defaultconstants: Universal constants Required for Calculating...

Description Usage Format Source See Also


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.




A list containing 20 constant values including:
lambda latent heat of evaporisationin = 2.45^-1 at 20 degree Celcius,
sigma Stefan-Boltzmann constant = 4.903*10^-9 MJ.K^-4.m^^-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^-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.


various references

See Also


Evapotranspiration documentation built on May 20, 2017, 4:11 a.m.
Search within the Evapotranspiration package
Search all R packages, documentation and source code

Questions? Problems? Suggestions? Tweet to @rdrrHQ or email at

Please suggest features or report bugs in the GitHub issue tracker.

All documentation is copyright its authors; we didn't write any of that.