The Santa Clara Valley Water District has engaged in a water banking and exchange program with the Semitropic Water Storage District since 1996 to help manage variability in the district’s State Water Project (SWP) and Central Valley Project (CVP) water supplies. Semitropic is a member district of the Kern County Water Agency, the second largest SWP contractor.
Established in 1958, Semitropic covers an area of more than 200,000 acres, and is located about 35 miles northwest of Bakersfield. The original Semitropic banking program was established in 1996 with a capacity of one million acre-feet of water. The water district is one of the original banking partners and has purchased a 35% share of the capacity, establishing the water district as one of the largest banking partners.
Semitropic provides a place to store surplus water supplies that may be inexpensively secured in wet years for later withdrawal and use in dry years. Water may also be stored and retrieved in the same year, depending on operational needs, to minimize the risk of losing SWP and CVP water allocated to the district.
Water to be stored in the Semitropic groundwater bank is delivered through the California Aqueduct to Semitropic in Kern County. Semitropic then either uses the water in lieu of groundwater pumping, or recharges it into the groundwater basin. Because Semitropic is located south, or “downstream,” of the district in the state’s water delivery system, the district must retrieve banked water by exchange with other SWP water being pumped at Banks Pumping Plant in Tracy.
Over the past 20 years, the district has stored about 425,000 acre-feet of water in Semitropic and withdrawn about 190,000 acre-feet of supply. During the current drought, over three years the district has withdrawn 97,000 acre-feet of dry year supply during a time when supplemental water supply is both scarce and extremely expensive. For example, in 2015, a drought year, the cost to withdraw an acre-foot of water from Semitropic was about $200 per acre-foot, while the cost to purchase an acre-foot of transfer supplies from north of the Delta was approximately $1,000 per acre-foot after taking into account conveyance losses.
The Semitropic groundwater bank has proven to be a valuable resource to help the district manage wet year water supplies, and to provide critically important dry year supplies to meet drinking water treatment plant demands and to protect Santa Clara County’s groundwater basin from overdrafting and subsequent land subsidence.