Christian Leibundgut

Prediction uncertainty of conceptual rainfall–runoff models caused by problems in identifying model parameters and structure

Abstract The uncertainties arising from the problem of identifying a representative model structure and model parameters in a conceptual rainfall-runoff model were investigated. A conceptual model, the HBV model, was applied to the mountainous Brugga basin (39.9 km”) in the Black Forest, southwestern Germany. In a first step, a Monte Carlo procedure with randomly generated parameter sets was used for calibration. For a ten-year calibration period, different parameter sets resulted in an equally good correspondence between observed and simulated runoff. A few parameters were well defined (i.e. best parameter values were within small ranges), but for most parameters good simulations were found with values varying over wide ranges. In a second step, model variants with different numbers of elevation and landuse zones and various runoff generation conceptualizations were tested. In some cases, representation of more spatial variability gave better simulations in terms of discharge. However, good results could...

Hydrograph separations in a mesoscale mountainous basin at event and seasonal timescales

[1]xa0The spatial and temporal (event and seasonal timescale) variability of major runoff components in the mountainous Brugga basin (Black Forest, Germany) were examined. The mesoscale (40 km2) study basin represented an extraordinary challenge as comparable studies have been undertaken mainly in smaller headwater basins. Discharge data, tracer concentrations of 18O, 3H, CFCs, and dissolved silica, and major anions and cations were analyzed during single events and over a period of 3 years. Three main runoff components were defined: event water with a residence time of several hours to a few days contributed up to 50% during flood peaks, quantified by a classical hydrograph separation technique using 18O. However, this component is of minor importance for longer periods, comprising ∼11.1% of total runoff as estimated for the period August 1995 to April 1998. The other two flow components originated from shallow and deep groundwater. Source areas for these are the upper drift and debris cover for the shallow groundwater and the deeper drift, weathering zone and hard rock aquifer for the deep groundwater. Mean residence times ranged from 28 to 36 months on the basis of 18O data for the shallow groundwater and from 6 to 9 years on the basis of 3H and CFC data for the deep groundwater. The importance of the upper drift and debris cover of the slopes for runoff generation at the test site was clearly demonstrated at the seasonal timescale, showing a contribution of 69.4% based on a mixing model with a monthly time step. The deep groundwater contribution was 19.5%. With this information a conceptual model of runoff generation for the study site was constructed.

Influence of vegetation structure on isotope content of throughfall and soil water

The stable isotope oxygen-18 is often used as a natural tracer in stream-flow separation studies in forested catchments. Knowledge about the 18 O content of throughfall and soil water is needed. The present study was conducted in order to assess the effect of vegetation structure on the stable isotope 18 O composition of rainwater input during its passage through the canopy and into the unsaturated zone. The research area was a small (0.1 km 2 ) forested catchment in the mountainous Black Forest region of southern Germany. During the 3-month periods (September to December 1995 and April to July 1996) several structural units were instrumented according to tree species (beech and spruce) and canopy density. Overall throughfall was enriched compared with open rainfall by +0.38‰ and +0.36‰ for spruce and beech, respectively. Considerable differences existed in the results of the crown centre and the crown periphery sites. Throughfall in the crown centre showed a higher degree of enrichment, with a mean value of +0.30‰ for beech and +0.37‰ for spruce. Enrichment and depletion were observed in the lower canopy density locations. Mean results of the crown periphery were similar to those of open precipitation (+0.15‰ for beech and - 0.01‰ for spruce). Soil water was sampled at five different depths (0, 15, 60, 120 and 180 cm). The signals of individual rainfall events could be traced down to a depth of 60 cm. The soil water in the upper layers followed the seasonal trend in the precipitation input. At a depth of 180 cm, soil water had a very constant δ 18 O value. The 18 O composition of the soil water at various depths at different locations showed a similar behaviour. No detectable differences could be found between the structural units in the different layers, except at 180 cm depth. This might be attributed to downslope directed flow at that depth.

Identification of runoff generation processes using combined hydrometric, tracer and geophysical methods in a headwater catchment in South Africa / Identification des processus de formation du débit en combinat la méthodes hydrométrique, traceur et géophysiques dans un bassin versant sud-africain

Abstract Classical hydrometric measurements and detailed 2-D electrical resistivity imaging (ERI) surveys were combined with tracer sampling to identify the hydrological processes in a semi-arid headwater catchment in the Eastern Cape Province of South Africa. The analysis of precipitation and runoff events emphasized the strong link between precipitation and runoff formation characteristics. Soil water tension and groundwater level observations demonstrated the development of a perched water table within the soil layer. These results are supported by tracer-based runoff component separations and illustrate the important role of the shallow subsurface component. The ERI investigation permitted further insight into the structure of the subsurface. Finally, the ERI survey, in combination with time domain reflectometry (TDR) measurements, allowed the extrapolation of selective soil water content measurements. To summarize, the application and combination of different field methods led to the development of a conceptual model of the hydrological functioning of this catchment. The dominant role of the subsurface mechanisms was evaluated.

Spatial and temporal characterisation of stable isotopes in river water as indicators of groundwater contribution and confirmation of modelling results; a study of the Weser river, Germany.

River water samples were analysed for stable isotopes (deuterium and oxygen-18) collected from 46 sites during spring 2008, and from monthly samples at the outlets of seven sub-basins of the River Weser (46,200 km2 basin area in total) over a five year period from 2003 to 2007, to characterise temporal and spatial isotope patterns of river water. Results indicate a pronounced elevation effect (0.2‰ and 1 to 2‰ per 100 m for δ18O and δ2H, respectively) as well as influence of seawater mixing for a few coastal locations. A lumped parameter modelling approach was used to compare residence times and relative amounts of direct flow, fast and slow groundwater with those derived from a combined water balance and tritium balance modelling approach. Residence times of direct runoff were estimated to be between one and three and a half months. Much longer groundwater residence times are necessary to explain tritium recession in river water. The modelling fits for stable isotope data in river water, derived with residence times and base flow amounts combined from a water and tritium balance approach, emphasise that beneath a characterisation of a direct flow component, seasonal variations of stable isotope values in river water carry information on groundwater contribution.

The significance of hydrological criteria for the assessment of the ecological quality in river basins

With the implementation of the European Water Framework Directive (WFD) the achievement of a good ecological status of surface waters and a good quantitative and qualitative status of groundwater has become obligatory. The ecological status is defined by biological, chemical and physical criteria. In the River Basin Management Plans required by the WFD, all human impacts on the aquatic environment shall be quantified and evaluated. For this purpose catchment related assessment methods which describe the physical and chemical predictors of the ecological status in surface waters and the status of groundwater are needed. They will have to be combined with monitoring programs and assessments of the water bodies themselves. n nTo complete the existing biological, chemical and morphological assessment methods a spatially orientated assessment procedure for the hydrological quality of meso-scale river basins was developed. In this procedure human impacts on hydrology, river morphology and water quality are quantified and assessed. The procedure is divided into the three assessment units “catchment properties”, “runoff dynamics” and “nutrient budget, water quality and solute dynamics”. It was applied to 22 meso-scale river basins in South-West Germany. The assessment results enable a quantification of the level of human impact on the aquatic environment.