What is the hydrologically effective area of a catchment?

Connecting human activities and climate change with their consequences for water availability is a prerequisite for sustainable water management, which is traditionally performed in topographically delineated catchments. However, inter-catchment groundwater flow results in effective catchment areas different from the areas suggested by surface topography. Here, we introduce the notion of effective catchment area quantified through an Effective Catchment Index (ECI), to understand the prevalence and significance of substantial differences between topographic and effective catchment areas in a global dataset. We validate our ECI analysis using hydraulic head simulations of a global groundwater flow model and the Budyko framework. We find that 1 in 3 catchments exhibit an effective catchment area either larger than double or smaller than half of its topographic area. These catchments will likely be affected by management activities such as groundwater pumping or land use change outside their topographic boundaries. Or they alternatively affect water resources beyond their topographic boundaries. The magnitude of the observed differences is strongly linked to aridity, mean slope, distance to coast, and topographic area. We show that our findings also hold for other large catchment databases. Our study provides a first-order identification of catchments where additional in-depth analysis of subsurface connectivity is needed for sustainable water management.