WSP04959

• File Number: 8460.500
• Description: Platte River Recovery Plan
• Basin: South Platte
• Date: 8/3/1999
• Author: Colo Div of Wildlife
• Title: Final Environmental Assessment for the Tamarack Managed Groundwater Recharge Project at Tamarack Ranch State Wildlife Area and Pony Express State Wildlife Area
• Water Supply Pro - Doc Type: Publication

I.
I
I
I

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 rechar.g~_
accretions to the river. USGS SDP maps giv~ SDP factors for recharge basins and
canal reaches along the South Platte River in Colorado. These SDP factors have units
of days and represent the lag time for recharge to return to the river. Prom the
mathematical solution of the SDP method, the SDP 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 SDP day
period. Figure 1 is a USGS map of SDP factors for the lowest reach of the South Platte
River in Colorado. The SDP 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. BISTORICAL ANALYSIS

Colorado has analyzed how the Tamarack Plan would have operated during the period
,~.. 1943-1994. Por the purpose of..this historic ~ysis.;'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 SDP values
of the new recharge basins which the wells pump into will vary from 60 days to

3

I
I
I
I
I
I
,
I
I
I
I
I
I
I
I
1
,
1