Rain-on-snow events and linkages to streamflow in the east branch of the Delaware Watershed, New York

Rain-on-snow (ROS) events are critical hydrometeorological triggers of flooding in snow-dominated and transitional watersheds. In this study, we investigated linkages between ROS events and streamflow in the historically flood-prone region of the East Branch of the Delaware Watershed (EBDW) in New York (USA). By integrating the University of Arizona 4 km daily gridded snow water equivalent (SWE) and snow depth data, PRISM precipitation and temperature data, and records from seven USGS streamflow gages, a 32-year climatology (1990-2022) of ROS events was developed to better understand the impacts of ROS events on peak streamflow. ROS events were identified based on thresholds for temperature (> 0°C), precipitation (≥ 10 mm), and snowpack thresholds (SWE and snow depth ≥ 10 mm). To quantify change and relationships, statistical analyses including Mann-Kendall trend tests, OLS, correlation analysis, and percentile-based thresholds (Q80 and Q90) to classify normal, high, and extreme streamflow events were used. Results indicate that most ROS events occur in winter (DJF) and spring (MAM), with MAM emerging as the season closely linked to peak streamflow. Precipitation had the strongest correlations with streamflow during and after ROS events, while higher SWE and snow depth values amplified discharge. Peak streamflow events were most frequent at the downstream gages with steep topography, in confined valleys, and in broad drainage areas, highlighting the influence of physiographic controls on flood generation. By providing a basin-scale climatology of ROS events and their influence on peak flows, this study aims to improve ROS forecasts that are critical to flood management and infrastructure planning in New York and similar regions.