Controls on hydrological connectivity in the Lower Oder river and its floodplain

Hydrological connectivity affects many ecological services (e.g., water storage, nutrients deposition, habitat provision) of landscape systems, especially in river-floodplain systems which generally show great variations at different spatial-temporal scales. Here, we present insights from the Oder river-floodplain system in Germany. Multiple potential contributions to flood plain inundation (i.e., from the river, rainfall, and groundwater) make this system complex and understanding the dynamics of connectivity and its controlling factors is still limited which has implication for floodplain management. We used a remote sensing data cube of harmonized Landsat and Sentinel-2 imagery to derive a temporally dense, 8-year NDWI times series to infer patterns of floodplain inundation and river-floodplain connectivity in two contrasting polders in the Lower Oder Valley National Park. Continuous wavelet transformation was used to investigate which timescale the hydrological variables present the pronounced variations. Wavelet coherency was employed to capture the factors contributing the hydrological connectivity. The upstream Polder A (14.4 km²) was extensively flooded for prolonged periods most winters and its strong seasonality was primarily driven by winter water levels in the river Oder (through 2 floodgates). Inundation of the downstream Polder 10 (17.7km²) was lower and had less marked seasonality, which reflected the impact of flood attenuation by storage in Polder A upstream, but also the greater connectivity (via 10 flood gates) to the Oder and a functional network of channels which facilitated rapid drainage. In Polder A, secondary periods of transient inundation could also occur in response to local intense summer rainfall, this was less evident in Polder 10. Groundwater recharge in and around Polder A is primarily induced by floodwater, whilst Polder 10 also reflects the influence of local rainfall-driven recharge. The hydrological connectivity regimes of the two polders showed marked inter-annual variation, largely dependent on flows from the upper Oder catchment. Understanding the hydrological connectivity in this system is important for sustaining and managing valuable wetland habitats within the National Park. Given projected climate change for this region and possible management alterations to the flow regime of the Oder, potential implications for these habitats needs urgent attention.