Evapotranspiration is a key hydrological process in Brazil's 1,000,000-km(2) tropical semiarid Caatinga biome, where monitoring is crucial, but data are scarce. We propose a method to estimate daily actual evapotranspiration (LE) based on the canopy-radiation balance, measuring air and canopy temperatures, relative humidity, wind speed, and global radiation. The method was applied at a location with preserved Caatinga forest [Aiuaba Experimental Basin, daily average net radiation (R-n) of 12 MJ/m(2)] during one hydrological year (2020-2021). The results, which agree with independently performed field measurements, identify the predominance of distinct evaporative processes throughout the year. In the dry season, net radiation is high (16.7 MJ/m(2)/day), but actual evapotranspiration is negligible (LE/R-n<0.01) because stress due to extremely low soil water content causes leave deciduousness and root shrinkage. Contrastingly, in the rainy season, soil water content increases, leaves recover, and roots expand, enhancing actual evapotranspiration (LE=6 mm/day and LE/R-n=0.89). Thus, in the Caatinga forest, during the dry and transition periods, actual evapotranspiration is ultimately ruled by the soil water content (source-limited), whereas during the rainy season, it is sink-limited (i.e., controlled by atmospheric demand).