Hydrologic data collected during the climatically near-average 6-year period of inventory 1957-62 provide the basis for making estimates of supply to and demand from the Lompoc subarea of the Santa Ynez River basin and changes in the quantity of water stored in the deposits of the basin. The hydrologic inventory presents gains or accretions to the water supply, as inflow, equated with loss by water demands, as outflow. The difference is the ground-water storage change. Items of inflow include precipitation, surface and subsurface inflow, irrigation return, and sewage effluent. Items of outflow include surface and subsurface outflow, evapotranspiration, and water pumped for irrigation and other uses. Ground-water storage changes occurred as depletions of the shallow water body beneath the eastern and central parts of the Lompoc plain and beneath the upland part of the Lompoc subarea to the north and east. The estimated annual depletion of storage averaged 3,000 acre-feet. A near balance between inflow and outflow is indicated by a calculated difference of 5,000 acre-feet of accretion and an observed depletion of ground water in storage of about 3,000 acre-feet. The difference of 8,000 acre-feet between the two values, considering the magnitude of total inflow and outflow--110,000 and 105,000 acre-feet, is not significant. For the period of inventory, more water was discharged from the basin by flow in the Santa Ynez River than by pumping from wells. The near balance between inflow and outflow for a period of near-average climatic conditions, in general, substantiates a previous estimate that perennial pumpage is as much as about 20,000 acre-feet. However, water in storage will be depleted if the progressive change in the ratio of irrigation pumpage to other pumpage continues. Even though the hydrologic balance is maintained, changes in chemical quality of the ground water, due to recycling of irrigation water and inflow of poor-quality connate water from the consolidated rocks, indicate that chemical equilibrium has not been reached. Perennial supply under the 1957-62 conditions of inventory is estimated to be between 24,000 and 26,000 acre-feet.