Comparing different field methods to quantify surface-groundwater interaction

Surface-groundwater interaction is an important link between hydrology and hydrogeology and can contribute considerably to groundwater recharge. However, quantification and continuous observation of water flows is challenging in practice. This may explain why surface-groundwater interaction is disregarded in many hydrogeological models so far.     
In this work we test several field methods in three medium-sized streams (average discharge at outlet: 1.9-10.9 m³/s) close to the city of Freiburg, south-west Germany. We subdivide the streams into sections and monitor gains and losses by a combination of different methods. Continuous discharge data is obtained by capacitance water level recorders combined with repeated runoff measurements by electromagnetic current meter. As an alternative, we apply particle tracking algorithms to drone footage and compute surface velocities and discharge volumes. Here we also analyse different types of seeding material. Additionally, thermal drone images show surface temperature anomalies which we combine with discharge measurements to estimate groundwater intrusion. Our first data shows that continuous data collection under field conditions is challenging and can suffer from drawbacks such as flooding or tree fall. We therefore recommend redundant methods. Discharge measurements via electromagnetic current meter are generally robust but limited to medium flow conditions. Here, remote sensing via drones can provide labour-efficient alternatives. With their help, discharge measurements are possible also during high flow periods but in turn limited to areas with little or no tree cover, whereas thermal imagery is more efficient during low flow periods. Then it is well suited to locate point sources of groundwater inflow, particularly during times of strong temperature gradients between rivers and aquifers in summer or winter. Quantification of these inflows remains uncertain, though.