Modelling Climate Change Impacts on Spring Runoff for the Rocky Mountains of Montana and Alberta II: Runoff Change Projections using Future Scenarios

Abstract

In Part I of this two-part study, the Simulated Grid microclimate model (SIMGRID) was modified and the new version validated on the St. Mary River watershed in northern Montana, using historical data. In Part II, future climate change scenarios are used to estimate spring streamflow (QS) for the 1961–2099 period. Relative to the base period (1961–1990), the model indicates median year QS decline of 3 – 8% by the 2020s, 8 – 17% by the 2050s, and 15 – 27% by the 2080s. Mean onset of the spring pulse is projected to occur in early March or late February for the 2080s, 36 to 50 days earlier than for the 1961–1990 reference period. Model results generally indicate increased precipitation, but spring runoff volumes will decrease substantially, because the higher rain:snow ratio and shorter accumulation period will decrease snowpack volume. Overall, the results of this study indicate that the increased winter temperature resulting from anthropogenically-induced climate change, will result in shorter winters, reduced snowpack volume, and earlier spring snowmelt and runoff onset, resulting in substantial reductions in spring discharge.