Ch. Leibundgut

Hydrograph separation in a mountainous catchment — combining hydrochemical and isotopic tracers

Runoff components of the Zastler catchment (18\4 km2, southern Black Forest, Germany) were analysed with hydrograph separations using stable oxygen isotopes and dissolved silica. It was shown that event water and components with low silica contributed only small amounts to total runoff. In addition, comparison of the two-component hydrograph separations showed that the low-silica components are generated by both event water and pre-event water fractions, depending on the state of the system. A modified three-component hydrograph separation method was introduced using dissolved silica and 18O. During storm events an interaction of three runoff components having distinct silica concentrations could be shown. Based on the geological and geomorphological genesis of the study site, it was appropriate to assign (i) the low silica component to the riparian zones and impermeable areas, (ii) the medium silica component to the periglacial debris cover and (iii) the high silica component to the crystalline detritus and crystalline hard rock. Exact quantification of the runoff components remained difficult. However, runoff components with medium silica concentrations reacted very sensitively and intensely. The contribution of this component to total runoff is comparatively large. This shows the important role of the periglacial debris to runoff generation of the study site and emphasizes the importance of runoff generation processes occurring in this reservoir. Copyright

Multi-criterial validation of TOPMODEL in a mountainous catchment

The need for powerful validation methods for hydrological models including the evaluation of internal stages and spatially distributed simulations has often been emphasized. In this study a multi-criterial validation scheme was used for validation of TOPMODEL, a conceptual semi-distributed rainfall–runoff model. The objective was to test TOPMODELs capability of adequately representing dominant hydrological processes by simple conceptual approaches. Validation methods differed in the type of data used, in their target and in mode. The model was applied in the humid and mountainous Brugga catchment (40 km2) in south-west Germany. It was calibrated by a Monte Carlo method based on hourly runoff data. Additional information for validation was derived from a recession analysis, hydrograph separation with environmental tracers and from field surveys, including the mapping of saturated areas. Although runoff simulations were satisfying, inadequacies of the model structure compared with the real situation with regard to hydrological processes in the study area were found. These belong mainly to the concept of variable contributing areas for saturation excess overland flow and their dynamics, which were overestimated by the model. The simple TOPMODEL approach of two flow components was found to be insufficient. The multi-criterial validation scheme enables not only to demonstrate limitations with regard to process representation, but also to specify where and why these limitations occur. It may serve as a valuable tool for the development of physically sound model modifications. Copyright

Multiscale calibration and validation of a conceptual rainfall-runoff model

Abstract Model calibration and validation is usually limited to comparing streamflows at the basin outlet. In this study observed runoff series from nested basins were used for calibration and validation on different spatial scales. A conceptual rainfall-runoff model was applied to nested basins of different sizes (15.2, 40 and 257 km2) located in the Black Forest in south-west Germany. The first step was to calibrate the model individually for each of the three basins and to use the runoff series from the other two basins for validation. Optimised parameter values were related to sizes and other properties of the basins. In the second step the model was calibrated simultaneously to the runoff series from all three catchments using a genetic algorithm and a fuzzy combination of the individual objective function values. It was not possible to obtain as good fits as those achieved by separately calibrating the model to each sub-basin. However, the fit between measured and observed streamflow for the individual basins were acceptable (model efficiency values around 0.8) and significantly better than those obtained using a parameter set optimised in just one of the other basins.

Application of a catchment water quality model for assessment and prediction of nitrogen budgets

Abstract The objective of this study is the assessment of a catchment water quality model towards its suitability for simulation of solute transport and nitrogen budgets and application in river basin management. The conceptual semi-distributed water budget and water quality model NPSM (Non Point Source Model) was applied in the meso-scale Brugga catchment (39,9 km 2 ) in southwest Germany. Spatial distribution was achieved by classifying sub-areas based on the concept of hydrological chemical response units (CHRUs). Parameter values for the runoff and nitrogen simulation were estimated from catchment properties, previous experimental and modelling studies, regional field studies and literature. Optimization of the parameter values, calibration of the model and validation of the simulation was achieved using the runoff curve, concentrations of nitrate and natural tracers. Sensitivity analysis revealed that only a few parameters were important for the simulation of runoff and the dynamics of the nitrogen fluxes. The simulated nitrate concentrations were strongly controlled by the nitrogen input, the water movements and the nitrogen reactions in the different sub-areas. Due to the lack of data, the parameterization of the nitrogen contents and reactions in the soil remained uncertain. As a result of this study NPSM proved to be applicable in a meso-scale catchment performing satisfactory results for the simulation of stream flow. However the short time dynamics of the measured nitrate could not be fitted by the nitrogen simulation. An improvement of the short time performance can only be achieved using a more detailed soil and agricultural data basis. To evaluate the suitability of the model as a tool for river basin management, simulations for longer time periods and with additional nutrients have to be performed.

