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Constants Required for Calculating Evapotranspriation
Description
This data set contains the universal constants, and examples of other variable constants required for calculating evapotranspiration in function ET, based on the climatic condition at Kent Town station in Adelaide, Australia.
Usage
data(constants)Format
A list containing 36 constant values including:
- 20 universal constants, which should be kept unchanged for most conditions:
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),
*Note: a re-calibrated value of 13.4 W.m^-2 was recommended to achieve achieve a Priestley-Taylor coefficient of 1.26 (Wang et al., 2009), rather the original value (14 W.m^-2) used by Morton that gave a Priestley-Taylor coefficient of 1.32;
b2 constant in Morton's procedure = 1.2 (Chiew and McMahon, 1991, Table A1),
*Note: a re-calibrated value of 1.13 was recommended to achieve achieve a Priestley-Taylor coefficient of 1.26 (Wang et al., 2009), rather the original value (1.2) used by Morton that gave a Priestley-Taylor coefficient of 1.32;
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.
- 16 variable constants, which are specific for the climatic condition at Kent Town station in Adelaide, Australia:
lat latitude = -34.9211 degrees for Kent Town station,
lat_rad latitude in radians = -0.6095 radians for Kent Town station,
as fraction of extraterrestrial radiation reaching earth on sunless days = 0.23 for Australia (Roderick, 1999, page 181),
bs difference between fracion of extraterrestrial radiation reaching full-sun days and that on sunless days = 0.5 for Australia (Roderick, 1999, page 181),
Elev ground elevation above mean sea level = 48m for Kent Town station,
z height of wind instrument = 10m for Kent Town station,
fz constant in Morton's procedure:
= 28.0 W.m^-2.mbar^-1 for CRAE model for T >= 0 degree Celcius;
*Note: a re-calibrated value of 29.2 W.m^-2.mbar^-1 was recommended to achieve achieve a Priestley-Taylor coefficient of 1.26 (Wang et al., 2009), rather the original value (28.0 W.m^-2.mbar^-1) used by Morton that gave a Priestley-Taylor coefficient of 1.32;
= 28.0*1.15 W.m^-2.mbar^-1 for CRAE model for T < 0 degree Celcius;
= 25.0 W.m^-2.mbar^-1 for CRWE model for T >= 0 degree Celcius;
= 28.75 W.m^-2.mbar^-1 for CRWE model for T < 0 degree Celcius (Morton, 1983a, page65).
a_0 constant for estimating sunshine hours from cloud cover data = 11.9 for Adelaide (Chiew and McMahon, 1991, Table A1),
b_0 constant for estimating sunshine hours from cloud cover data = -0.15 for Adelaide,
c_0 constant for estimating sunshine hours from cloud cover data = -0.25 for Adelaide,
d_0 constant for estimating sunshine hours from cloud cover data = -0.0107 for Adelaide,
gammaps product of Psychrometric constant and atmospheric pressure as sea level:
= 0.66 mbar. degree Celcius^-1 for CRAE model for T >= 0 degree Celcius;
= 0.66/1.15 mbar. degree Celcius^-1 for CRAE model for T < 0 degree Celcius.
PA annual precipitation = 285.8mm for Kent Town station,
alphaMo constant in Morton's procedure:
= 17.27 when T >= 0 degree Celcius;
= 21.88 when T < 0 degree Celcius.
betaMo constant in Morton's procedure:
= 237.3 degree Celcius when T >= 0 degree Celcius;
= 265.5 degree Celcius when T < 0 degree Celcius.
lambdaMo latent heat of vaporisation in Morton's procedure:
= 28.5W.day.kg^-1 when T >= 0 degree Celcius;
= 28.5*1.15W.day.kg^-1 when T < 0 degree Celcius.
References
McMahon, T., Peel, M., Lowe, L., Srikanthan, R. & McVicar, T. 2012. Estimating actual, potential, reference crop and pan evaporation using standard meteorological data: a pragmatic synthesis. Hydrology and Earth System Sciences Discussions, 9, 11829-11910.
Allen, R. G., Pereira, L. S., Raes, D. & Smith, M. 1998. Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage. paper 56. FAO, Rome, 300, 6541.
Szilagyi, J., & Jozsa, J. 2008. New findings about the complementary relationship-based evaporation estimation methods. Journal of Hydrology, 354(1-4), 171-186.
Brutsaert, W., & Stricker, H. 1979. An advection-aridity approach to estimate actual regional evapotranspiration. Water Resources Research, 15(2), 443-450.
Chiew, F. H. S., & McMahon, T. A. 1991. The applicability of Morton's and Penman's evapotranspiration estimates in rainfall-runoff modelling. JAWRA Journal of the American Water Resources Association, 27(4), 611-620.
Frevert, D.K., Hill, R.W.Braaten, B.C. 1983, Estimation of FAO evapotranspiration coefficients, Journal of Irrigation and Drainage Engineering, vol. 109, no. 2, pp. 265-270.
Roderick, M. L. 1999. Estimating the diffuse component from daily and monthly measurements of global radiation. Agricultural and Forest Meteorology, 95(3), 169-185.
Wang, Q. J., McConachy, F. L. N., Chiew, F. H. S., James, R., de Hoedt, G. C., & Wright, W. J. 2009. Maps of Evapotranspiration. Retrieved from Melbourne, Australia: http://www.bom.gov.au/climate/averages/climatology/evapotrans/text/et-description.pdf
Morton, F. I. 1983. Operational estimates of areal evapotranspiration and their significance to the science and practice of hydrology. Journal of Hydrology, 66(1-4), 1-76. doi:http://dx.doi.org/10.1016/0022-1694(83)90177-4