Concentrations of dissolved organic carbon (DOC) in runoff from catchments are often subject to substantial short-term variations. The aim of this study was to identify the compartmental sources of DOC in a forested catchment and the causes for short-term variations in runoff. Furthermore, we investigated the implication of short-term variations for the calculation of annual runoff fluxes. High frequency measurements (30 min intervals) of DOC in runoff, of discharge and groundwater table were conducted for one year in the 4.2 km² forested Lehstenbach catchment, Germany. Riparian wetland soils represent about 30% of the catchment area. The quality of DOC was investigated by three dimensional fluorescence excitation-emission matrices in samples taken from runoff, deep groundwater and shallow groundwater from the riparian wetland soils. The concentrations of DOC in runoff were highly variable at an hourly to daily time scale, ranging from 2.6 mg L⁻¹ to 34 mg L⁻¹ with an annual average of 9.2 mg L⁻¹. The concentrations were positively related to discharge, with a counter clockwise hysteresis. Relations of DOC to discharge were steeper and the degree of hysteresis larger in the summer/fall than in the winter/spring period. Dynamics of groundwater table, discharge, DOC concentrations and DOC quality parameters indicated that DOC in runoff originated mainly from the riparian wetland soils, both under low and high flow conditions. The annual export of DOC from the catchment was 84 kg C ha⁻¹ yr⁻¹ when calculated from the high frequency measurements. If the annual export was calculated by simulated samplings of >2 days intervals substantial deviations resulted. Predicted changes in precipitation and discharge patterns as well as generally increasing temperatures likely will cause raising DOC exports from this catchment.

Denitrification is the main process removing nitrate in river drainage basins and buffer input from agricultural land and limits aquatic ecosystem pollution. However, the identification of denitrification hotspots (for example, riparian zones), their role in a landscape context and the evolution of their overall removal capacity at the drainage basin scale are still challenging. The main approaches used (that is, mass balance method, denitrification proxies, and potential wetted areas) suffer from methodological drawbacks. We review these approaches and the key frameworks that have been proposed to date to formalize the understanding of the mechanisms driving denitrification: (i) Diffusion versus advection pathways of nitrate transfer, (ii) the biogeochemical hotspot, and (iii) the Damköhler ratio. Based on these frameworks, we propose to use high-resolution mapping of catchment topography and landscape pattern to define both potential denitrification sites and the dynamic hydrologic modeling at a similar spatial scale (<10 km²). It would allow the quantification of cumulative denitrification activity at the small catchment scale, using spatially distributed Damköhler and Peclet numbers and biogeochemical proxies. Integration of existing frameworks with new tools and methods offers the potential for significant breakthroughs in the quantification and modeling of denitrification in small drainage basins. This can provide a basis for improved protection and restoration of surface water and groundwater quality.

Assessing the potential for transfer of pollutants and nutrients across catchments is of primary importance under changing land use and climate. Over the past decade the connectivity/disconnectivity dynamic of a catchment has been related to its potential to export material; however, we continue to use multiple definitions of connectivity, and most have focused strongly on physical (hydrological or hydraulic) connectivity. In contrast, this paper constantly focuses on the dynamic balance between transport and material transformation, and defines material connectivity as the effective transfer of material between elements of the hydrological cycle. The concept of exposure timescales is developed and used to define three distinct regimes: (i) which is hydrologically connected and transport is dominated by advection; (ii) which is hydrologically connected and transport is dominated by diffusion; and (iii) which is materially isolated. The ratio of exposure timescales to material processing timescales is presented as the non-dimensional number, N_E, where N_E is reaction-specific and allows estimation of relevant spatial scales over which the reactions of interest take place. Case studies within each regime provide examples of how N_E can be used to characterise systems according to their sensitivity to shifts in hydrology and gain insight into the biogeochemical processes that are signficant under the specified conditions. Finally, we explore the implications of the N_E framework for improved water management, and for our understanding of biodiversity, resilience and chemical competitiveness under specified conditions.