[1] Distributed temperature sensing (DTS) was used to map spatial and temporal changes in temperature on a 25 m by 6 m lakebed area in the winter (February), spring (May), and summer (August) of 2012. A constant and high discharge of groundwater with the average temperature of around 8 C to the lake will result in either lower (summer) or higher (winter) daily temperatures and reduce temperature variability at the sediment-water interface (SWI). DTS data were used as a proxy for groundwater discharge using three metrics; daily minimum temperature, diel amplitude, and daily standard deviation of temperature. During the seasons, the daily minimum temperatures at the SWI indicate a discharge zone 4-6 m offshore. From winter to summer, the extent of this zone changes and the SWI temperatures also show a shift of discharge locations toward the shore. Fluxes estimated on the basis of vertical temperature profiles from the top 50 cm of the lakebed and seepage meters in August compare well with the locations of the high-discharge zones detected by the DTS in the same period, giving confidence in the ability of the method to map both the areas and spatial variability of groundwater discharge to lakes. Compared to February, the DTS was able to detect new relatively cold temperature zones at the SWI in May and August indicating that groundwater discharge to the lake changes over time and that DTS can be used to monitor temporal variability in areas of discharge.Citation: Sebok, E., C. Duque, J. Kazmierczak, P. Engesgaard, B. Nilsson, S. Karan, and M. Frandsen (2013), High-resolution distributed temperature sensing to detect seasonal groundwater discharge into Lake Vaeng, Denmark, Water Resour. Res., 49, 5355-5368,

Abstract:Streambed hydraulic conductivity is one of the main factors controlling variability in surface water-groundwater interactions, but only few studies aim at quantifying its spatial and temporal variability in different stream morphologies. Streambed horizontal hydraulic conductivities (K h ) were therefore determined from in-stream slug tests, vertical hydraulic conductivities (K v ) were calculated with in-stream permeameter tests and hydraulic heads were measured to obtain vertical head gradients at eight transects, each comprising five test locations, in a groundwater-dominated stream. Seasonal small-scale measurements were taken in December 2011 and August 2012, both in a straight stream channel with homogeneous elevation and downstream of a channel meander with heterogeneous elevation. All streambed attributes showed large spatial variability. K h values were the highest at the depositional inner bend of the stream, whereas high K v values were observed at the erosional outer bend and near the middle of the channel. Calculated K v values were related to the thickness of the organic streambed sediment layer and also showed higher temporal variability than K h because of sedimentation and scouring processes affecting the upper layers of the streambed. Test locations at the channel bend showed a more heterogeneous distribution of streambed properties than test locations in the straight channel, whereas within the channel bend, higher spatial variability in streambed attributes was observed across the stream than along the stream channel.

Abstract:Analytical modelling of heat transport was used to address effects of uncertainty in thermal conductivity on groundwater-surface water exchange. In situ thermal conductivities and temperature profiles were measured in a coastal lagoon bed where groundwater is known to discharge. The field site could be divided into three sediment zones where significant spatial changes in thermal conductivity on metre to centimetre scale show that spatial variability connected to the sediment properties must be considered. The application of a literature-based bulk thermal conductivity of 1.84 Wm À1°CÀ1 , instead of field data that ranged from 0.62 to 2.19 W m À1°CÀ1, produced a mean overestimation of 2.33 cm dÀ1 that, considering the low fluxes of the study area, represents an 89% increase and up to a factor of 3 in the most extreme cases. Incorporating the uncertainty due to sediment heterogeneities leads to an irregular trend of the flux distribution from the shore towards the lagoon. The natural variability of the thermal conductivity associated with changes in the sediment composition resulted in a mean variation of ±0.66 cm d À1 in fluxes corresponding to a change of ±25.4%. The presence of organic matter in the sediments, a common situation in the near-shore areas of surface water bodies, is responsible for the decrease of thermal conductivity. The results show that the natural variability of sediment thermal conductivity is a parameter to be considered for low flux environments, and it contributes to a better understanding of groundwater-surface water interactions in natural environments

Abstract. Detecting, quantifying and understanding groundwater discharge to streams are crucial for the assessment 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 leaching of nutrients to streams. Here we aim to combine hydraulic 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 Denmark. At the point-scale, groundwater fluxes to the stream were quantified based on vertical streambed temperature profiles (VTPs). At the reach scale (0.15-2 km), the spatial distribution of zones of focused groundwater discharge was investigated by the use of distributed temperature sensing (DTS). Groundwater discharge to the stream was quantified using differential gauging with an acoustic Doppler current profiler (ADCP). At the catchment scale (26-114 km 2 ), runoff sources during main rain events were investigated by hydrograph separations based on electrical conductivity (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 responsive 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 precipitation events. This shows a large variability in groundwater discharge to the stream, despite the similar lowland characteristics of sub-catchments indicating the usefulness of environmental tracers for obtaining information about integrated catchment functioning during precipitation events. There were also clear spatial patterns of focused groundwater discharge detected by the DTS and ADCP measurements at the reach scale indicating high spatial variability, where a significant part of groundwater discharge was concentrated in few zones indicating the possibility of concentrated nutrient or pollutant transport zones from nearby agricultural fields. VTP measurements confirmed high groundwater fluxes in discharge areas indicated by DTS and ADCP, and this coupling of ADCP, DTS and VTP proposes a novel field methodology to detect areas of concentrated groundwater discharge with higher resolution.