This digital-map data set consists of a grid of mean annual precipitation, in inches, based on the period 1961-90, for Oklahoma. The data set was derived from the PRISM (Parameter-elevation Regressions on Independent Slopes Model) mean annual precipitation grid for the United States, developed by Daly, Neilson, and Phillips (1994, "A statistical-topographic model for mapping climatological precipitation over mountainous terrain:" Journal of Applied Meteorology, v. 33, no. 2, p. 140-158). The precipitation grid was used to develop peak-flow regression equations by Tortorelli, (1997, "Techniques for estimating peak-streamflow frequency for unregulated streams and streams regulated by small floodwater retarding structures in Oklahoma," U.S. Geological Survey Water-Resources Investigations Report 97-4202.)