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Accurate quantification of evapotranspiration (ET) is important to understanding its role in the global hydrological cycle of terrestrial ecosystems and feedbacks to the climate system. This study improves ET quantification in North America using a data assimilation technique and a process-based Terrestrial Ecosystem Model as well as in situ and satellite data. ET is modeled using the Penman-Monteith equation with an improved leaf area index (LAI) algorithm in a biogeochemistry model, the Terrestrial Ecosystem Model (TEM). The evaluated TEM was used to estimate ET at site and regional scales in North America from 2000 to 2010. The estimated annual ET varies from 420 to 450 mm year−1 with the improved model, close to Moderate Resolution Imaging Spectroradiometer monthly data with a root-mean-square error less than 10 mm month−1 for the study period. Alaska, Canada, and the conterminous US account for 33%, 6%, and 61% of the regional ET, respectively. Water availability, the difference between precipitation and ET, is 181 mm month−1, averaged from 2000 to 2010. Under IPCC Representative Concentration Pathway (RCP) 2.6 and RCP 8.5 scenarios, the regional ET increases by 11% and 24%, respectively. Consequently, the water availability decreases in the region by 2.4% and 23.7%, respectively. For the period of 2020–2100, due to uncertain parameters, TEM versions integrated with three different ET algorithms estimated that the regional ET in the USA are 430.5 ± 10.5 mm year−1, 482.1 ± 11.2 mm year−1, and 489.7 ± 13.4 mm year−1, and the available water is − 105.3 ± 8.7 mm year−1, − 20.3 ± 11.9 mm year−1, and − 126.2 ± 15.4 mm year−1, respectively, by the end of the twenty-first century. Our analysis suggests natural terrestrial ecosystem soils in North America will get drier under future climate conditions, which will impact the regional water resource and the climate system. Based on our ET simulation under three climate change scenarios, our study suggests that the RCP 2.6 is the optimum trajectory for preserving freshwater resources in North America and other regions in the globe.