Groundwater–surface-water interactions: perspectives on the development of the science over the last 20 years

Freshwater Science published a special series of papers on groundwater–surface-water (GW-SW) interactions in this issue (2015), marking the anniversary of an earlier special series of papers on the hyporheic zone published in 1993. In this concluding paper, I compare the 2 special series of papers and use this comparison to examine the development of the science in the years between 1993 and 2015. The 1993 papers marked the beginning of a period of exponential growth in the study of, and publication of, papers on GW-SW interactions. The 1993 papers tended to be forward looking, proposing conceptual models of GW-SW interactions across stream networks and identifying critical gaps. The 2015 special series of papers contrasts sharply with that of 1993. Broad issue papers are mostly lacking from the current special series. Instead, the special series is dominated by papers focusing on process-based or descriptive studies using empirical approaches. This difference probably stems from major methodological advancements over the past 2 decades that make it possible to study GW-SW interactions with ever greater detail and, thus, allows more complete understanding of specific processes. In contrast, the authors of the 1993 special series were acutely aware of the paucity of GW-SW studies and, thus, posed their conceptual models as hypotheses. Surprisingly, these hypotheses have not been rigorously tested in the decades since their publication. Perhaps it is time to re-examine such broad conceptual models. There remains a critical need for a holistic understanding of how GW-SW interactions vary among streams types and sizes and with changes in discharge among seasons or over storm events and how these GW-SW interactions influence stream ecosystem processes.