High resolution temperature observations to identify different runoff processes

Headwater catchments are important contributors to streamflow. They are small, but all combined they influence river flow significantly. To be able to make proper runoff predictions under different climate conditions and changing land use, it is important to have detailed understanding of the discharge processes in the headwater catchments. In this thesis we explore the possibilities of fibre optic Distributed Temperature Sensing (DTS) to obtain more insight in temporal and spatial discharge dynamics during stormflow. DTS is a technique capable of measuring temperature with high spatial and temporal resolution. It was developed in the 1980’s by the oil industry but only recently (in 2006) it has been introduced into the field of hydrology. The technique relies on short laser pulses that are sent through a fibre optic cable. Throughout the fibre optic cable small parts of the pulses are reflected by disturbances in the glass fibre, of which the exact position is obtained by measuring the travel time of the reflected light. The frequency shifts of the reflected light are then used to calculate the temperature of the fibre optic cable at the point of reflection. The DTS system used in this thesis has a spatial resolution of 2 m and a temporal resolution of 3 min. With these settings a precision of 0. 1_C is obtained. All experiments described in this thesis were done in the Maisbich: a 565 m long, first order stream in Central Luxembourg. The lithology of the Maisbich consists of schist which is covered with a layer of loose rock clasts with variable clay content of a few meters thick and a thin layer of fine sediments. Summer baseflow in the Maisbich can be <1 l s? 1, while the annual maximum discharge is in the order of 50-100 l s? 1. The stream reach has six distinct sources and two areas of significant infiltration losses. In order to be able to quantify the hydrological fluxes within the Maisbich headwater using temperature as a tracer, we had to calculate the temperature of the stream water independently. Therefore, we coupled an energy balance model with a routing and advection-dispersion model. Through this coupling we could treat temperature as a high resolution conservative tracer. The difference between observed and simulated temperature should then be caused by different exchange fluxes, which subsequently can be quantified. For an adequate temperature model many processes need to be included and described. Therefore a large part of this thesis consists of the quest for a perfect temperature model. During this quest a couple of new insights were gained. First of all, heat exchange with in-stream rock clasts appeared to be important. The many abundant rock clasts present on top of the streambed store heat, resulting in retarded heat transport. We showed that during winter flow the heat was retarded for more than 30 min after being transported over 565 m. By using a pin-meter we were able to observe the amount of rock clasts present in the cross-sectional area of the stream. Due to the fast heat exchange between water and rock clasts, we could simplify the model by assuming that the temperature of in-stream rock clasts is always the same as the water temperature. During the quest for the perfect model, we also developed a new method to quantify surface water-groundwater interactions (or hyporheic exchange). Stream water infiltrates into the subsurface (hyporheic zone) where it remains for a while before it returns to the stream. This process influences in-stream temperature, making it possible to quantify this flux. Because of the high resolution temperature observations, we were able to show the large spatial variability in hyporheic exchange. Compared to many other in-stream tracer studies that use conventional stream tracers, we could quantify hyporheic exchange at a high resolution with more flexibility in experimental design. After the inclusion of heat exchange with in-stream rock clasts and hyporheic exchange, observed daily temperature fluctuations could be adequately simulated during steady state discharge conditions. In the last step we extended the model to unravel spatial and temporal dynamics in discharge during a short but intensive summer rainstorm. Using the model as a learning tool, we showed that for such an event, gains of water remained constant over the event, stream losses increased with increasing discharge, and hyporheic exchange appeared to increase with discharge for part of the stream. From the modelling results, we also concluded that a side channel becomes active several hours after the start of the rainfall event. Although we were able to quantify discharge dynamics at high temporal and spatial resolution, which was the main objective and trigger for this research, we were only able to do this for low flow conditions during summer. During higher flows, larger rainstorms or less favourable meteorological conditions, observed temperature differences appeared to be too small to distinguish between different fluxes. A higher accuracy in temperature observation would decrease this problem. At this moment, DTS systems with a spatial resolution of 25 cm are already available while advanced calibration techniques improve the accuracy significantly. These developments make it possible to quantify sources even when there are small temperature differences in the stream. Another way forward is to artificially heat up or cool down the stream water although this would require large amounts of energy which may have their practical limitations as well. This study has shown that high resolution temperature observations can be instrumental in understanding detailed processes in hydrological systems. With the advance of this and other high resolution tracer techniques more new breakthroughs in understanding hillslope processes