Does catchment nestedness enhance hydrological similarity?

This contribution analyses how the topological relationship between two river basins affects the correlation of the flows at their outlets. A pair of river basins can have two distinct topological configurations. They can either be nested (if the smaller catchment is part of a larger one), or they can be disjointed (or non-nested), if their contributing areas are not overlapping.  Nested catchments tend to be considered as hydrologically more similar as they share a fraction of their contributing area and, consequently, a fraction of their streamflows (i.e. they are hydrologically connected).  Nonetheless, using a large dataset of catchments spanning a wide range of scales and geomorphoclimatic conditions, we show that – as inter-catchment distance increases – the correlation between daily flows at the outlet of nested sites experiences a faster decline as compared to non-nested sites. By using a recently developed analytical model we are able to highlight that the enhanced loss of streamflow correlation in nested sites is primarily due to a sharp decrease in the frequency of simultaneous runoff-generating rainfall in the two contributing areas, and to a larger loss of correlation between their magnitudes. This surprising effect can be explained by the fact that, as distance increase, nested catchments tend to become systematically more heterogeneous in hydrologically-critical features such as in their size, elevation and slope. Acknowledging and understanding the enhanced hydrological variability of nested catchments across scales can help to better capture the spatial variability of river flows, with benefits for streamflow regionalization, ecological modelling and processes interpretation.