Ecosystem Evapotranspiration (ET) is one of the main components in the water cycle affecting the local-to-regional water balance. Furthermore, ET links the long-term feedbacks between vegetation and climate including precipitation. Understanding the controls on ecosystem ET is crucial to comprehend present and potential future vegetation and water availability. This study investigated ET and the regulatory mechanisms in two semiarid shrub-dominated ecosystems in the Sierra Madre (SM, Wyoming, USA) and the Kubuqi Desert (KD, Inner Mongolia, China). The controlling mechanisms were analyzed based on ET and climate data from two eddy-covariance towers at SM and KD for the years 2004-2005 and 2006-2007, respectively. Climate variables were examined for their individual correlation with daily ET. Subsequently, the FAO Penman-Monteith model (FAO-PM syn. FAO-56) was stepwise modified to test the biophysical control of individual parameters in concert with other climate variables.
Located at opposite sites of the globe, the two ecosystems shared various similarities in vegetation and climate (e.g., mean temperature and amount of precipitation). At both sites, ET showed positive correlations with available energy (Rn - G), soil volumetric water content (VWC), and soil water potential (Ψs), but lacked positive correlation with vapor pressure deficit (VPD). Daily ET during growing seasons also correlated with air temperature (Ta) at KD, but not at SM. The important modifications to the FAO-PM included the insertion of Ψs and the removal of VPD and wind speed (u). These modifications improved daily estimates of growing season ET as they reduced deviations between modeled and measured ET to 1-33%. Model tests corroborated that available energy and soil water potential were the main drivers of ET during growing seasons. VPD and u influenced ET mainly after precipitation events and at low Ψs (i.e., Ψs close to zero). The conditional sensitivity of ET to VPD during growing seasons may be regarded as an important difference between the control of ET in shrub-dominated and other ecosystems such as forests. This characteristic may be also considered in land management and conservation with respect to interferences that can potentially affect the water cycle and the water availability in areas that are naturally covered by shrub-dominated ecosystems.