PUB00004

• Year: 2003
• Title: Gunnison Basin Water: No Panacea for the Front Range
• CWCB Section: Administration
• Author: Land and Water Fund
• Description: Gunnison Basin Water: No Panacea for the Front Range
• Publications - Doc Type: Other

.44.

The land and Water Fund of the Rockies

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Meeting Water Demand Without Gunnison Water

The estimated total pumping of groundwater from the Denver Basin Aquifers
in 1996 was 56,000 AF, virtually all in the South metropolitan areas of Douglas and
Arapahoe Counties. Just over half of this use (24,500 AF) was for municipal purpos-
es.'" Pumping 56,000 AF is less than 2% of the annual allowable pumping quantity
assuming a 100-year aquifer life.'"' Though there has been some localized well
pumping drawdowns and loss of hydrostatic pressure, the overall lifetime of the
Denver Basin aquifer at this level of production (56,000 AF/yr) may exceed 1000
years. Simplified projections of well development and population growth to the year
2100 result in groundwater production estimates of about 300,000 AF per year,
approximately 10% of the annual allowable pumping from the Denver Basin
aquifers. '"

Despite the potentially long life of Denver Basin groundwater supplies, there
are several factors that caution against the use of groundwater as a sole source of
municipal supply. First, pumping costs tend to increase over time. As basins are
depleted, underground water levels and hydrostatic pressure drop, leading to
increased electricity consumption for the same water harvest. Second, the chance of
surface subsidence grows as groundwater tables are lowered. Third, pumping water
from aquifers drains a non-renewable water supply.'"

To help deal with the south metro area's long-term water needs, Douglas
County, the Denver Water Board, and the Colorado River Water Conservation District
have been exploring the South Metro Conjunctive Use and Interruptible Supply
Project. The project potentially would link groundwater systems serving parts of
Douglas and Arapahoe Counties with Blue River and South Platte River water sup-
plies conveyed by the Denver Water Department, with a resulting net reduction in the
total volume removed from the Denver Basin aquifers.'" If carried out, excess surface
water available through storage and delivery infrastructure managed by Denver Water
would, in average and wet years, supply water to the Douglas County Water Resource
Authority (DCWRA) and, in some cases, recharge Denver Basin aquifers. In dry years,
groundwater sources would be available to satisfy DCWRA demands in peak summer
months and used to payback "borrowed" water to Denver Water in the fall.
Conjunctive use essentially allows storage underground, minimizing evaporative loss
and obviating the need for new surface supply structures.

Preliminary studies showed that conjunctive use could yield up to 60,000 AF
of supply. More recent studies, based on new modeling of the speed of groundwater
recharge, estimate a yield of 27,000 AF. Whatever the ultimate yield produced, a stip-
ulation requiring multiple re-use of any new surface supplies delivered to DCWRA
members could leverage the yield by a factor of 1.7 or 2.'"' For our MWSI plus sce-
nario, this reort uses a factor of 1.5.

Conjunctive use may depend partly on increased imports for the mainstem
Colorado River. As such, there may be legitimate opposition to some portion of the
conjunctive use potentials we estimate.