Isotope Tracers in Catchment Hydrology

The drainage basin has been a fundamental unit of study in hydrology and geomorphology throughout the modern era of these Earth sciences. The basin, or catchment, is a convenient unit because it typically is well defined topographically, can be studied as a series of nested units of increasing size, and is an open system for which inputs and outputs of mass and energy can be defined and measured. Small catchments have been the “outdoor laboratories” for hydrologists interested in rainfall‐runoff relationships, for geochemists interested in the export of weathering products from the landscape, and for ecosystem ecologists interested in biogeochemical cycling. Research studies on small catchments have evolved from a strong focus on strictly input‐output relationships (a “black‐box” approach) to a focus on understanding flow paths, residence times, and biological and chemical reactions within the catchment. Tracers are essential tools for such work. In catchment studies, tracers that move with the water but do not interact with biota, soils, and rocks are sought so that water itself can be followed. Other tracers are used to follow selected biogeochemical reactions. A multitude of stable and radioactive isotopes has been used as tracers in catchment studies.