Effects of 20th Century Warming and Climate Variability on Flood Risk in the Western U.S.

Flood risk in the western U.S., although frequently assumed for design purposes to be static, is in reality a function of non-stationary regional climate and climate variability. During the 20th century, the western U.S. has warmed at a rate of about 1 degree C per century overall, and changes in cool season precipitation variability are also evident after about 1975. Using the Variable Infiltration Capacity (VIC) hydrologic model and a long term driving data set from 1916-2003, we examine the implications of these 20th century changes in climate on natural flooding regimes in the West. We find that changes in flood risk that accompany large scale warming are a complex function of mid-winter temperature regimes, which are related to seasonal changes in antecedent snowpack and contributing basin area during storms that cause flooding. In strongly snowmelt dominant river basins, simulated flood risk in most cases has declined with 20th century warming, because of reductions in peak spring snowpack. For river basins near the freezing level in mid-winter the effects of warming on flood risk vary widely and increases or decreases in flood risks can occur in response to warming depending on the relative importance of changes in effective basin area and antecedent snow conditions that are coincident with storms. Relatively warm basins close to the Pacific Coast tend to show increased flood risk, while somewhat colder inland areas tend to show decreased flood risk overall. Flood risk in rain-dominant basins, although affected by small systematic changes in soil moisture, is largely stationary in response to warming. Changes in cool season precipitation variability (increased variance, autocorrelation, and regional synchronicity) since about 1975 are shown to increase flood risk over much of the western U.S., however it is currently unclear whether these precipitation changes are systematic in nature (e.g. related to warming) or whether this is simply one mode of precipitation variability which has been coincident with late 20th century warming.