The study moves from a concern for maximizing crop production on a single plot, or farm, to evaluating large-area agricultural systems in order to maximize economic gains under the constraint of minimizing pollution resulting from return flows. To accomplish this objective may require a change in the physical system, but much can be accomplished by more efficient use of materials (e.g., water, fertilizers, pesticides, and herbicides). In studying any particular irrigation system, the first step is identifying the extent of water pollution problems, which is usually done by analyzing inflow-outflow data for the irrigated area. Then, the sources of these pollutants must be established, which may require extensive field investigations. The next step is a cost-effectiveness analysis, where the cost of implementing a technology to various levels is compared against its effectiveness for improving water quality. These concepts are illustrated for three irrigation systems in the western United States; namely, Yakima Valley, Wellton-Mohawk Irrigation and Drainage District and Grand Valley.