On Complementing the Tracer Toolbox for Quantifying Hydrological Connectivity: Insights Gained from Terrestrial Diatom Tracer Experiments

Abstract

Prior to the widespread use of tracers, early forest hydrology studies relied mostly on hydrometric measurements (soil moisture content, groundwater levels, discharge, precipitation inputs). A dominant pre-tracer paradigm was that high precipitation intensities resulted in rapid runoff responses, when infiltration rates were exceeded by precipitation rates, as deduced from hydrometric data. Early hydrologic tracer applications included performing hydrograph separations to understand the fractions of event and pre-event water in streams. Using additional tracers, the end-member mixing analysis model was introduced for quantifying contributions of multiple geographic stormflow sources. With longer series of tracer measurements, more sophisticated inferences became possible, such as transit time distributions and young water fractions. A limitation to the common toolbox of hydrological tracers is that stable isotopes and geochemical tracers do not specifically show surface connectivity, allowing for ambiguity in our understanding of how ‘event water’ actually reaches streams. Furthermore, there are challenges in aggregating these tracer measurements across diverse landscapes to understand larger scale connectivity patterns. Here, we provide a primer on tracer applications in studying the hydrological connections in watersheds and forests. After providing an overview of general basics in tracer applications, we focus on a new type of tracer—terrestrial diatoms—and give some insights into their use for tracing the onset/cessation of surface hydrological connectivity. Given that this is a newly developing tracer method, we demonstrate the process of exploring and testing tracer applications. Furthermore, we discuss the differences among and complementarity between diatoms and more traditional tracers (e.g. solutes).