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In 2008 the TERENO project was initiated with the general aim to conduct integrated and long-term observation studies of climate change and global change impacts on the terrestrial system across Germany. Central elements of the TERENO network are "terrestrial observatories" at the catchment scale which were selected in climate sensitive regions of Germany. Within these observatories small scale research facilities and test areas were placed in order to accomplish energy, water, carbon and nutrient process studies across the different compartments of the terrestrial environment. Following a hierarchical scaling approach (point-plot-field) these detailed information and the knowledge gained is transferred to the regional scale using integrated modelling approaches. In addition, mobile ground-based and airborne measurement platforms are used to monitor spatial patterns of dynamic processes at the regional scale. By combining the observatories within Germany, large-scale atmospheric feedbacks and impacts can be investigated, and thus a more pronounced general link to the atmospheric research community can be established. A data policy statement was developed in a common approach by all TERENO partners. As a rule, all TERENO data are freely accessible for the scientific community and a data portal facilitates the online data provision. In the ten years since its foundation, TERENO has become a key infrastructure in the German and international research landscape and has made a significant contribution to the development of integrated approaches in long-term environmental research. In this contribution the concept of TERENO will be illustrated with example key findings of the ongoing interdisciplinary research work aiming to advance the understanding of complex hydrological and biogeochemical processes and interactions. [ABSTRACT FROM AUTHOR]

The time precipitation needs to travel through a catchment to its outlet is an important descriptor of a catchment's susceptibility to pollutant contamination, nutrient loss and hydrological functioning. The fast component of total water flow can be estimated by the fraction of young water (Fyw) which is the percentage of streamflow younger than three months. Fyw is calculated by comparing the amplitudes of sine waves fitted to seasonal precipitation and streamflow tracer signals. This is usually done for the complete tracer time series available neglecting annual differences in the amplitudes of longer time series. Considering inter-annual amplitude differences, we here employed a moving time window of one-year length in weekly time steps over a 4.5-years 18O tracer time series to calculate 189 Fyw results. The results were then tested against the following null hypotheses, defining 2% difference in Fyw as significant based on results of previous studies: (1) Fyw does not deviate more than ±2% from the mean of all Fyw results indicating long-term invariance. Larger deviations would indicate either flow path changes or a change in the relative contribution of different flow paths; (2) for any four-week window Fyw does not change more than ±2% indicating short-term invariance. Larger deviations would indicate a high sensitivity of Fyw to a 1-4 weeks shift in the start of a one-year sampling campaign; (3) for a given calendar month Fyw does not change more than ±2% indicating seasonal invariance of Fyw. In our study, all three null hypotheses were rejected. Thus, the Fyw results were time-variable, showed a high variability in the chosen sampling time and had no pronounced seasonality. Based on high short-term variability of Fyw when the mean adjusted R² was below 0.2 we recommend that a low R² should be regarded as indicating potentially highly uncertain Fyw results. Furthermore, while investigated individual meteorological factors could not sufficiently explain variations of Fyw, the runoff coefficient showed a moderate negative correlation of r = -0.54 with Fyw. This indicated that when annual runoff exceeded precipitation the catchment received the water deficit from storage which is old water causing a decrease in Fyw. The results of this study suggest that care must be taken when comparing Fyw of catchments that were based on different calculation time periods. [ABSTRACT FROM AUTHOR]

The exchange of water between atmosphere, biosphere, and hydrosphere can be viewed as a result of complex interactions of dynamic feedback mechanisms, where soil moisture content acts as the key state variable. Approaches are required to handle land surface complexity and scale dependency of water fluxes. State-of-the-art observations of soil moisture content are ranging from continuous point-scale measurements via field-scale snapshots to remote sensing products on the basin scale and beyond. They have to deal with a space-time trade-off since the measurement frequency typically decreases with the covered spatial scale. Cosmic-ray neutron sensing is one of the growing techniques to move soil hydrological observations forward. We have established a research unit (DFG FOR 2694) named "Cosmic Sense" to jointly improve such measurements with or in combination with cosmic-ray neutron sensing, especially in annual joint field campaigns at small catchment scale. Key aims are to- bridge existing gaps between scales through additional techniques and sources of information about the soil water storage. Cosmic-ray neutron sensing measures the presence of water by sensing changes in neutron density above the ground. We want to develop a quantitative, adaptable, and transferable approach for observing representative soil moisture content values on the field scale while accounting for other dynamic water pools on the land surface, such as biomass, interception, and snow. Furthermore, by sensor clusters and mobile neutron detectors the mapping of soil moisture can be transferred to larger scales. - bridge gaps between disciplines. Cosmic-ray neutron sensing shall function as a unique combination of invasive and non-invasive observations in joint field campaigns. The measurement and interpretation of soil moisture by Cosmic-ray neutron sensing requires advanced knowledge of soil hydrology as well as particle physics including neutron transport modeling. Efforts will be made for detector development that aims to substantially improve the temporal resolution and spatial coverage. - foster an approach that is especially tailored to resolve the discrepancy in support volume and timing between hydrological models and field observations. Comprehensive observations by cosmic-ray neutron sensing, remote sensing and complementary methods will be used concertedly with hydrological and land-surface models at the regional scale to infer also groundwater recharge and atmospheric fluxes. Thereby, models and observations together will allow for identifying patterns and processes at the scale of small catchments with improved spatio-temporal resolution. [ABSTRACT FROM AUTHOR]