The model simulates discharge using rainfall, temperature and potential evaporation as input. Precipitation is simulated to be either snow or rain depending on whether the temperature is above or below a threshold temperature, TT [ฐC]. All precipitation simulated to be snow, i.e. falling when the temperature is bellow TT, is multiplied by a snowfall correction factor, SFCF [-], which represents systematic errors in the snowfall measurements and the `missing' evaporation from the snow pack in the model. Snowmelt is calculated with the degree-day (degree-Δt in case of a non-daily time step) method (Equation 1). Meltwater and rainfall is retained within the snowpack until it exceeds a certain fraction, CWH [-], of the water equivalent of the snow. Liquid water within the snowpack refreezes according to a refreezing coefficient, CFR (Equation 2). Rainfall and snowmelt (P) are divided into water filling the soil box and groundwater recharge depending on the relation between water content of the soil box (SM [mm]) and its largest value (FC [mm]) (Equation 3). Actual evaporation from the soil box equals the potential evaporation if SM/FC is above LP [-] while a linear reduction is used when SM/FC is below LP (Equation 4). Groundwater recharge is added to the upper groundwater box (SUZ [mm]). PERC [mm Δt-1]defines the maximum percolation rate from the upper to the lower groundwater box (SLZ [mm]). For the lakes, precipitation and evaporation is added and subtracted directly from the lower box. Runoff from the groundwater boxes is computed as the sum of two or three linear outflow equations depending on whether SUZ is above a threshold value, UZL [mm], or not (Equation 5). This runoff is finally transformed by a triangular weighting function defined by the parameter MAXBAS (Equation 6) to give the simulated runoff [mm Δt-1].
If different elevation zones are used the changes precipitation and temperature with elevation are calculated using the two parameters PCALT [%/100 m] and TCALT [บC / 100 m] (Equation 7 and 8).
The long-term mean of the potential evaporation, Epot,M for a certain day of the year can be corrected to its value at timestep t , Epot(t), by using the deviations of the temperature, T(t), from its long-term mean, TM , and a correction factor, CET [ฐC-1] (Equation 9).