hauto: Auto-calibrate Hargreaves model
In sirad: Functions for Calculating Daily Solar Radiation and Evapotranspiration

Description Usage Arguments Details Value Author(s) References See Also Examples

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

1 2	hauto(lat, lon, days,extraT = NULL, Tmax, Tmin, tal, Ha_guess = 0.16, Hb_guess = 0.1, epsilon=0.5, perce = NA)

`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'

data(Metdata)
Tmax <- Metdata$meteo$TEMP_MAX
Tmin <- Metdata$meteo$TEMP_MIN
days <- Metdata$meteo$DAY
lat <- Metdata$LATITUDE
lon <- Metdata$LONGITUDE
hauto(lat,lon,days,extraT=NULL,Tmax,Tmin,tal=0.76)