The objective of this study was to estimate, using an empiric formula, the daily reference evapotranspiration in the city of Campina Grande, Paraiba State, Brazil (EToT), using the daily thermal amplitude. The ETo was also estimated using the methods of FAO-Penman-Monteith (EToPM), Hargreaves (EToH) and Linacre (EToL), being correlated later with EToT proposed in this research by season. For that, daily values of temperature, air relative humidity, solar radiation, precipitation, atmospheric pressure and wind speed at 2 m, were taken using the Automatic Meteorological Station installed in the Universidade Federal de Campina Grande (UFCG), over the period of January 1 to December 31, 2004. These values were registered at intervals of 10 min. It was verified that the correlations of EToT with EToH had better adjustments where the coefficients were: R² = 0.7882 (for the year), R² = 0.9796 (for the summer), R² = 0.8253 (for the autumn), R² = 0.878 (for the winter) and R² = 0.914 (for the spring). The methods of EToPM and EToL did not adjust well for Campina Grande, which indicates that EToT can be used there in the form proposed for estimate of ETo. The only restriction among the proposed equations is EToT for summer, in which case the use of the annual equation is recommended.
precipitation; climatic demand; thermal amplitude
