Water resource partitioning among three co-occurring species of California annual grassland was investigated. Plantago erecta, Clarkia rubicunda and Hemizonia luzulifolia differ in lifespan. The lifespan of Plantago is coincident with the October-May rainy season, but the other two species reproduce during summer when no precipitation occurs, and thus depend on stored water. Field studies indicated differential access to stored water commensurate with the phenology of each species. The authors concluded that Hemizonia , the longest lived species, survives on water stored in decomposed rock below the soil layer. Clarkia is restricted to cooler slope faces where a slightly longer growing season appears just sufficient to complete reproduction.