Alzette River Basin Research Articles

The Weierbach experimental catchment in Luxembourg: A decade of critical zone monitoring in a temperate forest ‐ from hydrological investigations to ecohydrological perspectives

AbstractThe Weierbach experimental catchment (0.45 km2) is the most instrumented and studied sub‐catchment in the Alzette River basin in Luxembourg. Within the last decade, it has matured towards an interdisciplinary critical zone observatory focusing on a better understanding of hydrological and hydro‐geochemical processes. The Weierbach catchment is embedded in an elevated sub‐horizontal plateau, characterized by slate bedrock and representative of the Ardennes Massif. Its climate is semi‐marine, with precipitation being rather evenly distributed throughout the year. Base flow is lowest from July to September, essentially due to higher losses through evapotranspiration in summer. The regolith is composed of Devonian slates, overlaid by Pleistocene slope deposits and entirely covered by forest with 70% deciduous and 30% coniferous trees. Since 2009, the Weierbach has been extensively equipped for continuously monitoring water fluxes and physico‐chemical parameters within different compartments of the critical zone. Additionally, these compartments are sampled fortnightly at several locations to analyze δ18O and δ2H isotopic composition of water including rainfall, throughfall, soil water, groundwater and streamwater. This ongoing monitoring and sampling programme is used for answering pressing questions related to fundamental catchment functions of water infiltration, storage, mixing and release in forest ecosystems. A recently started research line aims at investigating interactions between forest eco‐hydrosystems with the atmosphere and understanding how catchments will respond to a non‐stationary climate.

Bedrock geology controls on catchment storage, mixing, and release: A comparative analysis of 16 nested catchments

AbstractThe bedrock controls on catchment mixing, storage, and release have been actively studied in recent years. However, it has been difficult to find neighbouring catchments with sufficiently different and clean expressions of geology to do comparative analysis. Here, we present new data for 16 nested catchments (0.45 to 410 km2) in the Alzette River basin (Luxembourg) that span a range of clean and mixed expressions of schists, phyllites, sandstones, and quartzites to quantify the relationships between bedrock permeability and metrics of water storage and release. We examined 9 years' worth of precipitation and discharge data, and 6 years of fortnightly stable isotope data in streamflow, to explore how bedrock permeability controls (a) streamflow regime metrics, (b) catchment storage, and (c) isotope response and catchment mean transit time (MTT). We used annual and winter precipitation–run‐off ratios, as well as average summer and winter precipitation–run‐off ratios to characterise the streamflow regime in our 16 study catchments. Catchment storage was then used as a metric for catchment comparison. Water mixing potential of 11 catchments was quantified via the standard deviation in streamflow δD (σδD) and the amplitude ratio (AS/AP) of annual cycles of δ18O in streamflow and precipitation. Catchment MTT values were estimated via both stable isotope signature damping and hydraulic turnover calculations. In our 16 nested catchments, the variance in ratios of summer versus winter average run‐off was best explained by bedrock permeability. Whereas active storage (defined here as a measure of the observed maximum interannual variability in catchment storage) ranged from 107 to 373 mm, total catchment storage (defined as the maximum catchment storage connected to the stream network) extended up to ~1700 mm (±200 mm). Catchment bedrock permeability was strongly correlated with mixing proxies of σδD in streamflow and δ18O AS/AP ratios. Catchment MTT values ranged from 0.5 to 2 years, based on stable isotope signature damping, and from 0.5 to 10 years, based on hydraulic turnover.

Rare Earth Elements as Hydrological Tracers of Anthropogenic and Critical Zone Contributions: A Case Study at the Alzette River Basin Scale.

Enrichments in Gd and LREE/HREE fractionation have been observed for the dissolved and colloidal fractions in waters of the Alzette River during low flow conditions. They can be related to the effluents of the waste water treatment plants (Gd/Gd*: 8-380 and LaN/YbN: 0.02-0.07). Mean daily flux balance calculations at the basin scale show that during low flow conditions Gd only comes from the anthropogenic effluents. When flood events occur, the Gd anomaly progressively disappears and gives way to a chemical water signature, which is closer to that of natural REE sources in the basin (Gd/Gd*<1.4 and 0.11<LaN/YbN<1.5).

