Dynamics of water table depths in forested wetlands under temperate climate conditions using multi-frequential periodogram analysis and wavelet analysis

The temporal variation of water-table depths (WTD) of wetlands has been acknowledged as their hydrological signature; it integrates the different inputs and outputs of their water budget and provides insights into their seasonal hydrological services at the watershed scale. In this study, we monitored the WTD fluctuations of six forested wetlands in the St. Charles River watershed, Quebec, Canada, from 2019 to 2022. A factor analysis of mixed data and a hierarchical classification of the principal components were performed to first characterize the wetlands of the study area and to guide the selection and in-situ monitoring of a few. Those selected were instrumented with wells equipped with a water level data logger, recording hourly total pressure and temperature. We applied a multi-frequential periodogram analysis to the observed time series to extract periodicities. The time series were also transformed using the Morlet wavelet, and WTD interactions with precipitations and lateral stream flows was explored using cross-wavelet analysis. Our results show that hydroconnectivity represents a good criterion for characterizing the diversity of wetlands at the watershed scale, as reflected in the distinct WTD patterns of isolated and riparian wetlands. Using wetland scale observed data, we also demonstrate the cascading effect between precipitation, wetland WTD increase, and lateral inflow to the river network, which illustrates the damping effect of these ecosystems on the transfer from precipitation to stream flows at local scale. These new insights provide guidance for improving the simulation of wetlands in hydrological models.