Terrestrial carbon export via inland aquatic systems is a key process in the global carbon cycle. It includes loss of carbon to the atmosphere via outgassing from rivers, lakes, or reservoirs and carbon fixation in the water column as well as in sediments. This review focuses on headwater streams that are important because their stream biogeochemistry directly reflects carbon input from soils and groundwaters. Major drivers of carbon dioxide partial pressures (pCO2) in streams and mechanisms of terrestrial dissolved inorganic, organic and particulate organic carbon (DIC, DOC, and POC) influxes are summarized in this work. Our analysis indicates that the global river average pCO2 of 3100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (> 500 km2). Because of their large proportion in global river networks (> 96% of the total number of streams), headwaters contribute large—but still poorly quantified—amounts of CO2 to the atmosphere. Conservative estimates imply that globally 36% (i.e., 0.93 Pg C yr−1) of total CO2 outgassing from rivers and streams originate from headwaters. We also discuss challenges in determination of CO2 sources, concentrations, and fluxes. To overcome uncertainties of CO2 sources and its outgassing from headwater streams on the global scale, new investigations are needed that should include groundwater data. Such studies would also benefit from applications of integral CO2 outgassing isotope approaches and multiscale geophysical imaging techniques.

Preferential flow has been hypothesized as an important factor for chemical leaching from tile‐drained agricultural fields with structured soils originating from glacial till sediments. Previous studies showed that one‐dimensional single‐porosity models (1D‐SPM) failed and that one‐dimensional dual‐permeability models (1D‐DPERM) were limited in explaining both Br leaching and residual Br distribution, although tile water outflow peaks could somehow be reproduced. The objective of this paper was to analyze the tile outflow and leaching patterns using a two‐dimensional (2D)‐DPERM and a standard 2D‐SPM for comparison. Flow and transport were simulated in a 2D vertical cross‐section of 5.9 m length and 2 m depth using previously tested parameters. Simulated drainage rates and Br‐effluent concentrations were made comparable with collector data from a field experiment by weighing results for irrigated and nonirrigated plots according to their area fractions. The 2D‐DPERM simulations for surface application of Br in dissolved form in both domains overestimated the observed initial outflow concentration peaks, in contrast to closer approximation of observations assuming Br application in the soil matrix domain only. The simulated 2D mass transfer rate distribution showed most intensive exchange between domains near the water table and in the topsoil. Results from the 2D‐DPERM analyses suggest that conditions at the soil surface, near the water table, and of the field‐scale mixing are significantly affecting leaching patterns, in addition to local nonequilibrium effects. Here, the description of preferential flow toward tile drain could be strongly improved with the 2D‐DPERM compared with the 2D‐SPM. Further improvements remain challenging with respect to DPERM numerical modeling and field experimentation, with special attention toward soil structure and soil surface conditions.

Soil water dynamics at an experimental hillslope site is studied by means of a one-dimensional dual-con nuum model. The model is based on Richards' equa on for ver cal soil water fl ow and the advec on-dispersion equa on for transport of the stable isotope 18 O. The water body contained in the soil-matrix pore space and the one transmi ed through the system of preferen al pathways are treated as two separate, mutually communicating soil water con nua. The 18 O isotope, monitored in precipita on, subsurface hillslope discharge, and soil water, was used as a natural tracer to study the role of preferen al fl ow in the forma on of shallow subsurface runoff . It is shown that the dual-con nuum approach can, in principle, explain the observed varia ons of 18 O content in the subsurface hillslope discharge. The model successfully describes mixing of new water, which refl ects the isotope signatures of the individual precipita on events, with old water, refl ec ng the seasonal variability of the isotope signal. Abbrevia ons: PF, preferen al fl ow; SM, soil matrix.Hydrological responses of hillslopes are determined by a number of factors associated with parent geological material, topography, climate, and vegetation. Diff erent runoff processes can be dominant in diff erent environments. One of these processes, which frequently dominates hillslope runoff in well-permeable, shallow soils of humid, temperate to cold climates, is the saturated subsurface fl ow along the soil-bedrock interface, oft en referred to as subsurface stormfl ow.

Disk infiltrometers are established as standard devices for measuring soil surface hydraulic properties. This study explored the validity of a semiempirical approach that is used to obtain estimates of the near‐saturated hydraulic conductivity from disk infiltrometer data. The approach was compared with two other estimation expressions. The analysis was based on three‐dimensional numerical modeling of the infiltration process, i.e., on synthetic data. The results of the validation procedure showed that the original expression performed best among the compared methods, but still failed for fine‐textured soils and the selected Cambisols. This is due to the overwhelming importance of lateral soil water movement by capillarity, which is not adequately addressed by any of the models. The study showed that improved estimates, specifically for fine‐textured soils and Cambisols and for small infiltrometer radii (minidisks), can be obtained by extending the original approach. This is achieved primarily (i) by using the modified van Genuchten parameterization of soil hydraulic functions instead of the original one, and (ii) by including a more representative set of soils in the objective function when optimizing the estimation formula.

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The phenolic profile of barley ( Hordeum vulgare L.) leaves, seeds, awns, and stems, collected in two different locations from Portugal, was determined by a high-performance liquid chromatography/diode array detector (HPLC/DAD). A total of 28 compounds were identified and quantified, which included 4 phenolic acids, 6 C-glycosylflavones, and 18 O-glycosyl-C-glycosyl flavones, with some of them acylated. Distinct profiles were noticed among the analyzed materials. The greatest diversity of compounds was found in barley leaves (26 flavonoids and 2 phenolic acid derivatives), which also exhibited the highest concentration of phenolics. Isoorientin-7-O-glucoside (lutonarin) was the major compound in leaves, while, in general, the pair isovitexin-7-O-rutinoside plus isoscoparin-7-O-glucoside were the main phenolics in the other materials. Thus, barley leaves may constitute an important dietary source of protective compounds, which could be used, for example, to take profit from the wastes resulting from alcoholic drink obtainment.

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The simulation of preferential flow in structured soil using dual‐porosity models requires separate sets of the hydraulic, transport, and mass transfer parameters and of the boundary conditions. Analyses of tracer experiments with two‐domain models are limited by constraints in specifying separate boundary conditions (BCs) for each pore domain. The appropriate boundary conditions for dual‐permeability models have not been systematically studied or addressed in experiments. The objective of this study was to numerically evaluate the effects on Br leaching from a tile‐drained field of different surface boundary conditions for a two‐dimensional dual‐permeability model. For the previously described Br tracer experiment at the Bokhorst site (Germany), effects of irrigation intensities, flux‐ and resident‐type solute BCs, and Br application domain on drain discharge and Br effluent concentrations were compared. The two‐dimensional dual‐permeability numerical model divides the soil into soil matrix (SM) and preferential flow (PF) domains. In case of ponding at the surface of the SM domain, water is redistributed toward the PF domain surface. The combination of detailed irrigation record, flux‐type solute BCs, and solute application to the PF domain resulted in the largest Br leaching. In contrast, the lowest Br leaching was predicted for averaged irrigation rates, flux‐type solute BCs and Br addition to the SM domain. For flux‐type (third‐type) solute BC application to both pore domains, enhanced Br leaching was obtained due to surface redistribution effects. Detailed irrigation patterns with realistic intensities predicted higher Br leaching than averaged intensities. In addition to surface effects, the temporal availability of Br in the preferential pathways seemed to control Br leaching patterns. The results suggest that the formulation of the upper BCs strongly affects two‐dimensional dual‐permeability Br leaching predictions. Proper experimental consideration of domain‐specific BCs may help improve descriptions of preferential flow.