Effects of 20th century warming and climate variability on flood risk in the western U.S.

[1] Using precipitation and temperature data for the 20th century in combination with a macroscale hydrologic model, we evaluate changes in flood risk in the western U. S. associated both with century-scale warming and interannual climate variations. In addition, we examine the implications of apparent increases in precipitation variability over the region since the mid-1970s. We use detrended temperature data representing early and late 20th century climate to force the variable infiltration capacity hydrologic model and show that spatially homogeneous temperature changes over the western U. S. in the 20th century on the order of +1 degrees C per century have resulted in substantial changes in flood risks over much of the region. Although changes specific to particular geographic areas are apparent in some cases, the overall changes due to observed warming trends are well categorized by midwinter temperature regimes in each watershed. Cold river basins where snow processes dominate the annual hydrologic cycle (<-6 degrees C average in midwinter) typically show reductions in flood risk due to overall reductions in spring snowpack. Relatively warm rain-dominant basins (> 5 degrees C average in midwinter) show little systematic change. Intermediate or transient basins show a wide range of effects depending on competing factors such as the relative role of antecedent snow and contributing basin area during storms that cause flooding. Warmer transient basins along the coast in Washington, Oregon, and California, in particular, tend to show increased flood risk. While the absolute value of simulated changes in flood risk is affected by basin scale, the nature of the relationship of flood risk to basin temperatures in midwinter is largely scale-independent. Climate variations associated with Pacific Decadal Oscillation (PDO) and El Nino Southern Oscillation (ENSO) also have strong effects on flood risks. In contrast to the effects associated with 20th century warming, the climate variability signal is characterized by regional scale patterns related to the geographic distribution of cool season precipitation also identified in many previous studies. In general, the largest changes in simulated flood risks are associated with years when PDO and ENSO are in phase,'' particularly in the southwest. Changes in the variability of cool season precipitation after about 1973, the causes of which are uncertain, are shown to result in increased flood risk over much of the western U. S. in the simulations.