Abstract:

Pervasive increases in temperature over the Western U.S. over the last half-century have been accompanied by hydrologic changes, most notably in streams where much of the annual flow is associated with winter snow accumulation. Maximum snow accumulation (mostly in March or early April) has seen widespread declines, especially at lower elevations. This has resulted in shifts in the timing of seasonal runoff, for instance as measured by the centroid of timing, which in many locations has trended earlier by several weeks over the same period. While these changes have been well documented for some time, subtleties such as changes in melt rates (which somewhat counterintuitively trend downward in a warmer climate) and changes in annual streamflow volume (which is critical in river basins such as the Colorado, where seasonal timing is less consequential than volume declines) have been less studied. Furthermore, there is evolving evidence that the rate of hydrologic change may actually be decelerating as the climate continues to warm, and snow-affected area in the Western Cordillera decreases.