Time-lapse photography &#8211; a tool for unravelling the intricate complexity of eco-hydrologic processes
Authors: Pfister Laurent , Enrico Bonanno , Ginevra Fabiani , Laurent Gourdol , Christophe Hissler , Viola Huck , Jean François Iffly , Richard Keim , Núria Martínez-Carreras , Xavier Mestdagh , Alessandro Montemagno , Daniele Penna , Stan Schymanski , Erwin Zehe 
Published: March 2024

For decades, field data collection has been largely in decline in favour of environmental modelling &#8211; the latter being considered less labour and cost-intensive. However, this trend goes against the grain with new observational field data having repeatedly been the source of breakthroughs in science (e.g., high-frequency measurements in stream water, in-situ monitoring of the isotopic composition of tree xylem water, imaging of infiltration pathways with electrical resistivity tomography, or time-lapse mapping of surface-saturation dynamics with thermal infrared imagery). Hypotheses generated from this type of novel, integrative, observations offer the potential to free hydrological concepts from the restrictions of typical datasets.However, recent technological developments in field instrumentation have also revealed an increasingly complex landscape heterogeneity. General organizing principles have been proposed to explain river basin complexity. Deciphering this heterogeneity remains very challenging &#8211; essentially because eco-hydrologic processes occur over a wide range of spatial and temporal scales and vary by multiple orders of magnitude. The dilemma here is that we could continue instrumenting our catchments to the point of littering, and still miss out on processes or features that we were simply not looking for.Here, we demonstrate the potential for time-lapse photography to unravel the complex organisation of eco-hydrologic processes at various temporal and spatial scales. This technique (also called undercranking) consists of taking regular frames with a camera and subsequently speeding up the action during playback. We installed a wildlife monitoring camera (RECONYX Hyperfire 2 Professional White Flash Camera) in the forested Weierbach experimental catchment (WEC) &#8211; an interdisciplinary Critical Zone observatory dedicated to the long-term study of hydrological, hydro-geochemical, and eco-hydrological processes. The rainfall-runoff response of the WEC is characterized by a strong seasonality, with pronounced summer low flows and winter high flows (resulting from a complex interplay of multiple eco-hydrological processes). The full time-lapse video of a hillslope-riparian zone-stream transect in the Weierbach catchment spans from December 2020 to July 2022 and is available online via https://youtu.be/74S7DfT7Uhs.The high-speed playback of pictures recorded between December 2020 and July 2022, combined with in-situ eco-hydrological measurements reveals a comprehensive view of contrasted seasons with gradually changing processes. In winter, snowfall events trigger a slow but gradual snow-fed groundwater recharge (recorded by soil moisture probes and groundwater wells). Balmy weather in spring announces the onset of leaf-out and a recession in groundwater levels and hydrographs. In summer, vegetation is highly dynamic and growing, while groundwater levels and discharge evolve between high and low levels along successive dry and wet sequences. With cooler autumn temperatures and wet weather, leaf senescence starts, and the groundwater system switches back to a rain-fed recharge state.The combination of a four-season-long time-lapse sequence of the pulse of the Weierbach with a high-frequency, multi-parameter dataset is offering an innovative opportunity for combining &#8216;soft&#8217; and &#8216;hard&#8217; data across multiple scales and eventually improving the dialogue between experimentalists and modelers. Such an alternative source of information may eventually become the starting point for a new cycle of hypothesis framing and testing.Further information on this study is available at https://doi.org/10.1002/hyp.15026.