An Integrated Natural and Social Science Decision Support Tool to Model the Potential Effects of Severe Water Supply Disruptions in Los Angeles

The result of this study will be a model that integrates the hydrology of the groundwater basins with the economies that rely on groundwater to meet their water demands. The integrative structure of this model will allow users to analyze the tradeoffs that need to be made in the event of a major imported water disruption in southern Los Angeles County.

Water use and water needs have been very closely tied to the development of greater Los Angeles, from its agricultural origins through its subsequent urbanization. Since the first water wells were drilled about 150 years ago, ground water has been a significant component of water supply in the region. In the Central and West Coast Basins of coastal Los Angeles, ground-water development through the first half of the 20th century resulted in large water-level declines and associated problems such as seawater intrusion. This led to the adjudication of the basins in the early 1960s and the initiation of ground-water management activities including injection, spreading, pumping restrictions, and delivery of surface water to replace some pumping. The Water Replenishment District of Southern California (WRDSC) was formed and given responsibility for protecting the ground-water resources for the 4 million people who live in the Central and West Coast Basins. Water to the area is supplied by imported surface water delivered directly to users (via aqueducts from northern California and the Colorado River) (360,000 acre-ft/yr) and groundwater pumped from the basins (250,000 acre-ft/yr).

California’s Department of Water Resources has reported that a 6.5 magnitude earthquake in Northern California could render water imported from this area unusable for at least 15 months. An earthquake in the southern region could also critically impact water supplies from Colorado River. The probability of a 6.7 or greater magnitude earthquake in the Los Angeles region is 60%. The USGS has developed a groundwater model to determine the physical impacts of an extended and severe water shortage on the region’s two aquifers. We are now in the early stages of expanding the model to include the economic impacts of groundwater/aquifer impairment in an extended imported water shortage. This tool attempts to answer the following questions: If the aquifers are not able to supply the region’s water demand in the event of an extended water shortage, what are the economic impacts? If, in a worst-case scenario, the structure of the aquifers is jeopardized and widespread subsidence and salt-water intrusion occurs, what are the socioeconomic impacts given communities’ different vulnerabilities to subsidence and intrusion? What are the ways (i.e. conservation measures, infrastructure construction, policy measures) that we can prepare for potential water disasters, and at what cost? What are the implications of preparing for a disaster versus not and what magnitude disaster are we prepared for?

Regional water authorities in Los Angeles are faced with the need to develop realistic water management contingency plans in the event of a catastrophic event (natural or otherwise) that would disrupt the imported water supply to the southern California region. The principal objective of the proposed effort is to provide water authorities with an integrated natural and social science framework to more effectively analyze the risks associated with catastrophic natural or human hazard that could cripple the region’s water supply.

In June 2006 the USGS had a workshop with over thirty water authorities in the region to talk about their disaster preparedness efforts and how this model can help them in these efforts. We are now developing the model and will have a second workshop in early 2007 to present the results to stakeholders.