Abstract
Abstract. In response to ongoing and future-projected global warming, mid-latitude, nival river basins are expected to transition from a snowmelt-dominated flow regime to a nival–pluvial one with an earlier spring freshet of reduced magnitude. There is, however, a rich variation in responses that depends on factors such as the topographic complexity of the basin and the strength of maritime influences. We illustrate the potential effects of a strong maritime influence by studying future changes in cold season flow variability in the Fraser River Basin (FRB) of British Columbia, a large extratropical watershed extending from the Rocky Mountains to the Pacific Coast. We use a process-based hydrological model driven by an ensemble of 21 statistically downscaled simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), following the Representative Concentration Pathway 8.5 (RCP 8.5). Warming under RCP 8.5 leads to reduced winter snowfall, shortening the average snow accumulation season by about one-third. Despite this, large increases in cold season rainfall lead to unprecedented cold season peak flows and increased overall runoff variability in the VIC simulations. Increased cold season rainfall is shown to be the dominant climatic driver in the Coast Mountains, contributing 60 % to mean cold season runoff changes in the 2080s. Cold season runoff at the outlet of the basin increases by 70 % by the 2080s, and its interannual variability more than doubles when compared to the 1990s, suggesting substantial challenges for operational flow forecasting in the region. Furthermore, almost half of the basin (45 %) transitions from a snow-dominated runoff regime in the 1990s to a primarily rain-dominated regime in the 2080s, according to a snowmelt pulse detection algorithm. While these projections are consistent with the anticipated transition from a nival to a nival–pluvial hydrologic regime, the marked increase in FRB cold season runoff is likely linked to more frequent landfalling atmospheric rivers in the region projected in the CMIP5 models, providing insights for other maritime-influenced extratropical basins.
Highlights
Rising air temperatures and changes in precipitation patterns are altering hydrological processes and states in river basins across the globe, including those in cold regions
Snowdominated river basins are sensitive to warming air temperatures, as these can lead to marked decreases in seasonal and longer-term water storage that otherwise provides a reliable source of streamflow generation during the spring and summer melt periods (Barnett et al, 2005)
To explore potential regime changes in the Fraser River Basin (FRB), we evaluate and compare the fraction of years for which snowmelt pulse (SP) are recorded within each analysis period (1990s, 2050s and 2080s) at each 0.25◦ grid cell and for all Coupled Model Intercomparison Project Phase 5 (CMIP5)-Variable infiltration capacity (VIC) simulations
Summary
Rising air temperatures and changes in precipitation patterns are altering hydrological processes and states in river basins across the globe, including those in cold regions. Snowdominated (nival) river basins are sensitive to warming air temperatures, as these can lead to marked decreases in seasonal and longer-term water storage that otherwise provides a reliable source of streamflow generation during the spring and summer melt periods (Barnett et al, 2005). This loss of storage is expected to lead to increased interannual streamflow variability (Fleming and Clarke, 2005; Fountain and Tangborn, 1985). The timing of the spring freshet advances with warming and declin-
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