Brian Bledsoe and Tim Stephens
The southeast U.S. is widely perceived as a water-rich region, yet recent droughts, highly variable streamflows, and rapidly growing demands for water have revealed vulnerabilities and the potential for widespread water scarcity. Additionally, recent studies suggest that the magnitude of low flows may be decreasing across the region. Power plants are fundamentally dependent upon reliable supplies of water for a variety of critical processes in electric power production. For example, water surface elevations during minimum river flows must remain above operational thresholds for plant intakes, and instantaneous streamflow must exceed required cooling water withdrawal rates. These risks could be exacerbated by projected increases in water and power demands across the region. Previous studies of regional water scarcity and vulnerability have been carried out at a very large scale and do not provide a clear assessment of differences in vulnerability among watersheds and river segments in the regions. Accordingly, there is a need to evaluate trends in low flows and the vulnerability of thermoelectric power production to water availability at a scale relevant to operational planning and decision-making. We analyze streamflow records at 340 gage locations across the South Atlantic hydrologic unit of Mississippi, Alabama, Georgia, and northern Florida to evaluate trends in low flows over the last century. Low flows are defined as the annual minimum seven-day mean streamflow (minimum Q7), and trends are identified utilizing the Mann-Kendall trend test. Decreasing trends in the magnitude of low flows and an increasing trend in the frequency of low flows are identified at approximately 80% of gage locations (Figure ES.1). Statistically significant trends are identified at approximately 30% of the locations analyzed with the vast majority of the other gages also having downward trends. The rate of decrease in the magnitude of low flows appears to be greater over the last 25 years compared to longer time periods. Statistical tests also indicate that a large portion of trends in the magnitude of low flows occur as an abrupt shift around 1975, consistent with national-scale studies of trends in streamflow
To evaluate trends in low flows and the vulnerability of thermoelectric power production to water availability at a scale relevant to operational planning and decision-making.