HURRICANE HYDROLOGY IN WESTERN NORTH CAROLINA – HOW HELENE IMPACTED THE STORM WATER BUDGET ACROSS THE REGION

In September 2024, the mountainous region of Western North Carolina (WNC) experienced widespread and unprecedented flooding following heavy rains brought by Tropical Storm Helene. Over the 3-day storm, most precipitation ranged from 8 to 25 inches equating to about 1/200-to-1/1000-year events. The resulting widespread, record setting floods also varied significantly, even within regions of similar rainfall totals. The differential response prompts questions about the relationships of flood buffering capacity of groundwater storage, basin morphometry, and land cover to the intensity of surface runoff from basin-to-basin. For example, “How many inches of runoff are ‘bought’ by dry antecedent conditions prior to hurricane rains?” “How much does basin morphometry and drainage efficiency matter in extreme events” and “What is the importance of land cover in extreme events?”. To address these questions, we investigate the water budget (surface runoff (RO), precipitation (P) and changes in storage (∆S)) as well as runoff efficiency (RO/P) for watersheds across the WNC region, including at different temporal and spatial scales. We source precipitation from a combination of ground-based rain gauges and from NOAA summary products for Helene. Most runoff data are sourced from US Geological Survey discharge gauging stations; where peak-discharge measurements are missing from the gage record, we compute preliminary peak discharge estimations. Changes in storage are computed as the residual of precipitation and discharge over short term. To understand how flood runoff magnitudes and basin storage varied across landscape features, we evaluate these water budget components (and runoff efficiency) in the context of basin landscape characteristics including land cover type, presence/absence of reservoirs and slopes. While our results are preliminary, our findings contribute to better characterization of “hurricane hydrology” and the flood regime in WNC and will inform future flood preparedness and planning efforts in the Southern Appalachian region.