Preferential pathways in catchments induce simple functioning but complex chemical transport

Abstract

Preferential pathways are omnipresent on Earth. They play a key but often confusing role in hydrology, being seen either as a source of “simplicity” by catchment hydrologists, or of “complexity” by groundwater hydrologists. Here we use thermodynamic and information theoretical arguments in combination with field data to unravel this paradox. We show that preferential pathways reduce frictional dissipation and production thermal entropy. As this causes faster fluid flow, preferential pathways equally enlarge dissipation of head gradients and related production of mixing entropy. For the catchment water balance, we show that preferential pathways lead to a faster but also more spatially uniform dissipation of the head gradients controlling streamflow generation. This explains the success of simplified, aggregated models. Second, we show that this simplicity is only apparent: preferential pathways strongly broaden the spectrum of fluid velocities, implying that bulk water fluxes consist of a complex mixture of waters of variable ages. This complexity cannot manifest when considering the pure fluid but requires examination of chemicals to trace the travel time distribution of the water leaving the system. We conclude that preferential pathways generally increase the complexity of hydrological systems, while consideration of bulk streamflow enables a convenient but possibly misleading simplification.