https://hal-insu.archives-ouvertes.fr/insu-01346018/document

Using multi-tracer inference to move beyond single-catchment ecohydrology

Temperature buffering by groundwater in ecologically valuable lowland streams under current and future climate conditions

Four sessions on "Monitoring Strategies: temporal trends in groundwater and surface water quality and quantity" at the EGU conferences in 2012, 2013, 2014, and 2015 and a special issue of HESS form the background for this overview of the current state of high-resolution monitoring of nutrients. The overview includes a summary of technologies applied in high-frequency monitoring of nutrients in the special issue. Moreover, we present a new assessment of the objectives behind high-frequency monitoring as classified into three main groups: (i) improved understanding of the underlying hydrological, chemical, and biological processes (PU); (ii) quantification of true nutrient concentrations and loads (Q); and (iii) operational management, including evaluation of the effects of mitigation measures (M). The contributions in the special issue focus on the implementation of high-frequency monitoring within the broader context of policy making and management of water in Europe for support of EU directives such as the Water Framework Directive, the Groundwater Directive, and the Nitrates Directive. The overview presented enabled us to highlight the typical objectives encountered in the application of high-frequency monitoring and to reflect on future developments and research needs in this growing field of expertise. © Author(s) 2016.

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High-resolution monitoring of nutrients in groundwater and surface waters: process understanding, quantification of loads and concentrations, and management applications

The delineation of groundwater discharge areas based on Distributed Temperature Sensing (DTS) data of the streambed can be difficult in soft-bedded streams where sedimentation and scouring processes constantly change the position of the fibre optic cable relative to the streambed. Deposition-induced temperature anomalies resemble the signal of groundwater discharge while scouring will cause the cable to float in the water column and measure stream water temperatures. DTS applied in a looped layout with nine fibre optic cable rows in a 70 × 5 m section of a soft-bedded stream made it possible to detect variability in streambed temperatures between October 2011 and January 2012. Detailed monthly streambed elevation surveys were carried out to monitor the position of the fibre optic cable relative to the streambed and to quantify the effect of sedimentation processes on streambed temperatures. Based on the simultaneous interpretation of streambed temperature and elevation data, a method is proposed to delineate potential high-groundwater discharge areas and identify deposition-induced temperature anomalies in soft-bedded streams. Potential high-discharge sites were detected using as metrics the daily minimum, maximum and mean streambed temperatures as well as the daily amplitude and standard deviation of temperatures. The identified potential high-discharge areas were mostly located near the channel banks, also showing temporal variability because of the scouring and redistribution of streambed sediments, leading to the relocation of pool-riffle sequences. This study also shows that sediment deposits of 0.1 m thickness already resulted in an increase in daily minimum streambed temperatures and decrease in daily amplitude and standard deviation. Scouring sites showed lower daily minimum streambed temperatures and higher daily amplitude and standard deviation compared with areas without sedimentation and scouring. As a limitation of the approach, groundwater discharge occurring at depositional and scouring areas cannot be identified by the metrics applied. Copyright © 2015 John Wiley & Sons, Ltd.

Application of Distributed Temperature Sensing for coupled mapping of sedimentation processes and spatio‐temporal variability of groundwater discharge in soft‐bedded streams

The Danish hydrological observatory was established in 2007 in a catchment in the western part of Denmark representing hydrological conditions in a temperate climate with groundwater-dominated streamflow. In the catchment, an experimental infrastructure has been established where measurements, experiments, and modeling are performed across a range of spatial and temporal scales. The primary research question is water balance closure at different scales, which has been addressed using improved measurement and modeling methods. In addition, an array of research questions are related to processes in the individual hydrological compartments as well as the interactions between them. We provide here an overview of long-term monitoring and observation, dedicated observations, experiments, and modeling. An overview of main research findings is also provided. The research has provided new insights into the dynamics of the individual hydrological processes and their interactions. The hydrological fluxes are now determined with greater certainty, and the water balance is better constrained at different spatial scales.

HOBE: The Danish Hydrological Observatory

Detecting, quantifying and understanding ground- water discharge to streams are crucial for the assess- ment of water, nutrient and contaminant exchange at the groundwater-surface water interface. In lowland agricultural catchments with significant groundwater discharge this is of particular importance because of the risk of excess leach- ing of nutrients to streams. Here we aim to combine hy- draulic and tracer methods from point-to-catchment scale to assess the temporal and spatial variability of groundwater discharge in a lowland, groundwater gaining stream in Den- mark. At the point-scale, groundwater fluxes to the stream were quantified based on vertical streambed temperature pro- files (VTPs). At the reach scale (0.15-2 km), the spatial dis- tribution of zones of focused groundwater discharge was investigated by the use of distributed temperature sensing (DTS). Groundwater discharge to the stream was quanti- fied using differential gauging with an acoustic Doppler cur- rent profiler (ADCP). At the catchment scale (26-114 km 2 ), runoff sources during main rain events were investigated by hydrograph separations based on electrical conductiv- ity (EC) and stable isotopes 2 H= 1 H. Clear differences in runoff sources between catchments were detected, ranging from approximately 65 % event water for the most respon- sive sub-catchment to less than 10 % event water for the least responsive sub-catchment. This was supported by the groundwater head gradients, where the location of weaker gradients correlated with a stronger response to precipita- tion events. This shows a large variability in groundwater discharge to the stream, despite the similar lowland char- acteristics of sub-catchments indicating the usefulness of

https://hess.copernicus.org/articles/19/1871/2015/hess-19-1871-2015.pdf

J. R. Poulsen

Papers

Detecting groundwater discharge dynamics from point-to-catchment scale in a lowland stream: combining hydraulic and tracer methods