Recent Changes in Cold Season Precipitation Patterns in Northeast Watersheds and Their Effects on Streamflow Dynamics

The ongoing climate warming has the potential to shift winter precipitation patterns from snowfall to rainfall, potentially altering streamflow behavior and increasing flood risks. Previous studies on extreme precipitation have generally not distinguished between snowfall and rainfall. Similarly, investigations into winter streamflow dynamics often overlook event scale flood analysis and fail to differentiate the impacts of rainfall from those of snowmelt. Consequently, it remains unclear how winter pluvial and fluvial floods have evolved and how these changes may be related to the alterations in cold-season precipitation under a warming climate.

In this study, we examined cold-season streamflow dynamics by analyzing the contributions of rainfall and snowmelt to potential flood events. This analysis utilized data from the United States Geological Survey (USGS) streamflow records, gridMET precipitation and temperature data, GLEAM root zone soil moisture, and datasets from the National Snow and Ice Data Center (NSIDC) over the period from 1981 to 2021. Potential flood events were identified by calculating the 90th percentile of winter streamflow across 40 years in various watersheds in the US Northeast. A flood event at each watershed was defined when streamflow exceeded the defined thresholds for each watershed. Temperature data, with a 0℃ threshold, was used to differentiate between rainfall and snowfall events. Out of 2,591 USGS gauges in the Northeast, 348 were selected based on the length of flow record and watershed drainage area, excluding very small watersheds (less than 100 square miles) and very large watersheds (more than 1,000 square miles) to focus on the local impacts of winter precipitation on flood dynamics. Statistical analyses, including correlation, regression, and trend analysis, were conducted to develop a process-based understanding of how precipitation characteristics influence winter streamflow dynamics. Preliminary results indicate that rainfall is the primary contributor to maximum storm runoff in Northeast watersheds during winter, with snowmelt playing a secondary role, especially over low to medium altitude with warmer climate compared to regions with higher altitudes. At higher altitudes (and latitudes) with colder climates, the antecedent snow melt had major contribution to the stream flow. Additionally, we observed a statistically significant increasing trend in maximum storm runoff discharge in recent years, particularly in higher latitude regions with colder climates. In lower latitude regions with warmer climates, the trends in runoff discharge were mixed, showing both non-significant increases and decreases