Climate variability, changing land use and management, and dynamic policy environments are the main reasons why long-term water quality data sets are needed to understand and predict possible water quality outcomes to alternative future scenarios. Such data sets were acquired by the USDA-ARS in three watersheds in Oklahoma: the Southern Great Plains Research Watershed (SGPRW), the Little Washita River Experimental Watershed (LWREW), and the Fort Cobb Reservoir Experimental Watershed (FCREW). Water quality data collection in the SGPRW began in the 1960s and continued through 1978, while that in the LWREW covered the 1960s to 1990 period. Data collection began in the FCREW in 2004 and continues through the present. The data were collected from streams, unit source watersheds, groundwater wells, and reservoirs. It should be noted that various forms of P—reactive P, total P, soluble P, water-soluble P, particulate P, bioavailable P, total water-soluble P—were measured and are described here as given in the original data sets. No effort was made to determine the similarity of these variables.