Stable isotopic compositions of locally derived and imported surface and groundwaters in southwestern California provide a quantitative background for assessment of a variety of hydrologic and meteorologic questions. Geographically controlled meteoric water isotope-ratio variations in the maritime climate zone of southwestern California show best fit ΔD dependence on latitude (-2.0? per degree), distance inland (-21? per 100 km), and elevation (-10? per kilometer). Meteoric and evaporative fractionations provide distinctive isotopic signatures that permit tracing of transported waters, as well as identification of watersheds for those precipitated locally. This regional framework of evaporative and meteoric isotope fractionation trends permits interpretation of local recharge and subsurface hydrology in the San Jacinto region. Similar techniques may be applied to other regions in southwestern California where differing source watersheds, surface or subsurface transport, and mixing provide significant variations in groundwater isotopic composition. ¿ 1997 American Geophysical Union