When it comes to supplies of water, many local cities are dependent on one far-away source: the San Francisco Regional Water System, which comes from the Sierra Nevada, mainly the Hetch Hetchy reservoir. Numerous Peninsula cities get 100% of their water from this supplier.

But the West's deepening drought and recent calls for Californians to voluntarily reduce their water use by 15% have residents starting to wonder: Just how resilient are local water systems in the event of a long-term drought or an emergency?

Data from the Bay Area Water Supply and Conservation Agency (BAWSCA) indicates that local cities have little in the way of alternative or local sources to their imported water supply. Storage is also limited.

Some communities are better diversified than others. Mountain View and Stanford have perhaps the greatest amount of diversity; East Palo Alto has no emergency source other than through tie-ins with surrounding cities who also rely on Hetch Hetchy supplies.

Even the well-diversified supplier Valley Water, also known as Santa Clara Valley Water District, is dependent on imported sources of water from the San Francisco Bay-San Joaquin Delta and Hetch Hetchy supplies.

Groundwater, local storage and recycled supplies would not be enough to offset a loss of water from these life-sustaining sources if they run dry. Valley Water's current concerns perhaps illustrate just how vulnerable the Bay Area's water supply can be. Anderson Reservoir, the largest in Valley Water's storage system, has been reduced to nearly a puddle since federal authorities mandated a seismic retrofit that will take a decade to complete. The water district's other major water supplies from the San Francisco Bay-San Joaquin Delta have also been vastly reduced by state and federal authorities to provide additional needed water for wildlife and natural fisheries.

California has experienced multiple, extended periods of dry weather since 1895, and one of the most exceptional occurred between 2011 and 2017, according to the National Oceanic and Atmospheric Administration's National Integrated Drought Information System.

Ask local water providers what they'll do in a deep drought or catastrophic failure of their own system and they look to the other cities. Most have interties: pipe systems that allow them to move water from one municipality to another. But in a catastrophic drought, would neighboring cities have extra water to spare?

Here's a look at the water capacity of local jurisdictions and alternative sources of water based on 2019-2020 data from BAWSCA's annual survey, which was published in March 2021.

Palo Alto

Palo Alto is among the more diversified local municipalities when it comes to water. In fiscal year 2019-2020, the city and its 67,082 residents used 10.5 million gallons of water per day. About 93% came from the San Francisco Regional Water System. Recycled water accounted for 7%.

Palo Alto uses recycled water to irrigate its golf course and a city park and to fill its duck pond. The recycled water is also used for the city's Emily Renzel Marsh enhancement project and as part of processing at the water quality control plant. Recycled water is not for drinking or swimming in.

Palo Alto has recently improved its water capacity through its Emergency Water Supply and Storage Project, which rehabilitated five existing wells, constructed three new wells and built a new 2.5-million-gallon emergency water-storage reservoir. The eight emergency wells can pump up to 15.5 million gallons per day if needed.

The city's seven storage reservoirs — Mayfield, Boronda, Corte Madera, Dahl, El Camino, Montebello and Park — have a total capacity of 13 million gallons.

The city now has adequate storage and pumping capacity to provide back-up should there be an interruption of San Francisco water service. The wells may also be available to meet limited dry year requirements, according to BAWSCA.

It has interties with East Palo Alto County Water District, Mountain View, Purissima Hills Water District and Stanford University.

Days of storage: 1.13

Stanford University

The Stanford Sustainability & Energy Management Department supplies water to the campus area and Stanford's unincorporated lands, serving 32,075 people, which is the university's average daytime population, according to BAWSCA.

Stanford's average daily water demand is 2.5 million gallons per day.

The university has five sources of water: purchased potable water from the San Francisco system, groundwater, non-potable surface water from the local watershed, stormwater and runoff capture, and recycled water, according to BAWSCA.

Stanford gets 57% of its water from the San Francisco system. Another 43% comes from "other" sources, including Stanford's surface-water diversions such as Searsville Lake and groundwater. Alternative sources include local groundwater, surface water, stormwater, construction dewatering and recycled water. The university now tracks its other supplies for use as irrigation water. The extent of groundwater used depends on the amount of rainfall and how much surface water is available.

Four wells on Stanford property could be used in an emergency. Three of the wells are in compliance with all drinking water standards; the fourth well is on "standby" since its manganese levels exceed current standards, BAWSCA noted. Wells can supply 3.7 million gallons per day in an emergency.

Stanford also has three storage reservoirs with a total capacity of 9.5 million gallons, according to BAWSCA. A recycled water plant completed in 2008-2009 was decommissioned in 2015 but could be used in the future, the BAWSCA report stated.

It has interties with Palo Alto.

Days of storage: 2.5 to 4

East Palo Alto

The city of East Palo Alto's water utility receives all of its potable water supply from the San Francisco system. In 2019-2020, the city used 1.5 million gallons of water per day with only about 311,000 gallons of water annually coming from groundwater.

The water utility, which serves 26,181 people, is operated and managed by a private contractor. Two privately owned water companies, O'Connor Tract Water Coop and Palo Alto Park Mutual Water Company, serve a small area of the city separate from the city's water supply.

