Low Flows over the Eastern United States: Variability, Trends, and Attributions (1962-2011)

Low flows are a seasonal hydrologic response generally during a drying period. Extreme low flows are a result of prolonged antecedent precipitation deficit and/or high evaporative demand, and can indicate hydrological droughts (water availability deficit) and ecological droughts (water quality degradation). Human impacts (e.g. dams, reservoirs, and power plants) also play a role in exacerbating the severity of low flow droughts. For drought mitigation, it is critical to better understand how low flows vary over time and their generating mechanisms. The goals of this study are to examine trends in low flows over the eastern U.S. and to assess their attributions and teleconnections in the context of climate change and variability. We selected 149 out of 4878 USGS stations over the eastern U.S., taking into account data availability and minimal human impacts. We analyzed annual 7-day low flows (Q7) from the series of daily streamflow records for 1962-2011. We also computed an antecedent precipitation (AP) over the corresponding basin for each station. We found a north-south (increasing-decreasing) dipole pattern in Q7 trends and a monopole (increasing) pattern in AP trends, which indicates a gap between the trends of Q7 and AP over the southern part of the study region (Virginia, North and South Carolina). We found that these regions show significant increasing trends in potential evapotranspiration (PET) as driven by increasing temperatures and vapor pressure deficit. We also examined teleconnections between detrended Q7 and nine atmospheric and oceanic climate indices. We found that the North Atlantic Oscillation (NAO) and Pacific North America (PNA) pattern show prediction skill for Q7 at one and two month lead time, respectively. Our findings suggest that the worst scenario for future droughts over the eastern U.S. is a combination of a response to an increasing trend in temperature driving PET with strong negative NAO and positve PNA during summer.