July 9, 2021

New regional regression equations for Puerto Rico have been implemented in StreamStats to estimate flow-duration and low-flow frequencies. Ordinary least-squares and generalized least-squares regression techniques were used to develop regional regression equations for flow-duration statistics at the 99th, 98th, 95th, 90th, 80th, 70th, 60th, and 50th percent exceedance probabilities and annual n-day low-flow frequency statistics for the 1-, 7-, 14-, and 30-day mean low flows with the 2-year (0.5 nonexceedance probability), 5-year (0.2 nonexceedance probability), and 10-year (0.1 nonexceedance probability) recurrence intervals. Further information can be found in the USGS Scientific Investigations Report (https://doi.org/10.3133/sir20215054).

The peak-flow frequency estimates and regional regression equations for Illinois flood frequency region 2 (northeastern Illinois) from Over and others (2016) have been revised to make use of weighted skew coefficients (see "Revision of Over and others (2016)" below). The revised equations were updated July 8, 2021. These revisions only affect flood frequency region 2 in Illinois as described in Over and others (2016). The revised report is available at USGS (https://doi.org/10.3133/sir20165050) and is also available at the Illinois Center for Transportation website (https://ict.illinois.edu/).