The city's water system has no storage facilities or alternate potable water supply sources in the event of an earthquake. The city has 3.6 million gallons of storage identified but approval and funding have not been secured, according to BAWSCA.

East Palo Alto has one emergency well that is not currently certified for drinking-water use.

It has interties with Palo Alto and Menlo Park.

Days of storage: No storage. East Palo Alto cannot sustain a loss of water without obtaining water through its interties in an emergency.

Menlo Park

The city purchases all of its water directly from the San Francisco system and uses nearly 3 million gallons of water per day. Menlo Park Municipal Water runs the city of Menlo Park's water system and serves 18,224 people. Two reservoirs supply the Sharon Heights area.

There is emergency storage in the areas supplying north and east of El Camino Real.

California Water Service and the city's storage well are the primary emergency sources of water for Menlo Park, according to BAWSCA.

The area has emergency interties with California Water Service Bear Gulch District, Redwood City, O'Connor Tract Water Coop and East Palo Alto.

Days of storage: 0.65

Mountain View

The city of Mountain View serves a population of 79,772. Its primary water supplier, San Francisco Regional Water System, provides 84% of the water. Valley Water supplies 10% treated water; 2% supply is from groundwater and 4% from recycled water. The city uses more than 9 million gallons per day.

California Water Service also provides water to a small part of Mountain View, according to BAWSCA.

The city has four water storage facilities, above and below ground, said Elizabeth Flegel, city water resources manager.

Mountain View has four active wells (and four are currently out of service). They are not currently operated at their maximum capacity due to various maintenance and operational issues.

Local storage in the reservoirs and the wells comprises a total of 17 million gallons, according to BAWSCA.

Flegel said the city is studying its recycled water distribution system and is updating a feasibility study for expansion. Mountain View's construction program requires contractors to do hydrant metering so that water trucks using hydrant water to keep construction dust down track how much is expended and are charged for the water, Flegel said.

The city has interties with Palo Alto, Sunnyvale, Valley Water and California Water Service.

Days of storage: If the city lost its San Francisco water supply only, it could utilize wells, Valley Water or storage to meet an eight-hour outage.

California Water Service

This sprawling water operator has multiple districts on the San Francisco Peninsula, including the Bear Gulch and Los Altos districts.

The Bear Gulch district is located in southern San Mateo County and serves Atherton, Portola Valley, Woodside, parts of Menlo Park, parts of unincorporated Redwood City and adjacent unincorporated portions of San Mateo County including West Menlo Park, Ladera, North Fair Oaks, and Menlo Oaks.

The Bear Gulch district receives 85% to 95% of its daily supply from the San Francisco system. The balance is supplied by surface water runoff from California Water Service Company's watershed.

In 2019-2020, the district used nearly 11.5 million gallons of water per day to serve 60,827 residents. Local surface water, local groundwater (in its Skyline system only) offer a limited additional supply.

The water is stored in the 215-million-gallon Bear Gulch reservoir, which is treated at a filtration plant before distribution, but the reservoir's capacity has been reduced by 6 feet to be in compliance with California Division of Safety of Dams safety requirements.

Although Bear Gulch reservoir is currently at 80% capacity, the treatment plant isn't running at this time, said Lee Blevins, production superintendent for the district. Currently, the district is using 100% of its water directly from the San Francisco system.

"Turning on the treatment plant would deplete the reservoir. The supply is seasonal," he said.

The reservoir water is used in late fall through early spring during the rainy season when it is replenished.

The district also has storage tanks ranging from 50,000 to 1 million gallons, Blevins said. The total storage capacity for the reservoir and tanks is 226 million gallons, according to the BAWSCA report. The system has Interties with Redwood City and Menlo Park.

Days of storage: If the Bear Gulch district lost all of its sources of water supply, it would have 0.92 days of water, according to the BAWSCA report.

The Los Altos district, which does not obtain water from the Hetch Hetchy system, provides water to Los Altos Hills, Los Altos, Mountain View and Sunnyvale through groundwater and water from Valley Water and serves more than 55,000 people, according to its 2010 Urban Water Management Plan, its most recent report. Water use was projected to reach 21 million gallons per day by 2020.

Between approximately 32% of its water supply comes from groundwater and up to 68% is purchased from Valley Water, but these numbers are variable based on the water supply from Valley Water. The district can pump out more water from the ground if needed and it encourages conservation.

The Los Altos district doesn't store water seasonally. In an extended drought, it would need to install new wells to meet demand, according to the water management plan.

Purissima Hills Water District

The Purissima Hills Water District provides service to two-thirds of the Town of Los Altos Hills and unincorporated county land on the southern boundary. Its largest customer is Foothill College, according to BAWSCA. The population served is 6,150.

The district receives 100% of its water from the San Francisco system or 1.75 million gallons per day. It doesn't produce any local water and has no alternative supply sources. The district has 11 gravity-fed storage tanks with a total capacity of 9.88 million gallons. It has no wells.

The district does have interties with the California Water Service in Los Altos and Palo Alto.

Days of storage: The system can meet an eight-hour supply in an emergency.