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The purpose of the emergent discipline of ecohydrology is to generate knowledge to understand processes that are fundamental to ecosystems in terms of the dynamics of the hydrological cycle. During the rainy season, which coincides with high temperatures in semi-arid zones, a variety of ecological processes are triggered involving land surface-atmosphere water exchange through evapotranspiration (ET). Although different methodologies exist to calculate ET, it is complicated to identify the proportion of its components, evaporation in soil (ES) and transpiration of the vegetation (T) at congruent scales. The objective of the present work is to identify the ratio T/ET for one day during the rainy season in a semiarid ecosystem in northwestern Mexico using stable isotopes as tracers of different components of ET. The value of the T/ET ratio was 59 +/- 6% on July 24, 2007, but a significant variation was shown between the morning and the afternoon, decreasing from 86 +/- 21% in the morning to 46 +/- 9% in the afternoon. These results indicate that the vegetation is more active in the morning, contributing more to ET than in the afternoon. By using stable isotopes, ET can be separated into its components at the ecosystem level, thereby contributing to knowledge about the ecohydrology.