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Seasonal ability to use precipitation by three shrub species with different root systems (dimorphic: Balbisia peduncularis and Senna cummingii; deep: Haplopappus parvifolius) was evaluated in Quebrada El Romeral, north-central Chile. The site has an and Mediterranean climate with coastal fog influence and an annual mean precipitation of 78 mm during the last 30 years. The use of artificial rain (i.e.. 25 mm in fall, winter and spring) by plants was estimated by the hydrogen isotope composition (delta(2)H) in the xylem water and the pre-dawn water potentials (psi(Pa)) before and after watering. The results show that the three species use a mixture of two water sources (shallow water coming from rainfall and a greater proportion of subterranean water). Except in winter, the species with dimorphic roots are able to reduce their water deficits after the artificial watering. The decrease in rainfall observed for the last 100 years would affect the productivity of the species with dimorphic root systems, especially the forage shrub Balbisia peduncularis.