Evaluation of hydrological models for scenario analyses: signal-to-noise-ratio between scenario effects and model uncertainty

International audienceMany model applications suffer from the fact that although it is well known that model application implies different sources of uncertainty there is no objective criterion to decide whether a model is suitable for a particular application or not. This paper introduces a comparative index between the uncertainty of a model and the change effects of scenario calculations which enables the modeller to objectively decide about suitability of a model to be applied in scenario analysis studies. The index is called "signal-to-noise-ratio", and it is applied for an exemplary scenario study which was performed within the GLOWA-IMPETUS project in Benin. The conceptual UHP model was applied on the upper Ouémé basin. Although model calibration and validation were successful, uncertainties on model parameters and input data could be identified. Applying the "signal-to-noise-ratio" on regional scale subcatchments of the upper Ouémé comparing water availability indicators for uncertainty studies and scenario analyses the UHP model turned out to be suitable to predict long-term water balances under the present poor data availability and changing environmental conditions in subhumid West Africa

Analysis of possible impacts of climate change on the hydrological regimes of different regions in Germany

In this study, the impact of climate change scenarios on the hydrological regimes of five different regions in Germany is investigated. These regions (Northwest Germany, Northeast Germany and East German basins, upper and lower Rhine, pre-Alps) differ with respect to present climate and projected climate change. The physically based SVAT-model SIMULAT is applied to theoretical soil columns based on combinations of land use, soil texture and groundwater depth to quantify climate change effects on the hydrological regime. Observed climate, measured at climate stations of the German Weather Service (1991–2007), is used for comparison with climate projections (2071–2100) generated by the regional scale climate model WETTREG. <br><br> While all climate scenarios implicate an increase in precipitation in winter, a decrease in precipitation in summer and an increase in temperature, the simulated impacts on the hydrological regime are regionally different. In the Rhine region and in Northwest Germany, an increase in the annual runoff and groundwater recharge is simulated despite the increase in temperature and potential evapotranspiration. In the Eastern part of Germany and the pre-Alps, annual runoff and groundwater recharge will decrease. Due to dry conditions in summer, the soil moisture deficit will increase (in Northeast Germany and the East German basins in particular) or remain constant (Rhine region). In all regions the seasonal variability in runoff and soil moisture status will increase. Despite regional warming actual evapotranspiration will decrease in most regions except in areas with shallow groundwater tables and the lower Rhine. Although the study is limited by the fact that only one climate model was used to drive one hydrologic model, the study shows that the hydrological regime will be affected by climate change. The direction of the expected changes seems to be obvious as well as the necessity of the adaptation of future water management strategies

Impact of spatial data resolution on simulated catchment water balances and model performance of the multi-scale TOPLATS model

International audienceThis paper analyses the effect of spatial input data resolution on the simulated water balances and flow components using the multi-scale hydrological model TOPLATS. A data set of 25m resolution of the central German Dill catchment (693 km2 is used for investigation. After an aggregation of digital elevation model, soil map and land use classification to 50 m, 75 m, 100 m, 150 m, 200 m, 300 m, 500 m, 1000 m and 2000 m, water balances and water flow components are calculated for the entire Dill catchment as well as for 3 subcatchments without any recalibration. The study shows that both model performance measures as well as simulated water balances almost remain constant for most of the aggregation steps for all investigated catchments. Slight differences occur for single catchments at the resolution of 50?500 m (e.g. 0?3% for annual stream flow), significant differences at the resolution of 1000 m and 2000 m (e.g. 2?12% for annual stream flow). These differences can be explained by the fact that the statistics of certain input data (land use data in particular as well as soil physical characteristics) changes significantly at these spatial resolutions, too. The impact of smoothing the relief by aggregation occurs continuously but is not reflected by the simulation results. To study the effect of aggregation of land use data in detail, three different land use scenarios are aggregated which were generated aiming on economic optimisation at different field sizes (0.5 ha, 1.5 ha and 5.0 ha). The changes induced by aggregation of these land use scenarios are comparable with respect to catchment water balances compared to the current land use. A correlation analysis only in some cases reveals high correlation between changes in both input data and in simulation results for all catchments and land use scenarios combinations (e.g. evapotranspiration is correlated to land use, runoff generation is correlated to soil properties). Predominantly the correlation between catchment properties (e.g. topographic index, transmissivity, land use) and simulated water flows varies from catchment to catchment. This study indicates that an aggregation of input data for the calculation of regional water balances using TOPLATS type models leads to significant errors from a resolution exceeding 500 m. A meaningful aggregation of data should in the first instance aim on preserving the areal fractions of land use classes

Analysis of the suitability of the German soil texture classification for the regional scale application of physical based hydrological model

