Yan Liu , Thorsten Wagener, Hylke E. Beck, Andreas Hartmann 

Linking human activities and climate change with their consequences for water availability is a prerequisite for sustainable water management, which is traditionally performed at topographically delineated catchments. However, inter-catchment groundwater flow results in effective catchment sizes other than sizes suggested by topography. Here, we introduce the notion of effective catchment size and analyze the relative prevalence of substantial differences between topographic and effective catchment sizes using a large global dataset. We find that 1 in 3 catchments exhibit an effective catchment size either twice as large or half as small as its topographic size. 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 observed differences are strongly linked to aridity, slope, distance to coast, and topographic size. We show that our findings also hold for other large catchment databases, e.g., GRDC, CAMELS-US, and MOPEX. Our study provides a first-order identification of catchments where additional in-depth analysis of subsurface connectivity is needed to achieve sustainable water management.