Climate Change Study on the Los Angeles Aqueduct System

CLIENT:  Los Angeles Department of Water and Power

Precipitation from the Eastern Sierra Nevada is a main water source, and one of the highest quality sources, for Los Angeles' more than 4 million people. Winter precipitation is stored as a natural reservoir in snowpack, and the resulting meltwater (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). Future availability of this water supply source under climate change is of critical importance to the city's growing population and large economy.

NHC climate hydrologist, Mariza Costa-Cabral (under her previous sole proprietorship company, Hydrology Futures), was contracted by Tetra Tech, Inc. to develop an approach for evaluating climate change scenarios. She developed and applied a distributed hydrologic model for the Owens Valley and Mono Lake watersheds under future climate scenarios, and analyzed the results in ways useful to LADWP.

The Variable Infiltration Capacity (VIC) hydrologic model was applied to the Eastern Sierra watersheds, and calibrated using observations-based estimates of daily temperature, precipitation, and wind over a 12-km spatial grid, and monthly unimpaired stream flow data provided by LADWP for 1950-2005. The calibrated VIC model was then run at daily time step over the 150-year period from 1950 to 2099, using as input 32 climatic time series (produced by 16 global climate models for 2 different emissions scenarios). The modeling produced a range of plausible future hydrological outcomes, most of which predicted declining snowpack. As a result of these projections, LADWP is able to evaluate the potential severity of future declines in snow water equivalent and summer water shortages from this key water source.