Climate Change Study on the Los Angeles Aqueduct System

Team: Bill Mills and Sujoy Roy (team lead, Tetra Tech, IInc, R&D, Lafayette CA); Roger Bales (Sierra Nevada Research Institute and U.C. Merced), Norm Miller (Lawrence Berkeley National Lab., U.C. Berkeley and U. Arizona), and Edwin Maurer (Santa Clara University).
  
Our role: Mariza Costa-Cabral in collaboration with Edwin Maurer, co-leads the hydrologic modeling component of the project, which evaluates impacts of climate change scenarios on the hydrology of Eastern Sierra Nevada.

Client: Los Angeles Department of Water and Power (LADWP)

Project Timeline: 2009-2011

Introduction

Precipitation from the Eastern Sierra Nevada is one of the main water sources, and the one of highest quality, for Los Angeles’ more than 4 million people.  Winter precipitation is stored in the large natural reservoir that is the snowpack, and meltwater (some 200 to 500 thousand acre-feet annually) is delivered to the city in the dry season by the 340-mile long Los Angeles Aqueduct (LAA).  The relative locations of the watershed, the aqueduct, and the supply area are shown in the figure below.  Seven reservoirs built along the aqueduct have a combined storage capacity of about 300 thousand acre-feet. Future availability of this water supply source is of critical importance to the city’s growing population and large economy.

Los Angeles Aqueduct area map

Traditionally water-limited, Los Angeles now faces renewed water pressures.  These include a Federal Court ruling that limits exports from the Sacramento-San Joaquin Delta; the city’s commitment in the last two decades to environmental mitigation and flow enhancements in Owens Valley and Mono Lake Basin (which have reduced the aqueduct’s average contribution to the city from a historical 2/3 to about 1/3 of total); the contamination of San Fernando valley groundwater; sustained drought in the Colorado River; and finally, uncertain climate change effects that loom on the horizon. 

These rising pressures led Mayor Antonio R. Villaraigosa to release in May 20081), a Water Supply Action Plan for the city2.  The plan represents an investment of more than $1.5 billion3 in infrastructure, water recycling, and conservation programs.




   










The Mayor’s plan includes exploration of opportunities for groundwater storage along the Los Angeles Aqueduct, in the Owens Valley and the Antelope Valley; the pursuance of a groundwater conjunctive use storage project in the L.A.  County groundwater basins; and the construction of a connection between the LAA and the California Aqueduct, located at their intersection point in Antelope Valley.  The interconnection, whose design is nearly complete and whose construction is planned for the present year, will allow water transfers and could be used to move water for groundwater storage.

The City of Los Angeles’ dependence on snowpack for water supply is mirrored by urban centers throughout the Western United States where over 70% of annual runoff originates from snowmelt (Serreze et al., Water Resources Research vol.35 no.7, 1999).  Indeed, such dependence on snowpack is shared by a full one-sixth of the world’s population (Barnett et al., Nature vol.438, 2005).  This is the reason why one of the most significant potential impacts of climate change is the loss of snowpack and a shift towards earlier annual melt time.

Aside from efforts to decrease water use through conservation and reuse/recycling, development of storage infrastructure to help make up for the loss of natural snow storage, and the development of infrastructure facilitating flexible water management are key adaptation strategies to investigate the changing hydroclimate.  Our project entailed  

1) Selection of the most current projections of future climate (16 GCMs from CMIP3 and greenhouse gas emissions scenarios A2 and B1), the downscaling of these projections (1/8-deg resolution), and use of a hydrologic model to simulate the resulting snowpack accumulation, snowmelt runoff and its timing; and a water quality model to simulate outcomes in LADWP’s Eastern Sierra Watershed,

2) Analysis of the response of the existing surface water system connecting the Owens Valley to Los Angeles, to each of the scenarios in (1)

3) Development of alternative adaptation options, and of objective evaluation criteria to compare among different options.

The climate scenarios in (1) have large and unquantifiable uncertainty and do not represent predictions, but are considered to be plausible under the current state of knowledge. Our results indicate future increases in the fraction of precipitation falling as rain rather than snow, from a historical value of about 20% to 20-30% by mid-century and 28-52% by end of century (depending on the GCM) for emissions scenario A2. As a result, the snowpack’s peak snow water equivalent (SWE) is projected to decline by most GCMs; and to shift toward earlier dates (by a few days by mid-century and by a GCM-average of 2 weeks by end of century under emissions scenario A2). The diminished SWE, earlier SWE peak and earlier melt associated with rising temperatures result in earlier hydrograph peaks, a shift in the date marking the passage of half of the year’s hydrograph volume (by more than one month by end of century for some GCMs under A2), and declining dry-season streamflows.

End-of-century projections
Figure: Projected changes in timing (x-axis) and magnitude (y-axis) of the center of mass of climatic and hydrologic variables, from the historical period (1950-1999) to the late 21st century period (2070-2099), for 16 GCMs under emissions scenario A2 (from Costa-Cabral et al., submitted)

Footnotes:

1.  May 2008 comes one year after the 2007 Mayor's GREEN LA Action Plan, which called for water conservation and recycling as well as smaller carbon footprint for the city. That year of 2007 saw the lowest Sierra Nevada snowpack on record.
2. Read the Mayor's Water Supply Action Plan here
3.  According to the plan's May 15, 2008 press release, funding is provided by a combination of fees on industrial polluters, grants, and already budgeted LADWP funds.

Publication:

Costa-Cabral M, Roy S, Maurer EP, Mills WB, Chen L  (2012) Snowpack and runoff response to climate change in Owens Valley and Mono Lake watersheds. Climatic Change.
doi: 10.1007/s10584-012-0529. The final publication is available from springerlink.com link
1. download paper pre-print
2. download supplementary materials