The 20-square-mile Winchester structural subbasin is an alluvium-filled paleocanyon that is as much as 900 feet deep. The alluvial aquifer is composed of detrital material that generally ranges in size from clay to fine gravel; the fine and coarse materials are mixed in some places and inter- bedded in others. The apparent lenticularity of fine- and coarse-grained materials and differing water quality with depth indicate that the aquifer is partly or locally confined. A ground-water divide exists east of the town of Winchester. West of the divide, ground water moves toward and into the South Perris and the Menifee subbasins. East of the divide, ground water moves toward and into the Hemet subbasin. The components of flow direction in the Winchester?Hemet subbasins border area are complex: along the border, some water moves from the southwest corner of the Hemet subbasin into the Winchester subbasin and then eastward subparallel to the border before moving back into the Hemet subbasin. The direction of ground-water movement between the Winchester and Hemet subbasins, and the position of the ground-water divide in the central part of the Winchester subbasin, have changed with time. Prior to about 1974, ground water moved both eastward from the divide and westward from the Hemet subbasin toward a local depression of the water table caused by pumping in the eastern part of the Winchester subbasin. Comparison of spring 1970 and spring 1993 ground-water levels indicates a net rise of as much as 150 feet in the east end of the Winchester subbasin. For this same period, water levels rose about 3 to 20 feet in the western and central parts of the subbasin. Ground-water chemistry in the Winchester subbasin and adjacent subbasins varies areally and vertically. In general, sodium, calcium, chloride, and sulfate are dominant ions. Water quality is generally poor: dissolved-solids concentration exceeded 2,000 milligrams per liter throughout much of the subbasin and was highest west of the town of Winchester. Eastward along the subbasin axis (toward the Hemet subbasin), the dissolved-solids concentration decreases and the pH increases (generally greater than 7.0). Samples from two multiple-well monitoring sites at the west and east ends of the subbasin indicate that the best quality water (dissolved-solids concentrations of 395 and 483 milligrams per liter) is from the deepest wells (perforated near the alluvium- bedrock contact). Samples from the deeper wells in the eastern part of the Winchester subbasin are similar in water type to a sample from a well in the western part of the Hemet subbasin, which suggests that the water may have flowed from the Hemet subbasin; alternatively, the chemistry may reflect the influence of good-quality water flowing from the fractured bedrock basement to the alluvium in the eastern part of the Winchester subbasin. In addition, the potential problem of poor-quality water moving from the Winchester subbasin into the Hemet subbasin may not exist at all depths; fair- to good-quality water may be present below a depth of about 450 feet. Dissolved-solids concentrations in the southwest part of the Hemet subbasin ranged from about 900 milligrams per liter at well 5S/1W-19Q1 about one-quarter mile north of the Winchester?Hemet subbasin boundary to about 3,500 milligrams per liter at well 5S/2W-24C2 near the bedrock outcrops southeast of the Lakeview Mountains. High dissolved-solids concentration in the vicinity of well 5S/2W-24C2 most likely is a result of dissolution of constituents from the aquifer matrix, evaporative processes, and agricultural practices that occur in that vicinity rather than a result of flow from the Winchester subbasin. Aquifer-test results indicate that the transmissivity is about 950 feet squared per day in the eastern part of the Winchester subbasin near the boundary with the Hemet subbasin and about 72 feet squared per day in the western part of the subbasin near the boundary with th