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The use of winter cover crops to improve the productivity and sustainability of agroecosystems in California has increased during the last decade. Little information exists however, on water use by winter cover crops. This 3-year study was conducted in the Central Valley of California to quantify changes in water storage in winter fallowed and cover cropped soils. Soil water depletions in the surface 210 cm were determined by neutron hydroprobe during the 1991-92, 1992-93 and 1993-94 winter seasons under barley (Hordeum vulgare), vetch (Vicia dasycarpa), and barley/vetch cover crops as well as in fallowed soils. With an average input of 50 mm of supplemental irrigation water for germination and establishment each fall, cover crop growth under typical winter conditions for this region was quite vigorous: the barley and barley/vetch mixes averaged about 9,000-11,000 kg of dry matter ha(-1) and the vetch crops yielded an average of 5,600 kg ha(-1). Soil water storage at the time of cover crop incorporation in mid-March was reduced by 7.4 +/- 1.0, 7.8 +/- 1.4 and 6.5 +/- 1.1 cm by the barley, barley/vetch and vetch cover crops respectively, relative to the storage in fallowed soils over the three seasons of this work. Soil water tension gradients indicated that drainage beyond 210 cm did not occur. Although depletions in stored water under barley and vetch that are grown during the winter as cover crops were relatively small, they may impact preirrigation needs and the growth and productivity of subsequent crops. Thus, long-term water budgeting at a cropping systems level is essential to successfully integrate cover crops into crop rotations in arid and semi-arid areas.