Data release for mean annual runoff and annual runoff variability map for Oklahoma, 1940-2007

The U.S. Geological Survey (USGS), in cooperation with the Oklahoma Water Resources Board, produced this data release that includes vector and raster geographic information systems layers used in the analysis and publication of a mean annual runoff and annual runoff variability map for 1940-2007 for Oklahoma. The data release covers all 69 8-digit hydrologic units with at least 1 square mile of area in Oklahoma; those 8-digit hydrologic units contain 2,870 12-digit hydrologic units that provided the geographic framework for the analysis described in the companion map report (USGS Scientific Investigations Map 3482). The mean annual runoff and annual runoff variability values presented in this data release are most representative of runoff conditions in rural, unregulated drainage basins at the 12-digit hydrologic-unit scale. As drainage basins increase in size, they are more likely to be (1) regulated by large dams or irrigation practices, (2) augmented by substantial groundwater contributions, and (3) monitored by streamgages with long period of record. Methods for computing or estimating runoff for gaged basins generally rely on the streamgage record when sufficient data are available. For an ungaged stream reach between two streamgages, the drainage-area-ratio method, rather than the GIS layers in this data release, can be used to obtain a more accurate mean annual runoff or annual runoff variability estimate. The drainage-area-ratio method, which assumes that streamflow generally increases in proportion to increased drainage area, equates the ratio of streamflow at two stream locations to the ratio of the respective drainage areas. The layers were developed by using regression equations formulated on streamgage data for the entire period of record through 2007, but those equations are biased to the period 1940-2007 when streamgages became more numerous and distributed across Oklahoma. Therefore, the data layers are likely most representative of runoff conditions during the period 1940-2007. Because runoff is a function of climate variables that can change over time, caution is warranted when using the information in this data release to project mean annual runoff and annual runoff variability conditions beyond 2007.