Paso Basin Supplemental Water Supply Study

Located in northern San Luis Obispo County, the Paso Robles Groundwater Basin (Paso Basin) has long served the municipal and industrial, domestic, and agricultural needs of communities within and surrounding the county. However, given the area’s heavy historical reliance on groundwater and its continued growth and development, groundwater levels in Paso Basin have declined severely over several decades, dropping over 50 feet from just 1997 to 2003.

The negative effects of ever-increasing water demands on local water supplies led the San Luis Obispo County Flood Control and Water Conservation District (District) to evaluate new groundwater management solutions that will secure a sustainable water supply for customers while replenishing the Paso Basin.

Carollo supported the District in conducting a supply options study that identifies new sources of supply that can be obtained to supplement the Paso Basin. The study’s primary goal was to define the quantity, quality, cost, and points of transfer of supplemental water options; the infrastructure needs at the transfer points; and the terms and conditions under which this new supply may be procured.

As part of this effort, Carollo determined that approximately 35,000 acre-feet per year (AFY) of water is needed to balance the Paso Basin. With this in mind, Carollo developed a list of the most viable options for procuring and delivering available water from the California Department of Water Resources’ State Water Project (whose pipeline currently runs through the basin), Lake Nacimiento (a local reservoir), and local exchanges and recycled water sources. Utilizing groundwater modeling, Carollo assessed the benefits of each supply option, evaluating where benefits can be maximized in the basin through direct use (i.e., offsetting pumping) or recharge.

Throughout this process, Carollo and the District worked closely with a diverse group of local stakeholders ranging from wine grape growers, rural residents, ranchers and farmers, and urban city representatives. These groups of interest collaborated to identify further options and offer feedback on the evaluation results.

Ultimately, Carollo concluded that the new, identified supplies could contribute up to approximately 20,000 AFY of water to stabilize Paso Basin; however, the District would need to continue managing demand to achieve sustainable conditions.