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 programme 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 modelling 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 modelling 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 emphasises the need to continue long-term integrated environmental monitoring programmes in the future.

The Critical Zone (CZ) evolves through weathering and erosion processes that shape landscapes and control the availability and quality of natural resources. Although many of these processes take place in the deep CZ ($\sim$10-100 m), direct information about its architecture remain scarce. Near-surface geophysics offer cost-effective and minimally-intrusive alternatives to drilling that can provide information about the physical properties of the CZ. We propose a novel workflow combining geophysics, petrophysics and geostatistics to characterize the architecture of the CZ (i.e., weathering front and water table depths) at the catchment scale, on the volcanic tropical island of Basse-Terre (Guadeloupe, France). Our results highlight two spatial organizations patterns for the weathering front and the water table, one along the stream and one transverse to it. This illustrates the robustness and strong potential of the proposed workflow to study hydrological and weathering processes in the CZ.

Few of the large Southern peri-alpine lakes have been studied with a sedimentological approach in their deep basin to understand the dynamics of their long-term sedimentation due, among other factors, to the high complexity of the coring in such deep lakes. In 2018, a 15.5 m-long sediment section was retrieved from the deep basin of Lake Iseo (Italy) at 251 m of water depth. Seismic survey associated to a multi-proxy approach with sedimentological and geochemical analyses, reveals a high number of event layers that corresponds to 61.4 % of the total sedimentation during the last 2000 years. The great heterogeneity of textures, colours, and grain-size distribution between the different types of event layers can be explained by the high number of potential sources of sediment inputs in this large lake system. By combining proxies for sediment source with transport processes, we were able to distinguish: i) flood events, and ii) destabilisations of slopes and deltas due to an increase of the sediment load and/or to seismic shaking. From a thorough comparison with both, the regional climatic fluctuations, and the human activity in the watershed, it appears that periods of high sediment remobilization can be linked to a previous increase in Critical Zone erosion in the watershed mainly under human forcing. Hence, even in large catchments, human activities play a key role on erosion processes and on sediment availability, disrupting the recording of the Critical Zone functioning in such lacustrine archive.