/*!
* \brief Use this method to choose model for ET0 calculation automatically depending on available data.
*
* \param climate Parameter to relay to matching ET0 calculation model.
* \param date Parameter to relay to matching ET0 calculation model.
* \param location Parameter to relay to matching ET0 calculation model.
* \param et0PmArgs Relayed to Et0CalcPm()
* \param et0HgArgs Relayed to Et0CalcHg()
*
* \return An Et0Result structure.
*/
public static bool Et0Calc(
Climate climate,
DateTime date,
Location location,
Et0PmArgs et0PmArgs,
Et0HgArgs et0HgArgs,
ref Et0Result result
)
{
var climateSet = climate.getValues(date);
//check availability of values
if (climateSet == null)
{
return(false);
}
if (
climateSet.max_temp.HasValue &&
climateSet.min_temp.HasValue &&
climateSet.humidity.HasValue &&
(climateSet.Rs.HasValue || climateSet.sunshine_duration.HasValue)
)
{
//data sufficient for Penman-Monteith
return(Et0CalcPm(climate, date, location, et0PmArgs, ref result));
}
//use Hargreaves
return(Et0CalcHg(climate, date, location, et0HgArgs, ref result));
}
/*!
* \brief Calculate ET0 with Penman-Monteith method as described in FAO paper 56.
*
* \param climate Use this dataset with climate data.
* \param date Date for ET0 calculation.
* \param location The location for calculation, latitude and altitude are used.
* \param et0Args Regression coefficients for Penman/Monteith calculation
* \param [in,out] result Result of calculation
*
* \return true on success
*/
public static bool Et0CalcPm(
Climate climate,
DateTime date,
Location location,
Et0PmArgs et0Args,
ref Et0Result result
)
{
var climateSet = climate.getValues(date);
//check availability of values
if (climateSet == null)
{
return(false);
}
if (
!climateSet.max_temp.HasValue ||
!climateSet.min_temp.HasValue ||
!climateSet.humidity.HasValue ||
(!climateSet.Rs.HasValue && !climateSet.sunshine_duration.HasValue)
)
{
return(false);
}
//initialize default values and replace missing ones
if (!climateSet.mean_temp.HasValue)
{
climateSet.mean_temp = (climateSet.min_temp + climateSet.max_temp) / 2;
}
//FAO recommended value default of 2m/s
if (!climateSet.windspeed.HasValue)
{
climateSet.windspeed = 2;
}
//copy altitude from climate, location cannot be null
if (!location.alt.HasValue)
{
if (climate.location.alt.HasValue)
{
location.alt = climate.location.alt;
}
else
{
location.alt = 0;
}
}
var raResult = Ra.RaCalc(date, location);
result.ra = raResult.ra;
if (result.ra == 0)
{
result.et0 = 0;
return(true);
}
var g = 0;
var e0Tmin = 0.6108 * Math.Exp(17.27 * (double)climateSet.min_temp / ((double)climateSet.min_temp + 237.3));
var e0Tmax = 0.6108 * Math.Exp(17.27 * (double)climateSet.max_temp / ((double)climateSet.max_temp + 237.3));
var es = (e0Tmin + e0Tmax) / 2;
var ea = climateSet.humidity / 100 * es;
var n = 24 / Math.PI * raResult.omegaS;
var rs0 = (0.75 + 0.00002 * location.alt) * result.ra;
var rs = climateSet.Rs;
if (rs == null)
{
rs = (et0Args._as + et0Args._bs * climateSet.sunshine_duration / n) * result.ra;
rs = Math.Min((double)rs, (double)rs0);
}
var rsDivRs0 = Math.Min((double)rs / (double)rs0, 1);
var rnl = 4.903e-9 * ((Math.Pow((double)climateSet.max_temp + 273.16, 4) + Math.Pow((double)climateSet.min_temp + 273.16, 4)) / 2) * (0.34 - 0.14 * Math.Sqrt((double)ea)) * (1.35 * rsDivRs0 - 0.35);
var rns = 0.77 * rs;
var rn = rns - rnl;
var p = 101.3 * Math.Pow((293 - 0.0065 * (double)location.alt) / 293, 5.26);
var gamma = 0.000665 * p;
var d = 4098 * (0.6108 * Math.Exp(17.27 * (double)climateSet.mean_temp / ((double)climateSet.mean_temp + 237.3))) / Math.Pow((double)climateSet.mean_temp + 237.2, 2);
var et0x = (0.408 * d * (rn - g) + gamma * (900 / ((double)climateSet.mean_temp + 273)) * (double)climateSet.windspeed * (es - ea)) / (d + gamma * (1 + 0.34 * (double)climateSet.windspeed));
result.et0 = Math.Max(0, (double)et0x);
return(true);
}