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;,'- ,,;;.'f_,,.,'j~''"_<(.' "~;_: _,"..t'~ ;".-.:."','--:"",,, <.."-,,,,,..,l, .",....,'-"..,.;,o~.."._,_., ,~~_>""I '~"""o _ ~ '~Lv i:"" r;.,i r:1"J 5 , .; (~;"J (.....,. Sedimentation in Rio Grande Proiect Reservoirs Sediment surveys were conducted in 1999 and 2000 for Elephant Butte and Caballo Reservoirs. On the basis of the sediment surveys, the U.S. Bureau of Reclamation (Reclamation) revised the area-capacity tables for the two reservoirs effective January 1,2001. Decreased project storage capacity due to sedimentation was 41,652 acre-feet (top of conservation pool) for Elephant Butte Reservoir and 4,838 acre-feet (top of conservation pool) for Caballo Reservoir. The Engineer Advisers recommend that the Rio Grande Compact Commission (Commission) adopt changes, effective January I, 200 I, to the Rules and Regulations of the Compact (Paragraph b of the section entitled "Actual Spill") to reflect the decrease in storage capacity in Elephant Butte and Caballo Reservoirs. In addition, the Engineer Advisers recommend that the remarks in the report of the Commission in the section entitled "Storage in Reservoirs, Reservoirs in Rio Grande Basin in New Mexico" for Elephant Butte and Caballo Reservoirs be revised to reflect the reduction in storage capacity. Evaooration Accounting Reclamation reported that a review of evaporation data at Elephant Butte, Heron, and EI Vado Reservoirs for the period 1993 through 1998 found arithmetic and transcription errors. The accounting procedures use gross evaporation rates and precipitation on the reservoir surface, in accordance with the Rules and Regulations of the Compact, to adjust the amounts of Colorado's and New Mexico's credit water in storage in Elephant Butte Reservoir for evaporative losses. Consequently, the calculations of credit water and usable water in Project Storage are in error for the periods 1993 to 1995 and 1997 to 1999. No credit water was in storage in 1996 due to spill in 1995. The total amount of error is still being quantified, but appears small for each state in relation to the amounts of credit water in storage at the end of 2000. The error did not affect Rio Grande operations or Compact administration in 2000. Reclamation did not honor its commitment to provide the Engineer Advisers in 2000 with documentation and quantification of the amount of the error. The Engineer Advisers will continue to work with Reclamation to quantify the amount of error through 1999 and will t...~ ,~.-",~,'l;'>-';"'.:_ (.ti:}1::~:';:fr~~ ti':"1'.:~~f.--:~..:t"i<"