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Surface waters, shallow groundwaters, and precipitation samples were collected along three traverses, which begin at the California coast and are oriented in an east–west manner, roughly parallel to the atmospheric flow path of meteoric water. The hydrogen isotope compositions from short-term precipitation records varied as much as 40‰ at one location and were not representative of the average annual isotopic composition of precipitation. The hydrogen isotope ratios of surface waters and shallow groundwaters distinguish segments with isotopic variations that correlate with measured vertical fluxes of meteoric water. One coastal segment appears to closely approximate an open system in which precipitation dominates over evapotranspiration. Most segments are described by rather regular variations in δD: 3–45‰ per 100 km, which represent different degrees of closure of the hydrologic system caused by variable partitioning of precipitation between runoff and evapotranspiration. All partially closed systems imply terrestrial recycling of water. A simple model estimates this amount to be about 20% across northern and central California. Three segments, representing the Great Basin, show virtually no geographic variation and appear to represent isotopically closed systems.