Are catchments leaky?

Catchments, generally understood as the drainage areas of low-order streams, are often regarded as closed hydrologic entities; that is, precipitation (P) minus evapotranspiration (ET) over a catchment equates stream outflow (Q_r). Here, we review evidence that catchments can be leaky due to groundwater outflow or inflow across topographic divides, based on catchment mass balance across a continent and several site-based studies across the globe. It appears that a catchment is more likely to be leaky with the combination of the following factors: small catchment size, positioned at either the high or low end of a steep regional topographic and climatic gradient, underlain by deep permeable substrates that extend beyond the study catchment, and in drier climate or dry seasons and droughts. Catchment leakage has hydrological, geochemical, and ecological implications. Thus, catchments are best framed as semiclosed hydrologic units perched on top of a larger, regional hydrogeological system with no real boundaries regarding the movement of water and solutes.