Description: Throughfall, i.e. the fraction of rainfall that passes through the forest canopy, is strongly influenced by rainfall and forest stand characteristics which are in turn both subject to seasonal dynamics. Disentangling the complex interplay of these controls is challenging, and only possible with long-term monitoring and a large number of throughfall events measured in parallel at different forest stands. We therefore based our analysis on 346 rainfall events across six different forest stands at the long-term terrestrial environmental observatory TERENO Northeast Germany. These forest stands included pure stands of beech, pine and young pine, and mixed stands of oak-beech, pine-beech and pine-oak-beech. Throughfall was overall relatively low, with 54-68% of incident rainfall in summer. Based on the large number of events it was possible to not only investigate mean or cumulative throughfall but also its statistical distribution. The distributions of throughfall fractions show distinct differences between the three types of forest stands (deciduous, mixed and pine). The deciduous stands have a pronounced peak at low throughfall fractions and a secondary peak at high fractions in summer, as well as a pronounced peak at higher throughfall fractions in winter. Interestingly, the mixed stands behave like deciduous stands in summer and like pine stands in winter: their summer distributions are similar to the deciduous stands but the winter peak at high throughfall fractions is much less pronounced. The seasonal comparison further revealed that the wooden components and the leaves behaved differently in their throughfall response to incident rainfall, especially at higher rainfall intensities. These results are of interest for estimating forest water budgets and in the context of hydrological and land surface modeling where poor simulation of throughfall would adversely impact estimates of evaporative recycling, water availability for vegetation and runoff.

Description: Lake-level reconstructions are a key tool in hydro-climate reconstructions, based on the assumption that lake-level changes primarily reflect climatic changes. Although it is known that land cover changes can affect evapotranspiration and groundwater formation, this factor commonly receives little attention in the interpretation of past lake-level changes. To address this issue in more detail, we explore the effects of land cover change on Holocene lake-level fluctuations in Lake Tiefer See in the lowlands of northeastern Germany. We reconstruct lake-level changes based on the analysis of 28 sediment records from different water depths and from the shore. We compare the results with land cover changes inferred from pollen data. We also apply hydrological modelling to quantify effects of land cover change on evapotranspiration and the lake level. Our reconstruction shows an overall lake-level amplitude of about 10 m during the Holocene, with the highest fluctuations during the Early and Late Holocene. Only smaller fluctuations during the Middle Holocene can unambiguously be attributed to climatic fluctuations because the land cover was stable during that period. Fluctuations during the Early and Late Holocene are at least partly related to changes in natural and anthropogenic land cover. For several intervals the reconstructed lake-level changes agree well with variations in modelled groundwater recharge inferred from land cover changes. In general, the observed amplitudes of lake-level fluctuations are larger than expected from climatic changes alone and thus underline that land cover changes in lake catchments must be considered in climatic interpretations of past lake-level fluctuations.

Description: Der Kronendurchlass, d. h. der Anteil des Niederschlags, der durch das Kronendach des Waldes dringt, wird stark von der Art des Niederschlags und den Eigenschaften des Waldbestands beeinflusst. Das komplexe Zusammenspiel dieser Faktoren inklusive ihrer jahreszeitlichen Veränderungen zu entschlüsseln, ist eine große wissenschaftliche Herausforderung und nur mit langjährigem Monitoring in verschiedenen Waldbeständen möglich. Das Langzeit-Umweltobservatorium TERENO Nord-Ost zur Erforschung der regionalen Auswirkungen des Globalen Wandels liefert hierfür ideale Voraussetzungen.

Description: The impact of rainfall events in terms of erosion, transport and flood generation strongly depends on the flow paths along which the water travels. These flow paths are influenced by initial conditions (wetness state) and driving forces (rainfall amounts and intensities) but also by the characteristics of the terrain and the subsurface. While we usually think of the first two as temporally variable, we often consider terrain and subsurface characteristics to be static. However, both surface and subsurface structures evolve over time. Knowledge of this evolution can help us to understand landscape patterns of flow processes. It is furthermore of special interest in alpine settings where the retreat of glaciers due to global warming leads to accelerated exposure of glacial moraines. We investigated how these “newborn” hillslopes respond to hydrological forcing and how structures and processes will change over time by making use of moraine chronosequences covering the first 10 millennia of landscape development. We analyzed two hillslope chronosequences in glacial forelands in the Swiss Alps, comparing siliceous and calcareous parent material. For each age class we determined soil properties and vegetation characteristics. We then investigated the hydrologic functioning of the slopes by analyzing soil moisture response times, soil water storage, dominant flow path types, and the frequency of preferential flow occurrence. Flow paths were identified by dye tracer experiments at the plot scale and irrigation experiments with deuterium-labelled water at the hillslope scale. A principal component analysis and clustering were used to identify how structure relates to hydrological responses.

