# defaultconstants

##### Universal constants Required for Calculating Evapotranspriation

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.

- Keywords
- datasets

##### Usage

`constants`

##### Format

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.

##### See Also

*Documentation reproduced from package Evapotranspiration, version 1.10, License: GPL (>= 2)*