Understanding catchment behavior through stepwise model concept improvement

Lack of data is one of the main limitations for hydrological modeling. However, it is often used as a justification for over simplifying, poorly performing models. If we want to enhance our understanding of hydrological systems, it is important to fully exploit the information contained in the available data, and to learn from model deficiencies. In this paper, we propose a methodology where we systematically update the model structure, progressively incorporating new hypotheses of catchment behavior. We apply this methodology to the Alzette river basin in Luxembourg, showing how stepwise model improvement helps to identify the behavior of this catchment. We show that the most significant improvement of the evolving model structure is associated to the characterization of antecedent wetness. This is improved accounting for interception, which affects vertical storage distribution, and accounting for rainfall spatial heterogeneity, which influences storage variations in the horizontal dimension. Overall, our results suggested that, due to the damping effect of the basin, the description of fast catchment response benefits more from spatially distributed information than that of slow catchment response.

Soft combination of local models in a multi-objective framework

Abstract. Conceptual hydrologic models are useful tools as they provide an interpretable representation of the hydrologic behaviour of a catchment. Their representation of catchment's hydrological processes and physical characteristics, however, implies a simplification of the complexity and heterogeneity of reality. As a result, these models may show a lack of flexibility in reproducing the vast spectrum of catchment responses. Hence, the accuracy in reproducing certain aspects of the system behaviour may be paid in terms of a lack of accuracy in the representation of other aspects. By acknowledging the structural limitations of these models, we propose a modular approach to hydrological simulation. Instead of using a single model to reproduce the full range of catchment responses, multiple models are used, each of them assigned to a specific task. While a modular approach has been previously used in the development of data driven models, in this study we show an application to conceptual models. The approach is here demonstrated in the case where the different models are associated with different parameter realizations within a fixed model structure. We show that using a "composite" model, obtained by a combination of individual "local" models, the accuracy of the simulation is improved. We argue that this approach can be useful because it partially overcomes the structural limitations that a conceptual model may exhibit. The approach is shown in application to the discharge simulation of the experimental Alzette River basin in Luxembourg, with a conceptual model that follows the structure of the HBV model.

A comparison of alternative multiobjective calibration strategies for hydrological modeling

A conceptual hydrological model structure contains several parameters that have to be estimated through matching observed and modeled watershed behavior in a calibration process. The requirement that a model simulation matches different aspects of system response at the same time has led the calibration problem toward a multiobjective approach. In this work we compare two multiobjective calibration approaches, each of which represents a different calibration philosophy. The first calibration approach is based on the concept of Pareto optimality and consists of calibrating all parameters with respect to a common set of objectives in one calibration stage. This approach results in a set of Pareto‐optimal solutions representing the trade‐offs between the selected calibration objectives. The second is a stepped calibration approach (SCA), which implies a stepwise calibration of sets of parameters that are associated with specific aspects of the system response. This approach replicates the steps followed by a hydrologist in manual calibration and develops a single solution. The comparison is performed considering the same set of objectives for the two approaches and two model structures of a different level of complexity. The difference in the two approaches, their reciprocal utility, and the practical implications involved in their application are analyzed and discussed using the Hesperange catchment case, an experimental basin in the Alzette River basin in Luxembourg. We show that the two approaches are not necessarily conflicting but can be complementary. The first approach provides useful information about the deficiencies of a model structure and therefore helps the model development, while the second attempts at determining a solution that is consistent with the data available. We also show that with increasing model complexity it becomes possible to reproduce the observations more accurately. As a result, the solutions for the different calibration objectives become less distinguishable from each other, indicating that calibration results become less dependent on the objective functions used when the model is a better representation of reality and has a higher potential to reproduce the observations.

Modelling subsurface storm flow with the Representative Elementary Watershed (REW) approach: application to the Alzette River Basin

Abstract. A new domain, the macropore domain describing subsurface storm flow, has been introduced to the Representative Elementary Watershed (REW) approach. The mass balance equations have been reformulated and the closure relations associated with subsurface storm flow have been developed. The model code, REWASH, has been revised accordingly. With the revised REWASH, a rainfall-runoff model has been built for the Hesperange catchment, a sub-catchment of the Alzette River Basin. This meso-scale catchment is characterised by fast catchment response to precipitation, and subsurface storm flow is one of the dominant runoff generation processes. The model has been evaluated by a multi-criteria approach using both discharge and groundwater table data measured at various locations in the study site. It is demonstrated that subsurface storm flow contributes considerably to stream flow in the study area. Simulation results show that discharges measured along the main river course are well simulated and groundwater dynamics is well captured, suggesting that the model is a useful tool for catchment-scale hydrological analysis.

