The HBV model

The model is a conceptual rainfall-runoff model. The runoff generating processes in nature are described by simplified relationships which transform precipitation into runoff. The sub-basin is the primary hydrological unit, and an area-elevation distribution and a basic  classification of land use (forest, open, lakes) are implemented for this unit. The model consists of three main components:

• Snow module for snow accumulation and melting


• Module for soil moisture accounting


• Hydrological response and river routing module.

The snow module keeps track of snow accumulation and melting in different elevation zones by using precipitation and temperature observations as input data. Storage of the precipitation which falls as snow during the winter season is decisive in determining the distribution of runoff during the year.

The soil moisture module controls the water balance in the upper soil layer. This zone dries out in periods of little rainfall/snow melt due to  evapotranspiration and fills up again in wet periods. When the soil water zone has a high water content, the catchment response to rainfall is relatively large and rapid. On the other hand, the same rainfall may produce only a negligible increase in runoff  when the conditions are dry.

The water which is not stored as snow or soil water moves  through the catchment. This water transport is delayed by seepage through the soil, throughthe groundwater zone and finally through bogs/marshes and small streams. This delay is treated in the response and river routing module.

The model can be adapted to river basins ranging from a few km² up to several thousand km². It can also be applied to larger areas represented by a gridded model structure. Many of the model parameters can be determined from catchment physiography, but the final adjustment of the model to a specific catchment is carried out by parameter calibration.

 

Figure: The main components of the HBV model.