TY - JOUR A1 - Musolff, Andreas A1 - Schmidt, Christian A1 - Selle, Benny A1 - Fleckenstein, Jan H. T1 - Catchment controls on solute export JF - Advances in water resources N2 - Dynamics of solute export from catchments can be classified in terms of chemostatic and chemodynamic export regimes by an analysis of concentration-discharge relationships. Previous studies hypothesized that distinct export regimes emerge from the presence of solute mass stores within the catchment and their connectivity to the stream. However, so far a direct link of solute export to identifiable catchment characteristics is missing. Here we investigate long-term time series of stream water quality and quantity of nine neighboring catchments in Central Germany ranging from relatively pristine mountain catchments to agriculturally dominated lowland catchments, spanning large gradients in land use, geology, and climatic conditions. Given the strong collinearity of catchment characteristics we used partial least square regression analysis to quantify the predictive power of these characteristics for median concentrations and the metrics of export regime. We can show that median concentrations and metrics of the export regimes of major ions and nutrients can indeed be inferred from catchment characteristics. Strongest predictors for median concentrations were the share of arable land, discharge per area, runoff coefficient and available water capacity in the root zone of the catchments. The available water capacity in the root zone, the share of arable land being artificially drained and the topographic gradient were found to be the most relevant predictors for the metrics of export regime. These catchment characteristics can represent the size of solute mass store such as the fraction of arable land being a measure for the store of nitrate. On the other hand, catchment characteristics can be a measure for the connectivity of these solute stores to the stream such as the fraction of tile drained land in the catchments. This study demonstrates the potential of data-driven, top down analyses using simple metrics to classify and better understand dominant controls of solute export from catchments. (C) 2015 Elsevier Ltd. All rights reserved. KW - Water quality KW - Catchment KW - Nutrient export KW - Tile drain KW - Export regime KW - Concentration-discharge relationships Y1 - 2015 U6 - https://doi.org/10.1016/j.advwatres.2015.09.026 SN - 0309-1708 SN - 1872-9657 VL - 86 SP - 133 EP - 146 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Trauth, Nico A1 - Schmidt, Christian A1 - Vieweg, Michael A1 - Oswald, Sascha A1 - Fleckenstein, Jan H. T1 - Hydraulic controls of in-stream gravel bar hyporheic exchange and reactions JF - Water resources research N2 - Hyporheic exchange transports solutes into the subsurface where they can undergo biogeochemical transformations, affecting fluvial water quality and ecology. A three-dimensional numerical model of a natural in-stream gravel bar (20 m x 6 m) is presented. Multiple steady state streamflow is simulated with a computational fluid dynamics code that is sequentially coupled to a reactive transport groundwater model via the hydraulic head distribution at the streambed. Ambient groundwater flow is considered by scenarios of neutral, gaining, and losing conditions. The transformation of oxygen, nitrate, and dissolved organic carbon by aerobic respiration and denitrification in the hyporheic zone are modeled, as is the denitrification of groundwater-borne nitrate when mixed with stream-sourced carbon. In contrast to fully submerged structures, hyporheic exchange flux decreases with increasing stream discharge, due to decreasing hydraulic head gradients across the partially submerged structure. Hyporheic residence time distributions are skewed in the log-space with medians of up to 8 h and shift to symmetric distributions with increasing level of submergence. Solute turnover is mainly controlled by residence times and the extent of the hyporheic exchange flow, which defines the potential reaction area. Although streamflow is the primary driver of hyporheic exchange, its impact on hyporheic exchange flux, residence times, and solute turnover is small, as these quantities exponentially decrease under losing and gaining conditions. Hence, highest reaction potential exists under neutral conditions, when the capacity for denitrification in the partially submerged structure can be orders of magnitude higher than in fully submerged structures. KW - in-stream gravel bar KW - groundwater-surface water interaction KW - aerobic respiration KW - denitrification KW - computational fluid dynamics KW - reactive transport model Y1 - 2015 U6 - https://doi.org/10.1002/2014WR015857 SN - 0043-1397 SN - 1944-7973 VL - 51 IS - 4 SP - 2243 EP - 2263 PB - American Geophysical Union CY - Washington ER -