Flood regions and annual exceedance probability flows for Alabama streams, data through 2015

To improve flood-frequency estimates at rural streams in Alabama, annual exceedance probability (AEP) flows at gaged streams in Alabama and regional-regression equations, used to estimate AEP flows for ungaged streams in Alabama, were developed using current geospatial data, additional statistical methods, and annual peak-flow data through the 2015 water year. The regional-regression equations were derived from statistical analyses of peak-flow data, basin characteristics associated with 217 streamgages, and the generalized skew from Bulletin 17B (Interagency Advisory Committee on Water Data, 1982). Four flood regions were identified based on residuals from the regional-regression analyses. Regional regression equations were also produced for small streams with drainage areas ranging from 0.13 to 14 mi2. Flood regions containing sites with similar basin and climatic characteristics yielded better regional-regression equations with lower error percentages. The generalized least squares method was used to develop the final regression models of AEP flows for each flood region. The peak-flow statistics were estimated by fitting a log-Pearson type III distribution to records of annual peak flows and then applying two additional statistical methods: (1) the expected moments algorithm to help describe uncertainty in annual peak flows and to better represent missing and historical record; and (2) the generalized multiple Grubbs-Beck test to screen out potentially influential low outliers and to better fit the upper end of the peak-flow distribution. Standard errors of prediction of the generalized least-squares models ranged from 9 to 58 percent. Pseudo coefficients of determination of the models ranged from 75 to 100 percent. Reference: Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood-flow frequency: Bulletin 17B, 183 p.