Projections of Increasing Flood Frequency and Intensity Across the Western United States Objective, Data, and Methods: The goal of this study is to quantify the future projected changes in flood frequency and intensity in the western United States. Data: Daily flow data was produced by use of statistically downscaled climate projections produced by 27 GCM runs from the 5th Coupled Model Intercomparison Project (CMIP5) Downscaling and hydrology model simulations performed by US Bureau of Reclamation (Reclamation, 2014) 421 gauge stations across the western United States for 30- year time periods: o , , , and Methods: Standard flood frequency analysis method (Bulletin 17B by the United States Interagency Committee on Water). Annual peak flow calculated at different recurrence intervals and, by inverting the process, the future recurrence intervals of historic flood levels were determined The effect of interpolated weighted skew (using USGS regional map skew) on future flood projections was analyzed Gretchen L. Kayser 1, Edwin P. Maurer 1, Laura Doyle 1, Andrew W. Wood 2 1 Civil Engineering Department, Santa Clara University, Santa Clara, CA National Center for Atmospheric Research, Boulder, CO Conclusions: Increases in severity of peak flows and return periods over most locations Widespread increases occur regardless of whether mean precipitation increases or decreases. Small effect of using USGS regional map skew versus local skew. Local Infrastructure planning can accommodate climate change impacts with a simple framework adjusting design return periods (using the  value above). A small number of cases of decreasing peak flow projections in mountainous areas with decreasing or small increases in precipitation. References: Mailhot, A., and Duchesne, S. (2010), Design Criteria of Urban Drainage Infrastructures under Climate Change, J. Water Res. Plan. Mgmt., 136, , /(asce)wr ,. Reclamation (2014), Downscaled CMIP3 and CMIP5 Hydrology Projections: Release of Hydrology Projections, Comparison with Preceding Information, and Summary of User Needs, Rep., 111 pp, available at Fig. 1: Change in precipitation between the years 1971–2000 and 2070–2099. Points are stream gauge locations used in this study. Fig. 2 and 3: 100 year and 25 year peak flow changes and future return period based on comparison on the years and using weighted skew (with USGS regional map skew from Bulletin 17B). Circles indicate statistically significant changes (at  =0.05) and squares are not significant. Future Peak Flow Values and Return Periods: Fig. 5: Values of  in Equation 1, representing the ratio of the return period to the return period. Historical return periods are 100-year (left) and 25-year (right). Motivation: The changing behavior of extreme hydroclimatic events increasing the frequency and intensity of extreme precipitation events in certain areas. Increased temperature effects on precipitation type, rain or snow, and rate of snow melt The potential of changing peak flood levels that could produce large losses to society Applying a single parameter to plan for changes: The simplified framework of Mailhot and Duchesne (2010) is applied to examine the prospect for using this technique for incorporating projected changes in flood flows to present planning of infrastructure. The simplified approach is described by the equation below: T 0 is the design recurrence interval to be used at the time of installation, T C is the critical return period describing the level of protection desired at some reference year in the future, t d is the design life of the infrastructure (useful life),  is the ratio of the reference year (at which the design level associated with T C is met) to t d, and  is the fractional change in the flood level over a 100-year period. The  value design criterion ensures that the level of service maintains the selected "acceptable" level over a predefined lifetime. Fig. 4: Difference (from to ) between using USGS regional map skew and local skew in the Bulletin 17B analysis for 100 year peak flow changes (left) and return period of current value 100 year flow (right) Expected Lifetime Critical Return Period Table 1: Design return periods for various critical return periods (T C ) and expected infrastructure lifetimes (t d ) as calculated from Equation 1. These are for a scenario in California where  0.2 for T C =100 years and  0.3 for T C =25 years. Assumes  =0.5. Poster GC51E-1124 Does only using station skew (no skew weighting) change the results? Example: if a levee is designed for a useful service life of 60 years, and must provide protection against a 25-year event in the future, the design should be for a 32-year recurrence interval.  =0.5 means the 25-year protection will be met through year 30. Projected Precipitation changes: