Applying many existing empirical or analytical relationships to estimate actual evapotranspiration in national-scale studies is constrained by the need for extensive meteorological and environmental data. This paper aims to develop a physically informed empirical relationship for calculating annual actual evapotranspiration at the national scale, using two annual climatic variables (precipitation and temperature) alongside two spatially constant characteristics (slope and geographic coordinates). The proposed relationship was developed using data from 1,185 hydroclimatological units, thirty secondary-order basins, and the national level. A key feature of the proposed relationship is the substitution of potential evapotranspiration with mean annual temperature as a proxy for energy capacity; this substitution eliminates the need for extensive climatic data. Furthermore, the effect of slope (topography) is incorporated, and to refine the temperature effect, a temperature sensitivity coefficient is used as spatial descriptors of the observed temperature-sensitivity pattern. Using the national water-balance reference estimates, the results indicate that, at the national scale, the proposed relationship exhibits a volumetric error of 0.016% in estimating actual evapotranspiration. At the scale of secondary basins, the Mean Absolute Relative Error is 4.69%. Furthermore, at the scale of hydroclimatological units, a coefficient of determination of 0.833 was obtained.