Regional runoff prediction through aggregation of first-order hydrological process knowledge: a case study

The dominant processes concept was used to develop a regionally applicable rainfall—runoff model. The first-order runoff processes are identified through a combination of field investigations, physico-geographical analysis of the research area, the Alzette River basin in the Grand-Duchy of Luxembourg, and discharge data series analysis. Lithology appeared to be the major source of discrepancy in hydrological behaviour over the total area. As a result, the hydrological behaviour of each lithological substratum was characterized and conceptualized into a parsimonious model structure. The runoff signals were calibrated against the hourly-recorded discharge series of eight sub-basins, with parameter sensitivity and correlation analysis outlining the need for minor corrections to the model structure. Validation against another set of 10 sub-basins showed good results for the regional parameter set, with an average loss in efficiency (Reff) of 0.04, compared to the reference model, with a mean Reff of 0.79. Due to an up-scaling effect, inducing variations in the dominance of particular runoff processes, some anomalies were found in the performance of individual runoff characteristics. In this respect, limiting the application of the model to a certain spatial scale gives a high reliability of the prediction of the dynamics of hourly runoff in ungauged basins within the study area.

Predicting storm runoff from different land‐use classes using a geographical information system‐based distributed model

AbstractA method is presented to evaluate the storm runoff contributions from different land‐use class areas within a river basin using the geographical information system‐based hydrological model WetSpa. The modelling is based on division of the catchment into a grid mesh. Each cell has a unique response function independent of the functioning of other cells. Summation of the flow responses from the cells with the same land‐use type results in the storm runoff contribution from these areas. The model was applied on the Steinsel catchment in the Alzette river basin, Grand Duchy of Luxembourg, with 52 months of meteo‐hydrological measurements. The simulation results show that the direct runoff from urban areas is dominant for a flood event compared with runoff from other land‐use areas in this catchment, and this tends to increase for small floods and for the dry‐season floods, whereas the interflow from forested, pasture and agricultural field areas contributes to recession flow. It is demonstrated that the relative contribution from urban areas decreases with flow coefficient, that cropland relative contribution is nearly constant, and that the relative contribution from grassland and woodland increases with flow coefficient with regard to their percentage of land‐use class areas within the study catchment. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Numerical simulations of runoff generation with surface water–groundwater interactions in the Alzette river alluvial plain (Luxembourg)

Detailed field measurements of water table levels and river stages operated on the Alzette river basin in Luxembourg have pointed out that the runoff response of the catchment to rainfall is strongly dependent on the groundwater level behaviour and on the water storage of the catchment. This dependency becomes even more pronounced for the topographic configuration of the Alzette basin that is characterized by narrowing valleys that hamper the downstream progression of alluvial groundwater. Modelling the hydrologic response of the Alzette catchment with a physically based approach requires the selection of a model that allows the simulation of an interaction between groundwater and surface water in the alluvial plain. For this reason a hydrological model based on the representative elementary watershed (REW) approach has been selected. The approach consists in discretizing a watershed in sub-watersheds or REWs, each subdivided into five zones according to dominant hydrological processes. Balance laws are averaged within each zone and hydrologic relations are derived directly at the REW scale. Each REW is considered as a spatial unit, represented by averaged watershed scale parameters and variables. The approach enables an interaction between saturated zone and channel reaches, which plays an essential role in the runoff generation in the Alzette floodplain. For this study a sub-catchment of the Alzette basin is selected that is located upstream of Luxembourg City. Results show that the model conceptualization is appropriate to describe the catchment dynamics in the alluvial plain, while the same cannot be assured for other regions of the study area. The model has been calibrated on channel flow data and piezometric observations provided by the Public Research Center—Gabriel Lippmann of Luxembourg.

