MODFLOW-USG model used to evaluate water management issues in the Los Angeles Coastal Plain, California

The U.S. Geological Survey (USGS), in cooperation with the Water Replenishment District of Southern California, developed a regional groundwater model of the Los Angeles Coastal Plain (LACP) incorporating new data, analyses, and modeling tools to better understand the hydrogeology of the groundwater basins. The LACP covers about 580 square miles and is the largest coastal plain of semiarid southern California. This aquifer is heavily developed with mostly residential, commercial, and industrial use and relies heavily on groundwater for its water supply. There has been a heavy reliance on groundwater from the LACP for many years. The need to replenish the groundwater basins within the LACP was recognized as far back as the 1930s, when spreading grounds were first used to replenish groundwater basins and store water underground during times of water surplus to meet demands in times of shortage. Seawater intrusion resulting from fresh-water pumping became significant in the 1940s. As a result, injection of imported water through wells at what is now the West Coast Basin Barrier Project began on an experimental basis in 1951. Managed aquifer recharge from the spreading grounds and barrier wells is an important part of the LACP's groundwater supply. This study, in cooperation with the Water Replenishment District of Southern California, involved an assessment of the historical and present status of groundwater resources in the LACP and the development of tools to better understand the groundwater system. The Los Angeles Coastal Plain Groundwater Model (LACPGM) developed for this study, represents a major update of a previous groundwater model developed by Reichard and others (https://doi.org/10.3133/wri034065). The LACPGM uses the USGS model code MODFLOW-USG, which enables the use of an unstructured finite-difference grid which contains nodes one-eighth mile (mi; 660 ft) in length and width. The LACPGM incorporates 12 layers for the 13 chronostratigraphic units, with the two bottom units modeled as a single layer. MODFLOW-USG was selected for its ability to effectively represent the discontinuous and faulted chronostratigraphic layers using an unstructured grid. The LACPGM simulates groundwater flow in the LACP using quarterly stress periods from 1971 to 2015. The model was used to predict water levels under future water-management scenarios, base case wet and dry scenarios, increased pumpage wet and dry scenarios, and optimized replenishment wet and dry scenarios. The scenarios include projected future pumping rates and estimate the increases in water spreading and injection that would be needed to maintain water levels. The scenarios simulate expected future pumping rates and estimate needed increases in water spreading and injection in order to maintain water levels. This USGS data release contains all the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20215088).

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