This data package contains pre-dawn water potential data for leaves collected from mesquite shrubs and black grama grasses at the Jornada Basin LTER site in 2007-2008. The purpose of this study was to quantify ecophysiological performance of shrubs and grasses in the context of precipitation manipulations. One mesquite and one black grama leaf from each plot (n = 20 plots; 5 PPT treatments X 4 replicate plots/treatment) were measured 5 times beginning July 2007 and ending in August 2008. The precipitation treatments consisted of +/-80%, +/-50%, and ambient/control. Leaves were excised from plants and immediately measured for water potential (MPa) with a pressure chamber at least 0.5 h before dawn. The data set is complete.