The influence of spring soil moisture on flood runoff in forest and grassland basins of the Haihe river mountainous region

Study region This study focuses on 10 typical basins in the Haihe River mountainous region, where runoff processes have undergone significant changes during the period of vegetation restoration. Study focus Based on hydrometeorological, vegetation, and soil moisture (SM) data from 1983 to 2020, this study analyzes runoff differences and their causes between forest and grassland basins under vegetation restoration. SEM combined with an LSTM was applied to identify the seasonal pathway among vegetation, SM, and runoff, and to quantify the hydrological sensitivity of flood season runoff to spring SM changes in the two basin types. New hydrological insights for the region Compared with the period before 2000, both annual and flood season runoff declined during the later stage of vegetation restoration, but at different rates, indicating a weakening of runoff seasonality. Grassland basins exhibited a larger discrepancy between flood season and annual runoff reductions, reflecting stronger seasonal sensitivity. The analysis indicates that spring SM depletion associated with vegetation restoration persists into the flood season through a SM carryover effect, and this is one of the key factors behind the different flood‑season runoff responses of forest and grassland basins. Compared with forest basins, grassland basins experienced larger spring SM declines and stronger impacts. Based on LSTM control experiments, a unit change in spring SM leads to an approximately 2.87% change in flood season runoff in grassland basins, which is 1.15 %age points higher than that in forest basins, indicating greater hydrological sensitivity.