Comparison of the Hargreaves and Samani equation and the Thornthwaite equation for estimating dekadal evapotranspiration in the Free State Province, South Africa

• Hargreaves and Samani versus Thornthwaite model in the Free State,South Africa.
• Evapotranspiration estimates from these models were evaluated against Penman Monteith.
• 8 Climate stations widely spread over the Free State region was used in the analyses.
• Hargreaves and Samani equation result in a better estimation.

Reference evapotranspiration (ET) is an important parameter that needs to be estimated accurately to enhance its utility in numerous applications. Although the widely recommended procedure for calculating this index involves using the FAO Penman–Monteith equation (ETo), the latter’s effectiveness is constrained by its considerable data requirements. To overcome this constraint, alternative methods using the limited data available have to be explored. In this study the ability of the Hargreaves and Samani (ETHS) and Thornthwaite (ETT) equations to estimate ET was investigated using multi-year data (1999–2008) from eight weather stations in the semi-arid Free State Province of South Africa. Results for non-calibrated equations are closely correlated, with ETHS tending to underestimate ET for the July to December period while ETT underestimates ET for all months of the calendar year. Although estimates from calibrated equations are also closely correlated, they have smaller deviations compared to the original equations with the calibrated Hargreaves and Samani equation (ETCHS) estimating reference evapotranspiration better than its calibrated Thornthwaite (ETCT) counterpart. The former’s better performance suggests that in data-scarce areas, the Hargreaves and Samani model is capable of giving results within acceptable ranges of accuracy.