International audienceRegional scale hydrological simulations are mostly based on the use of standard data sets such as soil maps which are based on soil texture classification schemes. This paper analyses the suitability of the German soil texture classification for the application of a physically based soil-vegetation-atmosphere-transfer scheme. Theoretical soil columns are defined to be able to represent the entire soil texture triangle by a 1% grid of the three particle size classes: sand, clay and silt. These theoretical soil columns are characterized by a homogenous soil texture and consist of two layers of increasing bulk density and decreasing content of organic matter with depth. Soil hydraulic parameterisation is derived by applying a pedotransfer function. Continuous water balance calculations are carried out for a ten year period for all grid cells of the 1% grid. The results of the water balance calculations are compared to the simulation results of the centre of gravity of the respective soil texture class. Texture class specific mean deviations and root mean squared deviations are calculated from the differences between the 1% pixels and texture class representatives. The results reveal that the loam and silt texture classes show only small deviations from the centres of gravity. For a few sand texture classes and most of the clay texture classes deviations are considerably large. Assuming an equal distributed probability of occurrence of all realisations within a soil texture class, an uncertainty of more than 100 mm/a with respect to runoff and actual evapotranspiration is detected for four clay texture classes, two sand texture classes and one silt texture class. These results are confirmed by a sensitivity analysis investigating the model response for a grid cell compared to the neighboured grid cells. High sensitivities mainly appear for sandy and clayey soils while the sensitivity of the model for loam and silt soils is smaller. Resuming it can be stated that most of the texture classes of the German texture classification scheme are suitable for the application of a physically based model, on regional scale in particular. Clay texture classes can be expected to cause high simulation uncertainties

The Peierls--Nabarro FE model in two-phase microstructures -- a comparison with atomistics

This paper evaluates qualitatively as well as quantitatively the accuracy of a recently proposed Peierls--Nabarro Finite Element (PN-FE) model for dislocations by a direct comparison with an equivalent molecular statics simulation. To this end, a two-dimensional microstructural specimen subjected to simple shear is considered, consisting of a central soft phase flanked by two hard-phase regions. A hexagonal atomic structure with equal lattice spacing is adopted, the interactions of which are described by the Lennard--Jones potential with phase specific depths of its energy well. During loading, edge dislocation dipoles centred in the soft phase are introduced, which progress towards the phase boundaries, where they pile up. Under a sufficiently high external shear load, the leading dislocation is eventually transmitted into the harder phase. The homogenized PN-FE model is calibrated to an atomistic model in terms of effective elasticity constants and glide plane properties as obtained from simple uniform deformations. To study the influence of different formulations of the glide plane potential, multiple approaches are employed, ranging from a simple sinusoidal function of the tangential disregistry to a complex model that couples the influence of the tangential and the normal disregistries. The obtained results show that, qualitatively, the dislocation structure, displacement, strain fields, and the dislocation evolution are captured adequately. The simplifications of the PN-FE model lead, however, to some discrepancies within the dislocation core. Such discrepancies play a dominant role in the dislocation transmission process, which thus cannot quantitatively be captured properly. Despite its simplicity, the PN-FE model proves to be an elegant tool for a qualitative study of edge dislocation behaviour in two-phase microstructures, although it may not be quantitatively predictive.Comment: 29 pages, 11 figures, 5 tables, abstract shortened to fulfill 1920 character limit, small changes after revie

Advances and visions in large-scale hydrological modelling: findings from the 11th Workshop on Large-Scale Hydrological Modelling

Large-scale hydrological modelling has become increasingly wide-spread during the last decade. An annual workshop series on large-scale hydrological modelling has provided, since 1997, a forum to the German-speaking community for discussing recent developments and achievements in this research area. In this paper we present the findings from the 2007 workshop which focused on advances and visions in large-scale hydrological modelling. We identify the state of the art, difficulties and research perspectives with respect to the themes "sensitivity of model results", "integrated modelling" and "coupling of processes in hydrosphere, atmosphere and biosphere". Some achievements in large-scale hydrological modelling during the last ten years are presented together with a selection of remaining challenges for the future

Harmonic behavior of metallic glasses up to the metastable melt

In two amorphous alloys ZrTiCuNiBe and ZrAlNiCu coherent neutron scattering has been measured over five decades in energy, including measurements in the metastable melt of a metallic alloy more than 80 K above Tg. In the vibrational spectra a pronounced "boson" peak is found: Even in crystallized samples the density of states exceeds the Debye ω2 model, and in the amorphous state low-frequency vibrations are further enhanced. The peak position shows no dispersion in q, while intensities are strongly correlated with the static structure factor. Over the full energy range the temperature dependence is strictly harmonic. From high-energy resolution measurements we establish lower bounds for the temperatures at which structural α and fast β relaxation become observable