[1] Recent studies suggest that ebullition of biogenic gas bubbles is an important process of CH 4 transfer from northern peatlands into the atmosphere and, as such, needs to be better described by models of peat carbon dynamics. We develop and test a simple ebullition model in which a threshold gas volume in the peat has to be exceeded before ebullition occurs. The model assumes that the gas volume varies because of gas production and variations in pressure and temperature. We incubated peat cores in the laboratory for 190 days and measured their volumetric gas contents and the ebullition flux. The laboratory results support the threshold concept and, considering the simplicity of the model, the calculated ebullition compared well with measured fluxes during the final 120 days with an r 2 of 0.66. An improved, more realistic description would also include temporal and spatial variations in gas production and bubble retention terms.

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Abstract. Increasing dissolved organic carbon (DOC) concentrations and exports from headwater catchments impact the quality of downstream waters and pose challenges to water supply. The importance of riparian zones for DOC export from catchments in humid, temperate climates has generally been acknowledged, but the hydrological controls and biogeochemical factors that govern mobilization of DOC from riparian zones remain elusive. A high-frequency dataset (15 min resolution for over 1 year) from a headwater catchment in the Harz Mountains (Germany) was analyzed for dominant patterns in DOC concentration (CDOC) and optical DOC quality parameters SUVA254 and S275−295 (spectral slope between 275 and 295 nm) on event and seasonal scales. Quality parameters and CDOC systematically changed with increasing fractions of high-frequency quick flow (Qhf) and antecedent hydroclimatic conditions, defined by the following metrics: aridity index (AI60) of the preceding 60 d and the quotient of mean temperature (T30) and mean discharge (Q30) of the preceding 30 d, which we refer to as discharge-normalized temperature (DNT30). Selected statistical multiple linear regression models for the complete time series (R2=0.72, 0.64 and 0.65 for CDOC, SUVA254 and S275−295, resp.) captured DOC dynamics based on event (Qhf and baseflow) and seasonal-scale predictors (AI60, DNT30). The relative importance of seasonal-scale predictors allowed for the separation of three hydroclimatic states (warm and dry, cold and wet, and intermediate). The specific DOC quality for each state indicates a shift in the activated source zones and highlights the importance of antecedent conditions and their impact on DOC accumulation and mobilization in the riparian zone. The warm and dry state results in high DOC concentrations during events and low concentrations between events and thus can be seen as mobilization limited, whereas the cold and wet state results in low concentration between and during events due to limited DOC accumulation in the riparian zone. The study demonstrates the considerable value of continuous high-frequency measurements of DOC quality and quantity and its (hydroclimatic) key controlling variables in quantitatively unraveling DOC mobilization in the riparian zone. These variables can be linked to DOC source activation by discharge events and the more seasonal control of DOC production in riparian soils.

Microtopography can have a large eff ect on fl ow processes at the soil surface and the composi on of soil water. Microtopography is o en represented by a roughness parameter in hydrological models. In areas without a strong topographical gradient, microtopography may be underes mated when accumulated in a single parameter, especially in shallow groundwater systems. This study reviews the intricate rela onships between microtopography, surface runoff , and ecohydrology in systems featuring shallow water tables. We specifi cally focus on rela ons between microtopography and runoff , impact of microtopography on response mes of shallow groundwater ecosystems, and microtopography and spa al distribu on of groundwater quality parameters and site factors. We advocate the use of microtopography in modeling approaches by examples that feature typical ecosystems with shallow groundwater under infl uence of microtopography. With a simple modeling approach, we show how microtopography could add fl exibility to the acrotelmcatotelm concept in raised bog hydrology. The classic acrotelm-catotelm concept hinders progress in understanding small scale hydrological varia ons and other ecohydrological rela ons. Furthermore, we illustrate possible self-organiza on proper es of wetlands. Finally, we show how microtopography and surface runoff aff ect the mixing of water with diff erent chemical signatures, resul ng in varia ons of the occurrence of plant species.