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The knowledge of the spatial and temporal variability of N2O concentrations in surface groundwater is the first step towards upscaling of potential indirect N2O emissions from the scale of localized samples to aquifers. This study aimed to investigate the spatial and the temporal variability of N2O concentrations at different scales in the surface groundwater of a denitrifying aquifer in northern Germany. The spatial variability of N2O concentrations in the surface groundwater was analysed at the plot (200 × 200 m) and at the transect scale (12 m). Twenty plots that were distributed across an area of 11 km2 and 6 transects were sampled. Sixty per cent of the spatial variance of N2O was located at the plot scale and 68–79% was located at the transect scale. This indicates that small-scale processes governed the spatial variability of N2O in the surface groundwater. A spatial upscaling of N2O from the transect to the aquifer scale might be possible with an adequate number of samples that represent important boundary conditions for N2O accumulation in the catchment (topography, groundwater level, land use). For the investigation of the temporal variability, 4 multilevel wells were sampled monthly over a period of 13 months. In two periods, a multilevel well was additionally sampled in 2-day intervals over 8 days. At the annual scale, N2O concentrations in the surface groundwater were higher during the vegetation period (median 87 μg N2O-N l−1) and could change rapidly on the day scale whereas the concentrations were smaller in winter (median 21 μg N2O-N l−1). Groundwater recharge events seemed to be crucial for the day scale variability. Capture of the temporal variations for upscaling might be achieved with a process-based sampling strategy with weekly sampling intervals during the vegetation period, the additional sampling after groundwater recharge events and monthly sampling intervals in winter. [ABSTRACT FROM AUTHOR]

Presents a study on developing a numerical solute-transport model based on the two-region flow hypothesis. Theoretical considerations; Differential equation used for the study; Numerical procedure; Model testing.

Hydrological models featuring root water uptake usually do not include compensation mechanisms such that reductions in uptake from dry layers are compensated by an increase in uptake from wetter layers. We developed a physically based root water uptake model with an implicit compensation mechanism. Based on an expression for the matric flux potential (M) as a function of the distance to the root, and assuming a depth-independent value of M at the root surface, uptake per layer is shown to be a function of layer bulk M, root surface M, and a weighting factor that depends on root length density and root radius. Actual transpiration can be calculated from the sum of layer uptake rates. The proposed reduction function (PRF) was built into the SWAP model, and predictions were compared to those made with the Feddes reduction function (FRF). Simulation results were tested against data from Canada (continuous spring wheat [(Triticum aestivum L.]) and Germany (spring wheat, winter barley [Hordeum vulgare L.], sugarbeet [Beta vulgaris L.], winter wheat rotation). For the Canadian data, the root mean square error of prediction (RMSEP) for water content in the upper soil layers was very similar for FRF and PRF; for the deeper layers, RMSEP was smaller for PRF. For the German data, RMSEP was lower for PRF in the upper layers and was similar for both models in the deeper layers. In conclusion, but dependent on the properties of the data sets available for testing, the incorporation of the new reduction function into SWAP was successful, providing new capabilities for simulating compensated root water uptake without increasing the number of input parameters or degrading model performance. [ABSTRACT FROM AUTHOR]