Storage selection functions: A coherent framework for quantifying how catchments store and release water and solutes

TLDR

We discuss a recent theoretical approach combining catchment‑scale flow and transport processes into a unified framework. The study aims to characterize catchment hydrochemistry using SAS functions and to promote their routine use while identifying future research needs. StorAge Selection functions (SAS) represent how catchment storage supplies outflows with water of different ages, regulating chemical composition, while biogeochemical processes are reflected in the evolving residence time distribution and age‑selection. We argue for routine use of SAS functions and highlight areas for further research.

Abstract

Abstract We discuss a recent theoretical approach combining catchment‐scale flow and transport processes into a unified framework. The approach is designed to characterize the hydrochemistry of hydrologic systems and to meet the challenges posed by empirical evidence. StorAge Selection functions (SAS) are defined to represent the way catchment storage supplies the outflows with water of different ages, thus regulating the chemical composition of out‐fluxes. Biogeochemical processes are also reflected in the evolving residence time distribution and thus in age‐selection. Here we make the case for the routine use of SAS functions and look forward to areas where further research is needed.