International audienceProtecting or restoring aquatic ecosystems in the face of growing anthropogenic pressures requires an understanding of hydrological and biogeochemical functioning across multiple spatial and temporal scales. Recent technological and methodological advances have vastly increased the number and diversity of hydrological, bio-geochemical, and ecological tracers available, providing potentially powerful tools to improve understanding of fundamental problems in ecohydrology, notably: 1. Identifying spatially explicit flowpaths, 2. Quantifying water residence time, and 3. Quantifying and localizing biogeochemical transformation. In this review, we synthesize the history of hydrological and biogeochemical theory, summarize modern tracer methods, and discuss how improved understanding of flowpath, residence time, and biogeochemical transformation can help ecohydrology move beyond description of site-specific heterogeneity. We focus on using multiple tracers with contrasting characteristics (crossing proxies) to infer ecosystem functioning across multiple scales. Specifically, we present how crossed proxies could test recent ecohydrological theory, combining the concepts of hotspots and hot moments with the Damköhler number in what we call the HotDam framework

Abstract. Surface saturation can have a critical impact on runoff generation and water quality. Saturation patterns are dynamic, thus their potential control on discharge and water quality is also variable in time. In this study, we assess the practicability of applying thermal infrared (TIR) imagery for mapping surface-saturation dynamics. The advantages of TIR imagery compared to other surface-saturation mapping methods are its large spatial and temporal flexibility, its non-invasive character, and the fact that it allows for a rapid and intuitive visualization of surface-saturated areas. Based on an 18-month field campaign, we review and discuss the methodological principles, the conditions in which the method works best, and the problems that may occur. These considerations enable potential users to plan efficient TIR imagery-mapping campaigns and benefit from the full potential offered by TIR imagery, which we demonstrate with several application examples. In addition, we elaborate on image post-processing and test different methods for the generation of binary saturation maps from the TIR images. We test the methods on various images with different image characteristics. Results show that the best method, in addition to a manual image classification, is a statistical approach that combines the fitting of two pixel class distributions, adaptive thresholding, and region growing.

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Surface saturated areas are key features in generating run‐off. A detailed characterization of the expansion and contraction of surface saturation in riparian zones and its connectivity to the stream is fundamental to improve our understanding of the spatial and temporal variability of streamflow generation processes. In this first contribution of a series of two papers, we used ground‐based thermal infrared imagery for characterizing riparian surface saturation seasonal dynamics of seven different riparian areas in the Weierbach catchment (0.42 km2), a small forested catchment in Luxembourg. We collected biweekly panoramic images of the seven areas over a period of 2 years. We identified the extension of saturation in each collected panoramic image (i.e., percentage of pixels corresponding to saturated surfaces in each riparian area) to generate time series of surface saturation. Riparian surface saturation in all areas was seasonally variable, and its dynamics were in accordance with lower hillslope groundwater level fluctuations. Surface saturation in the different areas related to catchment outlet discharge through power law relationships. Differences in these relationships for different areas could be associated with the location of the areas along the stream network and to a possible influence of local riparian morphology on the development of surface saturation, suggesting a certain degree of intracatchment heterogeneity. This study paves the way for a subsequent investigation of the spatio‐temporal variability of streamflow generation in the catchment, presented in our second contribution.