Digital model of the unconsolidated aquifer system in the Modesto area, Stanislaus and San Joaquin Counties, California

A digital mathematical model of the unconsolidated alluvial aquifer system in the Modesto area, Calif., has been developed which can be used to determine the effects of increased pumping and water use of future water levels in the aquifer system. The model is divided into two units. The lower unit is confined in the western part of the study area by a confining clay bed; elsewhere in this unit the aquifer is unconfined. The upper unit represents an unconfined aquifer and lies above the clay bed or its extension. Where the clay bed is absent the upper and lower units are considered as a single aquifer. The model, as calibrated, can evaluate with reasonable accuracy the effects on water levels of changing stresses and stress patterns only within the area of primary interest for the upper unit. In other areas of the upper unit and for the lower unit, predicted changes should be looked upon as, at best, representing only general trends. (USGS)