The new Swedish guidelines for the estimation of design floods for dams and spillways are presented, with emphasis on high-hazard dams. The method is based on a set of regional design precipitation sequences, rescaled for basin area, season and elevation above sea level, and a full hydrological model. A reservoir operation strategy is also a fundamental component of the guidelines. The most critical combination of flood generating factors is searched by systematically inserting the design precipitation sequence into a ten year climatological record, where the initial snowpack has been replaced by a statistical 30-year snowpack. The new guidelines are applicable to single reservoir systems as well as more complex hydroelectric schemes, and cover snowmelt floods, rain floods and combinations of the two. In order to study the probabilities of the computed floods and to avoid regional inconsistencies, extensive comparisons with observed floods and frequency analyses have been carried out.

General principles in development and application of hydrological models are discussed and related to the confidence in the results. The presentation is mainly based on the experience from the work with the HBV and PULSE models at the Swedish Meteorological and Hydrological Institute between 1971 and 1990 but has also been influenced by other modelling work. It covers a discussion on the optimal complexity of models, use of observations, calibration, control and sensitivity analysis. Special attention is given to the uncertainties encountered when using hydrological models for the simulation of extreme floods and long-term scenario simulations. Finally a few ethical problems in modelling are mentioned.

The experiences of conceptual runoff modelling at the Swedish Meteorological and Hydrological Institute are surnmarized in the present work. The basic philosophy and the methodology when developing the HBV-model are discussed. The structure of the model is described with a discussion of its physical relevance and examples of alternatives. The sensitivity of the model to changes in parameter values is studied through reappings of the response surfaces of a sum of squares criterion of fit. Applications to a variety of catchments in Sweden and Norway are presented and the performance of the model is verified by a numerical criterion of fit, plottings of computed hydrographs and recorded ones, scatter diagrams of peak flows and flow duration curves. Examples of both short range and longrange hydrological forecasting are given.

A general conclusion is that the HBV-model can be used for the reconstructionof the discharge in catchments of the presented type, if it is properly calibrated. The model can also be used for hydrological forecasting, if combined with meteorological forecasts or recorded climatic series.

Erfarenheterna av begreppsmässiga avrinningsmodeller vid Sveriges Meteorologiskaoch Hydrologiska Institut summeras i föreliggande arbete. Den grundläggande filosofin och metodiken vid utvecklingen av HBV-modellen diskuteras. Modellstrukturen beskrives med en presentation av den fysikaliska bakgrunden och exempel på alternativ. Modellens känslighet förstörningar av parametervärden studeras genom kartläggning av ett minsta kvadratkriteriums responsytor. Tillämpningar av modellen på ett antal avrinningsområden i Sverige och Norge redovisas och simuleringen utvärderas med ett numeriskt anpassningskriterium, uppritningar av den beräknade och observerade hydrografen, jämförelser mellan simulerade och beräknade flödestoppar samt varaktighetsdiagram. Exempel ges på hydrologiska lång- och korttidsprognoser.

En allmän slutsats är att HBV-modellen är användbar för rekonstruktion av vattenföringsserier i den typ av områden, där den tillämpats, under förutsättning att den kalibreras på ett riktigt sätt. Modellen kan också användas för hydrologiska prognoser med hjälp av meteorologiska prognoser eller observerade klimatserier.

The application of the model in the Filefjell Basin led us to some conclution concering tho model structure and the relative importance of its parameters.

The original HBV-2 model had to be modified with the introduction of a third runoff component. A oompletely lumped modifioation proved to be better than a more distributed one.

Once the model was modiified the calibration was relatively simple.The most important parameters were those in the snow-routine and the recession coefficiants in the responee function. One of the biggest problems when trying to apply the model to ungauged catohments seems to bo the empirical coefficiente that we are forced to use when extrapolating preoipitation observations to areal values.

The soil moisture routine is less important in this alpine catohment because evaporation is low. A fictive value of the available water, Fc, of 150 mm was assumed but the maximum deficit in the soil moisture zone in the model was less than 50 mm during the seven years in the period.

The conflict between subjective visual inspection and optimum parameters in the least squares-sense in a strong indication that fittingsoriterious must not be used blindly, unless their capability of representing tbe agreement between tbe observed and computed hydrographs is explored rlgorously.