Hydrological modelling of alpine river catchments using output variables from atmospheric models

Over the last decade, flooding events on the southern side of the European Alps have claimed over 100 lives and property damage has amounted to more than 30 billion Swiss Francs. Accordingly, policy-makers and the public are strongly interested in precise and early forecasts in order to improve the options for control actions which are needed for warning and for damage prevention and minimization. Commonly used methods of runoff and flood prediction based on observed data are strongly limited with respect to their capabilities of providing the necessary lead time for small to medium scale alpine river catchments. Hence, there is a great need for advanced forecast methods. The aim of this thesis is to investigate the current potential of such an advanced approach for a medium scale mountain watershed. For this purpose, series of high-resolution numerical weather predictions are used to drive a detailed catchment model. Target areas of the investigations are the river catchments of Ticino, Verzasca and Maggia. The catchments cover a total area of 2627 km2 and drain directly from the north into the Lago Maggiore.Due to its particular location in the European Alps, these pre-alpine to high-alpine catchments are relatively frequently affected by heavy precipitation and corresponding flooding events. The first part of the thesis presents the hydrological Water Flow and Balance Simulation Model WaSiM-ETH.The theoretical basics of this highly detailed catchment model are described as well as the possible model applications and the potential for future model Upgrades. In the second part of the thesis, spatially distributed simulations investigate the hydrological water balance of the Ticino-Verzasca-Maggia watershed. In these simulations,the physically based WaSiM-ETH catchment model is driven by meteorological data from surface Station networks. The simulations are performed in hourly time Steps using a spatial grid resolution of 500 m x 500 m. The calibration and Validation of WaSiM-ETH are carried out using measured runoff data for the periods 1993-1996 and 1997-2000, respectively. The impact of reservoir management and artificial runoff redistributions on the natural catchment water balance is considered. Using the calibrated model WaSiM-ETH,seven relevant flooding events leading to considerable damages in the period between 1993 and 2000 are thoroughly analysed in the third part of the thesis. The model experiments are focused on runoff simulations using different meteorological model inputs. On the one hand, WaSiM-ETH is driven by meteorological data of either surface Station networks or weather radar. On the other hand, the Simulation results of five high-resolution numerical weather prediction models (3-14 km mesh) are used as model input into WaSiM-ETH in order to reproduce the selected flood events. The experimental design of this model coupling is described.Particular attention is paid to the "scale problem", i.e. the disaggregation of the coarser grid data of the weather models (e.g. 10 km mesh) to the finer grid of the hydrological model (0.5 km mesh). Although the results of the flood forecasting experiments (leading times up to 36 hours) are of varying accuracy, they clearly suggest that coupled atmospheric-hydrological modelling has a great potential. Nevertheless, there are also some obvious problems and limitations concerning this technique which are particularly evident in sensitivity studies. The latter make clear that relatively small inaccuracies in precipitation forecasting often grow into considerable errors in flood forecasting. The contribution of hydrological models to the process of verification and evaluation of high resolution atmospheric simulations is discussed in the last part of the thesis. This is done by concatenating daily forecasts of the former operational weather prediction model of Switzerland to a 3.5-year forecast sequence (1996-1999) which is subsequently provided as meteorological input for the hydrological WaSiM-ETH model. The results of these coupled model runs are compared to measured runoff data and to simulated catchment water balances which are based on computations with the Swiss weather model itself and on observation-driven WaSiM-ETH model runs. In addition, high-resolution weather predictions of another model are used for modelling the river runoff by means of coupled simulations for a 2.5-monthsequence (MAP-SOP 1999). The Simulation results agree in general with the observed and simulated runoff sequences. Nevertheless,they also make clear the need to adjust the precipitation forecasts with respect to their bias before using them for hydrological flood forecasting