<p>Interactions between groundwater and surface water are of crucial importance for the ecological functioning of wetland systems since they control groundwater levels in the wetlands, water temperature in the river and exchange of solutes. Anthropogenic impacts such as the construction of drainage systems in the vicinity of wetlands can completely change the magnitude and direction of groundwater – surface water exchange, often negatively affecting the ecological functioning of the wetlands.</p><p>Management practices aiming to conserve the ecological status strongly depend on estimates of groundwater – surface water exchange. However, currently established methods to estimate groundwater flow (i) rely on point measurements, missing the effect of crucial short term events (e.g. precipitation), (ii) rely on differences in physical characteristics between the groundwater and surface water (e.g. temperature and/or conductivity), which are not always present or (iii) require extensive modelling.</p><p>In this presentation, we present a newly developed sensor, the iFLUX sensor. Two versions of this sensor exist, for measuring horizontal and vertical flow, respectively. The sensor probe for horizontal flow consists of two bidirectional flow sensors that are superimposed and is installed in a monitoring well with dedicated pre-pack filter, allowing for measurement of both groundwater flux magnitude and direction. The probe measuring vertical flow can be installed directly in the soil, in the riverbed or in a monitoring well. With a broad measuring range of groundwater fluxes from 0.5 cm/day to 2000 cm/day and measurements every second, this setup can map rapidly changing flow conditions.</p><p>Here, we show a selection of results from a case study in North-East Poland. In the Biebrza National Park, high groundwater levels resulting from subsurface runoff from the uplands protect the highly valuable peatland system. During most of the year, the river is gaining, with a sharp increase in upward groundwater flux in the hyporheic zone during summer months. In the valley surrounding the river, groundwater flows towards the river, as expected. However, the data show a remarkable diurnal pattern of both flow magnitude and direction, with the highest flow velocity occurring in the late afternoon, suggesting a relation with evapotranspiration. After large precipitation events, the flow direction reverses, suggesting infiltration of surface water into the aquifer.</p><p>Since these events occur on a small temporal scale, they were never measured before in the area with traditional methods. As such, our sensors provide new insights in groundwater – surface water interactions and will become an invaluable tool in ecohydrological studies worldwide, ultimately leading to more integrated management strategies to protect our remaining wetlands.</p>