Snowmelt Contribution to Streamflow in a Heavy-Snowfall Basin in Japan: A SWAT+ Modeling Study

In heavy-snowfall mountain basins, seasonal snowpack acts as an important reservoir for winter precipitation, yet applications of SWAT+ to such basins on the Sea of Japan side remain limited. We developed a SWAT+ model for the Tedori River Basin (691 km2) in Ishikawa Prefecture, Japan, and evaluated how the precipitation lapse rate (PLAPS) affected daily streamflow simulation and how the hydrological role of snowmelt varied within the basin during 2016–2023. Ten cases combining different winter and non-winter PLAPS values were compared, and seasonal specification produced only limited improvement. The adopted case achieved an NSE of 0.734 and a PBIAS of −8.6%. Simulated mean annual precipitation was 4178 mm, of which 1438 mm (34.4%) occurred as snowfall. The snowmelt input ratio—the fraction of the liquid water input supplied by snowmelt—was 33.8% for the whole basin and 34.2% for the high-elevation Tedorigawa Dam Basin, but only 20.4% for the lower-elevation Dainichi River Basin. Snow/No-Snow scenario analysis showed that snow processes had little effect on annual runoff volume but redistributed streamflow from winter to spring and beyond. This influence ended 61 days later at the Tedori River Basin outlet than at the Dainichi River Basin outlet. Combining the snowmelt input ratio with scenario analysis therefore provides complementary information on how much snowmelt enters the basin and on the timing and duration of its influence on streamflow.