Diagnosing Anthropogenic Contributions to Heavy Colorado Rains 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 eastern tropical Pacific and Gulf of Mexico 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 observation and models found that, given very little change in the large-scale weather pattern, there is an increase in local atmospheric water vapour under anthropogenic climate warming, with a secondary dynamical feedback drawing in moisture from further afield, leading to a substantial increase in the magnitude and odds of heavy rainfall occurring over northeast Colorado in September 2013. Here we develop that study by including a hydrological modeling component to investigate any anthropogenic influence on the actual flood occurrence in the South Platte basin during that time. We use rainfall generated by that study – in both anthropogenic / non-anthropogenic configurations for September 2013 – to drive the high-resolution WRF-Hydro model over the basin and generate river runoff. Thus by comparing changes in peak simulated 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.