1.1. Groundwater-Surface Water Exchange Fluxes at the Aquifer-River Interface
 The mixing of groundwater and surface water can have substantial impact on stream and streambed thermal patterns [Malcolm et al., 2002; Hannah et al., 2004, 2009; Krause et al., 2011b], the availability of dissolved oxygen, nutrient cycling and carbon respiration [Pinay et al., 2009; Mulholland et al., 2000, 2008; Krause et al., 2008a, 2008b; Pinay et al., 2009; Zarnetske et al., 2011a, 2011b], transport and transformation of contaminants [Conant et al., 2004; Ellis and Rivett, 2007; Rivett et al., 2011], as well as the ecohydrological functioning of the riverine environment [Brunke and Gonser, 1997; Dole-Olivier et al., 1997; Boulton et al., 1998; Malcolm et al., 2002; Stubbington et al., 2009; Robertson and Wood, 2010; Krause et al., 2011b, 2011c]. A major challenge for quantifying the impact of physical streambed controls on the biogeochemical and ecohydrological functioning is the difficulty to identify temporal dynamics and detailed spatial patterns of groundwater-surface water exchange at stream reach and larger scales [White, 1993; Krause et al., 2011a]. Recent studies have successfully investigated aquifer-river exchange fluxes by using temperature differences between groundwater and surface water as a tracer [Constantz et al., 2003; Constantz, 2008; Conant, 2004; Anderson, 2005; Keery et al., 2007; Schmidt et al., 2007; Hatch et al., 2010].
1.2. Fiber-Optic Distributed Temperature Sensing
 In particular, advances in distributed sensor technologies such as fiber-optic distributed temperature sensing (FO-DTS) have supported the monitoring of thermal patterns at aquifer-river interfaces at increasing spatial and temporal resolution [Selker et al., 2006a, 2006b; Tyler et al., 2009]. FO-DTS technology, which is based on the monitoring of thermal patterns along a fiber-optic cable of up to several kilometer length, allows for continuous measurements of temperatures at a spatial resolution between currently 1–2 m and measurement precision of 0.05–0.1°C for sampling intervals of 30 s [Selker et al., 2006a, 2006b; Hausner et al., 2011; Van de Giesen et al., 2012].
 In addition to successful studies of spatial patterns and temporal dynamics of exchange fluxes between groundwater and surface water [Lowry et al., 2007; Westhoff et al., 2007; Vogt et al., 2010; Slater et al., 2010; Briggs et al., 2012; Krause et al., 2012a], FO-DTS has experienced a rapid increase in interdisciplinary applications for thermal monitoring in hydrology and hydrogeology, glaciology, soil sciences, civil engineering, and meteorology [Selker et al., 2006a, 2006b; Tyler et al., 2008, 2009; Henderson et al., 2009; Steele-Dunne et al., 2010; Slater et al., 2010; Keller et al., 2011; Suárez et al., 2011; Krause et al., 2012a, 2012b]. Different experimental designs and strategies have been deployed in order to optimize the FO-DTS setup to accommodate the specific monitoring requirements at the aquifer-river interface, including the installation of coiled fiber-optic cables for a 5–10 times increase in spatial sampling resolution along a vertical profile [Vogt et al., 2010; Briggs et al., 2012].
 The tracing of groundwater-surface water exchange represents a complex challenge for FO-DTS surveys where uncertainties related to sampling design or monitoring mode are still largely unknown. Spatial patterns in river or streambed temperatures caused by aquifer-river exchange can involve either gradual alteration of temperatures or rather discrete temperature changes [Selker et al., 2006a, 2006b; Westhoff et al., 2007; Lowry et al., 2007; Krause et al., 2012a]. Furthermore, groundwater-surface water mixing can cause temperature changes that propagate downstream [Selker et al., 2006a, 2006b; Westhoff et al., 2007] or localized temperature anomalies of limited spatial extent [Lowry et al., 2007; Krause et al., 2012a]. As shown by Rose et al. , in particular, the precise detection of smaller signals that do not exceed the respective FO-DTS sampling resolution by at least three to four times can be critically limited, causing concern for the accurate interpretation of FO-DTS survey results under such conditions.
