BOARD00363

• Board Meeting Date: 5/24/2004
• Description: Artificial Recharge of Ground Water in Colorado - A Statewide Assessment
• Board Meetings - Doc Type: Memo

Artificial Recharge of Ground Water in Colorado
A Statewide Assessment

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San Luis Valley
The San Luis Valley is an intermontane basin in south-central Colorado that covers
approximately 3,200 square miles within five counties (Figure VI-3). Although this high
mountain desert has an average annual precipitation of only eight-to-ten inches per year, it is also
home to a thriving agricultural community second only to that in the lower South Platte River
basin. Historically, croplands covering 617,000 acres were irrigated using surface water diverted
from rivers emerging from the San Juan Mountains on the west and the Sangre de Cristo
Mountains on the east. The valley relies on a vast network of distribution canals and laterals to
distribute the surface water across the flat valley floor with over 2 million ac-ft of water per year
diverted for irrigation (Topper and others, 2003).

In the 1950s and 1960s farmers began to rely more heavily on ground water for irrigation both in
response to drought and improvements in sprinkler technology. Currently, the vast majority of
irrigation is done with center-pivot sprinklers driven by well water. The principal
unconsolidated aquifers in the San Luis Valley are referred to as the upper unconfined aquifer
and the lower confined aquifer ofthe late Cenozoic Alamosa formation (Topper and others,
2003). Reported aquifer transmissivity values reach as high as 225,000 gallons per day per foot
(gal/day/ft) and well yields can be as high as 3,000 gpm, giving the unconfined aquifer optimum
characteristics for large-scale irrigation exploitation. As of the 2151 century, approximately 0.8
million ac-ft of ground water per year are withdrawn for irrigation (CWCB, 2003).

With the increased reliance on ground water for irrigation, water levels in the unconfined aquifer
began to decline rapidly. During the period from 1969 to 1980 areas in the valley experienced
declines of up to 40 feet (Topper and others, 2003). In the late 1970s and early 1980s the water
community in the San Luis Valley realized that AR could help maintain water levels in the
unconfined aquifer and, thus, AR became a tool for managing surface and ground-water
resources (Allan Davies, oral commun., 2003). Since that time, farmers and ditch companies
have been converting surface-water rights to include AR as a beneficial use so that recharged
water could be used for diversion at a later date using wells.

The combined surface- and ground-water resource has evolved into a large-scale conjunctive-use
system, and the primary objective for recharge is to regulate supply through the irrigation season.
In effect, the unconsolidated aquifer is utilized as a large storage reservoir allowing farmers to
continue to irrigate, even during times oflow surface-water flow. The water in storage in the
unconfined aquifer also provided a cushion for the 2002 drought. Farmers were able to irrigate
during the summer of2002 even though there was very little surface flow.

There are hundreds of individual recharge sites in the valley and the number continucs to grow
(Allan Davies, oral commun., 2003). Many of the AR sites are excavated pits in the comers of
land grid quarter-sections that are difficult to irrigate with the center-pivots. Water is directed
from the canals to the pits for recharge. Artificial recharge is also accomplished through leaky
ditches and canals with many canals and laterals actually maintained to enhance leakage. As a
check of the balance between well usage and recharge, the Rio Grande Water Conservancy
District monitors water levels in the valley. Results of this monitoring indicate that the
management system appears to be very effective (Steve Vandiver, oral commun., 2003).

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