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some non-consumptive uses of the reservoir which may have an effect on its operation, such as recreation, are not considered in this analysis. Nor is the variance of economic benefits from year to year considered. Whether the strategy of running the reservoir dry is acceptable from a standpoint of risk management is beyond the scope of this study. Tualatin Project Oregon Economic theory would suggest the optimal size of a multi-purpose reservoir is where the marginal benefits of the reservoir, summed across all uses, are equal to the marginal cost of the reservoir development costs. The modeling system is capable of estimating marginal benefits of alternative reservoir sizes for proposed projects. This is illustrated in an ex post analysis of the Tualatin Project in Northwest Oregon. The Tualatin Project area lies near the City of Portland, Oregon. The main project features are Scoggins Dam, which forms Henry Hagg Lake. The reservoir has an active capacity of 59,170 acre-feet. Of approximate annual releases of 62,000 acre-feet, about 25,000 acre-feet are dedicated to M&I uses in suburban Portland, 20,000 acre-feet are used to maintain water quality in the Tualatin River, and the remaining 17,000 acre-feet are tagged for irrigation purposes. Due to the difference in climate and related hydrology the Tualatin can deliver more water than Lake Cachuma with a reservoir less than one-third the size. Little modification of the Operations model was necessary to accommodate the Tualatin case study. Historical inflows were estimated for the period of record 1929 through 1952. Annual and monthly demands were obtained from the Bureau's 1970 Definite Plan Report (DPR), and updated with information published in their annual project reports. Physical coefficients, such as area capacity and pan evaporation were obtained form the DPR also. The next cheapest single purpose alternative for suburban Portland to acquire additional M&I supplies is contracting directly with the city itself. The suburbs face a delivered price of S180 per acre-foot for Portland city water of similar quality. The economic benefit of instream flow releases is the avoided cost of advanced water treatment. The Tualatin DPR estimated the avoided cost to be 513.60. Updating this . cost with a construction cost index results in a 1988 avoided cost of $45 per acre-foot. Ten crops are considered in the irrigation component: alfalfa, grass hay, pasture, corn silage, processing beans, sweet corn, onions, potatoes, seed crops (clover and bluegrass), and berries (strawberries and blackberries). Unlike the Cachuma case study, the perennial crops considered in the Tualatin Project do not suffer the longer term drought impacts such as those seen with avocados and citrus. Therefore, little modification of the model was necessary. The rainfall volume in the study area is more than adequate to provide crop consumptive water use requirements. However, rainfall is seasonal. Supplemental irrigation is required in the months of May through August to ensure acceptable crop yields. Active capacity of the reservoir is reduced by 30,000 acre-feet in the months of October and November for flood control purposes. Occurring after the irrigation season, this can result in nearly draining the reservoir. Flood space requirements are relaxed to