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In arid and semiarid lands worldwide, vegetation cover is being transformed to savannas dominated by exotic grasses. However, the ecohydrological and biogeochemical changes associated with this process remain little understood. For more than 3 years, we studied soil moisture dynamics in natural semiarid shrubland patches and compared those with changes in an adjacent site that had been transformed to an exotic buffelgrass (Cenchrus ciliaris) savanna. We found that volumetric soil moisture was higher in patches with some kind of cover. For both tree and bare patches, volumetric soil moisture and plant-available water in the 60- to 100-cm depth interval were significantly lower in the savanna than in the natural site. In buffelgrass patches, volumetric soil moisture and plant-available water were higher at almost all depths and in wet years and increased significantly during the summer monsoon season, suggesting rapid infiltration mechanisms under buffelgrass. During years with wet winters, these mechanisms also led to brief accumulations of water in the soil profile (0-200 cm). Such changes in soil profile water dynamics brought about adaptive responses in perennial plant species. The ability of species to change their patterns of soil water use in the profile may be an important mechanism enabling them to compete when novel tree-grass interactions are established by invasion of grasses. Our results provide new insights for understanding the changes caused by grass invasion in arid and semiarid lands, as well as how these changes may impact ecohydrological dynamics in global change scenarios. Copyright (C) 2016 John Wiley & Sons, Ltd.