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<br />predicted by the model may be over-estimated. particularly at the higher tax <br />rates. <br /> <br />~ <br />H:llo Taxina Irriaation Water. An alternative to taxing a nonpoint effluent of <br />.... <br />~ uncertain origin and magnitude is taxing an important input to the process <br />yielding the pollutant (the so-called "influent tax") (Pfeiffer and Whittlesey; <br />Radosevich and Skogerboe). In the case of Grand Valley salinity, the most <br />appropriate input to tax is irrigation water. Reducing water use would reduce <br />.the deep percolation of water into the sal1ne aquifers. and thus reduce salt <br />loading. Different water rate structures must distinguish between revenues <br />required to cover the costs of maintaining and operating the water distribution <br />system and revenues derived from the influent tax. The benchmark case <br />incorporated the increasing block water rate used by the Grand Valley Water <br />Users Association. In 1982, growers were assessed $11.20 for up to four acre- <br />feet per acre, with additional water costing $4.20 per acre foot. For the <br /> <br />initial test of the influent tax. a fixed charge of $12 per acre ($600,000 <br /> <br />total) is assumed to cover the distribution system costs of the water <br /> <br /> <br />districts, and additional revenue is considered an influent tax. <br /> <br />Parametric changes on the water constraint influence the model's choice of <br />farm practices and provide shadow prices on irrigation water. These shadow <br />prices were modeled as water rates: the results appear in part C of table' 2. <br />About 58.000 tons of salt are removed from the river at a net social cost of <br />$7.50 per tan. This net social cost is lower than the cost with irrigation <br />system subsidies, yet it is significantly higher than the cost under a salt <br /> <br />discharge tax. However, net farm income completely disappears before <br /> <br />significant salt reductions occur. <br /> <br />13 <br />