Reference evapotranspiration (ETo) defines the maximum evapotranspiration that can occur in a basin. Its accurate estimation is therefore very important in accurately estimating how much water is available in any Basin for use since the actual Evapotranspiration (AET) that occurs depends on the ETo estimated. AET is a parameter that is difficult to measure on the field and therefore is mostly estimated from ETo. Different empirical equations are used in estimating ETo with different levels of accuracy mostly defined by the set of input data. FAO-56 Penman-Monteith (FAO-56 PM), Priestley-Taylor, and 1985 Hargreaves equation were used in computing ETo for a station in Koforidua and Accra airport and the results were compared with a measured ETo estimated from pan evaporation data. When the FAO-56 PM ETo was compared with the ETo from Priestley-Taylor and 1985 Hargreaves, the average root-mean-square errors were 0.8659 and 1.1770 mm/d respectively. The Index of Agreement (d) between FAO-56 PM ETo values and ETo values from Priestley-Taylor and 1985 Hargreaves were 0.79 and 0.59 respectively. The estimated ETo values were however improved through local calibration of the Priestley-Taylor and 1985 Hargreaves equation. The results generally indicate that the Priestley-Taylor equation better estimates ETo values from the FAO-56 PM equation than the 1985 Hargreaves equation. When ETo values were estimated with FAO-56 PM, Priestley-Taylor and Hargreaves equations were compared to measured ETo values from pan evaporation data, FAO-56 PM equation performed better followed by the Priestley-Taylor and then the Hargreaves equation. This means that if enough data is available for ETo estimation then the FAO-56 PM equation is best in estimating ETo values followed by the Priestley-Taylor equation and then the Hargreaves equation. However, where only maximum and minimum temperature are the only available data, then Hargreaves is recommended for the estimation of the ETo values.

 

Key words: Evapotranspiration, Densu basin, Hargreaves, Priestley-Taylor, penman-monteith.