Description: Salt dilution is a well-established streamflow measurement method in creeks, which works particularly well downstream of turbulent flow sections as the mixing of the salt tracer is enhanced. Usually, salt dilution measurements are performed manually, which considerably limits the observations of rare peak flow events. These events are particularly important for constructing robust rating curves and avoiding large uncertainties in the extrapolation of streamflow values. An additional challenge is the variability of the river cross section, especially after larger discharge events, leading to nonstationary rating curves. Therefore, discharge measurements well distributed over time are needed to construct a reliable streamflow–water level relationship and to detect changes caused by erosion and deposition processes. To overcome these two issues, we used an automated streamflow measuring systems at three different sites with contrasting hydrological and hydraulic characteristics in the Alps. This system allowed us to measure discharge at nearly maximum flow of the observation period (2020–2021) at all three sites and to detect abrupt changes in the rating curve by performing event-based salt injections. The uncertainty in the measurements was quantified, and the streamflow was compared with official gauging stations in the same catchment. Based on a very large dataset of almost 300 measurements, we were able to evaluate the reliability of the system and identify the primary sources of uncertainty in the experimental setup. One key aspect was the site selection for the downstream electrical conductivity sensors, as measurement location strongly controls the signal-to-noise ratio in the recorded breakthrough curves.

Description: Tree-specific canopy conductance (Gc) and its adjustment play a critical role in mitigating excess water loss in changing environmental conditions. However, the change of Gc sensitivity to environmental conditions due to drought remains unclear for European tree species. Here we quantified the environmental operational space of Gc, i.e., the water supply (soil moisture, tree water deficit) and demand conditions (vapor pressure deficit) under which Gc ≥ 50% is possible (Gc50OS), at two sites with different soil water availability for three common European tree species. We collected sap flow and dendrometer measurements for co-occurring Pinus sylvestris, Fagus sylvatica and Quercus petraea growing under different soil hydrological conditions (drier/wetter). These measurements were combined with meteorological variables and soil moisture conditions in five depths. Dendrometer measurements were used to confirm soil water availability patterns. For all analyses, the contrasting soil hydrology between sites was the main driver of Gc response. At the drier sites, F. sylvatica and P. sylvestris reduced their water consumption in response to decreasing soil water supply earlier in the growing season than Q. petraea. However, our analysis on the Gc50OS revealed that at the drier sites, F. sylvatica and Q. petraea reduced the extent of their Gc50OS to a higher degree than P. sylvestris. This indicates a higher level of Gc50OS adjustment to the drier site conditions for the two broadleaved species. These differences were more pronounced when using the dendrometer-derived tree internal water status as proxy for tree water supply. Our results provide preliminary evidence for diverging short-term Gc responses when temperate trees are exposed to prolonged reduction in water availability. These findings suggest that Gc50OS can help to constrain species-specific predictions of water use by mature trees, especially when combined with high-resolution water potential measurements.

Description: Form and function are two major characteristics of hydrological systems. While form summarizes the structure of the system, function represents the hydrological response. Little is known about how these characteristics evolve and how form relates to function in young hydrological systems. We investigated how form and function evolve during the first millennia of landscape evolution. We analyzed two hillslope chronosequences in glacial forelands, one developed from siliceous and the other from calcareous parent material. Variables describing hillslope form included soil physical properties, surface, and vegetation characteristics. Variables describing hydrological function included soil water response times, soil water storage, drainage, and dominant subsurface flow types. We identified links between form and hydrological function via cluster analysis. Clusters identified based on form were compared in terms of their hydrological functioning. The comparison of the two different parent materials shows how strongly landscape evolution is controlled by the underlying geology. Soil pH appears to be a key variable influencing vegetation, soil formation and subsequently hydrology. At the calcareous site, the high buffering capacity of the soil leads to less soil formation and fast, vertical subsurface water transport dominates the water redistribution even after more than 10,000 years of landscape evolution. At the siliceous site, soil acidification results in accumulation of organic material, a high water storage capacity, and in podsolization. Under these conditions water redistribution changes from vertical subsurface water transport at the young age classes to water storage in the organic surface layer and lateral subsurface water transport within 10,000 years.

