Process separation using models of stream transport: A look back and a look forward

The fate of nutrients in stream ecosystems is controlled by a suite of interacting hydrologic and biogeochemical processes. The relative roles of individual processes may be quantified using stream solute transport models that provide separate, quantitative descriptions of the predominant processes. Model application is thus an exercise in process separation in which model simulations are fit to data from field-scale experiments. Two recent publications describing techniques for process separation are reviewed here. The first publication presents Fmed, the median travel time that is attributable to the hydrologic process of transient storage. Use of Fmed is preferable to the use of As/A (storage-zone area/main-channel area) because it considers the interaction between stream velocity and the rate of exchange with the storage zone. Future research efforts should examine the relation between Fmed and the experimental Damkohler number, in an effort to develop a unified measure of transient storage. The second paper presents a transport-based approach to the problem of nutrient spiraling. Under the transport-based approach, time-series data are used to separate the amount of uptake attributable to the storage zone from the amount of uptake occurring in the main stream channel. The transport-based approach thus presents an advantage over the previous regression-based approach to nutrient spiraling, where only the aggregate amount of uptake is determined. Future research on nutrient spiraling should include field efforts that provide data for implementation of both the transport- and regression-based approaches. These field efforts will allow for a direct comparison of the two approaches, highlighting the advantages and disadvantages of each.