GIS-gestützte Ausweisung von hydrologischen Umsatzräumen und Prozessen im Löhnersbach-Einzugsgebiet (Nördliche Grauwackenzone, Salzburger Land)

KurzfassungAm Beispiel des mesoskaligen Einzugsgebietes des Löhnersbaches (Kitzbüheler Alpen, ca. 16 km2) wird erläutert, wie auf der Basis von ExpertInnenwissen zur Landschaftsgenese und allgemein verfügbarer Daten mit Hilfe eines Geographischen Informationssystems (GIS) hydrologische Homogenbereiche (Hydrotope) rasterbasiert ausgewiesen werden können. Es wurde ein Regionalisierungsansatz für die flächenhafte Abschätzung der Struktur- und Lithovarianz des quartären Hangschutts entwickelt, welcher auf der Hangentwicklungsgenese basiert. Jeder der ausgewiesenen hydrologischen Homogenbereiche ist sowohl durch die gleichen prozessrelevanten Umsatzräume, als auch durch die gleichen dominanten Abflussbildungsprozesse gekennzeichnet. Die Ausweisung erfolgt prozessebenenspezifisch, so dass Prozesse der Geländeoberfläche, Zwischenabflussprozesse der Deckschichten, sowie basisabflussbildende Prozesse der Kluftgrundwasserleiter unterschieden werden. Die Plausibilität der für das mesoskalige Löhnersbach-Einzugsgebiet erzielten hydrologischen Raumgliederung konnte auf Basis einer empirischen Raumgliederungskarte geprüft werden.SummaryA method to delineate hydrological response units (HRU) within a GIS environment using generally available data sets and expert knowledge on landscape genesis is presented for the meso-scale Löhnersbach basin, located in the Austrian alps (Kitzbühler Alpen, 16 km2). Based on the genesis of the hillslope material a regionalization approach was developed which delineates the spatial structure and the lithological variance of the quaternary drift covers. Each of the delineated HRUs is characterized by the same runoff source areas and the same dominating runoff generation processes. The delineation was conducted using different process levels, differentiating between near surface processes, interflow processes in the drift cover, and base flow generating processes in the fractured hard rock aquifer. The delineation approach applied to the mesoscale Löhnersbach basin and the plausibility was proofed by a comparison with an empirically generated map.

Development and validation of a process oriented catchment model based on dominating runoff generation processes

Abstract The conceptual rainfall runoff model TAC (tracer aided catchment model) has been developed based on the experimental results of a tracer hydrological investigation for the mountainous Brugga basin (39.9 km2). The model contains an extensive physically realistic description of the runoff generation, which includes seven unit types each with characteristic dominating runoff generation processes. These processes are conceptualised by different linear and non-linear reservoir concepts. The model is applied to a period of 3.2 years with good success. In addition, the model is validated with additional information, e.g. tracer concentrations, beside to the simulated discharge.

Determination of spatially and temporally highly detailed groundwater recharge in porous aquifers by a SVAT model

Abstract An extended version of the SVAT model TRAIN is used to model groundwater recharge grid based (500 × 500 m) on a daily time step for porous aquifers without lateral flow components. The conceptual soil model of the HBV rainfall runoff model was integrated into the SVAT model. One empirical soil parameter was calibrated on measured percolation of 20 lysimeters in Baden-Wurttemberg, one of the federal states in the south-west of Germany. The model proved suitable to simulate monthly percolation (groundwater recharge) under very different conditions. After the calibration of the model at the point sites the model was applied to a study area. The study area is part of an extensive porous aquifer in the Upper Rhine Valley. The nature of the conceptual soil model allows an easy parameterisation from digital soil data sets of the scale 1 : 200 000 together with land use data. Different land uses are modelled separately within each pixel. The model can provide monthly groundwater recharge as input data for groundwater modelling as well as long term groundwater recharge, as a basis for sustainable management of groundwater resources.

Hydrological assessment of flow dynamic changes by storm sewer overflows and combined sewer overflows on an event-scale in an urban river

In addition to assessing the impacts of water quality changes in urban rivers caused by storm water sewer overflows (SWO) and combined sewer overflows (CSO), the extent to which flow dynamics are changed by these structures must be understood in order to define hydrological assessment criteria to guide sustainable water management strategies as required by the European Community (EC) Water Framework Directive. In this study, the quantitative impacts of SWOs and CSOs on the flow dynamics of an urban river and their variability are investigated. For four single runoff events, hydrological measurements were accomplished in the River Dreisam, upstream and downstream of the city of Freiburg, in southwest Germany. As the catchment is widely free of urban areas upstream of the city, comparison with downstream locations allowed quantification of Freiburgs effects on the changes in the hydrograph on an event scale. The proposed hydrological parameter—flow acceleration, peak discharge, and discharge dosage—were shown to be appropriate to assess the impacts of SWOs and CSOs on flood hydrographs in urban rivers.