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Daily reference crop evapotranspiration with reduced data sets in the humid environments of Azores islands using estimates of actual vapor pressure, solar radiation, and wind speed

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Abstract

Reference crop evapotranspiration (ETo) estimations using the FAO Penman-Monteith equation (PM-ETo) require a set of weather data including maximum and minimum air temperatures (T max, T min), actual vapor pressure (e a), solar radiation (R s), and wind speed (u 2). However, those data are often not available, or data sets are incomplete due to missing values. A set of procedures were proposed in FAO56 (Allen et al. 1998) to overcome these limitations, and which accuracy for estimating daily ETo in the humid climate of Azores islands is assessed in this study. Results show that after locally and seasonally calibrating the temperature adjustment factor a d used for dew point temperature (T dew) computation from mean temperature, ETo estimations shown small bias and small RMSE ranging from 0.15 to 0.53 mm day−1. When R s data are missing, their estimation from the temperature difference (T maxT min), using a locally and seasonal calibrated radiation adjustment coefficient (k Rs), yielded highly accurate ETo estimates, with RMSE averaging 0.41 mm day−1 and ranging from 0.33 to 0.58 mm day−1. If wind speed observations are missing, the use of the default u 2 = 2 m s−1, or 3 m s−1 in case of weather measurements over clipped grass in airports, revealed appropriated even for the windy locations (u 2 > 4 m s−1), with RMSE < 0.36 mm day−1. The appropriateness of procedure to estimating the missing values of e a, R s, and u 2 was confirmed.

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Acknowledgements

The first author thanks the postdoctoral fellowship (SFRH/BPD/102478/2014) provided by FCT. The support of FCT through the research unit LEAF (UID/AGR/04129/2013) is acknowledged. The support to the third author by the project PROAAcXXIs (PO Açores 01-0145-FEDER-000037) is also acknowledged. Data were provided through the PROAAcXXIs project from Secretaria Regional do Ambiente e do Mar, Azores, Instituto Português do Mar e da Atmosfera (IPMA) and from the Eastern North Atlantic (ENA) Graciosa Island facility from the Atmospheric Radiation Measurement (ARM) Program sponsored by the US Department of Energy, Office of Science, Office of Biological and Environmental Research, Climate and Environmental Sciences Division.

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Correspondence to L. S. Pereira.

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Paredes, P., Fontes, J.C., Azevedo, E.B. et al. Daily reference crop evapotranspiration with reduced data sets in the humid environments of Azores islands using estimates of actual vapor pressure, solar radiation, and wind speed. Theor Appl Climatol 134, 1115–1133 (2018). https://doi.org/10.1007/s00704-017-2329-9

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  • DOI: https://doi.org/10.1007/s00704-017-2329-9

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