Technical Abstract: The use of distributed parameter models to address water resource management problems has increased in recent years. Calibration is necessary to reduce the uncertainties associated with model input parameters. Manual calibration of a distributed parameter model is a very time consuming effort. Therefore, more attention is given to automated calibration procedures. This paper describes the development and demonstration of such an automated procedure developed for a national/continental scale assessment study called Conservation Effects Assessment Project (CEAP). The automated procedure is developed to calibrate spatial variation of annual average runoff components for each USGS eight-digit watershed of the United States. It uses nine parameters to calibrate water yield, surface runoff and sub-surface flow respectively. If necessary, the procedure uses a linear interpolation method to arrive at a better value of a model parameter. When tested for the Upper Mississippi river basin of the United States, the automated calibration procedure gave satisfactory results. Other test results from the procedure are very encouraging and show potential for its use in very large-scale hydrologic modeling studies.