4 Summary and Conclusions
STUDY 3 a possible response of wetland biogeochemistry to the complex subsurface flow patterns and the
dynamic shifts in hydrological and biogeochemical boundary conditions induced by the hummocky topographies typical for wetland systems. These processes and the resulting heterogeneous distribution of redox- sensitive solutes including the formation of local hot spots can be simulated using the rill storage concept.
On the plot scale, the rill storage concept can be an efficient way to represent the impact of micro-topography on hydrological processes. As shown for the synthetic wetland model, grid resolution can be significantly reduced by using spatially distributed rill storage zones resulting in increased computational efficiencies and significantly reduced simulation times while important aspects of micro-topography induced surface and subsurface flow processes are being preserved. However, surface runoff in our test case model is generated predominantly because of saturation excess where the local groundwater level rises above the land surface. Whether the rill storage concept can also be applied to systems with micro-topography where surface runoff is generated due to infiltration excess, like for example in arid system as described by Solé-Benet et al. (1997), remains to be tested. In larger scale models (e.g for entire watersheds) where it is impossible to explicitly account for micro-topography because of the coarser grid resolutions, the rill storage concept may provide a viable means to account for micro-topography. First results along those lines look promising. However further work is needed to more rigorously test, which aspects of micro-topography driven surface and subsurface flow processes can be adequately mimicked at larger scales by applying the rill storage concept and which ones not.
Acknowledgments
This study was funded by the German Research Foundation (DFG, grant FL 631/6-2). Their financial support is greatly appreciated. The authors also thank Rob McLaren, Young-Jin Park and Ed Sudicky at the University of Waterloo, Canada for their invaluable help with the ins and outs of the numerical code HGS.
STUDY 3
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Study 4
Concentrations and fluxes of dissolved organic carbon in runoff from a forested catchment: Insights from high frequency measurements
By Stefan Strohmeier, Klaus-Holger Knorr, Martin Reichert, Sven Frei, Jan H. Fleckenstein, Stefan Peiffer and Egbert Matzner
Submitted to Biogeosciences Discussions
STUDY 4
Submitted to Biogeosiences Discussions
Concentrations and fluxes of dissolved organic carbon in runoff from a forested catchment: Insights from high frequency measurements
S. Strohmeier1, K.-H. Knorr2, M. Reichert2, S. Frei2, J.H. Fleckenstein3, S. Peiffer2 and E. Matzner1
1 Department of Soil Ecology, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, 95448 Bayreuth, Germany
2 Department of Hydrology, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, 95440 Bayreuth, Germany
3 Department of Hydrogeology, Helmholtz Center for Environmental Research (UFZ), 04318 Leipzig, Germany Abstract
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 spatial sources of DOC and the causes for short term variations in runoff from a forested catchment. Furthermore, we investigated the implication of short term variations for the calculation of annual runoff fluxes. High frequency measurements (30 minute 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 pronounced, counter clockwise hysteresis. Relations of DOC to discharge were steeper in the summer/fall than in the winter/spring season. 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 random fortnightly samplings, the range was 47 to 124 kg C ha-1 yr-1. Calculations of DOC export fluxes might result in significant errors when based on infrequent (e.g. fortnightly) sampling intervals. Future changes in the precipitation and discharge patterns will influence the DOC dynamics in this catchment, with largest effects in the summer season.
STUDY 4
1 Introduction
The importance of dissolved organic carbon (DOC) for the functioning of terrestrial and aquatic ecosystems is widely known. DOC plays an important role in the C cycle, in the acid-base chemistry of soils and surface waters, it influences nutrient cycling, and affects the mobility and availability of metals and contaminants (Bolan et al. (2011[5]), Kalbitz et al. (2000)[23]). Although numerous studies on DOC in soils and catchments have been published in the last decade, sources and sinks of DOC in soils and the transition of DOC from terrestrial to the aquatic ecosystems are still poorly understood in their quantitative response to driving factors, like climatic conditions, flow paths, vegetation and soil conditions. DOC in runoff from forested catchments originates mostly from soil organic matter (Degens et al. (1991)[11]). Depending on precipitation, flow paths and catchment characteristics, different soil types and soil horizons from different parts of the catchment may feed the runoff with DOC, resulting in temporal variations of DOC quality and quantity in runoff. In general, the DOC export from forested catchments in Skandinavia was found to be positively related to the area of wetland soils (Laudon et al. (2011)[29]). To identify the spatial sources of DOC in runoff, quality parameters of DOC can be used, like fluorescence spectroscopy (Ishii and Boyer (2012)[20], Fellman et al. (2009)[13], Austnes et al. (2010)[3]). This is a highly sensitive method to determine changes in DOC quality and can be applied to a large number of samples. DOC concentrations in runoff from forested catchments are often subjected to temporal variations of one order of magnitude at time scales ranging from hours to seasons. This can be attributed to the large differences in DOC concentrations in the two dominant flow components contributing to individual discharge events, i.e. groundwater (baseflow) versus shallow groundwater and surface runoff from riparian wetland soils (high flow) (McGLynn and McDonnell 2003[36], Hood et al. 2006[18]).
Ludwig et al. (1996)[33] related the DOC fluxes to drainage intensity, basin slope, and the amount of carbon stored in soils. Interestingly, they found a negative relationship between basin slope and DOC concentrations as steeper slopes may cause a restricted contact between soil and water, and thus lead to lower DOC concentrations in runoff.
Hysteretic relationships between discharge and solute concentrations have been observed by inter alia Hornberger et al. (1994)[19], Evans and Davies (1998)[12], Butturini et al. (2006)[7], Raymond and Saiers (2010)[43], Pellerin et al. (2012)[41], and Jeong et al. [22]. Evans and Davies (1998)[12]
proposed the hysteretic concentration/discharge relationship for the analysis of episode hydrochemistry. Using a three component model, they categorized the concentration/discharge relationships into clockwise and counter-clockwise hysteresis with a positive, negative or no trend.
This categorization allows the distinction of flow components that are drained during the rising or falling limbs of the hydrograph. Generally, high concentrations during the rising limb of the hydrograph lead to clockwise hysteresis, while high concentrations during the falling limb imply
STUDY 4