The fifth, PM-FAO with missing data of solar radiation and replaced with data estimated by local extraterrestrial radiation
and solar radiation from a nearby meteorological station (Eq.
has constant data for a year as there are no many obstacles such as those present in the earth's atmosphere (Figure 1).
Variables Correlation to [(HDI).sub.D] Mean temperature ([T.sub.D]) 0.520 Sunshine duration ([S.sub.D]) 0.927 Declination angle ([delta]) 0.65 Relative humidity ([Hr.sub.D]) -0.669 Wind speed ([WS.sub.D]) 0.687 Global solar radiation ([G.sub.D]) 0.940 Daylight hours (DH) 0.558 Extraterrestrial radiation
([ER.sub.D]) 0.556 Sunshine fraction ([S.sub.F]) 0.865 Table 5: Best performance of FFNN architectures.
Category (4): Extraterrestrial radiation
and solar declination-based models:
Where PET is the potential evapotranspiration (mm/day), [T.sub.mean] is the daily mean air temperature ([degrees]c), Ra is the Extraterrestrial Radiation
for Daily Periods (mm/day), [E.sub.t] is the saturation vapour pressure (mb), [e.sub.1] is vapor pressure related to the mean minimum temperature in Hottest month of the year (mb), [e.sub.2] is vapor pressure related to the mean maximum temperature in Hottest month of the year (mb), E is the elevation of the site (m).
Hourly global radiation was collected from a CASTNET weather station within the ASP on the Huntington Wildlife Forest (HWF187) and extraterrestrial radiation
(i.e., above the atmosphere) was calculated based on the earth's distance from the sun in a model developed for the Solar Energy Research Institute (Bird and Hulstrom 1991).
on a horizontal Surface.
It is a dimensionless number between 0 and 1.It is defined as the surface radiation divided by the extraterrestrial radiation
. The clearness index has a high value under clear, sunny conditions, and a low value under cloudy conditions.
Hence, the extraterrestrial radiation
, [G.sub.on], measured on the plane normal to the radiation on the nth day of the year is given by
Daily extraterrestrial radiation
on a horizontal surface H0 can be calculated as a function of the solar constant ([I.sub.sc]), the latitude of the site ([lambda]), the eccentricity correction factor of the Earth's orbit ([E.sub.0]), the solar declination ([delta]) and the mean sunrise hour angle ([w.sub.s]); using the equation
The direct radiation is empirical established depending on the extraterrestrial radiation
, the medium solar constant, the day number during a year, the distortion factor, the solar altitude angle, the solar declination, the latitude angle, the solar hour angle, and the local solar time.
where z in the elevation above sea level in meters and [R.sub.a] is the extraterrestrial radiation
in MJ [m.sup.-2] [day.sup.-1] and given by