Topographically sketched catchment areas are a spatial unit based on the shapes of the earth's surface. They show how human activities and climate change influence the available quantities of water. Knowledge of these units is fundamental to sustainable water management. However, due to underground connections, some catchment areas accumulate water from areas beyond their topographic boundaries, while others are effectively much smaller than their surface topography would suggest. Currently, most hydrological modeling strategies do not take these groundwater connections into account, but assume that the catchments are independent of their surroundings. For this reason, Dr. Yan Liu and Assistant Professor Dr. Andreas Hartmann from the Chair of Hydrological Modeling and Water Resources at the University of Freiburg, together with a team of researchers from the University of Bristol in England and Princeton University in the US, have introduced the Effective Catchment Index (ECI). Using this new metric, they were able to determine how topographic and actual catchment areas differ when analyzing a global data set. The team recently published the results in the journal Environmental Research Letters.