The CBP is integrating nutrient-source, delivery, and load data through the implementation of a deterministic watershed model that describes water movement and nutrient transport throughout the watershed. The CBP watershed model is based on software referred to as the Hydrologic Simulation Program - FORTRAN (HSPF) (Donigian and others, 1994). The CBP HSPF model is a temporally and spatially variable model that allows the simulation of nutrient loading on the basis of information collected in the watershed. The CBP is using the model to simulate nutrient loading under various land-cover and land-management scenarios to evaluate the effects of environmental factors in the watershed, and to design nutrient-management strategies. The CBP HSPF model is spatially defined by 87 watershed segments that are on average more than 700 square miles in area. Calibration is performed for 14 of the 87 watershed segments on the basis of loading information collected at the downstream end of the segments (Donigian and others, 1994).