Partitioning Water Storage in Stream Reaches: Implications for Solute Transport Under Varying Hydrological Conditions

Abstract Current models on solute transport often fail to reproduce discharge‐dependent behavior of solute transport in stream reaches because they rely on the assumption of well‐mixed conditions and fail to account for the complex coupling between in‐stream and subsurface flow. StorAge Selection (SAS) functions describe outflow as a mixture of waters of different ages, providing a framework to overcome the well‐mixed assumption in “traditional” transport models. In this study, we applied SAS functions to model solute transport from 13 slug tracer experiments conducted under varying hydrological conditions in a headwater stream reach. Using SAS function parameters (expressed in units of volume) together with measurements of groundwater (GW) levels and streambed microtopography, we partitioned the total water storage within the study reach into distinct components: advective storage, in‐stream transient storage, tracer‐derived hyporheic storage, and GW level‐derived hyporheic storage. This partitioning assumes that transport processes and subsurface water flow in stream reaches are associated with different storage volumes. We found positive linear relationships between discharge and age‐ranked, advective, and tracer‐derived hyporheic storage. In‐stream transient storage increased with discharge up to 17 L s −1 , corresponding to the discharge threshold above which streambed sediments became completely submerged, and declined at higher flows. This pattern likely reflects the contribution of eddies at lower discharge levels and highlights the importance of in‐stream transient storage for solute transport. Our results demonstrate that partitioning the total water storage in a reach–enabled only through applying SAS functions–is essential for understanding and modeling solute transport under varying hydrological conditions.