Assessing floodplain porosity for accurate quantification of water retention capacity of near-natural riparian ecosystems—A case study of the Lower Biebrza Basin, Poland

• Wetland porosity (ε) is an overland fraction of volume which can be filled with water.
• ε depends on morphology of vegetation and flooding depth.
• We assessed ε of the most common vegetation types of a temperate riparian wetland.
• Assessed values of ε reached from 0.882 to 0.993 (average 0.968).
• ε in a sample flood induced reduction of floodplain water retention by 0.75 mn m3.

Near-natural lowland floodplains, which nearly disappeared in temperate climates due to the vast anthropopression, play a critically important role in shaping biodiversity in regional scales. The other important aspects of floodplains such as their role in flood mitigation for downstream reaches of rivers, catchment-scale ecosystem services, and agriculture emphasize the need for research of these ecosystems, with special focus on the flood-vegetation interactions. In our study we performed an experiment oriented at the accurate quantification of water retention capacity of the densely vegetated floodplain located in NE Poland, referring to the floodplain porosity (ε). We conducted measurements of the geometry of selected types of the floodplain vegetation, with a special focus on the communities dominated by the common reed (Phragmites australis (Cav.) Trin. ex Steud.), the reed-manna grass (Glyceria maxima (Hartm.) Holumb.) and dominant representatives of loosely structured sedges (Carex acuta L.) and of tussock sedges (Carex appropinquata Schumach.)., which allowed us to derive the volume of these species and eventually the floodplain porosity coefficients in a function of various flood depths. Estimated values of ε equalled from 0.882 to 0.993. Average value of ε for all vegetation communities analysed for all flood depths considered equalled 0.968. Consideration of the derived ε values in the case study on the floodplain water retention capacity estimation in hypothetical conditions of a standard flood (2-year recurrence interval) resulted in the reduction of the total water storage volume of the floodplain by 0.75 mn m3 of water. Results of our research indicated that the studies oriented at quantification of water storage within the floodplain as an asset for ecosystem services should consider floodplain porosity due to its significance in the accurate estimation of floodplain capacity.