A lengthening vernal window: hydrologic impacts of changes in the timing of snow melt and budburst in northeastern North America

The vernal window, or the interval between snowmelt and canopy leaf out, is a key period for the functioning of seasonally snow-covered, forested ecosystems. Despite the ecological importance of the vernal window, few studies have examined how it has changed historically, how it might lengthen in the future, and how its duration impacts ecosystem function and hydrology. We simulated historical and future changes in the vernal window using a hydrologic model and a phenology model under lower and higher climate forcing. We found that the window will lengthen by more than two weeks by the end of the century under higher climate forcing, largely driven by earlier snow disappearance. Loss of snow cover under a high emissions scenario also causes the complete elimination of the vernal window over 43 - 59% of the study domain area by the end of the century. In addition, the timing of key vernal window transitions such as the spring runoff center of volume date advances by -1.2 to -2.0 days per decade, making the timing of the spring runoff center of volume in seasonally snow-covered areas resemble that of snow-free regions. Further, longer lags between snow disappearance and the spring runoff center of volume, and the spring runoff center of volume and budburst, have implications for ecosystem function, changing ecosystem energy, carbon, and water balances.