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Although dew is an unimportant source of moisture in humid areas, plants and arthropods living in some arid regions depend on it for survival. Further, this moisture is occasionally available during the dry summer months when plants are experiencing the greatest stress. To assess dew contribution to the annual water balances in semi-arid desert valleys, we conducted two experiments; one in a heterogeneous semi-arid desert valley in north-eastern Nevada, U.S.A. (Site A), and the other one in an irrigated homogeneous alfalfa (Medicago sativa L.) field of a semi-arid desert valley in central Utah, U.S.A. (Site B). Both sites are located in the Great Basin, Western U.S.A. Site A is considered a closed desert valley with precipitation as the only source of water. There is no supply of water from adjacent areas or from below the root zone (due to the existence of hard pans). On the other hand, precipitation, irrigation, and a limited amount of ground-water are sources of water at Site B. Results indicate the cumulative amount of evapo-transpiration (ETa) at the end of the 1993-1994 water year (starting 1 October 1993) at Site A was greater than the amount of precipitation (about 160 mm year(-1) vs 131 mm year(-1)). Knowing that Site A is a closed desert valley, the only source of additional ETa (about 29 mm) is related to the formation of dew or trace precipitation (which is not recorded). Our analyses show that dew contributed about 14 mm to the annual water balance of the semi-arid desert valley at Site A. The cumulative ETa, precipitation, and irrigation amounted to about 1004 mm year(-1), 320 mm year(-1), and 595 mm year(-1), respectively, during 1996 (starting 1 January) at Site B. The ETa was about 89 mm greater than (precipitation + irrigation) at this site. Dew and frost contribution to the annual water balance at this site nas about 29 mm year(-1). Contribution from the ground-water (about 50 mm year(-1)) and trace precipitation closed the water balance at this site. The proposed study can be applied to any ecosystem to assess dew (amount, duration, frequency) and its impacts on plants and animals lives.