Evapotf anspiration in Hong Kong — Ramage 
absence of an atmosphere, converted 
to an evaporation equivalent by put- 
ting 59 cal/cm^^l mm. evaporation. 
r = reflection coefficient, to be taken as 
0.05. For standard evaporation tanks 
with light walls and clean water it may 
be very much greater. 
n/N = actual/possible hours of sunshine. In 
the first term in H it is a factor limiting 
incoming short-wave radiation; in the 
second it is a transform of a cloudiness 
factor limiting outward long-wave 
radiation. 
o-Ta^ = theoretical black-body radiation at 
mean air temperature Ta; this too is in 
evaporation units like Ra. 
Cd — saturation vapor pressure at dewpoint 
(mm Hg). 
ea = saturation vapor pressure at mean air 
temperature; hence (Ca— Cd) is the 
mean saturation deficit. 
U 2 == average wind speed in miles/day at 
2 m. above the ground. 
Fig. 2. Monthly means of potential evapotranspira- 
tion measured at King’s Park (1), and calculated ac- 
cording to Thornthwaite’s formula (2), Penman’s 
formula (3), and the new formula (4). Based on five 
years’ data. 
85 
Since the formula incorporates both hu- 
midity and wind parameters it appeared 
likely to give more reasonable results than 
Thornthwaite’s formula when applied in 
Hong Kong. The data listed in Table 1 have 
been used to calculate mean monthly P.E. 
according to Penman’s formula and the val- 
ues are plotted in Figure 2. They approach 
the observed values more closely than those 
determined by Thornthwaite’s formula but 
err in the same sense, overestimating summer 
and underestimating winter P.E. 
POTENTIAL EVAPOTRANSPIRATION 
FORMULA FOR HONG KONG 
Deriving the Formula 
In Penman’s formula, the expression 
Ea = 0.35 (Ca-ed) ( 1 +U 2 X 10 “ 2 ) mm/day 
incorporates humidity and wind parameters. 
Eigure 3 shows a plot of both Ea and mean 
daily observed P.E. at Hong Kong derived 
from the monthly means for five years. Erom 
October through January and from February 
(mm) 
Fig. 3. Mean daily potential evapotranspiration 
measured at King’s Park (1), and Ea (2). Based on five 
years’ data. 
