Groundwater Discharge to Streams: Baseflow Estimation in the Middle Magdalena River and its Tributaries, Drivers and Human Impacts

Baseflow is defined as the groundwater contribution to any streamflow and it can be considered the largest interaction between surface water and groundwater. It is the main reason for streamflow during dry seasons and it also contributes to nutrient transport from porous media to the streamflow. In this research we aimed to identify baseflow in different parts of the Middle Magdalena river and some of its more important tributaries using different analytical techniques found in literature, estimating the volume of water entering the river from the subsurface. Then, baseflow was compared with the ONI index to assess the influence of climatic variability on the study area, with the area's geomorphology, topography and land cover to evaluate the impact of these features in different baseflow scenarios. Our results show that geological settings, topographical features, and human activity influence baseflow to different degrees. In particular, a dam construction can be evidenced not only by the flow normalization but by the difference between baseflow index values before and after its construction. In addition, baseflow tends to be more constant as the stream size grows, showing that some of the analytical techniques used consider as baseflow some of the long time runoffs in a large basin. Finally, the study of baseflow is an important tool to assess river corridor water availability in dry seasons, help groundwater modelers to estimate large scale groundwater-surface water interactions and enhace our understanding of rivers as complex systems.