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satisfy Mexico. Most importantly, the Upper Basin would argue, it should be entitled to withhold any water that might have otherwise gone to Mexico, with the entire Mexico Burden being met by the Lower Basin. The Upper Basin should not be penalized, the argument continues, because the Lower Basin is consuming Colorado River water in excess of its compact entitlements and is gobbling up any basin-wide Rsurplus" before it gets to Mexico. The Lower Basin's counter argument is that the "surplus" under Article III(c) is unambiguously defined as any Colorado River water in excess of that amount required to satisfy the consumptive use apportionments under Article III paragraphs (a) and (b) of 16.0 MAP. Again, assuming a basin-wide virgin flows of 17.0 MAF (virgin flows at Lee Ferry plus Lower Basin tributary virgin flows) and excluding any evaporation or channel losses, there would only be a surplus of 1.0 MAP over the Article III apportionments, and a deficiency in meeting the Mexico Burden of .5 MAF, to be borne equally by the Upper and Lower Basins. The Lower Basin has even gone further and suggested that the Lower Basin tributaries should not be counted in deter- mining whether a "surplus" exists. This argument relies on the decision in Arizona v. California which excluded the Lower Basin tributaries in apportioning the Lower Colorado River among California, Arizona, and Nevada. It should be noted, however, that this decision is not a strong precedent for the Lower Basin in that it was based on the implicit Congressional apportionment of the Lower Colorado River in the 1928 Boulder Canyon Project Act, and did not construe the 1922 Compact. Nonetheless the exclusion of the Lower Basin tributaries under the 1928 Boulder Canyon Project Act was unprecedented, and it is possible that the 1922 Compact could be similarly construed. If the Lower Basin tributaries are excluded from the accounting, and the rest of the Lower Basin definition of "surplus" again applied with the same assump- tions, there would not be any "surplus" at all for Mexico, and each basin would have to contribute .75 MAF. The Upper and Lower Basin's disagreement over the defi- nintion of "consumptive use" could also in turn affect the definition of "surplus" and the Upper Basin's obligation to Mexico. The Lower Basin defines "consumptive use" as diver- ~~ ' sions less returns to the river. This definintion could be understood to include a great deal of evaporation and con- veyance losses. In 1975, mainstem evaporation and conveyance losses totalled 1.408 MAP in the Lower Basin, and .607 MAF in the Upper. ("Consumptive Uses and Losses Report"). In applying this definition to Article III(c), the Lower Basin would add these amounts to the 16.0 MAF apportioned by Article III paragraphs (a) and (b) to determine whether there -13-