Cumulative Effect Report on Plan for Future Depletions ‎(2)‎

• File Number: 8461.250
• Description: Water Issues
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 3/31/2007
• Author: Ted Kowalski, Alan Berryman
• Title: Cumulative Effect Report on Plan for Future Depletions
• Water Supply Pro - Doc Type: Board Memo

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MEMORANDUM
TO: Governance Committee-GC, Platte River Recovery Implementation Program (PRRIP)
FROM: Ted Kowalski, State of Colorado GC Member
Alan Berryman, Colarado Water User GC Member
DATE: March 31, 2007
REF: Cumulative Effect Report far Colorado's Plan for Future Depletions
As required by the PRRIP, Attachment 5, Section 9, and Paragraph E of Colorado's Plan for
Future Depletions (CPFD) dated October 24, 2006, this report (due 90 days from the inception of
the Program on January 1, 2007) provides information on the average monthly distribution of the
Cumulative Effect for anticipated new water related activities in the South Platte River Basin in
Colorado. This report also reviews the assumptions in computing this distribution, proposes
modifying the % water supply source mix by region due to the ongoing drought, and discusses the
initial water retiming efforts in the lower South Platte River in Colorado for mitigating periods of
net depletions.
Included with this Memorandum is the letter report from the Colorado State Demography Office as
required by Paragraph A of Colorado's Plan for Future Depletions. This report lists a population
increase of 676,697 from July 1, 1997 to January 1, 2009 or for this 11.5-year period, an annual
population increase of 58,843 per year. Therefore by January 1, 2008, the projected population
increase is 617,854 and this value is used in Colorado's future depletion spreadsheet to compute
net accretions and depletions for the current year.
Figure 1 shows the results for the monthly net cumulative effect far the current year. The
assumptions for this analysis are those stated in Colorado's Plan for Future Depletions (CPFD)
approved by the Governance Committee in PRRIP Attachment 5, Section 9. At the bottom of
Figure 1 is the % water supply source mix by region to be initially assumed as stated in the CPFD,
Paragraph B. The transit loss assumptions used in this analysis are those stated in the CPFD,
Paragraph C. The results of this analysis show that May and June are months of net depletions
while the other 10 months have net accretions. Colorado's mitigation commitment in the PRRIP is
to undertake re-regulation projects within Colorado to shift water flows from periods of net
accretion to periods of net depletion. The main period of re-regulation through managed
groundwater recharge in the lower South Platte River in Colorado when recharge water rights are
in-priority in Colorado is the winter period of typically October through April. Figure 1 lists a
total of 30,259 acre-feet of net accretions far these winter months and lists a total of net depletions
in May and June of 5,304 acre-feet.
Throughout the Platte Basin, there has been an extended drought beginning in 2002. The use of
native South Platte flows with a priority date junior to July l, 1997 (start date for new water
related activities) has been minimal during this drought. Colorado requests that the Governance
Committee consider a modification to the % water supply from native South Platte flow
development used for new water related activities. A good indication and basis for this
modification is the frequency of river calls on the South Platte River. Figure 2 lists the monthly
percents for April through July that a Jlily 1, 1997 priority date would be out-of-priority far the
period 1998 through 2006. The April through July period is the spring runoff period from
snowmelt and is the time when South Platte flow development would take place under junior water
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