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.

Abstract. In this study, we explored the spatio-temporal variability of surface saturation within a forested headwater catchment using a combined simulation–observation approach. We simulated the occurrence of surface saturation in the Weierbach catchment (Luxembourg) with the physically based model HydroGeoSphere. We confronted the simulation with thermal infrared images that we acquired during a 2-year mapping campaign for seven distinct riparian areas with weekly to biweekly recurrence frequency. Observations and simulations showed similar saturation dynamics across the catchment. The observed and simulated relation of surface saturation to catchment discharge resembled a power law relationship for all investigated riparian areas but varied to a similar extent, as previously observed between catchments of different morphological and topographical characteristics. The observed spatial patterns and frequencies of surface saturation varied between and within the investigated areas and the model reproduced these spatial variations well. The good performance of the simulation suggested that surface saturation in the Weierbach catchment is largely controlled by exfiltration of groundwater into local topographic depressions. However, the simulated surface saturation contracted faster than observed, the simulated saturation dynamics were less variable between the investigated areas than observed, and the match of simulated and observed saturation patterns was not equally good in all investigated riparian areas. These mismatches between observations and simulation highlight that the intra-catchment variability of surface saturation must also result from factors that were not considered in the model set-up, such as differing subsurface structures or a differing persistence of surface saturation due to local morphological features like perennial springs.

Diatoms are remarkable organisms. They are present in almost all habitats containing water (e.g., lakes, streams, soils, bark) and rank among the most common algal groups in both freshwaters and marine ecosystems. The ubiquitous character of aquatic diatoms has triggered countless applications as environmental tracers for studies in water quality, paleoclimate reconstruction and sediment tracing. However, diatoms also occur in the terrestrial environment. It is this plethora of diatom life-forms that has recently triggered interest in their use as tracers of hydrological processes. The use of diatoms in catchment hydrology has been very limited. Part of the reason is that until recently, the taxonomy and ecology of terrestrial diatom assemblages were largely unknown. However, in the past decade, much work has been done to quantify terrestrial diatom reservoir size, dynamics, and potential depletion following precipitation events. Therefore, such terrestrial diatoms now hold promise for use in catchment hydrology-for tracing runoff flow sources and pathways across a wide range of spatial scales. Here we review the literature on terrestrial diatoms and describe the various sampling protocols that have been designed and tested for specific applications in hydrological processes research. We review and summarize the work on terrestrial diatom reservoir characterization, transport mechanisms and pathways to show how such diatom-based tracer work might be possible at the catchment scale for rainfall-runoff studies. Finally, we present a vision for future work that might take advantage of terrestrial diatoms in catchment hydrology and discuss the main challenges going forward. © 2017 The Authors. WIREs Water published by Wiley Periodicals, Inc. How to cite this article:WIREs Water 2017Water , 4:e1241. doi: 10.1002Water /wat2.1241 INTRODUCTION D iatoms (Bacillariophyta) are present in almost all habitats containing water (e.g., lakes, streams, soils, litter, bark). They count among the most common algal groups in both freshwaters and marine ecosystems. 1 Despite their microscopic size (10-200 microns), 2 diatoms generate an impressive amount of carbon through photosynthesis: almost as much as all rainforests combined. 3 Identification of diatoms date back to the early 1700s as noted by Round et al. 1 Decades later, Otto Friedrich Müller published the first description of a diatom in 1783 (Vibrio paxillifer ≡ Bacillaria paradoxa). Since then, This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. more than 64,000 diatom species have been described-and new species are added to that list almost daily with the number of extant species extrapolated to ca 100,000. 4,5 The identification of diatoms (from the Greek 'diatomos' or 'cut in half') commonly relies on the highly differentiated cell wall (or frustule) that is mostly co...

Investigating the spatio‐temporal variability of streamflow generation is fundamental to interpret the hydrological and biochemical functioning of catchments. In humid temperate environments, streamflow generation is often linked to the occurrence of near stream surface saturated areas, which mediate hydrological connectivity between hillslopes and streams. In this second contribution of a series of two papers, we used salt dilution gauging to investigate the spatio‐temporal variability of streamflow in different subcatchments and for different reaches in the Weierbach catchment (0.42 km2) and explored the topographical controls on streamflow variability. Moreover, we mapped stream network expansion and contraction dynamics. Finally, we combined the information on the spatio‐temporal variability of streamflow with the characterization of riparian surface saturation dynamics of seven different areas within the catchment (mapped with thermal infrared imagery, as presented in our first manuscript). We found heterogeneities in the streamflow contribution from different portions of the catchment. Although the size of the contributing area could explain differences in subcatchments' and reaches' net discharge, no clear topographic controls could be found when considering the area‐normalized discharge. This suggests that some local conditions exert control on the variability of specific discharge (e.g., local bedrock characteristics and occurrence of perennial springs). Stream network dynamics were found not to be very responsive to changes in catchment's discharge (i.e., total active stream length vs. stream outlet discharge relationship could be described through a power law function with exponent = 0.0195). On the contrary, surface saturation dynamics were found to be in agreement with the level of streamflow contribution from the correspondent reach in some of the investigated riparian areas. This study represents an example of how the combination of different techniques can be used to characterize the internal heterogeneity of the catchment and thus improve our understanding of how hydrological connectivity is established and streamflow is generated.

Abstract. The inundation of flood-prone areas varies in space and time and can have crucial impacts on runoff generation and water quality when the surface saturated areas become connected to the stream. In this study, we aimed to investigate and explain the variability of surface saturation patterns and dynamics within a forested headwater catchment. On the one hand, we mapped surface saturation in seven distinct riparian areas of the Weierbach catchment (Luxembourg) with thermal infrared images, taken weekly to bi-weekly over a period of two years. On the other hand, we simulated the surface saturation generation in the catchment with the integrated surface subsurface hydrologic model HydroGeoSphere over the same period. Both the observations and simulations showed that the saturation dynamics were similar across the catchment, but that small differences between the dynamics at different areas occurred. Moreover, the model reproduced the observed saturation patterns well for all seasonal and hydrologic conditions and at all investigated locations. Based on the observations and simulation results and the matches and mismatches between them, we concluded that the generation of surface saturation in the Weierbach catchment was largely controlled by exfiltration of groundwater into local depressions. However, we also illustrate that the entire variability of the patterns, dynamics and frequencies of surface saturation within the different riparian areas of the catchment can only result from additional controlling factors to microtopography and groundwater exfiltration, such as differing hysteretic behaviour, differing subsurface structures, or additional water sources.