Identifying Spatial Patterns of Hydrologic Drought over the Southeast US Using Retrospective National Water Model Simulations

Given the sensitivity of natural environments to freshwater availability in the Southeast US, as well as the reliance of many municipal and commercial water consumers on surface water supplies, specific issues related to low river streamflow are apparent. As a result, the need for quantifying the spatial distribution, frequency, and intensity of low flow events (a.k.a., hydrologic drought) is critical to define areas most susceptible to water shortages and subsequent environmental and societal risk. To that end, daily mean discharge values from the National Water Model (NWM) retrospective data (v. 2.0) are used to assess low flow frequency, intensity, and spatial distribution within the Southeast US. Low flow events are defined using the US EPA 7Q10 approach, based on the flow duration curve (FDC) developed using a 1993–2018 period of record. Results reflect the general climatological patterns of the region, with a higher probability of low flow events occurring during the warm season (June–August) while low flow events in the cool season (January–March) are generally less common and have a higher average discharge. Spatial analysis shows substantial regional variability, with an area from southeastern Mississippi through central South Carolina showing higher low flow event frequency during the cool season. This same area is also highlighted in the warm season, albeit along a more expansive area from central Alabama into the piedmont region of North Carolina. Results indicate that the NWM retrospective data are able to show general patterns of hydrologic drought across the Southeast US, although local-scale assessment is limited due to potential issues associated with infiltration and runoff during periods of warm-season convective rainfall.