Natural and anthropogenic drivers of the water table dynamics in a riparian fen peatland

Riparian fens are peatlands that are fed by precipitation, groundwater, and surrounding surface water bodies. They can therefore be influenced by meteorological conditions, emphasised by global warming, and anthropogenic constraints such as flow regulation of the nearby rivers. In this paper, field monitoring and numerical modelling were used to identify the main drivers of water table dynamics at the site scale in a riparian fen peatland located along a regulated river. To this end, water table levels were recorded during three years in a riparian fen located in the alluvial plain of the Essonne River in France. A 2D transect hydrological model was built using the physically-based code HydroGeoSphere. It was composed of two superimposed soil layers identified in undisturbed soil cores taken from two distinct locations. Laboratory and field experiments revealed contrasting properties between these two layers, with amore decomposed peat layer at the surface (one metre deep) showing higher hydraulic conductivity compared to the deeper, more organic layer. Both measured and simulated results showed that the strong fluctuations of the water table in the peatland were mainly due to the seasonal life cycle of the vegetation. The water supply to the riparian peatland from the nearby river was essential when the vegetation was active by limiting the water table drawdown during very dry periods. Modelling results highlighted the importance of the river stage on the amplitude of the water table since a 26 cm drop led to a decrease of up to 38 cm of the piezometric levels. Both data and simulations also proved that the precipitation seasonal distribution plays amore critical role than the total annual precipitation on the water table depth, particularly in summer. This paper demonstrates the importance of precipitation seasonality and river hydraulic regime on the sustainability of a riparian peatland.