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Understanding the regulation of ecosystem ET is crucial to comprehend present and potential future vegetation and water availability in arid environments. We compared ET data from eddy-covariance flux measurements in two semiarid shrub-dominated ecosystems. Located in the Kubuqi Desert (KD, Inner Mongolia, China) and the Sierra Madre (SM, Wyoming, USA), the ecosystems shared similarities in latitude, climate and vegetation. We analyzed ET at KD and SM for the years 2006-2007 and 2004-2005, respectively, and tested regulatory functions of climate variables through modifications to the widely used FAO-Penman-Monteith model. Important modifications included the insertion of soil water potential (Psi s) and the removal of vapor pressure deficit (VPD) and wind speed (u). The modifications improved daily estimates of growing season ET as they reduced deviations between model and measurement to 1-33%. According to model results, available energy (R-n - G) and Psi s were the main drivers of ET during growing seasons. VPD and u influenced ET mainly after precipitation events and at low Psi s (i.e., Psi s close to zero). Although our empirical model still requires validation through data from other sites, the results suggest that the regulatory power of Psi s and the conditional sensitivity to VPD are characteristic of ET in shrub steppes. (C) 2010 Elsevier Ltd. All rights reserved.