Simulation of flood reduction by natural river rehabilitation using a distributed hydrological model

Abstract. The effects of river rehabilitation on flood reduction in the Steinsel sub-basin of the Alzette River basin, Grand-Duchy of Luxembourg, are discussed; the rehabilitation measures include planting and changing riparian and in-stream vegetation, and re-meandering of channelised reaches, etc. in the headwater streams. To simulate flood reduction by river rehabilitation, the streams have been classified into different orders and by assessing the response of the stream channels to the resistance or obstruction of flows. Based on this assessment, the roughness to the flow in the first and second order streams is adjusted in line with the river rehabilitation while the roughness of higher order channels downstream is unchanged. The hydrological analysis utilises the WetSpa distributed model based on spatial information on topography, soil type and land use. The increased channel roughness in the headwater channels delays the flows, so that peak discharges at the outlet of the basin are reduced. The simulation indicates that, after river naturalisation, the reduction in peak flow can be as much as 14% and the time of concentration may be delayed by as much as two hours. Also, an impact analysis has assessed the possible flood reduction for a changed climate scenario. Keywords: river rehabilitation, flood reduction, distributed hydrological modelling, WetSpa

Analysing the effect of climate changes on streamflow using statistically downscaled GCM scenarios

This paper discusses the potential impact of climate changes on the streamflow of the Alzette river basin in the Grand‐duchy of Luxembourg. A statistical relationship between observed daily temperature and precipitation to surface and upper‐atmospheric circulation predictor variables from the National Centre for Environmental Prediction (NCEP) reanalysis data is formulated for the period of 1961 to 1990. Future implementation is performed with predictor variables from the coupled HadCM3A2a atmosphere‐ocean general circulation model experiments from the U.K. Meteorological Office, for the periods 2036 to 2065 and 2070 to 2099. To analyse the impact of climate change on the streamflow of the study area, downscaled future local scenarios are applied as input to WetSpa, a spatially distributed hydrological model in which runoff is simulated as a function of rain intensity, soil moisture and terrain characteristics. The results of the hydrologic simulation indicate that there will be a significant increase in the magnitude and frequency of future flooding in the basin.

Simulating the spatio-temporal variability of streamflow response to climate change scenarios in a mesoscale basin

A continuous rainfall-runoff simulation was performed to assess the potential effect of climate changes on the streamflow regimes and water resources of tributaries of the Alzette river basin extending over 1176 km 2, mainly in the Grand Duchy of Luxembourg, characterized by various hydrological patterns. Global climate change scenarios for the 2050 horizon, based on GCM projections from the KNMI and UKHI synoptic runs, were disaggregated into mesoscale daily PET and rainfall series. Seasonal expected PET changes were proportionally applied to present daily values, whereas future hyetographs were empirically constructed according to observed trends in rainfall time series for the study area. The various ways of applying the mesoscale rainfall scenarios exert a significant influence on the magnitude and spatial distribution of streamflow responses. The comparison of future and present hydrographs also shows that the impact of mesoscale climate change is extremely variable with regard to the considered hydrological variable. The spatial variability of streamflow responses is largely conditioned by climatic and physiographical characteristics of the sub-basins. The winter period is most affected by altered climate conditions and some sub-regions appear to be particularly sensitive in terms of changes in low or high flows.

Development of regionalized hydrological models in an area with short hydrological observation series

AbstractHigh spatio‐temporal resolution monitoring has only been progressively developed in the Rhine–Meuse basins over the last few decades. As a consequence, basic hydrological information can be very scarce in some areas. In regions which are homogeneous from a hydroclimatological and physiogeographical point of view, hydrographs can be reproduced via regionalized hydrological models, provided that climatological observation series are available.The Alzette river basin, monitored since the mid‐1990s by a very dense hydroclimatological observation network, had been chosen in the framework of the IRMA‐SPONGE project FRHYMAP for transposing the conceptual hydrological models HRM and SOCONT and regionalizing their parameters. The regionalized models were to be used both for extending the currently available runoff series and evaluating runoff in neighbouring non‐monitored basins.The 16 monitored sub‐basins of the Alzette, reflecting the physiogeographical diversity of the study area, were divided into two subsets, serving for both the calibration and the validation procedures. Once the transposition of the models to the Alzette basin had been successfully assessed, their parameters were linked to the physiogeographical characteristics of the sub‐basins. The performance of the thus regionalized models was assessed via a validation on a subset of basins that had not been retained for the elaboration of the regional parameter sets.The transposition of the HRM and SOCONT model to the Alzette river basin was completed successfully. Results overall proved to be satisfying, with the HRM model performing equally well for low flows and high flows, while the SOCONT model showed best results for high flows and a systematic overestimation of the mean discharge. Both models proved to be adequate for evaluating daily runoff in non‐monitored basins of the Grand‐Duchy of Luxembourg, helping thus to counterbalance the considerable lack of hydrological observation series in this part of the Rhine basin. Copyright © 2004 John Wiley &amp; Sons, Ltd.

