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'.. ~. ~ Colorado River Compact Symposium James S. Lochhead Page 23 Fortunately, the negotiators left some room in which to work. They based their discussions on what they felt were the reasonable water supply needs each basin -- 7.5 maf/yr. -- and gave the Lower Basin an additional million acre-feet to allow for Lower Basin tributary use. Thus, they took considerable comfort in their view that they had not allocated all the water in the river. They provided in Article III (c) that Mexican Treaty delivery obligations would be handled out of the surplus, and in Article III (fl and (g) for a further apportionment of the surplus after 1963 if either basin reached its allocation. It can be said that the Lower Basin has exceeded its total apportionment of 8.5 maf/yr., but it is not at all clear that there is a surplus. The Neqotiators Left Unanswered Questions of Interpretation In addition to its 75 million acre foot delivery obligation, the Upper Basin potentially faces the added burden of contributing one-half of the 1.5 million acre foot Mexican. Treaty obligation, and possibly the transit losses between Lee Ferry and the Mexican border. The Compact provides that obligations to Mexico will be met out of water surplus to that allocated under Article III (a) and (b). If no surplus exists, the Upper and Lower Basins are to share in any such deficiency equally. Some argue that there is no such surplus, and the upper Basin therefore bears the burden to share in the delivery obligation. The Upper Basin states assert that they are under no obligation to contribute any water to Mexico. The chart on the previous page shows Lower Basin mainstem uses of 9.3 maf/yr. If Lower Basin tributary uses are added in, even deducting for groundwater overdraft, total Lower Basin consumptive use is currently on the order of 12.8 maf/yr., far in excess of the 8.5 maf/yr. allocated to it. The Upper Basin states argue this excess use eliminates any burden the Upper Basin might otherwise have to bear toward Mexican Treaty deliveries. Resolution of the question may depend on how Lower Basin tributary use is accounted under the Compact. The question remains unresolved, at least until Upper Basin uses begin to approach the total allocated to it. But how much water is that? If the Upper Basin has a firm obligation to let an average of 7.5 million acre-feet go to the Lower Basin each year, then its use is limited only to what is left in the river. As noted above, the amount left depends upon hydrologic cycles. Based on its hydrologic analysis and interpretation