COLORADO'S PARTICIPATION IN THE

PLATTE RIVER BASIN RECOVERY IMPLEMENTATION PROGRAM

I. PROJECT DESCRIPTION

The Tamarack Plan involves the use of participating existing and future wells and other water facilities in Colorado to reregulate flows that are in excess of legal rights to and physical demands for water in Colorado in a manner that is consistent with the flow-related goals of the Platte River Recovery Implementation Program. As a result of the geographic location of the Tamarack Plan near the state line, groundwater recharge that results from the Tamarack Plan is estimated to increase flows at the Julesburg gage during the period of April through September by an average of approximately 10,000 acre-feet over the flows that would otherwise occur during that period. As stated in the Program description, all signatories have agreed that the combined operations of the Tamarack Plan and the other reregulating projects in the Proposed Program shall score and be credited with reducing flow shortages by 70,000 acre-feet. Water rights for the operation of the components of the Tamarack Plan will be obtained and exercised under Colorado law for beneficial uses in Colorado.

Participating wells, ditches or other facilities, and associated water rights, may also be operated for purposes other than those associated with the Program, for example but not by way of limitation, augmentation purposes and protection and enhancement of native species and wildlife. Such operations are not part of the Tamarack Plan, and references to this Plan do not include such operations.

The components of the Tamarack Plan will be developed within the 40 miles above the state line beginning at about the Tamarack Ranch State Wildlife Area owned by the Colorado Division of Wildlife near Crook, Colorado. These facilities will include wells located adjacent to the South Platte River that divert groundwater from the alluvial aquifer and canals that divert water from the South Platte River. Water that percolates into the groundwater alluvium from these facilities will return to the South Platte River at a later time. Inflows to canals and recharge basins will be identified as for Program or other purposes, and inflows for Program purposes will be measured and recharge or seepage will be computed as inflows minus evaporation. Evaporation in acre-feet will be determined by using available weather station data and the surface areas of the recharge sites. Recharge basins are typically located in sandy upland areas with high infiltration rates such that free water surface areas are minimal, resulting in low evaporation amounts. The evaporation computed for existing recharge projects in the lower South PLATTE RIVER BASIN in Colorado is typically less than one percent of gross flows.

By selecting the correct location of recharge basins with respect to their distance from the river, the return flows occurring in the summer can be optimized. The Stream Depletion Factor (SDF) method developed by the U.S. Geologic Survey (USGS) is a standardized procedure used for analyzing the timing of well depletions and recharge accretions to the river. USGS SDF maps give SDF factors for recharge basins and canal reaches along the South Platte River in Colorado. These SDF factors have units of days and represent the lag time for recharge to return to the river. From the mathematical solution of the SDF method, the SDF factor in days represents the time when 28 percent of the amount recharged has returned to the river. The remaining 72 percent of the recharge returns to the river in the time following the SDF day period. Figure 1 is a USGS map of SDF factors for the lowest reach of the South Platte River in Colorado. The SDF values for canal systems and recharge basins in the lower South PLATTE RIVER BASIN in Colorado range from 60 days to 1,500 days. Observation wells will be located between the recharge basins and the river so that groundwater gradients and return flows to the river from the recharge basins' seepage can be monitored. Estimates of return flows to the river from the operation of the Tamarack Plan may be reviewed by the Governance Committee during the term of the Cooperative Agreement.

II. HISTORICAL ANALYSIS

Colorado has analyzed how the Tamarack Plan would have operated during the period 1943-1994. For the purpose of this historic analysis, periods and amounts of excess flows for diversion by the Tamarack Plan to recharge facilities in Colorado were assumed to occur when the following two conditions were satisfied: (1) South Platte River Compact requirements were satisfied and (2) flows exceeded the Fish and Wildlife Service (FWS) year round target flows at the Grand Island gage on the Platte River in Nebraska. Existing target flows for every month were used in this analysis and the monthly target values varied with hydrologic conditions of wet, average, and dry.

This analysis assumed that pumping of new groundwater wells located next to the river to recharge basins could occur during the winter because wells can operate during freezing periods due to warmer groundwater temperatures. This analysis also assumed pumping to recharge basins that reflects potential operations at the 10,000 acre Colorado Division of Wildlife's Tamarack Ranch State Wildlife Area. Colorado plans to install approximately twenty new large capacity wells and associated pipelines and construct necessary recharge basins and related monitoring features. The SDF values of the new recharge basins which the wells pump into will vary from 60 days to 270 days. For the 1943-1994 study period, the average annual diversion by the wells for recharge in the Tamarack Plan would have been 22,950 acre-feet.

Recharge from canal systems is accomplished during periods when there is unused canal capacity. These periods occur in the fall after the irrigation season until freeze-up, typically through the month of November, and during spring runoff when there are excess river flows. The average annual diversion for the 1943-1994 study period by canals for recharge in the Tamarack Plan Project would have been 6,690 acre-feet for a total of 29,640 acre-feet by both wells and canals.

