Weakening Response of Groundwater to Rainfall Variability: A Case Study of Ganga Basin

Abstract:

The Indo-Gangetic plain is the most groundwater stressed region in the world. In the light of changing characteristics of rainfall frequency and intensity, there is possibility of altered relation between precipitation and groundwater dynamics. The objective of the present study is to develop a framework to determine whether the relationship between rainfall and groundwater is weakening over the past few decades and identify the underlying spatio-temporal patterns over different hydro-geological formations. The rainfall and groundwater data for Ganga Basin are obtained from Indian Meteorological Department (0.25° X 0.25°) and Central Groundwater Board of India, respectively. Advanced techniques of Singular Value Decomposition (SVD) and Cross Wavelet Transform (XWT) are used to analyze the exponentially decaying effect of rainfall on groundwater levels. Nearly 14% wells show significant declining trend (1996-2016) in shallow water table during the monsoon season. The decline in 64% of these wells is not related to rainfall fluctuations. Similarly, for post-monsoon kharif season, decline in 86% of the wells has no influence of precipitation. The rate of decline ranges from 0.01-3.45 m yr^-1 as against rise of 0.02-0.6 m yr^-1 in some wells. Four spatial patterns of relationship between rainfall and groundwater are identified based on the first major SVD mode. These patterns show a weakening correlation in most of the areas and even negative in some. Contrasting response of groundwater levels to rainfall is observed in the western, and in central and eastern parts of the basin. The temporal patterns reveal a lag or lead of rainfall variations with respect to groundwater level fluctuations. For some wells, on an average groundwater levels are out of phase with precipitation. Also, the presence of human factors such as urbanization, pumping, and urban heat island effect overshadow the role of water table depth on groundwater response. Thus, our study outlines distinct regions in the basin where human influence can be investigated further.