What are Tetens coefficients?

Question

Tetens coefficients appear in a few meteorological/climatological models that I've been looking at,for example APSIM:

svp_A = 6.106
svp_B = 17.27
svp_C = 237.3

These same values appear in at least two other models, but I can't find any documentation about what they represent, or where they come from. What are these numbers?

Edit: Oroud, I. M., 2001, Dynamics of evaporation from saline water bodies uses these constants and references Tetens, O. 1930 "Uber einige meteorologische Begriffe." Z. Geophys. 6: 297-309, but that text doesn't appear to be available online, or in my university's library.

Answer 1

The Tetens coefficents are the empirically derived coefficients of an equation for calculating the saturation vapour pressure of water. From wikipedia:

a very good approximation can usually be made using the August-Roche-Magnus formula (usually called the Magnus or Magnus-Tetens approximation, though this attribution is historically inaccurate):

$e_s(T)= 6.1094 \exp \left( \frac{17.625T}{T+243.04} \right)$

e_s(T) is the equilibrium or saturation vapor pressure in hPa as a function of temperature T on the Celsius scale. Since there is only a weak dependence on temperature of the denominator of the exponent, this equation shows that saturation water vapor pressure changes approximately exponentially with T.