 Alongside the increasing numbers of interdisciplinary applications of FO-DTS and problem-specific adaptations of the monitoring design, several studies have started to investigate the implications of different calibration techniques and experimental design for the quality and accuracy of FO-DTS monitored temperature patterns [Tyler et al., 2009; Hausner et al., 2011; Van de Giesen et al., 2012]. The recent works by Hausner et al.  and Van de Giesen et al.  provided benchmark analyses of the specific advantages and restrictions as well as calibration demands of single-ended and double-ended FO-DTS monitoring modes. While the experimental setup of single-ended or double-ended monitoring modes has been shown to have an impact on the accuracy of signal detection, with a degradation of precision toward the cable ends or at the center of a fiber-optic cable [Tyler et al., 2009; Hausner et al., 2011; Van de Giesen et al., 2012], the monitoring mode does not affect the sampling resolution of temperature measurements. However, the quantitative impact of FO-DTS monitoring modes on the accuracy of detected temperatures and temperature patterns has yet to be identified. As the majority of previous FO-DTS publications did not report on the applied monitoring modes, it still has to be established to what degree the accuracy of earlier reported FO-DTS survey results has been affected by the chosen monitoring mode.
 In order to adopt a sampling design and FO-DTS monitoring mode that are best suited to accurately detect site-specific signal patterns, it is required to identify the FO-DTS sampling mode dependent limitations and uncertainties in signal detection. While many recent studies have adopted double-ended FO-DTS monitoring modes in order to increase the robustness of signal detection, it is unknown how monitoring mode decisions affect the detection of signals of smaller spatial extent or the accuracy of predictions of discrete signal locations and signal changes.
 Moreover, the strength of the investigated temperature signal has been shown to have substantial impact on the detection accuracy of thermal patterns in a FO-DTS survey [Rose et al., 2013]. As FO-DTS surveys of aquifer-river exchange fluxes use the temperature difference between groundwater and surface water as a tracer signal, seasonal variability of the signal strength has to be taken into account. With the seasonal differences in groundwater and surface water thermal regimes, also the signal strength (as given by the temperature difference between groundwater and surface water) often varies on a seasonal basis [Lapham, 1989; Bartolino and Niswonger, 1999; Stonestrom and Constantz, 2003], presenting a challenge for accurate FO-DTS surveys. FO-DTS surveys at aquifer-river interfaces have been carried out during summer and winter conditions, utilizing the seasonally warmer as well as colder groundwater than surface water temperatures. The impacts of seasonal differences in detected signal strength as well as short-term (i.e., diurnal) signal variability on the accuracy of FO-DTS surveys have yet to be identified. In particular, for dynamic systems with small-scale variability and discrete signal patterns as well as for the quantitative interpretation of FO-DTS survey results, detailed quantitative understanding of the implications of monitoring modes and seasonal signal differences on the accuracy and uncertainty of FO-DTS surveys is paramount and a prerequisite for adapting the monitoring design to best accommodate site-specific survey demands.
1.3. Aims and Objectives
 This study quantifies the impact of (i) the seasonal variability of signal strength and (ii) the FO-DTS sampling design and monitoring mode on the accuracy and limitation of FO-DTS surveys at aquifer-river interfaces. Its specific objectives are therefore to (1) analyze the seasonally variable signal strength represented by the temperature differences between groundwater and surface water and its diurnal variation; (2) quantify the impact of seasonal variation in signal strength on the detection accuracy of groundwater-surface exchange flow patterns by FO-DTS and identify seasonally variable uncertainties and limitations in signal detection; (3) compare the impact of standard single-ended and double-ended monitoring modes on the accuracy of the detection of size, location, and spatial extent of exchange flow patterns at the aquifer-river interface; (4) and present an alternative approach that combines the advantages of single-ended and double-ended monitoring modes for optimizing the detection accuracy of FO-DTS surveys.
 This study uses the investigation of exchange fluxes between aquifer and river in a well-investigated field site as a model system for quantifying the accuracy and uncertainty of FO-DTS application in dependency of seasonal signal variation and experimental design. The findings of this study, however, are transferable to other situations and also to FO-DTS applications in systems other than aquifer-river interfaces.