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We provide an overview of the Rur hydrological observatory, which is the main observational platform of the TERENO (TERrestrial ENvironmental Observatories) Eifel/Lower Rhine Valley Observatory. The Rur catchment area exhibits distinct gradients in altitude, climate, land use, soil properties, and geology. The Eifel National Park is situated in the southern part of the Rur catchment and serves as a reference site for the hydrological observatory. We present information on general physical characteristics of the Rur catchment and describe the main features of the multi-scale and multi-compartment monitoring framework. In addition, we also present some examples of the ongoing interdisciplinary research that aims to advance the understanding of complex hydrological processes and interactions within the Rur catchment.

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Accurate estimates of soil hydraulic parameters and dispersivities are crucial to simulate water flow and solute transport in terrestrial systems, particularly in the vadose zone. However, parameters obtained from inverse modeling can be ambiguous when identifying multiple parameters simultaneously and when boundary conditions are not well known. Here, we performed an inverse modeling study in which we estimated soil hydraulic parameters and dispersivities of layered soils from soil water content, matric potential, and stable water isotope (d 18 O) measurements in weighable lysimeter systems. We used different optimization strategies to investigate which observation types are necessary for simultaneously estimating soil hydraulic and solute transport parameters. Combining water content, matric potential, and tracer (e.g., d 18 O) data in one objective function (OF) was found to be the best strategy for estimating parameters that can simulate all observed water flow and solute transport variables. A sequential optimization, in which first an OF with only water flow variables and subsequently an OF with transport variables was optimized, performed slightly worse indicating that transport variables contained additional information for estimating soil hydraulic parameters. Hydraulic parameters that were obtained from optimizing OFs that used either water contents or matric potential could not predict non-measured water flow variables. When a bromide (Br − ) tracer experiment was simulated using the optimized parameters, the arrival time of the bromide pulse was underestimated. This suggested that Br − sorbed onto clay minerals and amorphous oxides under the prevailing geochemical conditions with low pH values. When accounting for anion adsorption in the simulation, Br − concentrations were well predicted, which validated the dispersivity parameterization.Abbreviations: 2SOS, two-step optimization strategy; AV-NSE, average Nash-Sutcliffe efficiency; BOS Bi-objective optimization strategy; BTC, breakthrough curve; D L , longitudinal dispersivity; ET, actual evapotranspiration; LAI, leaf area index; MOS, multi-objective optimization strategy; NSE, Nash-Sutcliffe Quantification of water fluxes and fluxes of dissolved substances in the vadose zone is important to resolve a number of environmental issues. These issues comprise (i) the protection of groundwater resources, which is the main source of drinking water in many regions of the world (Aeschbach-Hertig and Gleeson, 2012;Taylor et al., 2013), both in terms of groundwater quantity and quality, and (ii) optimizing crop production by making efficient use of water, fertilizers, and plant protection products. Simulation models are used to link known fluxes at the upper boundary of the vadose zone with fluxes at different depths in the vadose zone and related state variables such as water contents, matric potentials, and solute concentrations. These simulation models require accurate and precise information about the properties of the vadose zone that link fluxes with sta...