Tables 1 and 2 list the reregulation results of the Tamarack Plan operations for this historical analysis of the 1943-1994 period. Table 1 lists the monthly additions or increases that would have occurred to the historic Julesburg gage flows as a result of the accretions or return flows to the river caused by the groundwater recharge of the Tamarack Plan. These are net values and occurred for months when river accretions exceeded the diversions to the recharge basins. Table 2 lists the monthly net depletions which would have occurred for months when the diversions to the recharge basins exceeded the accretions in that month. From Table 1, the average annual net addition or accretion is 16.1 kAF while the average annual net addition for the April through September period is 10.0 kAF. From Table 2, the average annual net depletion is 20.0 kAF. The difference between the average annual accretion of 16.1 kAF and the average annual depletion of 20.0 kAF is due to evaporation and some of the accretions to the river not being accounted for because they would have occurred after 1994, which is the last year of the modeled period.

III. CRITERIA FOR OPERATION OF THE TAMARACK PLAN

A. The components of the Tamarack Plan include participating existing and future wells or other water facilities which are operated to increase flows at the Julesburg gage during the period of April through September by an average of approximately 10,000 acre-feet over the flows that would otherwise occur during that period. All such facilities will be operated by Colorado and its water users in compliance with the requirements of the South Platte River Compact.

B. Colorado will also operate the Tamarack Plan, after consultation with the manager of the Environmental Account Account in Kingsley Reservoir, in a manner that does not cause an increase in target flow shortages at the critical habitat unless requested otherwise by the Environmental Account Manager, as measured at the Grand Island gage and using FWS target flows which are then in effect, so long as (a) any target flow shortage is not related to the operation of the Environmental Account in Kingsley Reservoir outside of the scope of the Annual Operations Plan for the Environmental Account adopted by the signatories, (b) the use of wells within Nebraska which are in existence at the date of the Cooperative Agreement is not expanded or enlarged, or modified in any manner that will cause an increase in target flow shortages, and (c) wells in Nebraska that are constructed, enlarged, improved, or placed into use in whole or in part after the date of the Cooperative Agreement are operated in a manner that does not cause an increase in target flow shortages at the critical habitat. Any replacement of depletions in Nebraska to flows at the critical habitat that is required to comply with (b) and (c) above shall be from water that is in excess to target flows and shall not rely upon flows that occur at the Colorado-Nebraska state line resulting from the operation of the Tamarack Plan or other re-regulation facilities for the benefit of the target species, or from accretions attributable to Colorado's new water-related activities. The analysis of the impacts from wells in Nebraska to flows at the critical habitat that is required by (b) and (c) above, and as provided in milestone 14-1, Att. I, shall be provided to the Governance Committee and shall be based on scientifically sound assumptions and data which are independently verifiable and made available to all signatories. If the depletions from these wells have been quantified as provided for above but are not fully replaced, Colorado will add these depletions back into Grand Island gage flows for purposes of determining when it may operate the Tamarack Plan without causing an increase in target flow shortages at the critical habitat. Any party may request that a dispute regarding the acceptability of information regarding these wells be resolved by a majority of a panel of three experts in groundwater hydrology, one member to be appointed by the State of Colorado, one member to be appointed by the State of Nebraska, and one member to be appointed by the Department of the Interior (DOI) who is an employee of the United States Geological Survey. Pending completion of the dispute resolution process regarding the acceptability of information concerning these wells, Colorado, in coordination with FWS, will continue to operate the Tamarack Plan consistent with and for the purpose of meeting Colorado's commitment under the Program, subject only to the requirements of state law and the South Platte River Compact. Such continued operation shall be considered adequate for meeting any Program milestones associated with the Tamarack Plan.

C. Components of the Tamarack Plan may also be operated to reregulate flows for the purpose of providing Colorado's contribution to the Program for new depletions as is described in Tab 3B Appendix A, of the Proposed Alternative. Participating wells or other water facilities which may contribute to the Tamarack Plan may also be operated for purposes other than the Program, consistent with Colorado's commitment under the Program and subject only to requirements of state law and the South Platte River Compact.

For the purpose of these criteria, an "increase in target flow shortages" is measured against the flow conditions which exist as of the date of the Cooperative Agreement. The Grand Island gage will be compared to routed amounts of water that would be diverted by the Tamarack Plan. This routed diversion will utilize the lag and loss factors that will be developed by the Water Technical Committee during the Cooperative Agreement. The routed amount shrunk by the loss factors will be subtracted from the expected (i.e., based on trends and scheduled operational releases from Lake McConaughy) Grand Island gage flow occurring for the number of days of lag in the future and if this computed Grand Island gage flow is still above a desired target then, assuming that the provisions of Paragraph III B. above regarding the replacement of depletions have been satisfied, diversions for the Tamarack Plan will take place to the extent that Grand Is land gage flows do not drop below targets.