Comparison of the Hargreaves and Samani equation and the Thornthwaite equation for estimating dekadal evapotranspiration in the Free State Province,South Africa |
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| Institution: | 1. Agricultural Research Council-Institute for Soil, Climate and Water, Private Bag X79, Pretoria 0001, South Africa;2. Department of Soil, Crop and Climate Sciences, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa;3. Department of GIS and Remote Sensing, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa;4. Risks and vulnerability assessment center, University of Limpopo, Private Bag x 1106, Sovenga 0727, South Africa;1. State Key Laboratory of Hydraulics and Mountain River Engineering & College of Water Resource and Hydropower, Sichuan University, Chengdu, China;2. State Engineering Laboratory of Efficient Water Use of Crops and Disaster Loss Mitigation/Key Laboratory for Dryland Agriculture, Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agriculture Sciences, Beijing, China;1. Centro de Investigação em Agronomia, Alimentos, Ambiente e Paisagem (LEAF), Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal;2. Departamento de Producción Agraria, ETSIAAB, Universidad Politécnica de Madrid, UPM, Avd. Puerta de Hierro, 2, Madrid 28040, Spain;1. Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences/Key Laboratory of Dryland Agriculture, MOA, 100081, Beijing, China;2. Beijing Research & Development Center for Grass and Environment, 100097, Beijing, China |
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| Abstract: | 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. |
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| Keywords: | Calibration Evapatranspiration models Penman–Monteith equation Temperature |
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