By Linda Taaffe

The Purissima Hills Water District is scheduled June 8 to set a formula for determining water usage allocations on customers as part of an emergency drought plan that would only go into effect if the San Francisco Public Utilities Commission declared a water shortage emergency. About two-thirds of Los Altos Hills households are within the water district.

The plan could cut usage to some customers as much as 70 percent if the San Francisco commission declares a 20 percent drought, according to district officials.

The district has been working on a plan to reduce its total water usage by 40 percent during water-shortage emergencies in anticipation of severe supply cutbacks from San Francisco, which provides all of the district’s water. San Francisco officials said Los Altos Hills residents will face larger cutbacks than other cities because the district has exceeded its usage by 34 percent over the past five years.

The district anticipates a 20 percent supply reduction from San Francisco during emergencies.

The plan up for approval is based on a formula that would take each customer’s usage history for the past five years and calculate it with the district’s lifeline summer and winter water allocations to determine a household’s monthly limit. A lifeline is the amount of water needed for “essential” uses. The district’s summer lifeline is 20 units, with 10 units for the winter.

The emergency cap is formulated for each customer but would limit the maximum use of any customer to 120 units, or 89, 760 gallons during the summer and 30 units, or 22,440 gallons in the winter.

Customers who use a greater amount of water will be most impacted by the plan, a water spokesman said.

If the formula is approved, a customer who historically has used 15 units of water each winter, could be forced to reduce usage to 13 units during a drought.

Those who do not comply could face fines.

Los Altos Hills resident Bill Krause said the formula is based on San Francisco residents, averaging one-eighth of an acre in mostly concrete environs.

“Los Altos Hills has a dramatically different profile,” he said.

Krause estimated that the property value for as many as 4,000 residents could be impacted due to landscaping issues stemming from such a decision.