Water Cycles - Hydrogeology: Objectives, Methods, Applications - page 27

where:
h: actual duration of insolation;
H: maximum possible duration of insolation (astronomical length of
day), given by tables, in function of latitude and season.
d) Primault's equation (1962)
Applicable over any length of time (month, decade, or day), this equation
takes into account the relative humidity, the elevation, and the season.
PET = c [A [(103 - hr)/100] (ts +2 tp) + B]
Where:
• PET (in mm);
• tp: length of the period under consideration in days (1 day, 10,
30,…);
• hr: relative humidity of the period in %;
• ts: duration of insolation in hours;
• c = -0.5068 sin [(2π/365) j + 0.5593] -0.0711 sin [(4π/365) j + 0.6112] +
0.6271
• where j: number of the day of the year;
• A = -0.12 + 0.00306 h - 2.83·10 -6 h 2 + 9.45·10 -10 h 3
• B = 0.5387 + 0.0003263 h - 6.525·10 -7 h 2
where h: elevation in m.
This equation is well-adapted to regions with high relief (300 to 1 200
m) such as the Jura.
Other equations, such as the Penman or the Penman-Monteith,
include additional parameters tied, for example, to wind or humidity,
for which data is not always available. They are generally used only by
agrometeorologists.
Table 3 Comparison of PET Values in Voinesti (Lower Carpathians, Romania) Calculated by
Different Methods (from Maftei, 2002).
Formula Thornthwaite Turc Penman FAO Penman-Monteith
Year value 639.77 743.87 1118.25 704.11
One notices great variability; Turc's equation and the FAO Penman-Monteith equation seem
to be the most well-adapted.
Figure 9 shows an estimate of PET in the different climate zones across
the planet, calculated according to the Priestley-Taylor method, derived
from Penman's equation.