The major June 2023 flood event in Central Chile: A rain-on-snow case study at the Achibueno en la Recova River catchment

After a decade long period of dry conditions in central Chile, the so called “Megadrought”, the winter of 2023 was an extraordinary wet season with two extreme precipitation events that led to two mayor floods. In June and August of 2023 (austral winter) two intense Atmospheric Rivers (AR) impacted the central region of Chile, resulting in high streamflow and flooding. The two hydrometeorological events produced significant infrastructure, social and economic damages in the region. The June 22-25 event occurred during a strong (Category 4) and persistent (~72 hrs) zonal AR. Intense precipitation was registered in several weather stations along the Central Andes Cordillera foothills, with total precipitation above 800 mm/event in several stations. Anomalous windy and warm temperature conditions were recorded, positioning the freezing level at higher-than-average elevations, and thus, creating a potential rain-on-snow (ROS) flood hazards in some catchments. We use the Achibueno en la Recova River (ARR) catchment as a case study as it had the highest recorded instantaneous peak flow in more than 30 years of records. Satellite images and data from a high elevation snow station show a persistent snowpack above the 2000 masl with about 200 mm of snow water equivalent, which began to melt at the beginning of the event, suggesting that a rain-on-snow event may have enhanced the flood. We implemented a physically based hydrological model using the Cold Regions Hydrological Model at the ARR catchment to reproduce the event, estimate the contribution of the ROS to the flood event and understand the controlling physical mechanisms. The hydrological model was compared against snow water equivalent and streamflow records, reasonably representing both the timing and the magnitude of these variables. Model results suggest that the ROS significantly contributed to the event, representing about 18% of the streamflow volume (1.1x10⁸ m³), primarily during the first 2 days. The ratio between the Terrestrial Water Input (i.e., snowmelt plus rainfall) to rainfall show values between 1.7 and 1.9 at elevations between 2000 and 3000 masl, with higher values at south-facing slopes. The energy balance shows that most of the energy to melt the snowpack comes from the advected energy from the rain (43%), followed by net radiation (37%), latent (10%) and sensible (10%) heat fluxes. This study is, to the authors knowledge, the first documented study of a ROS event in the Chilean Andes with a significant societal and economic impact, which may help to better understand the potential of future ROS floods in The Andes.