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Drought 9.1.3 Defined Drought Stages in California During critically dry years, the California State Water Resources Control Board can mandate water entitlements on water right holders to address statewide water shortages. Table 9-1 shows the state drought management program stages mandated to water right holders. Table 9-1. State Drought Management Program Drought Stage State Mandated Customer Demand Reduction Rate Impacts Stage 0 or 1 <10% Normal rates Stage 2 10 to 15% Normal rates; Drought surcharge Stage 3 15 to 20% Normal rates; Drought surcharge Stage 4 >20% Normal rates, Drought surcharge 9.1.4 Secondary Hazards The secondary impact most commonly associated with drought is wildfire. A prolonged lack of precipitation dries out vegetation, which becomes increasingly susceptible to ignition as the duration of the drought extends. In addition, lack of sufficient water resources can stress trees and other vegetation, making them more vulnerable to infestation from pests, which in turn, can make them more vulnerable to ignition. Prolonged droughts can impact underground aquifers, thus impacting groundwater supplies. Algae blooms can occur in surface water reservoirs that are stressed by drought impacts. 9.2 HAZARD PROFILE 9.2.1 Planning Area Water Supply and Drought Response Water Supply Infrastructure San Mateo County receives 92 percent of its water through the regional Hetch Hetchy Water System, with the remainder of the County’s water supply coming from surface, ground, and recycled water (San Mateo County OES, 2015). The water system was so-named because 85 percent of the water supply comes from the Sierra Nevada snowmelt stored in the Hetch Hetchy reservoir along the Tuolumne River in Yosemite National Park; the remaining 15 percent comes from runoff in the Alameda and Peninsula watersheds. The Hetch Hetchy Water System (see Figure 9-2) was approved in 1913 under the Raker Act, which allowed use of federal lands in the Sierra Nevada Mountains to build that water system. The system was constructed by San Francisco over 20 years, with first delivery of water in 1934. Although San Francisco owns the system, it was designed from the beginning to serve as a regional water supply system. In 2002, the San Francisco Public Utilities Commission (SFPUC) adopted a $2.9 billion capital improvement plan to enhance the water system. Need for the improvements had been recognized after the Loma Prieta earthquake in 1989 and drought in the 1990s. Much of the water supply system is 75 to 100 years old and does not meet modern seismic codes, and major pipelines cross earthquake faults. A 2000 SFPUC study found that a major earthquake could cripple the water supply system for up to 30 days. SFPUC has highlighted nine priority projects for implementation, completion of which should help ensure relative continuity of operations of the water supply system following a large seismic event. 9-5