sirad (version 2.0-2)

hauto: Auto-calibrate Hargreaves model

Description

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

Usage

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

Arguments

lat

Latitude in decimal degrees.

lon

Longitude in decimal degrees.

days

Vector of class 'Date' of length n.

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.

Value

• Vector of length 3 containing:
• Ha
  Hargreaves 'A' coefficient
• Hb
  Hargreaves 'B' coefficient
• Hr2
  Coefficient of determination of performed linear regression

Details

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.

References

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

See Also

'hacal'

Examples

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,Tmax,Tmin,tal=0.76,Ha_guess=0.16,Hb_guess=0.1,epsilon=0.5,perce=NA)