This data set contains the universal constants required for calculating evapotranspiration in function ET, which should be kept unchanged for most conditions. Please note that additional constants may be ET models - check the manual for individual ET models for details.
data(defaultconstants)A list containing 20 constant values including:
lambda latent heat of evaporisationin = 2.45 MJ.kg^-1 at 20 degree Celcius, sigma Stefan-Boltzmann constant = 4.903*10^-9 MJ.K^-4.m^-2.day^-1, Gsc solar constant = 0.0820 MJ.m^-2.min^-1 Roua mean density of air = 1.2 kg.m^-3 at 20 degree Celcius Ca specific heat of air = 0.001013 MJ.kg^-1.K^-1 G soil heat flux negligible for daily time-step = 0 (Allen et al., 1998, page 68) alphaA Albedo for Class-A pan = 0.14
alphaPT Priestley-Taylor coefficient: = 1.26 for Priestley-Taylor formula (Priestley and Taylor, 1972, Sect. 6; Eichinger et al., 1996, p.163); = 1.31 for Szilagyi-Jozsa formula (Szilagyi and Jozsa, 2008); = 1.28 for Brutsaert-Strickler formula (Brutsaert and Strickler, 1979),
ap constant in Penpan formula = 2.4, b0 constant in Morton's procedure = 1 (Chiew and McMahon, 1991, Table A1), b1 constant in Morton's procedure = 14 W.m^-2 (Chiew and McMahon, 1991, Table A1), b2 constant in Morton's procedure = 1.2 (Chiew and McMahon, 1991, Table A1), e0 constant for Blaney-Criddle formula = 0.81917 (Frevert et al., 1983, Table 1), e1 constant for Blaney-Criddle formula = -0.0040922 (Frevert et al., 1983, Table 1), e2 constant for Blaney-Criddle formula = 1.0705 (Frevert et al., 1983, Table 1), e3 constant for Blaney-Criddle formula = 0.065649 (Frevert et al., 1983, Table 1), e4 constant for Blaney-Criddle formula = -0.0059864 (Frevert et al., 1983, Table 1), e5 constant for Blaney-Criddle formula = -0.0005967 (Frevert et al., 1983, Table 1), epsilonMo Land surface emissivity in Morton's procedure = 0.92, sigmaMo Stefan-Boltzmann constant in Morton's procedure = 5.67e-08 W.m^-2.K^-4.