# hacal: Calibrate Hargreaves model In sirad: Functions for Calculating Daily Solar Radiation and Evapotranspiration

## Description

Function estimates Hargreaves model coefficients 'A' and 'B' based on reference data

## Usage

 `1` ```hacal(lat, days, rad_mea, extraT=NULL,tmax, tmin) ```

## Arguments

 `lat` Latitude in decimal degrees. `days` Vector of class 'Date' of length n. `rad_mea` Vector of length n containing reference (e.g. measured) solar radiation [MJm-2]. `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].

## Details

Function estimates Hargreaves model coefficients 'A' and 'B' based on reference (e.g. measured) solar radiation data. It performs a linear regression.

## Value

Vector of length 3 containing:

 `Ha` Hargreaves 'A' coefficient `Hb` Hargreaves 'B' coefficient `Hr2` Coefficient of determination of performed linear regression

## Author(s)

Jedrzej S. Bojanowski

## References

Hargreaves, G.H., Samini, Z.A. 1892. Estimating potential evapotranspiration. J. Irrig. Darin. Eng., ASCE 108 (3), 225-230.

 ```1 2 3 4 5 6 7``` ```data(Metdata) tmax <- Metdata\$meteo\$TEMP_MAX tmin <- Metdata\$meteo\$TEMP_MIN days <- Metdata\$meteo\$DAY lat <- Metdata\$LATITUDE rad_mea <- Metdata\$meteo\$RAD_MEA hacal(lat=lat,days=days,rad_mea,extraT=NULL,tmax=tmax, tmin=tmin) ```