A Comparison of Precipitation Downscaling Procedures

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A Comparison of Precipitation Downscaling Procedures: In relatively small basins with arid climates, rainfall characteristics are highly variable andstream flow is tightly coupled with the nuances of rainfall events (e.g. hourly precipitationpatterns). Reclamation’s climate change assessments in these basins typically employ CMIP5projections downscaled with Bias Corrected Statistical Downscaling and BiasCorrection/Constructed Analogs (BCSD-BCCA) methods, but these products do not always meetmanagement needs. This project, funded through Reclamation’s Science and Technology GrantProgram, consists of a rigorous comparison of the BCSD-BCCA methods with alternativeapproaches to guide Reclamation staff directed to incorporate climate change impacts intohydrologic studies. These include rainfall output from dynamical downscaled Regional ClimateModels (RCM), a stochastic rainfall generator forced by either Global Climate Models (GCM) orRCM, and projections using historical records conditioned on either GCM or RCM.This comparison will be performed for two sites. The Lower Colorado Region’s RiverOperations Group identified the Bill Williams River drainage to Alamo Lake, upstream of LakeHavasu, as an area in need of improved flood projections. The Upper Santa Cruz River nearNogales, Arizona on the Arizona-Mexico border is the site of an on-going Reclamation waterresources appraisal study. Our objectives are to: 1) evaluate the suitability of each precipitationprojection method for each study type; 2) develop a test to select the preferred method for agiven type of study; and 3) quantify the impact of each method on Reclamation’s planningactivities.