The significance of model structure in one-dimensional stream solute transport models with multiple transient storage zones – competing vs. nested arrangements

Abstract

Transient storage models are commonly used to simulate solute transport in streams to characterize hydrologic controls on biogeochemical cycling. Recently, 2-storage zone (2-SZ) models have been developed to represent in-channel surface transient storage (STS) and hyporheic transient storage (HTS) separately to overcome the limitations of single storage zone (1-SZ) models. To advance biogeochemical models, we seek to separate the effects of these storage zones on solute fate and cycling in streams. Here we compare and contrast the application and interpretations from two model structures that include STS and HTS storage: a competing model structure, where both zones are connected to the stream at the same location and the stream interacts with the STS and HTS separately, and a nested model structure, where STS is an intermediary between the stream and HTS. We adapt common residence time metrics used to compare single transient storage models for the competing and nested 2-SZ models. As a test case, we investigated the transient storage characteristics of a first-order stream in Pennsylvania, using 1-SZ, nested 2-SZ, and competing 2-SZ model configurations at several different flow conditions. While both 2-SZ models fit the observed STS and in-stream breakthrough curves well, calibrated model parameters and solute molecule travel paths differ, as evident by the faster exchange rate displayed by the nested model, and therefore so does the interpretation of associated transient storage metrics and its relationship with biogeochemical cycling processes. In addition, a study of hypothetical zone-specific reaction rates was very illustrative of the differences in discrimination characterized by each model structure, particularly for the case where reactions would predominantly occur in the STS (i.e. photochemical reactions), because of the compounding effects to the HTS for the nested 2-SZ; however, for the case where reactions would predominantly occur in the HTS, the influence of model structure was found to be relegated only to the HTS.