze the periodic and stochastic/random dynamics in the water storage changes at Visakhapatnam, Andhra Pradesh, India. We used time-variable gravity data in terms of Equivalent Water Thickness (EWT) measured from Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On satellite missions for the period 2002 to 2021 along with average rainfall data to assess the natural and anthropogenic impacts on water storage changes. We employ Wavelet Spectrum and Singular Spectrum Analysis (SSA) methods to analyze the non-stationary variation of spectral power and principal components. The water storage in the study area shows a significant positive trend with a peak correlation of 0.52 with the rainfall data at a lag of 3 months. The first, second, third, fourth, and fifth principal components depicting the monotonic trend and oscillations together contribute 69.48% to the total water storage changes. The wavelet spectrum of the SSA reconstructed signal from the first four principal components revealed non-stationary annual and 1.3 to 8 years periodicities associated with natural solar and El-Nino Southern Oscillations (ENSO) respectively. The phase plot of the residual signal of ~ 30% variance suggests the random dynamics. Thus the study suggests: (i) an increasing groundwater trend in the study area, (ii) nearly 70% of the water storage changes are linked with natural solar and ENSO variations, and (iii) 30% of water storage changes are with random dynamics possibly linked with anthropogenic activities and catastrophic climatic episodes of shorter duration at Visakhapatnam, Andhra Pradesh, India.