New Methodology to Develop Future Flood Frequency under the Changing Climate by Means of Physically-Based Numerical Atmospheric-Hydrologic Modeling

Effect of climate change on hydrologic flow regimes, particularly extreme events, necessitates modeling of future flows in order to best inform water resources management. In this study, the future flows were simulated using global climate models (GCMs) and a regional atmospheric model in tandem with a watershed model to make a hydro-climate model (WEHY-HCM), over the Cache Creek Watershed in Northern California. The future climate projections, based on four emission scenarios simulated by two GCMs (ECHAM5 and CCSM3) under several initial conditions, were dynamically downscaled using MM5, a regional climate model. The downscaled future precipitation data were bias-corrected before being input into the WEHY model to simulate the detailed flow at hourly intervals along the main Cache Creek branch and its tributaries during 2010–2099. The results suggest an increasing trend in flood magnitudes and their intensities at the outlet of the study region throughout the 21st century. Similarly, estimates of the 100 and 200-year floods increased throughout the study period. The observed differences in the estimated future flood frequencies between the first half and the second half of 21st century may be an evidence of the non-stationarity in the 21st century hydrological regime over the study region.