August TTic SB 76 water right claims total 5,709,376 acre- 

 feet, 53-timcs more than the 30 percentile flows. 



Currently, non-tribaJ irrigators have storage contracts 

 to 32,500 acre-feet and Northern Cheyenne Trib>e to 7,500 

 acre-feet of water from the Tongue River Reservoir. Also, 30 

 cubic feet per second (9,075 acre-feet from May to Septem- 

 ber) with a priority date of March 24, 1 909 was allocated to 

 the Northern Cheyenne Tribe in the Miles City Decree. 

 However, the Tribe does not recognize the decree as a legiti- 

 mate quantification of its water right. The Northern Chey- 

 enne Reservation was established by an Executive Order 

 issued on March 19, 1900; it is presumed by the RWRCC 

 negotiating team that this would be the likely priority date 

 assigned to the Tribe in an adjudication of its reserved right 



The present system wi th the reservoir works well for the 

 other existing users primarily because the Tribe doesn't fully 

 use its contraa water or its share of the Miles City Decree 

 water (around 500 acres are presently irrigated on the Res- 

 ervation). Shortages are none or minimal even during the 

 driest years (Mobley, 1990). If the irrigation season flow 

 volume difference between the dam and Miles City is used 

 as a surrogate for water demand, then 50 percent of the time 

 water demand doesn't exceed 44,000 acre-feet and 80 per- 

 cent of time it doesn't exceed 56,000 acre-feet (see Figure 5) . 



6. Water Availability and Modeling 



The water availability issue on the Tongue River is 

 complicated by the existence of the Yellowstone Compact, 

 which apportions water between Montana and Wyoming. 

 Under the terms of the Compact, Wyoming is en ti ded to 40 

 percent of the "unallocated flow" (the water left after servic- 

 ing all Montana and ^oming pre- 1950 rights) at Miles 

 City. Wyoming initially assened a right to 26,900 acre-feet 

 of "supplemental water" for its pre- 1 950 projects with par- 

 tial irrigation service. 



In 1 984 Systems Technology, under a contraa with the 

 DNRC, developed a water allocation model and a projea 

 yield analysis model to determine Montana's share of allo- 

 cable water and firm annual yield from the Tongue River 

 Reservoir. The project yield analysis model was updated 

 in 1 990 to correspond to different scenarios of water alloca- 

 tion between the private water users represented by the 

 Tongue River Water Users Association and the Northern 

 Chevenne Tribe. 



Initial computer runs used a hypothetical scenario in 

 which W'oming used all of its allocable and claimed supple- 

 mental water (29,000 acre-feet), the existing Montana de- 

 mand was estimatedat 83,200 acre-feet, and the fimi annual 

 yield from the enlarged, 80,300 acre-feet reservoir was 

 predicted to be around 5 5,000 acre-feet (Anderson, Bucher, 

 1990). The estimate of the existing demand was a high, 

 worst-case scenario; the hypothetical demand of 83,200 

 acre-feet of water used in the model is higher than the 

 72,500 acre-feet highest recorded flow volume diflFerence 

 between the dam and Miles City. Eighty percent of the time 

 the depletions do not exceed 56,000 acre-feet and it is 

 improbable that Wyoming would develop all 29,000 acre- 

 feet of its claimed water. Therefore, on a probability basis, 

 more water would be available most of the time because of 

 a lower demand. 



Wyoming's unused water could be available as well (see 

 Figures 7 and 8). In a hypothetical scenario in which Wyo- 

 ming does not use any of its claimed water and using 80th 

 percentile depletions (56,000 acre-feet), the reservoir firm 

 annual yield would significandy increase, depending on 

 uses. 



The firm annual yield also changes with different sce- 

 narios of water use; for example, a year-round industrial use 

 puts less demand on the reservoir than agricultural use 

 during a 5-month irrigadon season (Anderson, Bucher, 

 1 990). Subsequent computer analysis that used Wyoming's 

 revi sed claim for supplemental depletions ( 1 8, 700 acre-feet) 

 and the highest recorded Montana irrigation demand of 

 72,500 acre-feet resulted in a new firm annual vield estimate 

 of 62,200 acre-feet of water (McDonald, 1 990). Use of 80 

 percentile flows and 80 percentile depletions in the calcula- 

 tions indicates a large water reserve (see Fgure 7); however, 

 it is not dear how much of that water would actually be 

 available: not all ofit can be stored, and timing and quantity 

 of uses throughout the year can makesignificant difl^erences. 

 The analysis merely indicates that significandy more water 

 may beavailable on a probability basis, dependingon timing 

 and kind of use. 



In July, even the pre- 1 900 existing uses from the Miles 

 City Decree exceed the 80th percentile state line flows by 

 4,686 acre-feet and by 1 1 ,779 acre-feet in August, even if a 

 35 percent irrigation reuse of return flows is assumed. Thus, 

 there is no direct flow left for the Tribe during July and 

 August if 1900 is used as a priority date for theTribe. Only 



18 



