A degree-day (temperature-index) snowmelt model, the same approach hydrologists use to forecast spring runoff. Melt only happens when the effective air temperature at the snowline is above freezing, and the rate scales with both that temperature excess and the solar radiation reaching the pack:
T_eff = T_sea − 6.5°C·(elevation / 1000 m) (environmental lapse rate)
melt = max(0, T_eff) · (solar/100) · (1 + rain_boost) [mm of SWE per day]
SWE(t+dt) = max(0, SWE(t) − melt·dt)
flow = melt · SWE_present_factor · catchment_constant [m³/s]
Raising the snowline elevation cools the pack through the lapse rate, exactly like hiking a mountain gets colder with altitude — a snowpack can sit well above freezing at the valley floor while still frozen higher up. The "warm rain pulse" button adds a temporary rain-on-snow boost, mimicking the sharp melt spikes that a warm frontal rainstorm produces on top of ordinary radiation melt. The hydrograph on the right traces river discharge over model time, showing the classic single-peaked spring melt pulse: rising as the pack melts, then tailing off once the snow reserve is exhausted.
- Snow cap — visually shrinks as SWE (snow-water equivalent) depletes.
- Meltwater streaks — animated particles flowing down-slope at a rate proportional to the current melt flux.
- Hydrograph — flow vs. model time; a real spring-pulse curve rises with melt and falls once the pack is exhausted.