Spatiotemporal Dynamics of the Active Perirheic Zone in a Natural Wetland Floodplain

The ecologically and biogeochemically significant perirheic zone is a part of the floodplain where waters originating from both the river and adjacent floodplain are present. In this study, we investigate the spatiotemporal dynamics of the active perirheic zone, that is, river and floodplain waters and their transient mixing extent. This is achieved by using the Hydraulic Mixing-Cell method, a complement to a fully integrated surface-subsurface hydrological model, in the Biebrza catchment, northeast Poland. The model performance is verified against hydrological and hydrochemical data sets. The simulations show that overbank flooding river water is unable to penetrate deeply into the floodplain due to the presence of floodplain water. However, the active perirheic zone moves toward the floodplain and back within a buffer of at least 1 km from its initial position located approximately 0 to 2.5 km from the Biebrza River. The active perirheic zone is also present further away from the river due to the discharge of tributaries and surface runoff. The active perirheic zone exhibits multidirectional movement and can reappear in different places after a period of time. Effectively, during the flooding period, the active perirheic zone moves over 38% of the floodplain area, while the maximum daily extent is 24% of the floodplain. These dynamics imply that biogeochemical processes related to the perirheic zone, for example, denitrification, also vary in space and time. Due to the strong correlation of the perirheic zone extent with the meteorologically dependent variables, especially the snowmelt water extent, it is vulnerable to climate change.