View File - Development Services - City of Oxnard

5.0 Historical Groundwater Flow ModelingThis section provides a summary of the USGS groundwater flow model of the Santa Clara-Calleguas Basin (USGS, 2003) and the UWCD update to that model (UWCD, 2003).5.1 USGS ModelThe USGS developed the groundwater flow model of the Santa Clara-Calleguas Basin aspart of the Southern California RASA Program. The modeling was performed to meet thefollowing objectives:• Better define the hydrogeologic framework of the regional groundwater flow system• Help analyze the major problems affecting water resources management of a typicalcoastal aquifer system, including groundwater overdraft, stream flow depletion,subsidence, seawater intrusion, and groundwater contamination.As part of constructing the model, the USGS compiled geographic, geologic, and hydrologicdata and estimated hydraulic properties and flows. The study included a re-evaluation ofthe basin structure and stratigraphy of the water-bearing rocks, and evaluation of thehydrologic system under predevelopment, historical development, and future developmentconditions. The model was calibrated to historical surface water and groundwater flowconditions for the period 1891 to 1993. The calibrated groundwater flow model was thenused to simulate future groundwater conditions based on:• Proposed water supply projects in the existing management plan for the SantaClara-Calleguas groundwater basin.• Potential alternative water supply projects in the Santa Clara groundwater basin thatwere proposed to help manage the effects of increasing demand and variable supply onthe groundwater overdraft conditions5.1.1 Groundwater Model Construction and CalibrationThe model was developed to simulate groundwater flow of the two regional aquifersystems, UAS and LAS, in the Santa Clara-Calleguas Basin. The model was developed tosimulate steady-state predevelopment conditions prior to 1891 and transient conditionsfor the development period from 1891 to 1993. The model was developed using thethree-dimensional finite difference groundwater flow model (MODFLOW) developed byMcDonald and Harbaugh (1988). Additional packages were incorporated into the model toallow simulation of options not available in MODFLOW.Transient simulations were calibrated for the period of historical systematic data collection,which generally spans from the 1920s through 1993. The most important calibration periodspanned the recent period of pumpage reported to the FCGMA, which began in 1984 withthe adoption in 1983 of Ordinances No. 1 and No. 2 by the FCGMA. These ordinancesrequired that well owners with extraction facilities within the boundaries of the FCGMAW112003002SCO LW1458.DOC/ 033390002 66