Hydrologic implications of 20th century warming and climate variability in the western U.S


Hamlet, A.F.  2006. Hydrologic implications of 20th century warming and climate variability in the western U.S. Ph.D. dissertation, University of Washington, Seattle.


Variations in 20th century climate across the western U.S. from 1915-2003 are characterized by regionally-specific, decadal-scale shifts in precipitation regimes, combined with systematic increases in temperature across essentially the entire West. In a series of integrated studies we use both observations and hydrologic models to examine the hydrologic implications of these climatic changes. In many areas of the West that have substantial snow accumulation in winter, warming has resulted in reductions in spring snowpack, earlier snowmelt, increased runoff in winter and less runoff in summer, earlier peak runoff and soil moisture recharge, and changes in natural flooding regimes. Evaporation, while apparently increasing somewhat in many areas, has so far had relatively minor effects in comparison with interannual variations in precipitation. Flood risks are affected by decadal and interannual variations in precipitation, by changes in precipitation variability apparent after the mid-1970s, and by warming via changes in antecedent snowpack and contributing basin area during storms. These hydrologic changes, although broadly characteristic of the West as a whole, vary spatially as a function of mid-winter temperature regimes. Downward trends in spring snowpack, and related timing shifts in streamflow, soil moisture recharge, and evaporation, for example, are strongest in areas along the coast with mid winter temperatures close to the freezing point, and are largely due to temperature related effects. Colder areas are also affected by warming to some degree, but variations in precipitation remain the dominant driver. Simulated changes in natural flood risks that accompany large scale warming are also a complex function of mid-winter temperature regimes. In strongly snowmelt dominant river basins, simulated flood risks are shown in most cases to decline with 20th century warming, because of reductions in spring snowpack. For river basins near the freezing level in mid-winter the effects of warming on flood risks vary widely and increases or decreases in flood risks can occur in response to warming depending on the relative importance of changes in contributing basin area and antecedent snow conditions that are coincident with storms. Flood risks in rain dominant basins are largely stationary in response to warming.