Evaluation of low-flow metrics as environmental instream flow standards during long-term average and 2016 drought conditions: Tombigbee River Basin, Alabama and Mississippi, USA

Environmental instream flows are a common tool for maintaining river flows that are required to sustain both ecosystem and societal needs. Many of the most widely adopted environmental flow standards are based on historical flow, mainly because of the relative simplicity of these methods. Few previous studies, however, have examined the ability of historical flow standards to protect low flows. Here, the low-flow protective ability of five different historical flow methods, using 35 gaging stations in the Tombigbee River Basin of Alabama and Mississippi, was analyzed. The minimum environmental flow thresholds were calculated using the five indices, and the number of times in a recent 32-year period flows fell below each threshold was determined. The Tennant-based threshold was reached most frequently, followed by the modified Tennant. Although other low-flow metrics, such as 7Q10, were triggered infrequently (9% of the time) over the whole period, triggering rates increased to 46% for 7Q10 during the drought of 2016, suggesting that even minimal low-flow standards may provide some benefit during drought. Analyzing historical flow methods to see how often they would result in management actions if implemented is a useful way of developing guidance on the adoption of minimum environmental instream flow standards.