86 
PACIFIC SCIENCE, VoL XIII, January 1959 
TABLE 3 
Measured and Calculated Monthly Potential Evapotranspiration at Hong Kong 
FOR 1953 and October through December 1956 (mm.) 
1953 
1956 
Jan. 
Feb. 
Mar. 
Apr. 
May 
Jun. 
Jul. 
Aug. 
Sep. 
Oct. 
Nov. 
Dec. 
Oct. 
Nov. 
Dec. 
Measured 
72 
49 
73 
72 
99 
130 
133 
100 
119 
122 
93 
81 
141 
116 
78 
Thornthwaite 
formula 
35 
33 
53 
59 
122 
165 
175 
168 
138 
135 
73 
45 
115 
58 
38 
Difference 
+ 37 
+ 16 
+ 20 
+ 13 
-23 
-35 
-42 
-68 
-19 
-13 
+ 20 
+36 
+26 
+ 58 
+40 
Penman formula 
64 
46 
65 
76 
106 
140 
160 
144 
114 
104 
69 
59 
125 
85 
57 
Difference 
+8 
+ 3 
+8 
-4 
-7 
-10 
-27 
-44 
+ 5 
+ 18 
+24 
+ 22 
+ 16 
+31 
+ 21 
New formula. . . 
81 
57 
78 
81 
106 
126 
127 
121 
129 
124 
105 
84 
142 
111 
81 
Difference 
-9 
-8 
-5 
-9 
-7 
+4 
+6 
-21 
-10 
-2 
-12 
-3 
-1 
+ 5 
-3 
through August the curves are similar al- 
though between these periods they undergo 
sharp relative displacements. Nevertheless, 
Ea seems to bear some intraseasonal relation- 
ship to measured P.E. This may explain why 
Penman’s formula approximates Hong Kong 
observations more closely than does Thorn- 
thwaite’s formula which does not explicitly 
embody humidity and wind parameters. 
Thornthwaite attaches great importance to 
temperature in the evapotranspiration process. 
This was allowed for ^ combining Ca with Ea 
in the form VEa“b V^a- When this expression 
is plotted against mean daily measured P.E. 
(Fig. 4), a good fit is provided by a straight 
Fig. 4. Mean daily potential evapotranspiration meas- 
ured at King’s Park plotted against VE^ + 
Based on five years’ data. 
line whose equation is 
P.E.= VEad" V^a—3.5 mm/day 
Values calculated from this equation and 
plotted in Figure 2 are in reasonable agree- 
ment with mean monthly measured P.E. 
Testing the Formula 
1953 was the most abnormal year of the 
five-year period. Table 3 and Figure 5 depict 
Fig. 5. Monthly potential evapotranspiration for 
1953 measured at King’s Park (1), and calculated ac- 
cording to Thornthwaite’s formula (2), Penman’s 
formula (3), and the new formula (4). 
