The submission to the WSAC by County Water Resources Director, John Ricker, discusses aquifer recharge by means of sending river water to Scotts Valley and Soquel Creek during winter months, allowing those districts to reduce their well pumping. The submission by Piret Harmon, General Manager of Scotts Valley Water District, discusses aquifer recharge by means of injection wells or surface water infiltration at the Hanson Quarry. These are two of the ten aquifer recharge strategies ranked by Kennedy/Jenks according to cost/benefit and feasibility. Both strategies allow Santa Cruz to “bank” water in the aquifers for use during critically dry years. This discussion is to offer additional information about the long-term potential yield of those strategies.
Aquifer recharge should be a principal adaptation to climate change
The Water Supply Advisory Committee has committed to do water supply planning in the context of an uncertain future climate. Climate modeling points towards a shorter wet season for this region, even in models that envision similar or increased total precipitation. A shorter wet season means reduced aquifer (groundwater) recharge. Lower aquifer levels mean lower stream flows, since the aquifers feed the springs that run off into creeks.
Recommendation— that the City commit to aquifer restoration as a principal strategy to prepare for climate change. The Purisima Aquifer, source water for the City’s Live Oak wells, is overdrafted and threatened with salt water intrusion. The Santa Margarita Aquifer, source water for the San Lorenzo River is 200 feet lower than historic levels in the Scotts Valley area.
Recharged aquifers yield more water in droughts
Restoring the Santa Margarita Aquifer would mean that the San Lorenzo River would have a higher base flow in summer months of drought years. Moreover, if the City and Scotts Valley can agree on the water transfer strategy, the Santa Margarita Aquifer will be the City’s new savings account. “Santa Cruz Water Department should be able to purchase the banked groundwater in periods of drought”, according to the Kennedy/Jenks study.
If the Purisima Aquifer can be restored to safe levels, Santa Cruz would be able to pump much more water from Live Oak wells during drought years than the City is able do today. As recently as 2003, the Integrated Water Plan assumed that production from Live Oak wells would be 2 million gallons per day. According to the City’s current agreement with Soquel Creek Water District, the City only draws half that much. If the aquifer could be restored enough to allow the City to pump 2 million gallons per day during drought years, the additional 1 million gallons per day represents 40% of the output of the proposed desal plant.
The additional water from Live Oak Wells could be augmented by water from Soquel Creek District wells if the water transfer plan is implemented. According to the submission by John Ricker, “With current infrastructure and the addition of a pump station at 41st Avenue, Soquel could pump 1.44 mgd to the City, or 172.8 million gallons (530 acre-feet) over a 4 month period.” Assuming that Santa Cruz would not purchase this water except during critically dry years, that means a water transfer from Soquel Creek District every 6.5 years. The amount of water coming back to Santa Cruz represents about 18% of the water Santa Cruz would supply the District over a 6.5 year period with existing infrastructure (an average 145 million gallons per year).
A new reservoir (underground) increases the yield of the old reservoir
Loch Lomond has always served two functions: provision of water during the dry season, and a reserve for future dry years. Because of Loch Lomond’s small size relative to the City’s water demand, the need to keep a safe water reserve for future dry years means that the City typically operates the system so that the reservoir remains at high levels through the dry season of normal years. Even in a dry year with Stage 2 restrictions such as 2009, Loch Lomond was over 90% full at the end of September. Given the current lack of underground storage, Desal Alternatives applauds the Water Department’s cautious approach in keeping the reservoir at high levels. At the same time we recognize that this approach misses the opportunity to capture more water in the reservoir.
A high lake level at the end of the dry season means that the lake will not capture much storm water in the winter. If the lake is full as of early January, any future storm runoff will spill over the dam.
In one of life’s few “if you spend more, you get more” situations, if the City would use more Loch Lomond water in the dry season, it could capture more winter water.
Using the aquifers as storage would allow the City the security to use more Loch Lomond water during the course of normal years.
The average water production from Loch Lomond during 2002-2011 was 484 million gallons per year. In all of those years the reservoir filled during the course of the rainy season. We estimate that the City could double the average reservoir production of recent years and the reservoir would still refill in most winters.
The California Department of Fish and Wildlife has recommended that the City petition to increase its current maximum water right to using 1 billion gallons per year from Loch Lomond. The CDFW recognizes that the reservoir is key to reducing the City’s need to divert water in the dry season from river and creeks. The recommendation to use the reservoir in this way would be a lot more viable if the City begins banking water in the aquifers. The benefits of this strategy increase over time.
 Kennedy/Jenks, Conjunctive Use and Advanced Aquifer Recharge Project (2011)