Incorporating Climate Change and Variability into Bulletin 17B LP3 Model

The current techniques for flood frequency analysis presented in Bulletin 17B assume annual maximum floods are stationary, meaning that the distribution of flood flows is not significantly affected by climatic trends or long-term cycles (i.e. decadal variations). Observed trends in stream flows raise concern as to whether or not this assumption is valid. This paper considers how the Bulletin 17B framework might be modified to account for nonstationarity in flood records due to climate variability. In order to improve estimates/forecasts obtained using the LP3 model, the effects of climate variability may be incorporated into updated estimates of the mean, standard deviation, and perhaps the skew by regressing the LP3 parameters on climatic indices describing the Pacific Decadal Oscillation and Northern Atlantic Oscillation. The effects of climatic cycles occurring over a shorter time frame, such as El Niño, are averaged into estimates made using the procedures of Bulletin 17B. However, the effects of El Niño are likely to affect the magnitude of annual maximum stream flows, and thus would impact flood risk in a given year. El Niño effects are incorporated into forecasts by regressing the LP3 parameters on sea surface temperatures.