Function estimates Hargreaves model coefficients 'A' and 'B' based on autocalibration procedure

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`lat` |
Latitude in decimal degrees. |

`lon` |
Longitude in decimal degrees. |

`days` |
Vector of class 'Date' of length n. |

`extraT` |
Optional. Vector of length n of extraterrestrial solar radiation [MJm-2]. If 'NULL' then it is calculated by the function. Providing extraterrestrial solar radiation speeds up the computation |

`Tmax` |
Vector of length n containing daily maximum temperature [C]. |

`Tmin` |
Vector of length n containing daily minumum temperature [C]. |

`tal` |
Clear sky transmissivity. |

`Ha_guess` |
Assumption of Hargreaves Ha coefficient. Default set to 0.16. |

`Hb_guess` |
Assumption of Hargreaves Hb coefficient. Default set to 0.1. |

`epsilon` |
A value of which potential radiation is decreased. See "details". |

`perce` |
Percent of clear days. Default set to 1. |

The auto-calibration method bases on the assumption that on the clear-sky days model should not overpredict potential values. To define those clear-sky days, we estimate daily solar radiation using Hargreaves model with default values of A = 0.16, B = 0.1 and tal = 0.72 and we select those days for which estimated daily solar radiation is the closest to the potential values (extra-terrestrial*tal). The number of clear-sky days is estimated based on the mean Cloud Fraction Cover map. Next, based on selected clear-sky days, we perform a non-linear least squares regression to derive A and B coefficients treating potential values decreased by 'epsilon' as a reference solar radiation values. The analysis of auto-calibration results showed clear correlation between optimal 'epsilon' and distance to the coast. We proposed simplified method in which 'epsilon' is equal to 0.1 MJm-2 or to 0.5 MJm-2 when distance to the coast is smaller or bigger than 15 km respectively.

Vector of length 3 containing:

`Ha` |
Hargreaves 'A' coefficient |

`Hb` |
Hargreaves 'B' coefficient |

`Hr2` |
Coefficient of determination of performed linear regression |

Jedrzej S. Bojanowski

Hargreaves, G.H., Samani, Z.A. 1892. Estimating potential evapotranspiration. J. Irrig. Darin. Eng., ASCE 108 (3), 225-230. Bojanowski, J.S., Donatelli, M., Skidmore, A.K., Vrieling, A., 2013. An auto-calibration procedure for empirical solar radiation models Environmental Modelling and Software 49, 118-128.

'hacal'

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