The City of San Luis Obispo has four primary water supply sources including Whale Rock Reservoir, Salinas Reservoir (Santa Margarita Lake), Nacimiento Reservoir, and recycled water (for irrigation). The Utilities Department manages its water supplies by following the City’s State-approved, governing documents: Urban Water Management Plan (UWMP), Water Shortage Contingency Plan (WSCP), as well as the Water and Wastewater Management Element of the General Plan.  

Currently, the City’s waters supply portfolio is in stable condition, with over 5 years of available water projected to be available for community use, putting the City in the Monitor Phase of its WSCP. 

In fact, the City of San Luis Obispo’s drinking water system is the least vulnerable to supply impacts in San Luis Obispo County, according to a report released by the San Luis Obispo County Flood and Water Conservation District. In response to previous drought conditions, and in anticipation of future drought conditions, the County developed the Regional Water Infrastructure Resiliency Plan, for mitigating vulnerabilities and enhancing reliability, resilience, and optimum utilization of existing and future regional water infrastructure. The report ranked the City of San Luis Obispo with the highest resiliency risk ranking of all water purveyors in the county. 

The California Department of Water Resources requires urban water purveyors to develop Urban Water Management Plans (UWMP) for long-term water resource planning to ensure that adequate water supplies are available to meet existing and future water needs.  

The City of San Luis Obispo's Urban Water Management Plan describes and evaluates current and projected water supply sources through the year 2040, encompassing and going beyond the City’s projected General Plan build out. The water needed to serve the population projected in the General Plan is known as the City’s “Primary Supply”. Water needs to meet the City’s Primary Supply are projected using computerized models. These models simulate anticipated growth and development, anticipated water use by residents and businesses, and projected reservoir levels, including impacts from climate change. Because there are limitations in the ability to predict water demand in the future, conservative estimates of total community water use are made, resulting in the modeled water demands being higher than what is likely to occur.  

In addition to the water needed to meet current and future water demand, City policy also requires water be set aside to act as a buffer for unforeseen or unpredictable long-term impacts. This is known as the City’s “Reliability Reserve”. As a result of several generations focused on water supply planning and the community’s conservation efforts, the City’s water supply portfolio exceeds both its Primary Water Supply and Reliability Reserve. This excess water is referred to as the City’s “Secondary Supply”. The City's water supply and demand projections help to ensure that there is enough water to support future community growth with a buffer to account for climate change and other unforeseen impacts to water supply availability.  

City staff understand that there may be increased concerns about development and growth in the City during periods of dry weather. However, it is important to remember that the large development projects currently being constructed within the City are a part of the City’s General Plan, which projects growth through 2035, and that the City’s “Primary Water Supply” ensures there is enough water to sustainably provide the water needed to support this increased population, even during periods of extended drought. Development projects are only approved when the City is certain adequate water supplies are available to serve the project.  

In addition to planning for future population growth, the City’s Water Projection Model projects future reservoir levels, the volume of water that can be safely withdrawn from reservoirs each year, and how climate change may impact water resource availability. These planning practices ensure the City’s water resources remain adequate as the City grows. More information on how the City calculates it’s “Primary Supply” can be found in the Planning & Policies section above. 

The City of San Luis Obispo’s multi-source water portfolio reduces the risk of a potential water shortage due to drought, loss of delivery infrastructure, or poor source water quality. The City has invested in  a diverse water supply including three surface water reservoirs, recycled water produced at the City’s Water Resource Recovery Facility (WRRF), and groundwater production wells. Today the City’s water supply portfolio includes over 10,000 acre-feet of available water each year. The City’s the total water use for all uses is currently about 5,000 acre-feet of water per year, approximately half of what can be sustainably utilized each year.

Surface Water

The majority of the City’s water comes from the three surface water reservoirs: Whale Rock Reservoir, Salinas Reservoir (Santa Margarita Lake), and Nacimiento Lake. Having three surface water supplies allows for greater overall water storage and resiliency if one reservoir is unable to supply water. Furthermore, because these reservoirs are located in different watersheds, the City is able to capture water from rainfall events in different geographic areas of the county. The City prioritizes using water from Nacimiento Reservoir because its large watershed allows it to fill substantially year to year, and its total storage volume is nearly six times that of Whale Rock and Salinas Reservoirs combined. Maximizing the use of Nacimiento allows also the City to preserve water in its other reservoirs for use during critical water shortage periods.

In 2006 the City began the production of recycled water at its Water Resource Recovery Facility (WRRF).  Today about 5% of the City’s total annual water use comes from drought-resilient recycled water. Offsetting the use of potable water by utilizing recycled water for irrigation at local parks, business parks, and in common areas of large developments reduces the demand on potable surface water supplies by approximately 250 acre-feet per year. 

Groundwater

The City overlies the San Luis Valley Subarea of the San Luis Obispo Valley Groundwater Basin. Once a major component of the City’s water supply portfolio, groundwater has not been utilized in significant volumes since the 1990s. All public supply production of groundwater ceased in 2015, however the City intends to renew its use of groundwater pending completion of several groundwater-related studies that will help the City understand the volumes of water that can be sustainably extracted from the basin each year and what treatment technologies would be most efficient to ensure high quality water is delivered to the community.  Initial estimates indicate that groundwater could increase the City’s water supply portfolio by approximately 20% once fully utilized. More information about the City’s groundwater basin can be found within the recently completed Groundwater Sustainability Plan. 

Other Sources 

The City understands the water security we experience today is a result of many years dedicated to water resource planning and water conservation. The City takes action by planning today to ensure there is adequate water supplies for generations to come. The City remains engaged in discussions related to regional water resiliency through participation on the Water Resources Advisory Committee (WRAC), the Integrated Regional Water Management Group (IRWM), and through working closely with the County of San Luis Obispo and other local water purveyors to maintain and expand local water supplies.  

Beyond expansion of local groundwater supplies, and sustainable management of surface water supplies, the City is nearing completion of a major upgrade to the Water Resource Recovery Facility (WRRF). The upgrade will help position the City to potentially utilize recycled water for groundwater recharge in the future. This will allow the City to recharge nearly 1,800 acre-feet of recycled water into the groundwater basin for storage and subsequent use.