Abstract
Precise quantification of reference evapotranspiration (ETo) is crucial for calculating crop water demand. Eight empirical methods based on temperature and six on solar radiation were evaluated against Penman-Monteith FAO 56 method based on: Mean Bias Error (MBE), Root Mean Square Error (RMSE), Willmott coefficient (“d”), determination coefficient (R2) and the Student’s t-test. The meteorological data of Vilankulo district (maximum, minimum and medium temperature, relative humidity, wind speed and sunshine hours) were used and collected in the National Institute of Meteorology of Mozambique from 1979 to 2006. The results showed that Mak solar radiation method had the best efficiency (MBE = -0.03 mm day-1; RMSE = 0.28 mm day-1; "d" = 0.97 and R2 = 0.98). When sunshine hours or global solar radiation are not measured in order to use Mak method, Schendel method can be an alternative which requires air temperature and relative humidity (MBE = -0.09 mm day-1; RMSE = 0.81 mm day-1; “d” = 0.84 and R2 = 0.74). Both methods were not statistically different with PMF 56 method. The merit of this study stems from the fact that no similar study was conducted in Vilankulo district. Key words: Reference evapotranspiration, empirical methods, Penman Monteith.  
Highlights
Agricultural activities demand large volumes of water, making their scarcity more and more worrying
In terms of absolute value of Mean Bias Error (MBE), the Sch method presented the closest value of zero (MBE = 0.09 mm day -1 = 1.8%), showing the tendency to be the best method for estimating ETo in the district of Vilankulo
The worst MBE result was observed in the Mc Cloud (McC) method (MBE = 1.97 mm day-1 = 39.4%)
Summary
Agricultural activities demand large volumes of water, making their scarcity more and more worrying. Water economics in agriculture can be obtained accurately by estimating crop evapotranspiration (ETc). According to Allen et al (2011), precise measurement of ETc is obtained using lysimeters or through imaging techniques. Valipour (2015) mentioned that both techniques carry high costs. Due to the high costs to obtain ETc, it is necessary to calculate them from reference evapotranspiration (ETo) and crop coefficient. Quite complex calculations are involved in this method
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