Is a simple model based on two mixing reservoirs able to reproduce the intra-annual dynamics of DOC and NO3 stream concentrations in an agricultural headwater catchment?

Agriculture disturbs the biogeochemical cycles of major elements, which alters the elemental stoichiometry of surface stream waters, with potential impacts on their ecosystems. However, models of catchment hydrology and water quality remain relatively disconnected, even though the observation that dissolved organic carbon (DOC) and nitrate (NO3-) have opposite spatial and temporal patterns seems relevant for improving our representation of hydrological transport pathways within catchments. We tested the ability of a parsimonious model to simultaneously reproduce intra-annual dynamics of stream flow, DOC and NO3- concentrations using 15 years of daily data from a small headwater agricultural catchment (AgrHyS observatory). The model consists of an unsaturated reservoir, a slow reservoir representing the groundwater and a fast reservoir representing the riparian zone and preferential flow paths. The sources of DOC and NO3- are assumed to behave as infinite pools with a fixed concentration in each reservoir that contributes to the stream. Stream concentrations thus result from simple mixing of slow and fast reservoir contributions. The model simultaneously reproduced annual and storm-event dynamics of discharge, DOC and NO3- concentrations in the stream, with calibration KGE scores of 0.77, 0.64 and 0.58 respectively, and validation KGE scores of 0.72, 0.58 and 0.43 respectively. These results suggest that the dynamics of these concentrations can be explained by hydrological transport processes and thus by temporally variable contributions from slow (NO3- rich and DOC poor) and fast reservoirs (DOC rich and NO3- poor), with a poor representation of the biogeochemical transformations. Unexpectedly, using the concentration time series to calibrate the model increased uncertainty in the parameters that control hydrological fluxes of the model. The legacy storage of NO3- resulting from agricultural history in the studied catchment supports the assumption that the main DOC and NO3- sources behave as infinite pools at the scale of several years. Nevertheless, reproducing the long-term trends in solute concentration would require additional information about DOC and NO3- trends within the reservoirs.

Trigger characteristics of torrential flows from high to low alpine regions in Austria.

Torrential processes like fluvial flows (flash floods with or without intensive sediment transport) and debris flows can represent a threat to people and infrastructure in alpine domains. Up to now the hydro-meteorological trigger conditions and their connection with geomorphic watershed characteristics that favor the initiation of either process are largely unknown. Based on modeled wetness states we determined the trigger types (long-lasting rainfall (LLR), short-duration storm (SDS) and intense snow melt (SM)) of 360 observed debris flow and fluvial flood events in six climatically and geomorphologically contrasting watersheds in Austria. Results show that the watershed wetness states play very distinct roles for triggering torrential events across the study regions. Hydro-meteorological variables have little power to explain the occurrence of fluvial flows and debris flows in these regions. Nevertheless, trigger type separation highlighted some geomorphic influences. For example, intense SM triggered more events in sub-watersheds (torrential watersheds in the study region) that are characterized by significantly higher Melton ruggedness numbers than LLR does. In addition, the data show that events triggered by LLRs occur in sub-watersheds of similar exposures (aspects) other than SDS. The results suggest that the consideration of different trigger types provides valuable information for engineering risk assessment.

Uncertainties in transpiration estimates.

arising from S. Jasechko et al. Nature 496, 347-350 (2013)10.1038/nature11983How best to assess the respective importance of plant transpiration over evaporation from open waters, soils and short-term storage such as tree canopies and understories (interception) has long been debated. On the basis of data from lake catchments, Jasechko et al. conclude that transpiration accounts for 80-90% of total land evaporation globally (Fig. 1a). However, another choice of input data, together with more conservative accounting of the related uncertainties, reduces and widens the transpiration ratio estimation to 35-80%. Hence, climate models do not necessarily conflict with observations, but more measurements on the catchment scale are needed to reduce the uncertainty range. There is a Reply to this Brief Communications Arising by Jasechko, S. et al. Nature 506, http://dx.doi.org/10.1038/nature12926 (2014).

Long-term monitoring of the Danube river--sampling techniques, radionuclide metrology and radioecological assessment.

Sampling techniques and radiometric methods, developed and applied in a comprehensive radioecological study of the Danube River are presented. Results and radiometric data of sediment samples, collected by sediment traps in Austria and additionally by grab sampling in the Danube during research cruises between Germany and the delta (Black sea) are shown and discussed. Goal of the investigation is the protection of public and environment, especially the sustainable use and conservation of human freshwater resources against harmful radioactive exposure.