The applicability of a parsimonious model for local and regional prediction of runoff

The Hydrological Recursive Model (HRM), a conceptual rainfall-runoff model, was applied for local and regional simulation of hourly discharges in the transnational Alzette River basin (Luxembourg-France-Belgium). The model was calibrated for a range of various sub-basins with a view to analysing its ability to reproduce the variability of basin responses during flood generation. The regionalization of the model parameters was obtained by fitting simultaneously the runoff series of calibration sub-basins after their spatial discretization in lithological contrasting isochronal zones. The runoff simulations of the model agreed well with the recorded runoff series. Significant correlations with some basin characteristics and, noticeably, the permeability of geological formations, could be found for two of the four free model parameters. The goodness of fit for runoff predictions using the derived regional parameter set was generally satisfactory, particularly for the statistical characteristics of streamflow. A more physically-based modelling approach, or at least an explicit treatment of quick surface runoff, is expected to give better results for high peak discharge.

Using 3D dynamic cartography and hydrological modelling for linear streamflow mapping

This paper presents a regionalization methodology and an original representation of the downstream variation of daily streamflow using a conceptual rainfall–runoff model (HRM) and the 3D visualization tools of the GIS ArcView. The regionalization of the parameters of the HRM model was obtained by fitting simultaneously the runoff series from five sub-basins of the Alzette river basin (Grand-Duchy of Luxembourg) according to the permeability of geological formations. After validating the transposability of the regional parameter values on five test basins, streamflow series were simulated with the model at ungauged sites in one medium size geologically contrasted test basin and interpolated assuming a linear increase of streamflow between modelling points. 3D spatio-temporal cartography of mean annual and high raw and specific discharges are illustrated. During a severe flooding, the propagation of the flood waves in the different parts of the stream network shows an important contribution of sub-basins lying on impervious geological formations (direct runoff) compared with those including permeable geological formations which have a more contrasted hydrological response. The effect of spatial variability of rainfall is clearly perceptible.

Use of regionalized stormflow coefficients with a view to hydroclimatological hazard mapping

An attempt was made to compensate for the lack of long hydrological time series and the lack of information on maximum streamflow in the Alzette River basin (Luxembourg) via the regionalization of stormflow coefficients. Streamflow data recorded since 1995 with a very dense streamgauge network allowed the determination of maximum stormflow coefficients in 18 sub-basins of the Alzette. The stormflow coefficients were then regionalized via stepwise multiple regression analysis for 83 different sub-basins of the Alzette. Combined with 10-year daily rainfall heights (statistical estimation), this regionalization allowed the spatial variability of storm runoff in the Alzette basin to be mapped, thus providing a view of hazard and risk-producing areas, as well as of risk-exposed areas. In a basin with little historical hydrological information this technique can help identify areas where storm runoff reducing measures should be applied from the outset.

Recent trends in rainfall-runoff characteristics in the Alzette River basin, Luxembourg

The effects of rainfall structure and atmosphericcirculation variability on streamflow in the AlzetteRiver basin (Grand Duchy of Luxembourg, Europe) wereinvestigated. Evidence is presented regarding the local-scalesensitivity of the Alzette River basin to large-scalechanges in atmospheric circulation. Since the1950s, there has been a marked increase in thecontribution of the westerly component of atmosphericcirculation to rainfall. These changes in atmosphericcirculation are on a par with an increase in rainfallintensity and duration, which has induced a significantincrease in the winter maximum daily storm flow in theAlzette River basin since the 1970s.