Reference evapotranspiration (ET_o) has a key role in irrigation scheduling. In this sense, the Hargreaves–Samani equation (HS) is a reliable and widely used method to estimate ET_o. The HS equation just requires temperature and solar radiation data, making it a suitable method for places that lack of wind speed and relative humidity information. However, literature shows that a local calibration of its empiric parameter is needed for its complete application. This work shows a calibration for the Maule region in central-southern Chile. For this purpose, the Penman–Monteith equation from FAO-56 (PM) was considered as a reference, using a network of 400 meteorological stations between the 32^° and 39^° of south latitude for the 1973–2011 period. The calibration was based on the computation of the ratio of ET_o calculated by HS and PM and the spatial behaviour of input variables and parameters. The spatial distribution was done by geographical weighted regression and ordinary Kriging with a linear variogram, assisted by a digital elevation model from the Shuttle Radar Topography Mission and surface reflectances from Moderate Resolution Imaging Spectroradiometer. The process of calibration was validated with daily data through all months, with comparative errors of 5% against PM.