bcauto: Auto-calibrate Bristow-Campbell model In sirad: Functions for Calculating Daily Solar Radiation and Evapotranspiration

Description

Function estimates Bristow-Campbell model coefficient 'B' based on auto-calibration procedure

Usage

 1 2 bcauto(lat,lon,days,extraT=NULL,Tmax,Tmin,tal,BCc=2, BCb_guess=0.13,epsilon=0.5,perce=NA,dcoast=NA)

Arguments

 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. BCc Bristow-Campbell model coefficient 'C' usually equaled to 2. BCb_guess Assumption of Bristow-Campbell coefficient. Default set to 0.13. epsilon A value of which potential radiation is decreased. See "details". perce Percent of clear days. In 'NA' then perce is estimated based on the Cloud Fraction Cover map. dcoast Distance to the coast [km].

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 Bristow and Campbell model with default values of B = 0.13 and tal = 0.72 and we select those days for which estimated daily solar radiation is the closest to the potential values (extraterrestrial*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 B coefficient 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.

Value

 BCb Bristow-Campbell 'B' coefficient

Author(s)

Jedrzej S. Bojanowski

References

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.

'bc' to use Bristow-Campbell model, and 'bccal' to perform calibration based on reference data.

Examples

 1 2 3 4 5 6 7 8 9 10 data(Metdata) tmax <- Metdata\$meteo\$TEMP_MAX tmin <- Metdata\$meteo\$TEMP_MIN days <- Metdata\$meteo\$DAY lat <- Metdata\$LATITUDE lon <- Metdata\$LONGITUDE rad_mea <- Metdata\$meteo\$RAD_MEA dcoast <- Metdata\$DCOAST bcauto(lat,lon,days,extraT=NULL,tmax,tmin,perce=NA,dcoast)

Example output

BCb_auto
0.1759843

sirad documentation built on May 1, 2019, 7:42 p.m.