The southern part of the Sonoran Desert has an average rainfall of only 350 mm per year and little is known about the seasonal variation in the latent and sensible heat fluxes of this area. Hourly measurements of the energy budget of rangeland vegetation were made by an eddy correlation system from March to October 1996, covering the dry season , the wet season (from July to September) and the following dry down season. The measurements showed that the evaporative fraction was 10-20% during the dry season rising to 30-70% in the wet season. The difference was studied in relation to differences in evaporative demand and surface resistance.