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I I I I I I I I I I I I I I I I I I I 8.2.2 IMPACT OF SOUTH PLATTE WATER RIGHTS ALLOCATED OF BIG THOMPSON STREAMFLOW The Big Thompson River lies in District 4 of Division I (South Platte River Basin) for purposes of administration of Colorado water law. Vater rights holders located downstream of District 4 may place a "call" against the streamflow in District 4 to satisfy their water rights allocations. The water commissioner administers calls according to the priority system. Records of historic call data for Division I have been maintained from 1941 to the present by the Division Engineer. These records were examined to determine the possible impact 'of downstream calls or allocation of Big Thompson River streamflows. The most senior call on District 4 during this period had a priority date of 12/12/1874; this priority is junior to No. 21.5 water in the Big Thompson Basin (see Appendix for listing of water rights for each ditch). During low-flow periods all available Big Thompson streamflow would be allocated to more senior water rights within the Big Thompson watershed. Consequently, we concluded that downstream calls would have negligible impact on the allocation procedures used in the simulation model. CDM/Jehn and Vood staff met with Mr. Don Brazelton (Water Commissioner for District 4) to verify these findings. Mr. Brazelton informed us that in his experience downstream calls may impact water rights holders on the Little Thompson River but such calls had never affected Big Thompson River water rights. For the purpose of this study, allocation of river water from Big Thompson direct-flow rights was therefore based solely on the relative priority of water rights within the Big Thompson watershed. 8.2.3 WATER RIGHTS CONSTRAINTS The computer model to allocate Big Thompson streamflows to the various water rights owners honored the constraints of the Transfer Decree and operated in accordance with the following assumptions/algorithms: 8-9