The total water and land resources system model of the Water Resources Group within the Rockefeller Project for Phosphorus Pollution Control is discussed. The scope of this work has been dominated by two major themes: Distributed pollution sources (carried to the waterbody primarily by runoffs) will play a critical role in the ultimate solution of the water pollution problem; and the level of pollution discharged to a waterbody, attributed both to point and distributed sources, depends on the economic activity of the region as well as on its hydrologic characteristics. Two principal submodels of the total system model are used for modeling the two major themes. Economic analysis is performed using various other submodels: capacity expansion 'cost' model; distributed source 'cost' model; ecosystem response 'benefit' model; and a management and coordination (cost-benefit analysis) model.