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This paper compares different procedures for mapping reference evapotranspiration (ETo) by means of regression-based techniques and geographical information systems (GIS). ETo is calculated following the method of Hargreaves (HG) from a dense database of meteorological stations in the northernmost semi-arid region of Europe, the Ebro valley. The HG method requires the calculation of estimates of extraterrestrial radiation (R-a). We calculated this parameter using two approaches: (1) the common approach that assumes a planar surface and determines the parameter as a function of latitude and (2) using a digital terrain model (DTM) and GIS modelling. The maps were made on a monthly basis using both approaches. We also compared possible propagations of errors in the map calculations for maps derived from modelled layers of maximum and minimum temperatures with those modelled using previously determined local ETo calculations. We demonstrate that calculations of R-a from a DTM and GIS modelling provide a more realistic spatial distribution of ETo than those derived by only considering latitude. It is also preferable to model in advance the variables involved in the calculation of ETo (temperature and R-a) and to subsequently calculate ETo by means of layer algebra in the GIS rather than directly model the local ETo calculations. The obtained maps are useful for the purposes of agriculture and ecological and water resources management in the study area. (C) 2007 Royal Meteorological Society