• About Purissima Hills Water District

    Purissima Hills Water District (District) is a County Special District serving drinking water to approximately 6,400 residents and 10 institutional customers in Los Altos Hills. We purchase our water from the San Francisco Public Utility Commission (SFPUC) which collects and stores rain and snowmelt from the Sierra Nevada Mountains. This source is high quality, soft water. The District serves roughly 2/3 of the Town of Los Altos Hills, along with some homes in unincorporated Santa Clara County.

    The District was incorporated on April 26, 1955, and a Board of Directors was installed. The District has steadily grown over the past 70 years and now serves a population of 6,400 and maintains roughly 80 miles of pipelines and 10 tanks. We have a maximum of 9.5 million gallons of available storage, although actual storage at any given time will be less than that maximum, depending on the time of day and seasonal weather fluctuation. By comparison, the entire City of Palo Alto, with 10 times more population, has 13 million gallons of maximum available storage.

  • How susceptible is the Town of Los Altos Hills / PHWD to urban wildfire?

    The latest maps provided by CalFire (dated February 24, 2025) indicate that about half of our service area is classified as having moderate fire hazard. There is a small portion with high fire hazard. For the most part, the moderate and high hazard areas are located West of interstate 280. Click here for map.

  • Who owns the fire hydrants and how many hydrants do we have in the District?

    The District service area has approximately 630 fire hydrants, which are owned, operated and maintained by the Los Altos Hills County Fire District (LAHCFD). The LAHCFD serves residents of Los Altos Hills and the unincorporated areas known as Loyola, Los Trancos, and San Antonio Hills. LAHCFD contracts its firefighting personnel, apparatus, and equipment from the Santa Clara County Fire Department.

  • Will District hydrants run dry? What is the District doing to prevent this from happening?

    Wildfires can potentially stress any water distribution system. A typical urban water distribution system is designed for a few active simultaneous structure fires, along with providing water to its customers, day in and day out. Long ago, the founders of the District realized that Los Altos Hills was not an ordinary suburban water system. With its 1-acre minimum lots and large, expansive open spaces, Los Altos Hills has always been at risk for fires. We’ve been thinking about this issue and working on it for the past 70 years. We have invested in the following infrastructure:

    • We have maximized available water storage. We have more storage per capita than most surrounding agencies.
    • We were one of the first agencies of our size on the Peninsula to invest in a hydraulic model, which is a digital twin and mirrors the actual water distribution. The fully calibrated model has been in continued operation since the mid 1990’s. The model is used to pinpoint bottlenecks and vulnerabilities and allows us to fix those vulnerabilities as quickly as possible.
    • We have worked collaboratively with the LAHCFD to increase available fire flows throughout our mutual service area.
    • We have invested in seismic resilience. Over the past 17 years we have replaced nearly 50,000 linear feet of water main with larger, seismically resilient ductile iron pipe, along with seismically upgrading virtually all our large diameter water tanks. These upgrades will ensure that we can continue to provide safe, reliable water for our customers every day and during emergencies.

    We continue to collaborate with the LAHCFD to prepare for potential fire emergencies in advance of the event. We are notified of potential high fire danger events and prefill our water tanks and prepare our pump stations for power disruptions.

  • In particular, what has the District been doing since the fires in Southern California?

    Since the fires in Southern California, we have been in touch with the LAHCFD to work through worst case urban wildfire scenarios in Los Altos Hills. Where will the fire come from? And how will fire personnel combat the fire? We will use this information, along with our hydraulic model, to develop scenarios and identify vulnerabilities in our storage and distribution systems. These efforts are ongoing and take time to implement, but we have been working on our infrastructure for years and will continue to do so, as budget allows.

  • What happens to the water distribution system when PG&E / Power is down?

    The District has invested in installing generators at all our key pump stations, along with our operation center and administrative office. This will allow us to continue to move water uphill even when the power is interrupted. All generators are inspected weekly.

  • How soon will the District be able to provide potable water after a disaster?

    After a natural disaster the District will focus all of our resources on restoring the water system as quickly and safely as possible and will work with other emergency responders to provide an alternative water supply for the health and safety of our customers. We will restore water to our customers only after conducting appropriate testing to be sure that it is safe to do so.

  • “What can you do now?” and “what should you do when there is a fire?”

    Please refer to the LAHCFD Emergency Preparedness page. If evacuation becomes necessary, please remember to turn off your irrigation system to help keep the distribution system pressurized. Also remember that it is a good idea to store five days’ worth of potable water in a safe, temperature-controlled location.

  • Why doesn’t the District keep its water tanks completely full during the high fire danger summer months to maximize water storage?

    When we decide how much water to store, we balance many factors, including water quality, energy costs, and emergency needs.

    The District operates 10 storage tanks, with a maximum of 9.5 million gallons of available storage. The tanks are never full; rather, they fluctuate through the day and by season and customer use conditions. Each tank has a set point that ensures safe and reliable water quality, availability, and pressure.

    Water tanks are typically located on top of the service area (zones) and water is pumped from the lower zones to the tank. It takes a tremendous amount of electrical energy to pump water, so most water purveyors (including the District) pump at night, when rates are less expensive. The tank is filled at night, and during the day gravity does all the work as water travels down from the tank to our customers. Nightly pumping also helps cycle the tank to make sure fresh, clean water is always available for our customers. We monitor water quality carefully to ensure that our customers are receiving the highest quality water.

    If there is an unexpected draw on the system during the day, due to an emergency or other unexpected event, the pumps are activated automatically when the water in the tanks drops below the set point. During the summer months we adjust the set points for the tank levels to increase available water for emergencies. This means that we sometimes pump water at peak electrical rates to achieve greater storage.

    We work closely with the Fire District, especially during fire weather watches and alerts, to increase pumping and maintain water in our tanks. However, keeping the tanks full at all times is virtually impossible and would prevent us from our mission of providing optimal water quality and availability to our customers.

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