Diagnosing Possible Anthropogenic Contributions to Colorado Floods in September 2013.

Unusually heavy rainfall occurred over the Colorado Front Range during the second week of September 2013, with record or near-record totals recorded in several locations. It was associated predominantly with a stationary large-scale weather pattern (akin to the North American Monsoon, which occurs earlier in the year) that drove a strong plume of deep moisture inland from the Gulf of Mexico and eastern tropical Pacific towards the Front Range foothills. The resulting floods across the South Platte River basin impacted several thousands of people and many homes, roads, and businesses. A recent study using observational-based re-analysis to drive the regional WRF model finds that, given very little change in the large-scale weather pattern, there is an increase in atmospheric water vapour over northeast Colorado under anthropogenic climate warming, with a positive dynamical feedback drawing in moisture from further afield. This leads to a substantial increase in the magnitude and odds of heavy rainfall occurring over northeast Colorado during the rainy week of September 2013. Here we develop this work by including a hydrological modelling component in order to investigate any anthropogenic influence on the actual flood magnitude and occurrence across the South Platte basin during that time. We use WRF precipitation output from the aforementioned study - in both anthropogenic and non-anthropogenic configurations for September 2013 - to drive the recently developed high-resolution WRF-Hydro model over the basin and generate river runoff. Thus by comparing changes in runoff under the anthropogenic / non-anthropogenic driving conditions we assess any influence on the magnitude and odds of flood occurrence. Integral to this, we test the sensitivity of our results to hydrological parameters, such as infiltration, base flow, and land use/cover.