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lib/Drought/PET/Thornthwaite.pm  view on Meta::CPAN

        my $sunset_angle      = acos(-tan($lat_radians)*tan($solar_declination));
        my $daylength         = (24/pi)*$sunset_angle;
        # Calculate alpha term
        my $alpha             = (6.75e-7)*($tei**3) - (7.71e-5)*($tei**2) + 0.01792*$tei + 0.49239;
        # Apply the original Thornthwaite Equation
        my $pet_gross         = 16*((10*$temperature)/($tei))**$alpha;
        return $pet_gross*($daylength/12)*($ndays/30);
    }

    else {
        # Apply the hot temperature formula described in Huang et al. 1996
        return (-415.85 + 32.25*$temperature - 0.43*($temperature**2))*($ndays/30);
    }

}

=head2 tei_thornthwaite

 my $tei = tei_thornthwaite($jan,$feb,$mar,$apr,$may,$jun,$jul,$aug,$sep,$oct,$nov,$dec);

Calculates and returns the temperature efficiency index (TEI, sometimes called the