Pre-whitened wavelet analysis to evaluate the relationship between environmental factors and groundwater responses at a delta plain in the downstream region of Nackdong river Basin, South Korea

Groundwater variation in a delta plain at estuarine rivers is important for agricultural productivity and the development of a suburban area. A time–frequency analysis is necessary for better understanding the causal relationship between groundwater level and salinity and hydrological and meteorological conditions. The conventional wave analysis requires further development to properly handle the common driver if two time series share identical stochastic structures. As part of this study, we developed an improved wavelet coherence analysis to delineate causality configurations using a pre-whitening scheme. Residual series of river water and groundwater level were obtained by using the structure of a time series model for air pressure. Pre-whitening wavelet analysis removed the majority of common coherences between river water level and groundwater responses within 6 h. Additionally, substantial air pressure wavelet coherences (between 10–100%) were eliminated by the proposed method for time intervals exceeding 6 h. As a result of applying the proposed method to groundwater level and salinity responses, it has been demonstrated that eliminating the stochastic structure can help identify time-dependent impacts of extreme events, as well as improve our understanding of coherence relationships across a wide range of time and frequency scales.