Background/Question/Methods Stable isotope tracers have been a pivotal tool for understanding the physical environment. In hydrology, their use in the 1970s sparked radical changes in perceptual models of streamflow generation, often showing the importance of stored water (termed old or pre-event water) in the channel hydrograph. From the 1970s until now, hydrologists have discussed and debated the assumptions implicit in hydrograph separations and gone beyond hydrograph separation to explore the age of the old water and the age distributions of that water in the headwater stream. In the past decade, the use of stable isotopes in ecohydrology has exploded focused on the other main catchment outflow, plant transpiration, with radically changes in perceptual models of where plants get their water. Yet unlike stream hydrology, their use in plant analysis has not been strictly guided by the assumptions in tracer analysis.
Results/Conclusions Here we describe how methodical testing of assumptions embedded in isotope tracing in early hydrological studies may be a valuable template for future isotope ecohydrological studies to follow. We then show how today’s work aimed at plant water age and age distributions of transpiration mirrors the discoveries in catchment hydrology and how similar analysis of the flow signal in streams is to the flow signal in the plant xylem. This reflection on how one might learn from early hydrological investigations may help with linking ecophysiological traits to patterns in tree water use, much like hydrologists have linked catchment traits to flow source, flow path and water age. We discuss aspects from both “eras” to help understand tree water sources and guide research by going back to the future for linking geoscience and ecology through water.
