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Groundwater flow modeling of the Ojai basin using the USGS 3 dimensional MODFLOW model
A groundwater flow model designed by the United States Geological survey is applied to the Ojai basin which is located in Ventura County, California. The model is calibrated using field data during an eight year period (1976-1984). After model credibility is established, it is used to predict the effects of drought, flood, and safe pumping levels for the basin. The Ojai basin covers approximately 5, 729 acres and consists of water bearing deposits ranging in thickness from approximately 100 to 400 feet. Pleistocene and Recent alluvial sands and gravels comprise the water bearing units, with specific yields ranging from 2.9% to 7.3%. A hydrologic balance for the basin is established in order to determine modeling inputs which include pumping and recharge. Groundwater storage in the basin is approximately 50,000 acre-feet, with yearly changes ranging from approximately 1, 000 acre-feet to 11,000 acre-feet. Groundwater in the Ojai basin is used for irrigation of agricultural land and domestic use in the city of Ojai. For the modeling procedure, the Ojai basin covers 5,278 acres which are divided into 14 cells consisting of 377 acres each. Modeling inputs include specific yield of each cell, permeability, recharge, pumping, elevation of the base of water bearing units, and the elevation of the water table. These parameters are obtained from previous studies, hydrologic data and well records. The model utilizes the finite difference method to calculate water levels in the basin based on the boundary conditions as well as the pumping and recharge stresses acting on the basin. The model was calibrated against actual field measurements for the 1976 through 1984 period. Average deviations between calculated and observed water levels were approximately 5 feet. The model was then used as a predictive tool, predicting water levels during a hypothetical drought and flood. Modeling can also be used to assist in the effective management of groundwater in the region by predicting safe yield pumping levels. It is found that for the model to be used to predict water levels in a basin, a hydrologic balance is essential for predetermining model inputs. Large margins of error in some of the elements of the hydrologic balance require caution be taken when analyzing modeling results. Uncertainties in pumping, subsurface outflow, and precipitation presented the largest source of deviation. Deriving more detailed parameters of the hydrologic balance is suggested for future studies.