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

Storm events can drive highly variable behavior in catchment nutrient and water fluxes, yet short‐term event dynamics are frequently missed by low‐resolution sampling regimes. In addition, nutrient source zone contributions can vary significantly within and between storm events. Our inability to identify and characterize time‐dynamic source zone contributions severely hampers the adequate design of land use management practices in order to control nutrient exports from agricultural landscapes. Here we utilize an 8 month high‐frequency (hourly) time series of streamflow, nitrate (NO3‐N), dissolved organic carbon (DOC), and hydroclimatic variables for a headwater agricultural catchment. We identified 29 distinct storm events across the monitoring period. These events represented 31% of the time series and contributed disproportionately to nutrient loads (42% of NO3‐N and 43% of DOC) relative to their duration. Regression analysis identified a small subset of hydroclimatological variables (notably precipitation intensity and antecedent conditions) as key drivers of nutrient dynamics during storm events. Hysteresis analysis of nutrient concentration‐discharge relationships highlighted the dynamic activation of discrete NO3‐N and DOC source zones, which varied on an event‐specific basis. Our results highlight the benefits of high‐frequency in situ monitoring for characterizing short‐term nutrient fluxes and unraveling connections between hydroclimatological variability and river nutrient export and source zone activation under extreme flow conditions. These new process‐based insights, which we summarize in a conceptual model, are fundamental to underpinning targeted management measures to reduce nutrient loading of surface waters.

Distributed Temperature Sensing (DTS) technology enables downhole temperature monitoring to study hydrogeological processes at unprecedentedly high frequency and spatial resolution. DTS has been widely applied in passive mode in site investigations of groundwater flow, in‐well flow, and subsurface thermal property estimation. However, recent years have seen the further development of the use of DTS in an active mode (A‐DTS) for which heat sources are deployed. A suite of recent studies using A‐DTS downhole in hydrogeological investigations illustrate the wide range of different approaches and creativity in designing methodologies. The purpose of this review is to outline and discuss the various applications and limitations of DTS in downhole investigations for hydrogeological conditions and aquifer geological properties. To this end, we first review examples where passive DTS has been used to study hydrogeology via downhole applications. Secondly, we discuss and categorize current A‐DTS borehole methods into three types. These are thermal advection tests, hybrid cable flow logging, and heat pulse tests. We explore the various options with regards to cable installation, heating approach, duration, and spatial extent in order to improve their applicability in a range of settings. These determine the extent to which each method is sensitive to thermal properties, vertical in‐well flow, or natural gradient flow. Our review confirms that the application of DTS has significant advantages over discrete point temperature measurements, particularly in deep wells, and highlights the potential for further method developments in conjunction with other emerging hydrogeophysical tools.

An Ac vely Heated Fiber Op cs (AHFO) method to es mate soil moisture is tested and the analysis technique improved on. The measurements were performed in a lysimeter uniformly packed with loam soil with variable water content profi les. In the fi rst meter of the soil profi le, 30 m of fi ber op c cable were installed in a 12 loops coil. The metal sheath armoring the fi ber cable was used as an electrical resistance heater to generate a heat pulse, and the soil response was monitored with a Distributed Temperature Sensing (DTS) system. We study the cooling following three con nuous heat pulses of 120 s at 36 W m −1 by means of longme approxima on of radial heat conduc on. The soil volumetric water contents were then inferred from the es mated thermal conduc vi es through a specifi cally calibrated model rela ng thermal conduc vity and volumetric water content. To use the pre-asympto c data we employed a me correc on that allowed the volumetric water content to be esmated with a precision of 0.01-0.035 (m 3 m −3 ). A comparison of the AHFO measurements with soil-moisture measurements obtained with calibrated capacitance-based probes gave good agreement for we er soils [discrepancy between the two methods was less than 0.04 (m 3 m −3 )]. In the shallow drier soils, the AHFO method underes mated the volumetric water content due to the longer me required for the temperature increment to become asympto c in less thermally conduc ve media [discrepancy between the two methods was larger than 0.1 (m 3 m −3 )]. The present work suggests that future applica ons of the AHFO method should include longer heat pulses, that longer hea ng and cooling events are analyzed, and, temperature increments ideally be measured with higher frequency.

Water retention curves approaching infinitely negative matric potentials at residual water content are widely employed to model soil moisture dynamics. When used in numerical simulations, these retention curves fail to satisfactorily describe evaporation from arid soil (moisture‐limited regime) because they do not allow the soil to dry below residual water content. We show that simple modifications can be introduced to prevent unrealistic water retention at residual water content and predict more physically sound moisture dynamics. Modified retention models that allow drying below residual predict a moisture‐limited regime characterized by a thin subsurface evaporation zone and produce vapor fluxes up to 3 times larger than classical retention models. This might reduce the need to introduce empirical enhancement factors and improve the capability of modeling evaporation into the atmosphere and runoff in arid regions.