Description: The data set was collected to identify hydrological processes and their evolution over it time. It consists of several individual files in tabstop delimeted text format. The data set contains the data obtained from deuterium and brilliant blue tracer experiments at two chronosequence studies in the glacier forefield of the Stone Glacier and the Griessfirn in the central Alps, Switzerland. Each chronosequence consisted of four moraines of different ages (from 30 to 13500 years). At each forefield sprinkling experiments with deuterium and dye tracer experiments with blue dye (Brilliant Blue) were conducted on three plots per moraine. The moraines at the forefield of the Stone Glacier developed from siliceous parent material and at the forefield of the Griessfirn from calcareous parent material. Data from the siliceous forefield are marked with (S) and data from the calcareous forefield are marked with (C). The data set consist of soil moisture time series and soil water isotope profiles of the sprinkling experiments with deuterium, as well as trinary images of stained vertical subsurface flow paths from the dye tracer experiment. The individual plots per moraine are distinguished via their position relative to one another on the moraine (left, middle, and right, looking upslope). The plots used for the sprinkling experiments were located in close vicinity to the plots used for the dye tracer experiments. For the sprinkling experiments with deuterium each plot (4m x 6m) per age class was equipped with 6 soil moisture sensors. Three of these sensors were installed as a sensor profile at one side of the plot about one meter downslope from the upper plot boundary. The sensors were installed at 10, 30, and 50 cm soil depth. On the other side of the plot, two sensors were placed in 10 cm depth, one opposite to the sensor profile and the second sensor one meter upslope from the lower plot boundary. The sixth sensor was placed at 10 cm depth in the center of the plot. The plots were irrigated on three consecutive days with three different irrigation intensities and deuterium concentrations. Per forefield, the soil moisture data are listed in one file per age class. The file contains for each plot, the time stamp and the soil moisture values of the 6 sensors.

Description: Above-ground cosmic-ray neutron sensing (CRNS) allows for the non-invasive estimation of the field-scale soil moisture content in the upper decimetres of the soil. However, large parts of the deeper vadose zone remain outside of its observational window. Retrieving soil moisture information from these deeper layers requires extrapolation, modelling or other methods, all of which come with methodological challenges. Against this background, we investigate CRNS for downhole soil moisture measurements in deeper layers of the vadose zone. To render calibration with in situ soil moisture measurements unnecessary, we rescaled neutron intensities observed below the terrain surface with intensities measured above a waterbody. An experimental set-up with a CRNS sensor deployed at different depths of up to 10 m below the surface in a groundwater observation well combined with particle transport simulations revealed the response of downhole thermal neutron intensities to changes in the soil moisture content at the depth of the downhole neutron detector as well as in the layers above it. The simulation results suggest that the sensitive measurement radius of several decimetres, which depends on soil moisture and soil bulk density, exceeds that of a standard active neutron probe (which is only about 30 cm). We derived transfer functions to estimate downhole neutron signals from soil moisture information, and we describe approaches for using these transfer functions in an inverse way to derive soil moisture from the observed neutron signals. The in situ neutron and soil moisture observations confirm the applicability of these functions and prove the concept of passive downhole soil moisture estimation, even at larger depths, using cosmic-ray neutron sensing.

Description: The need to develop and provide integrated observation systems to better understand and manage global and regional environmental change is one of the major challenges facing Earth system science today. In 2008, the German Helmholtz Association took up this challenge and launched the German research infrastructure TERrestrial ENvironmental Observatories (TERENO). The aim of TERENO is the establishment and maintenance of a network of observatories as a basis for an interdisciplinary and long-term research program to investigate the effects of global environmental change on terrestrial ecosystems and their socio-economic consequences. State-of-the-art methods from the field of environmental monitoring, geophysics, remote sensing, and modeling are used to record and analyze states and fluxes in different environmental disciplines from groundwater through the vadose zone, surface water, and biosphere, up to the lower atmosphere. Over the past 15 years we have collectively gained experience in operating a long-term observing network, thereby overcoming unexpected operational and institutional challenges, exceeding expectations, and facilitating new research. Today, the TERENO network is a key pillar for environmental modeling and forecasting in Germany, an information hub for practitioners and policy stakeholders in agriculture, forestry, and water management at regional to national levels, a nucleus for international collaboration, academic training and scientific outreach, an important anchor for large-scale experiments, and a trigger for methodological innovation and technological progress. This article describes TERENO's key services and functions, presents the main lessons learned from this 15-year effort, and emphasizes the need to continue long-term integrated environmental monitoring programmes in the future.