Hydrologic implications of projected changes in rain-on-snow melt for Great Lakes Basin watersheds

Abstract

Abstract. Rain-on-snow (ROS) melt events reduce the amount of water stored in the snowpack while also exacerbating flooding. The hydrologic implications of changing ROS events in a warming climate, however, are still uncertain. This research used a calibrated and validated Soil and Water Assessment Tool (SWAT) hydrologic model, modified with energy budget equations to simulate ROS melt and forced with a climate model ensemble representing moderate greenhouse-gas concentrations, to simulate changes to ROS melt in the North American Great Lakes Basin from 1960–2099. The changes to ROS events between the historic period (1960–1999) and mid-century (2040–2069) represent an approximately 30 % reduction in melt in warmer, southern subbasins, but less than 5 % reduction in melt in colder, northern subbasins. Additionally, proportionally more rainfall reduces the formation of snowpacks, with area-weighted winter+spring rain-to-snow ratios rising from approximately 1.5 historically to 2.0 by the end of the 21st century. Areas with historic mean winter+spring air temperatures lower than -2 °C have ROS regimes that are resilient to 21st century warming projections, but ROS occurrence in areas that have mean winter+spring temperatures near the freezing point are sensitive to changing air temperatures. Also, relationships between changes in the timing of ROS melt and water yield endure throughout the spring but become weak by summer. As the influence of ROS melt events on hydrological systems is being altered in a changing climate, these conclusions are important to inform adaptive management of freshwater ecosystems and human uses in regions of the globe that are sensitive to changes in ROS events.

Winter/Spring Runoff Is Earlier, More Protracted, and Increasing in Volume in the Laurentian Great Lakes Basin

“…Stream gauge data for the entire Great Lakes Basin were collected from US Geological Survey (USGS) and Environment and Climate Change Canada (ECCC) public data. We downloaded stream gauge data for all stream gauges, organized by Hydrologic Unit Codes (HUC 8 level in USA) or watersheds (tertiary watershed level in Canada) in the Great Lakes Basin following a published list of HUCs and watersheds (Table 4 in Neff et al., 2005; see also Meyers et al., 2023) from the inception of gauging programs through 2019. The USGS HUC 8s and ECCC tertiary watersheds are hereafter referred to as “sub‐watersheds” and they include several stream gauges each.…”