A distributed version of the HBV-model using 1 km 2 grid cells and daily time step was used to simulate runoff from the entire land surface of Norway for the period . The model was sensitive to changes in small scale properties of the land surface and the climatic input data, through explicit representation of differences between model elements, and by implicit consideration of sub-grid variations in moisture status. A geographically transferable set of model parameters was determined by a multi-criteria calibration strategy, which simultaneously minimised the residuals between model simulated and observed runoff from 141 Norwegian catchments located in areas with different runoff regimes and landscape characteristics. Model discretisation units with identical landscape classification were assigned similar parameter values. Model performance was evaluated by simulating discharge from 43 independent catchments. Finally, a river routing procedure using a kinematic wave approximation to open channel flow was introduced in the model, and discharges from three additional catchments were calculated and compared with observations. The model was used to produce a map of average annual runoff for Norway for the period 1961-1990.

Runoff prediction in ungauged basins (PUB) is a challenging but essential research topic from scientific and practical perspectives, and the International Association of Hydrological Sciences (IAHS) was motivated to establish a "Decade on Predictions in Ungauged Basins (PUB): 2003-2012". Regionalization methods are currently regarded as the most common solution for PUB, coupled with hydrological models. Regionalization methods are generally categorized into three classes: spatial proximity, physical similarity and regression. However, knowledge of the applicability of regionalization methods is not clear or consistent, despite the number of extensive studies that have been conducted and considerable progress that has been achieved in recent decades. Due to the nonstationary climate, the reliability of results from historical data for future applications cannot be used undoubtedly. Even worse, PUB studies in cold regions (e.g., Norway) are limited.

This study examines the impact of climate change on droughts in Norway. A spatially distributed (1 3 1 km 2 ) version of the Hydrologiska Byrå ns Vattenbalansavdelning (HBV) precipitation-runoff model was used to provide hydrological data for the analyses. Downscaled daily temperature and precipitation derived from two atmosphere-ocean general circulation models with two future emission scenarios were applied as input to the HBV model. The differences in hydroclimatological drought characteristics in the summer season between the periods 1961-90 and 2071-2100 were studied. The threshold level method was adopted to select drought events for both present and future climates. Changes in both the duration and spatial extent of precipitation, soil moisture, runoff, and groundwater droughts were identified. Despite small changes in future meteorological drought characteristics, substantial increases in hydrological drought duration and drought affected areas are expected, especially in the southern and northernmost parts of the country. Reduced summer precipitation is a major factor that affects changes in drought characteristics in the south while temperature increases play a more dominant role for the rest of the country.

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