A procedure was devised to predict the quantity and quality of cattle feedlot runoff entering a runoff retention basin whenever precipitation occurs in excess of the surface holding capacity of the lot or whenever accumulated snow would melt. These predictions are based upon size and location of the feedlot, previous conditions, and daily climate data. Runoff retention basin liquid content and quality are predicted on a daily basis. The model considers inputs and outputs as well as chemical and biological changes which are occurring on a continuous basis. The quality parameters which are predicted on a daily basis include: chemical oxygen demand, total Kjeldahl nitrogen, ammonia nitrogen, total phosphorus, total solids, fixed dissolved solids, total coliforms, fecal coliforms and fecal streptococcus. A cattle feedlot in Illinois was sampled intensively to adjust the various coefficients of the predictive tool.