California Water Science Center (CAWSC) - Temecula Hydrogeology Project (THP)

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Temecula Hydrogeology

Welcome to the U.S. Geological Survey (USGS) Temecula website. This site provides hydrologic data collected or compiled by the USGS for the Temecula area; some additional data may be available from the USGS database National Water Information System (NWIS).

Project Chief: Wes Danskin
Phone: 619-225-6132
Email: wdanskin@usgs.gov

Background

Problem

The Santa Margarita Watershed, located in southern California near the town of Temecula, provides most of the water to local residents living in the upper basin and to Camp Pendleton Marine Corps Base in the lower basin. The upper and lower basins are separated by a coastal range of mountains. The upper basin is composed of rolling hills and broad valleys surrounded on the east and south by steep mountains. Depth of alluvium and poorly consolidated deposits in the upper basin commonly exceeds 800 feet. The lower basin is composed primarily of heavily dissected hills with narrow valleys filled with river-channel deposits generally less than 200 feet thick. Rain falling on the bedrock hills in the upper basin fills several small creeks, including Temecula and Murrieta Creek, and the Santa Margarita River, which merge and flow out of the upper basin through a gap in the coastal mountains, referred to as "the gorge." Downstream of the gorge, the Santa Margarita River flows through the lower basin in a narrow channel deeply incised in the surrounding bedrock, recharges a small ground-water basin used for domestic supply by Camp Pendleton, and eventually empties into the Pacific Ocean.

During the early 1900's, agriculture and ranching developed in the upper basin through the use of water diverted from the Santa Margarita River and water extracted from wells near the river. By 1940, base flow in the river had been reduced to such an extent that water users in the lower basin filed a lawsuit against water users in the upper basin. The result of this lawsuit was the "1940 stipulated judgment," which partitioned water in the upper basin between uses in the upper basin and flow to the lower basin. The judgment involves both surface-water diversions and ground-water extractions.

Since the 1940's, population and water use in the upper basin have continued to increase. Urbanization, especially since about 1970, has transformed the area from large ranches to tract homes with many residents commuting to work in either Los Angeles or San Diego County. Most of the water supplied to these homes is provided by the Rancho California Water District, which covers most of the upper basin. In the lower basin, the modest agricultural use of land and water, primarily for citrus and avocado trees, has stayed relatively constant. The creation of Camp Pendleton Marine Base in 1942, which covers most of the lower basin, added a relatively small, constant use of water for base operations. Base officials, however, are concerned that future water demands in the lower basin could be significantly greater to support a major military mobilization or to provide adequate habitat for endangered species.

The continued urbanization and increased pumpage in the upper basin of the Santa Margarita Watershed has caused concern by military officials at Camp Pendleton that surface-water flow to the lower basin has decreased and will continue to decline in the future. To enforce the provisions of the 1940 stipulated judgment between water users in the upper and lower basins and to prevent further jeopardizing of the water supply to Camp Pendleton, the U.S. Marine Corps and the U.S. Justice Department are involved in a water-rights negotiation with, and possible future litigation against, the Rancho California Water District (RCWD). Central to this negotiation is a quantitative understanding of the hydrogeologic system in the upper basin and its affect on surface-water flow to the lower basin.

Objectives

The primary objective of this project is to develop a more complete understanding of the hydrogeologic system in the upper basin of the Santa Margarita Watershed as an aid in achieving an amicable resolution of the present water-rights controversy without resorting to expensive litigation. This hydrogeologic understanding needs to encompass both the surface-water and ground-water systems in the upper basin, and the analysis needs to involve both historical conditions and likely future water operations.

Relevance and Impact

This project provides a number of benefits to the public, for example, it will: Advance knowledge of the hydrologic system in the upper basin of the Santa Margarita Watershed. Much of this knowledge will be gained from development and calibration of a linked surface-water and ground-water flow model of the upper basin, a task requested at the outset of the project by the attorneys group.

Advance the methodology of applying constrained optimization techniques to help manage a surface-water/ground-water system. A major issue in this area is how to manage pumpage in the upper basin so that streamflow at the gorge is maintained at acceptable levels. Presently, public-domain simulation/optimization computer codes do not have this capability. This project will help define how to best add this important capability.

Provide data and results that will be used by the U.S. Department of Justice, the U.S. Marine Corps, and the Rancho California Water District in resolving conflicts over water-rights and water-use in the Santa Margarita Watershed. Provide water-resources information that will be used by the Rancho California Water District and Camp Pendleton to help plan and operate their water systems. Additional public entities that likely will use the new information are the Pechanga Indian Tribe and Eastern Municipal Water District.

Provide hydrologic data and information that will contribute to protecting life and property in the Santa Margarita Watershed. In particular, the improved understanding of the surface-water system will allow for better protection against damaging floods, which have occurred as recently as 1993. The improved understanding of the relation between ground-water pumpage and ground-water levels in the upper basin will enable water managers to prevent a reoccurrence of land subsidence, which occurred in the 1980's and 1990's partly as a result of ground-water extractions.

Strategy and Approach

To resolve the water-rights issues in an efficient manner, at least two working committees are suggested to guide the specific tasks and to facilitate the negotiations. An attorneys group, composed of representatives of Camp Pendleton, the United States Justice Department, and the Rancho California Water District, would provide general direction for a technical advisory committee. The technical committee, composed of representatives of Camp Pendleton, the Rancho California Water District, and the United States Geological Survey (USGS), would investigate those hydrologic issues deemed necessary by the attorneys group.

The role of USGS scientists, as described and budgeted in this proposal, is to act solely as technical advisors and reviewers to facilitate the hydrologic research and negotiations by other members of the technical advisory committee and attorneys group. If hydrologic investigations are to be conducted by USGS scientists or if USGS reports are to be prepared and published, then this proposal will be revised to reflect the expanded scope of work and additional funding requirements.