Evapotranspiration (ET) is an important variable in the hydrologic cycle. As such, improved understanding of the spatial and temporal variability of ET is critical to weather and climate analysis and prediction, water management resources, agriculture, land-use and land-change projects, and ecological research. Eddy covariance flux towers were deployed over grasslands and winter wheat at the United States Department of Agriculture site near El Reno, Oklahoma and at the Marena Oklahoma In-Situ Sensor Testbed (MOISST). Ten total years of surface energy, water, and carbon fluxes were paired with fifty total years of data from twelve additional flux towers across the Southern Great Plains (SGP) for a regional land-surface analysis using the Breathing Earth System Simulator (BESS). BESS is a land-surface model that couples land-atmosphere processes to estimate ET and gross primary production (GPP). The study results show that BESS and the observations yield good agreement with R2 values of 0.74. Further, BESS outperformed other ET estimates including a two-source surface energy balance model (Gowda et al. 2013), an empirical model (Wagle et al. 2016), and the MODIS ET product. ET decreases from southeast to northwest across the SGP, ranging from 300-1000 mm/year. ET varies more in the southwest portion of the SGP (50-100 mm/year) and less in the northeast (10-40 mm/year). Using BESS to analyze long-term ET, it was determined that, on average, 74% of the precipitation received in the SGP is re-distributed by ET. However, the results also noted that BESS lacks the ability to accurately depict ET patterns during flash drought conditions, as seen during 2012, but can depict drought and pluvial conditions when soil moisture and near-surface atmospheric conditions are in equilibrium.